Mike's Weather Blog -- April 2009

Chief Meteorologist Explains Colorado's April Weather Events

Springtime weather can be a wild ride in Colorado! If you would like to learn much more about our often wild and crazy spring weather, check out a copy of my book, THE COLORADO WEATHER ALMANAC. The book is available at all local bookstores, or you can order it on-line from this website MIKE'S COLORADO WEATHER ALMANAC!

With the recent storm, I have had a number of questions about the weight of snow. The main thing that meteorologists look at to determine how much snow will fall is the ration between inches of snow and the actual amount of liquid water. The higher the ratio, the lighter and fluffier the snow. The typical ratio is a 10:1 - meaning that 10 inches of snow is produced from 1 inch of liquid precipitation. The "champagne powder" that our ski resorts like to brag about is sometimes a 20:1 ratio - really dry and fluffy! In the Midwest and the East, the warm, wet snowstorms can bring a slushy 6:1 ratio - that is the stuff that really gives shovelers a sore back.

In the really big storm of March 18-20th of 2003, the official Denver total for snow (measured by the National Weather Service at a site near the old Stapleton Airport) was 31.8 inches of snow. Many other spots around the city had much more - up to nearly double that total in the foothills between 6500 and 8500 feet. The melted total for the storm was 2.80" for Denver's official total - so a little closer to an 11:1 ratio from this storm.

An inch of rain on an acre of land equals 27,000 gallons of water. The storm total of 2.80 inches means that the 31.8" of snow equaled 76,500 gallons of water on an acre. Since a gallon of water weighs about 8.33 lbs, that snowfall would weigh approximately 638,000 pounds per acre. If you convert that over to tons, the total weight of the snow was about 320 tons per acre. That is for about 32 inches of snow, so for an average 10:1 or 11:1 snow/water ratio, the snow weighs about 10 tons per inch per acre. That might explain why your back hurts so much from shoveling and why all those roof collapses occurred, as well as the tree damage.

When storms set up just right to dump on Denver and the Front Range, they need to be in southeastern Colorado. A low pressure area stalled over southeastern Colorado will swirl moisture into the high plains all the way from the Gulf of Mexico! This type of system acts as a funnel to bring all the moist air our way, press it up against the mountains and create a literal "snow machine".

The biggest storm in Denver history, is still held by an amazing storm back in December, 1913. That system was similar to this past storm (a slow moving low over southeastern Colorado, but it stalled for an extra 24 hours and as such, dumped even more snow on Denver and the Front Range. Denver tallied 46 inches from that storm system, while Georgetown had about 80 inches of snow!

The biggest storm in a 24 hour period on record for the USA was in April 1921 at a small locale in Boulder County called Silver Lake (it no longer is found on most maps). They picked up over 76 inches of snow in just 24 hours!

The greatest annual snowfall on record for the USA is from the Paradise Ranger Station in the Cascades of Washington state. In 1955-56, they tallied an amazing 1000 inches of snow!

Here is some information about winter weather and the storms that can affect Colorado. I hope that you find it interesting!

Of all the seasons of the year, winter is the one that most people around the country would identify with Colorado. Our high elevation means that it is winter for most of the year on our mountain peaks, with the snow-free time lasting only a few weeks.

The winter season often begins well ahead of the calendar, as the first snows in Denver often come as early as mid-September. Newscasts from around the nation show us digging out of the deep drifts well before Halloween. Yet the big secret that we keep is that we are not left snowbound for months by these big storms. Instead, the rest of the country stops paying attention to us while the snow quickly melts, and we are out playing golf again by the end of the week!

Colorado serves as a breeding ground for some of the most powerful storm systems that bomb the rest of the central and eastern United States with heavy snow. Low pressure storm systems frequently develop just to the east or to the "lee" of the Rockies. Strong winds aloft at the jet stream level (about 25,000 to 30,000 feet) squeeze over the mountain peaks and then out over the plains. As the air moves out away from the mountains, the pressure falls, creating a low pressure area. If conditions are right, this low will begin to swirl moisture into the region from the Gulf of Mexico. This moisture spins into clouds, rain and snow, and before long, a storm is born. These storm systems first dump heavy snow over eastern Colorado and then churn toward the northeast. Heavy, wet snow, sleet, freezing rain and even thunderstorms mark the path of these storms as they roll on toward the Great Lakes. The soggy systems that spin out of eastern Colorado can give a false impression to the rest of the nation. Those eastern storms often dump heavy snow on Denver, Colorado Springs, Limon or other cities on the plains, but leave the mountains with a much smaller accumulation. Television weather reports in other cities show the snow in Denver and the folks in those cities think, "Wow, if Denver got a foot of snow, the mountains must have really been hammered!" In fact, eastern storms tend to be blocked off by the mountains of the Front Range, and the ski areas may have only scattered clouds while Denver gets the big snow. This was the case back in October 1984, when the Broncos played the Packers in a blizzard at Mile High Stadium. The national television audience saw all that snow in Denver and inundated the ski resorts with calls for reservations. Being good businesspeople, ski resort personnel didn't mention the fact that at the time, skies were clear in Vail and Aspen!

An old saying about Colorado weather is very helpful when trying to figure out if a storm system will bring heavy snow. Remember - Pacific front, mountains bear the brunt; southeast low, Denver gets the snow. As I mentioned earlier, storm systems on the eastern plains spin their heavy snow over the Front Range and adjacent plains, but may miss the mountains. In contrast, a moist storm front from the Pacific Coast may dump very heavy snow on the mountains, but have very little moisture remaining by the time the storm slips down into Denver. Those Pacific fronts have a rough time staying intact as they first hit the Cascades and Sierra Nevada Mountains, then have to cross the Wasatch Range in Utah and finally struggle over the Continental Divide. When the once mighty storm finally tops the last of three or four mountain ranges, moisture is so depleted that Denver doesn't even get a flake of snow! Sometimes these storms regenerate into a major system again once they get farther to the east of Colorado. The leftover circulation of the storm will start to tap into the rich supply of moisture available from the Gulf of Mexico and the storm will once again become a powerful precipitation producer! Our neighbors in Oklahoma, Kansas, Nebraska and points farther east get a messy winter storm - but Denver and adjacent cities in the I-25 corridor have just some clouds and gusty north winds on the backside of the storm. These storms are frustrating to forecast as they usually feature headline making weather from the West Coast, with huge waves, heavy rain, wind and mountain snow. The storm takes a couple of days to get here, so there is plenty of build up and anticipation. First our mountains get heavy snow, but then the storm seems to jump right over Denver, before bringing nasty winter weather to extreme eastern Colorado and most of the Midwest. Such a storm scenario makes it seem like Denver has a big clear plastic dome over us!

The winter weathercasts are sometimes filled with the term "blizzard", which is a derivative of the German word for lightning. Early European settlers on the high plains were astounded by the lightning-fast changes in the weather, especially when a powerful cold front roared through.

The true definition of a blizzard has more to do with wind and visibility than with snow - a storm earns blizzard status when winds of thirty-five miles per hour or higher are accompanied by falling or blowing snow that drops visibility to a quarter mile or less. The sky above can be clear, but blowing snow at ground level can produce blizzard conditions. A severe blizzard is bad - winds over fifty miles per hour with visibility near zero due to blowing or falling snow. Certainly our pioneer ancestors saw some very scary winter storms, but even today strong winter storms can bring peril to travelers. The wide-open eastern plains of Colorado offer no resistance to the wind and can allow howling storms to swirl snow into drifts five to ten feet high during the worst blizzards.

The passage of a strong Canadian cold front is a major weather story on the eastern plains, but often not for the mountains. The wall of granite to the west of Denver serves as a pretty good barrier for the cold, dense air. Quite literally, the cold air is just too heavy to climb up and over the mountains. Many times a surge of Arctic air will blanket the eastern plains of the state and put us in the deep freeze, while in the mountains, temperatures stay much milder as the cold front stalls in the foothills west of Denver.

The very soggy spring storm system currently moving over Colorado with rain, thunderstorms and heavy snow. This system originally came in from the Pacific Ocean, bringing with it ample amounts of moisture. The system is now tapping the Gulf of Mexico for an added boost. It has taken up residence over southeastern Colorado and northeastern New Mexico and will stay for the next 12 hours, allowing for it to draw up more moisture from the Gulf. As this storm continues to swirl to our south, it will still be able to produce prodigious precipitation. There is about 2 to 3 inches of what we call "precipitable water" within this storm. What that means is the amount of water vapor available to leave the storm in the form of rain, snow, hail, sleet, etc. The amount of anticipated snow accumulation is the tough call at this point, as it depends on temperatures dropping into the low to mid 30s to make the switch from rain to snow.

The average water to snow ratio is 1:10, meaning that one inch of rain could potentially produce ten inches of snow as it "fluffs out" into snowflake crystals. Sometimes in the spring season, the warmer temperatures can drop that ratio to 8:1 or even 6:1 - that is really wet, slushy stuff.

Therefore, temperature is crucial in analyzing what mother nature has in store for Denver. Temperatures were in the 40s at DIA this afternoon, while colder conditions in the mid 30s held a bit farther to the west. The amount of snow that accumulates will depend on the amount of rain...more rain equates to less snow.

At this point, we are anticipating another 4-6" inches of snow in Denver by Saturday morning. This could be more if the storm lingers over the area for longer than the models are currently showing, or if temperatures drop a little bit later this afternoon. Be sure to stay tuned to the 24/7 Weather Center for the most recent and up-to-date forecast as these storms are ever-changing. This volatile time of year is notorious for the twists and turns these storm systems tend to throw at us!

The mountains along and east of the Divide remain under Winter Storm Warnings through early Saturday with the expectation of 12-24" of snow making it a great weekend up at the ski resorts that remain open! On the plains, there will be plenty of energy to stir up some severe weather near the border of Kansas. The far eastern plains and western Kansas may be looking at their first significant severe weather outbreak of the spring. Our stormchasers will be "on the chase" for the next few afternoons. if you would like to follow their stories click on our Stormchaser Blogs STORM CHASERS!

We will enjoy another mild and breezy day today, with just a chance for a few afternoon showers and thunderstorms. In the mountains, the weather will begin to turn this afternoon as rain and snow increases thanks to an approaching storm. The storm is a pretty warm one, keeping the snow level above 10,000 feet.

The storm is also a slow mover, so it will take time for the system to swirl across Colorado over the next few days. The best chance for rain in the Denver area will be late Thursday into Friday. By Friday, enough chilly air will work into northern Colorado to bring Denver and the foothills a chance for snow. It will likely be pretty wet snow, so much of it will melt as it hits in the metro area. The foothills above 7,000 feet could be looking at several inches of snow.

Farther east, on the plains, there will be plenty of energy to stir up some severe weather Thursday and Friday. The far eastern plains may be looking at their first significant severe weather outbreak of the spring. Our stormchasers will be "on the chase" for the next few afternoons. If you would like to follow their stories click on our Stormchaser Blogs STORM CHASERS!

Springtime weather can be a wild ride in Colorado! If you would like to learn much more about our often wild and crazy spring weather, check out a copy of my book, THE COLORADO WEATHER ALMANAC. The book is available at all local bookstores, or you can order it on-line from this website MIKE'S COLORADO WEATHER ALMANAC!

We can expect a windy, warm and pleasant day today, but the mild weather will not last much longer. Another storm will head toward Colorado starting Wednesday. The mountains and western counties will begin to get a few showers later this afternoon, but these will be pretty widely scattered. By Wednesday afternoon, the moisture will really begin to flow into the western half of the state with snow over the higher elevations. The eastern plains should stay warm and windy on Wednesday, with scattered showers and thunderstorms possible late in the day. Right now, it still appears that Thursday and Friday will be cool and wet, with heavy snow likely in the mountains and rain on the plains. The storm will be a fairly slow moving affair, so it may pull in enough cold air to bring rain and snow to Denver and across the eastern plains, but this storm is pretty late in the spring, so much of the snow will melt as it hits the ground.

The soggy storm system that dumped a foot of snow on the mountains and up to an inch of rain on the southeast plains has moved out of Colorado. Sunny skies will be the rule for Monday and Tuesday along with mild temperatures. The moisture was very handy for farmers and ranchers on the eastern plains, adding beneficial water to the topsoil as we get closer to spring planting. In the high country, the ski areas have mostly closed for the season, with the exception of the few that stay on into May, such as Loveland and A-Basin. Nonetheless, the recent storm has helped keep the snowpack nice and high in the mountains and that bodes well for our spring and summer water supplies. Here is a link to see the latest snowpack numbers in the state. STATE SNOWPACK.

The weather will stay quiet and pleasant for the next 48 hours, but we will be watching the next storm system carefully for later this week. This next storm will move into the southwestern US and spin toward Colorado starting Wednesday. The mountains and western counties will begin to get showers and some thunderstorms Wednesday afternoon, with snow over the higher elevations. The eastern plains should stay warm and windy on Wednesday, with scattered showers and thunderstorms possible late in the day. Thursday and Friday will be cool and wet, with heavy snow likely in the mountains and rain and snow on the plains. This late in the spring, we tend to get more rain than snow in Denver and across the eastern plains, but this storm may pull enough cold air into it to bring us snow Thursday night and Friday.

It has been a tough spring on forecasters - so I will reserve judgement on the snow potential for a couple of days, but the potential is there. If you have not had a chance to get your lawn fertilized, the next two days might be a great time to do so. The wet weather later this week will really help to soak into the ground and green things up.

It is the time of year when the weather can turn on a dime. If you would like to learn much more about our often wild and crazy spring weather, check out a copy of my book, THE COLORADO WEATHER ALMANAC. The book is available at all local bookstores, or you can order it on-line from this website MIKE'S COLORADO WEATHER ALMANAC!

I often speak of "upslopes" and "downslopes" during my weather reports on 7News, but what exactly do these terms mean?

Much of the weather in Colorado is determined by the topography. Our spine of tall mountains that divide the state can create all kinds of havoc with our weather.

The winds that blow across Colorado cannot go under the mountains or through them, so the air must move over the mountains. As the winds blow against a mountain, the air is forced to rise up and over the peak. This rising air is moving "up" the slope of that mountain. When air rises, it cools at a rate of about 5.5 degrees Fahrenheit for each 1,000 feet that it goes up. This temperature change is called the "dry adiabatic lapse rate". The cooler air cannot "hold" as much water vapor as warm air, so the vapor condenses out to form clouds. If the air continues to rise up the mountain, more and more moisture will condense and the cloud will thicken and eventually produce precipitation.

To put this into an example, let’s take a situation where a westerly wind is pushing into western Colorado; the air will be forced to rise up as it travels into the mountains. This rising air will cool, the moisture will condense out, clouds will form and rain or snow will develop over the west facing slope of the mountain.

A "downslope" is just the opposite, as the air reaches the crest of the mountain and begins to move downhill, it will begin to warm up, due to the air being compressed as it descends (the molecules of air are being pushed closer together as the air moves down into the thicker atmosphere at lower elevations). This compression warms the air; warmer air can "hold" more moisture than cold air so the humidity drops and the clouds evaporate. Thus a downsloping wind will generally bring clearing skies and warmer temperatures.

The same weather pattern that can bring snow to the western side of a mountain can bring dry and mild weather to the opposite side of the mountain. That is why it is sometimes snowing on the west side of the Eisenhower Tunnel, but not on the east side. In fact, it can be sunny, breezy and warm in Denver with a strong westerly wind, while the mountains west of the Divide are getting hammered with snow.

In Denver, our "upslope" winds are from the east. An easterly wind will have to travel "up the hill" from the 4,000 foot elevations near the Kansas border, to our Mile High elevation here in Denver. Easterly winds will bring an increase in clouds and often a chance for rain or snow to the Denver area, especially since that air moves west of Denver and backs up against the mountains, gradually forming the clouds and precipitation over the foothills and then out across the eastern plains. In this instance, the upslope winds will often cause rain or snow here, but dry weather will hold west of the Continental Divide (remember, they would have an easterly "downsloping" wind there as the air blows down from the Divide).

It may sound a little confusing, but that is why Colorado weather is such a challenge to predict! Just remember, "upslopes" tend to bring clouds and precipitation - "downslopes" tend to bring windy, dry and mild weather.

A good source for general weather information is the National Weather Service homepage. You may also wish to check out the homepages for the Colorado Climate Center and the High Plains Climate Center. For long term outlooks, try the Climate Prediction Center's homepage. The various addresses are listed as follows...

www.crh.noaa.gov/den/

http://climate.atmos.colostate.edu/

www.cpc.ncep.noaa.gov/

You may also wish to check out the information in my book - THE COLORADO WEATHER ALMANAC. The book is available at all local bookstores, or you can order it on-line from this website MIKE'S COLORADO WEATHER ALMANAC!

We certainly did enjoy a warm and pleasant day yesterday, with highs in the mid 70s in Denver. A weak storm system is now moving across Colorado, with snow likely in the mountains and a chance for showers, mixed with a little snow on the plains. Today will be quite a bit colder, with highs in the 30s in the mountains and low 50s on the plains.

By tomorrow, opening day for the Rockies at Coors Field, the weather will be dry and mild again, with highs near 60 degrees. Another weak storm may impact the state on Easter Sunday, with some snow in the mountains. At this point, the Sunday storm does not look to be much of a concern for eastern Colorado - just a few showers, with highs in the 50s. The weather for sunrise on Sunday will be mostly cloudy with temperatures in the mid 30s.

Springtime in the Rockies can mean snow, cold, warmth and sunshine, thunderstorms, tornadoes and hail - sometimes all in the same afternoon!! The early spring is often the most turbulent time of year in Colorado as the retreating chill of winter does battle with the ever increasing warmth of spring. The temperature contrasts from north to south can create and tremendous amount of instability in our atmosphere and the result is often a very wild ride!

The heaviest snows of the year for the Front Range usually occur in late March through early May. Some of the really big snowstorms can dump 2-3 feet of soggy snow on the foothills west of Denver. Severe thunderstorms make their first appearances on the eastern plains by late April and really get going over the next 4-5 weeks. The peak for severe thunderstorms, tornadoes and hail in Denver and over eastern Colorado comes in late May and early June.

Gardeners have learned (sometimes the hard way) to ignore the temptation of a warm day in April or early May, even if the highs soar into the 80s, DO NOT PLANT that garden!! The most experienced gardeners will all tell you to wait until after Mother's Day. In fact, just a few years ago, we had 6 inches of snow on the 20th of May - tough on the tomatoes!

The much advertised storm system that spun across Colorado on Saturday may have been a dud for Denver in terms of snowfall, but it still packed a punch for most of the state. Heavy snow fell in the mountains, with one to two feet of powder falling on the peaks. On the plains of northeastern Colorado, snowfall ranged from two to six inches, with strong winds of 60 mph creating whiteout conditions for much of the afternoon.

The storm rolled out of the mountains and onto the eastern plains early on Saturday as expected, but at mid-levels of the atmosphere (about 20,000 feet), a tongue of drier air swirled into the system and moved across the Denver area. This effectively cut off the precipitation over Denver for much of the morning and early afternoon. As a result, snow totals were indeed quite meager over the metro area. The foothills and the northeast plains did get their share of snow - check this link for a listing of snow totals from the National Weather Service. SNOW TOTALS!

For most folks in the Denver area, the storm must have seemed like a poorly forecast event, and indeed it was in terms of the number one feature - the snow! However, the system was a massive one, shutting down southeastern Wyoming, most of Nebraska, northwest Kansas and South Dakota with a severe blizzard. Had the drier not moved over Denver early Saturday, we would have had several inches of snow and that would have been blown around by the 30-50 mph winds that raked the region through the afternoon. The result would have been similar to conditions seen just to our north and east.

As it was, the strong winds did cause numerous power outages due to the power lines bouncing from the force of the wind. There had been enough freezing drizzle to coat the power lines with a thin layer of ice. This icy cover acts like a wing on the power line and that makes the wire fly around in the wind. I used to forecast for the power utilities and they have a term for this - "galloping conductors". When the lines dance about in the wind, they can come close enough together for the resistance of the air to be broken and the electricity arcs from one line to another, causing them to short out. The lines have a circuit breaker, so they are not damaged, but the power utility needs to reset each breaker. Usually these outages are brief, because the wires are not broken and the poles are not down, but they can still range from minutes to several hours - depending upon whether the breaker can be reset manually or needs to be reset on site.

For the folks who were either simply disappointed in not getting 4 to 8 inches of snow or to those who truly had a major change in plans due to the expected snowfall, I am very sorry that the forecast was not more accurate. None of us in the business like to be wrong, but sometimes these storms are pretty complex and the forecast does not pan out. This storm was a major system, but it moved just far enough to the northeast that the big snow did not materialize for the Denver area. Had the storm tracked 100 mile farther to the south, we would have been pretty paralyzed by it's fury. Close counts in horseshoes, but not in snowfall prediction, so we missed this one.

Frankly, the past several storms have been humbling for weather forecasters in the area. Several of these storms have either brought more snow than expected or just the opposite. They seem to zig, when we think they will zag. That is the nature of forecasting just east of a 14,000 foot wall of granite. The mountains take the lion's share of the moisture from approaching storms and the plains just to the east need special circumstances to get a big snow. If things do not set up properly, the storm passes us by. If things come together, heavy snow can fall. No doubt, Mother Nature has had some fun with local forecasters in the past several weeks.

I think we can all agree it has been an interesting winter season for Colorado. We had spring-like weather in February and most of March. Now that it is officially spring, we have had a couple of winter-like storms. What is going on? Well, if you've lived in Colorado for a while, you know how unusual our weather can be. Sometimes it's more than just the seasons that determine temperature and precipitation patterns for Colorado. Most 'weather' can be attributed to global circulation patterns, terrain and local wind flow. For most of the winter season, our weather has been dominated by a large scale west-to-east wind flow in the mid-levels of the atmosphere. This usually means dry weather for areas east of the mountains.

As often happens with the changing seasons, the global air flow has recently become a bit more favorable for precipitation and storm systems across Colorado. However, citing this weekend's storm as an example, global patterns don't always do the trick. The amount of precipitation we receive greatly depends on the exact track of low-pressure systems, and the configuration of the storm itself. Occasionally, all we receive is strong wind. This is due to the local surface winds generated by the storm. Northeast winds usually mean heavier precipitation, while northwest winds can mean, well, just wind! With this weekend's storm, most of the heavy snow fell to the north and east of Denver, while we had wind gusts in excess of 60 mph at DIA. Had this low pressure tracked a few hundred miles to the south, it would have been a different story, with our snow shovels getting a big workout.

The upcoming week should be a little more settled over the region. There will be a storm coming through midweek, but it looks to be rather mild, so only a light amount of rain and snow can be expected about Wednesday. Temperatures are going to moderate, so April will begin to feel more like April and less like February. Springtime is often fast changing and stormy though, so I am sure that this respite will be fairly brief, and we will get some more forecast challenges before too long.!

Thunder rumbled over the Denver Metro Area Wednesday evening as a strong storm system dropped heavy snow over the western suburbs. We may be under some thunder again late today as another intense storm moves into the area.

It is not especially common to get thunder and lightning during the winter months, but it does happen periodically. The winter air is usually too cold and heavy to be lifted high into the sky to produce thunderstorm clouds, but under certain conditions, there is enough instability in the air to form some small, but intense snow squalls that can produce thunder and lightning. When these squalls develop, they are similar to summer thunderstorms in that they can drop very heavy snow over local areas in just an hour or two. The snow squalls typically last an hour or so before moving on to the east, much like a thunderstorm. When we get a really big storm, like the one in March of 2003, sometimes the thunder snow will last several hours and dump a foot or more of snow.

It does seem that an awful lot of our snow tends to occur at night, especially in Denver and along the Front Range foothills (elevations from 6,000 to 9,000 feet).

I have had folks ask "why 75% of snow falls at night" - that may be an overstatement, but I put the matter to Nolan Doesken our State Climatologist at CSU. Nolan has received this question often, and one of the graduate students at CSU actually has done a research paper on this matter. Some of this is a little technical, but I thought that I would throw it in anyway!

Lew Grant is a meteorology professor who has directed a study of this effect in western Colorado. The study attempts to explain the differences in observed snowfall between day and night. There is a possibility of enhanced cloud growth after sunset and overnight due to cooling from cloud tops. This is known to create upward motion as the cloud material below the top can then become moist-convectively unstable. The resultant turbulence and cloud height increase remotes precipitation growth.

What we learned was that some areas get a higher percentage of nighttime snow than others -- but it is not as high as 75%, maybe more like 60% if you really divide the day into 12-hour halves. If, however, you count the hours from about 4:30 PM to 7:30 AM -- which seem in mid-winter to be "night" then it is close to 75% of the precipitation -- but that's in 63% of the hours of the day.

As for causes, it is likely a combination of radiational cooling, and mountain drainage wind patterns that raise relative humidity and enhance condensation. It is most apparent from the little data we have in Nov-Feb. From late Feb on, then convective processes kick in and begin to favor the afternoon and early evening hours for the greatest snowfall.

Superimposed on this, is the fact that snow that falls during the day accumulates less (melting, settling, drifting) than snow that falls at night, so we often have more snow to see when we get up in the AM than when we come home at night. We've measured 7 AM and 7 PM 12-hour precipitation totals and 12-hour snowfall totals at CSU for 114 years and we see about the same amount of water content in the 2 periods in the mid winter months, but we see more nighttime snowfall -- probably just means more daytime melting and lower density snows at night.

Another definite possibility is that a significant amount of our snow, especially in spring, is related to either upright or slantwise convection - this is enhanced rising motion in the atmosphere. Our Convective Available Potential Energy (CAPE) values related to upright are probably maximized in mid-afternoon, but it takes a while for convection to get organized and produce widespread snow, in some cases a few hours. CAPE is a measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Not unlike a summer time MCS (large clusters of thunderstorms) in my opinion but lacking a squall line (typically) of course. So I think maximum snow rates would lag the maximum CAPE time by at least an hour...putting the maximum rates at sundown or later.

There is probably another factor related to how synoptic waves and fronts move related to the diurnal cycle. Certainly Canadian fronts move southward much faster at night, thus potentially creating larger areas of upslope at night. However I can't think of a good explanation of how waves relate to the sun, perhaps they slow down during late afternoon or evening due to convective processes playing a bigger role at that time, thus potentially dumping more snow in favored areas, winter convection typically doesn't move very fast. This latter point is really just hand-waving, but it makes for interesting discussions.

I have compiled a variety of information about snow and snowstorms; I hope that you will find it useful and interesting...

How big can snowflakes get?

Snowflakes are agglomerates of many snow crystals. Most snowflakes are less than one-half inch across. Under certain conditions, usually requiring near-freezing temperatures, light winds, and unstable, convective atmospheric conditions, much larger and irregular flakes close to two inches across in the longest dimension can form. No routine measure of snowflake dimensions are taken, so the exact answer is not known.

Why is snow white?

Visible sunlight is white. Most natural materials absorb some sunlight which gives them their color. Snow, however, reflects most of the sunlight. The complex structure of snow crystals results in countless tiny surfaces from which visible light is efficiently reflected. What little sunlight is absorbed by snow is absorbed uniformly over the wavelengths of visible light thus giving snow its white appearance.

What causes the blue color that sometimes appears in snow and ice?

Generally, snow and ice present us with a uniformly white face. This is because most all of the visible light striking the snow or ice surface is reflected back without any particular preference for a single color within the visible spectrum. The situation is different for that portion of the light which is not reflected but penetrates or is transmitted into the snow. As this light travels into the snow or ice, the ice grains scatter a large amount of light. If the light is to travel over any distance it must survive many such scattering events, that is, it must keep scattering and not be absorbed. The observer sees the light coming back from the near surface layers (mm to cm) after it has been scattered or bounced off other snow grains only a few times and it still appears white. However, the absorption is preferential. More red light is absorbed compared to blue. Not much more, but enough that over a considerable distance, say a meter or more, photons emerging from the snow layer tend to be made up of more blue light and red light. Typical examples are poking a hole in the snow and looking down into the hole to see blue light or the blue color associated with the depths of crevasses in glaciers. In each case the blue light is the product of a relatively long travel path through the snow or ice. So the spectral selection is related to absorption, and not reflection as is sometimes thought. In simplest of terms, think of the ice or snow layer as a filter. If it is only a centimeter thick, all the light makes it through, but if it is a meter thick, mostly blue light makes it through.

Is it ever too cold to snow?

No, it can snow even at incredibly cold temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur with relatively warm air temperatures near the ground - typically 15F or warmer since air can hold more water vapor at warmer temperatures.

More About Blue Snow and Ice...

It is a common misconception that the blue color exhibited by glaciers, old sea ice, or even holes poked into a snow bank is due to the same phenomenon that makes the sky blue-light scattering. But nature has more than one recipe for producing the color blue. In frozen water and in the sky the processes are almost the reverse of each other.

A blue sky results when light bounces off molecules and small dust particles in the atmosphere. Because blue light scatters more than red does, the sky looks blue except in the direction of the sun (particularly when the sun is near the horizon and the blue light is scattered out of the sunlight, leaving the red color of sunrises and sunsets).

When light passes through ice, however, the red light is absorbed while the blue is transmitted. Were the operating process scattering as in the atmosphere, then the transmitted light would be red, not blue. However, because of the large size of snow grains and ice crystals, all wavelengths of visible light are scattered equally. Scattering therefore does not play an appreciable role in determining the color of the transmitted light.

It takes an appreciable thickness of pure ice to absorb enough red light so that only the blue is transmitted. You can see the effect in snow at fairly shallow depths because the light is bounced around repeatedly between ice grains, losing a little red at each bounce. You can even see a gradation of color within a hole poked in clean, deep snow. Near the opening, the transmitted light will be yellowish. As the depth increases, the color will pass through yellowish-green, greenish-blue and finally vivid blue. If the hole is deep enough, the color and light disappear completely when all the light is absorbed.

The color of ice can be used to estimate its strength and even how long it has been frozen. Arctic Ocean ice is white during its first year because it is full of bubbles. Light will travel only a short distance before it is scattered by the bubbles and reflected back out. As a result, little absorption occurs, and the light leaves with the same color it had when it went in.

During the summer, the ice surface melts and new overlying ice layers compress the remaining air bubbles. Now, any light that enters travels a longer distance within the ice before it emerges. This gives red end of the spectrum space enough to be absorbed, and the light returned at the surface is blue.

Arctic explorers and mountain climbers know that old, blue ice with fewer bubbles is safer and stronger than white ice. An added bonus for explorers is knowing that floating camps built on blue ice will last longer.

The storm during the week of March 17th, 2003 was one of the biggest in Denver history, but still came up short of setting the all-time record for the city for a single storm. That title is still held by an amazing storm back in December, 1913. That system was similar to this past storm (a slow moving low over southeastern Colorado, but it stalled for an extra 24 hours and as such, dumped even more snow on Denver and the Front Range. Denver tallied 46 inches from that storm system, while Georgetown had about 80 inches of snow!

The biggest storm in a 24 hour period on record for the USA was in April 1921 at a small locale in Boulder County called Silver Lake (it no longer is found on most maps). They picked up over 76 inches of snow in just 24 hours!

The greatest annual snowfall on record for the USA is from the Paradise Ranger Station in the Cascades of Washington state. In 1955-56, they tallied an amazing 1000 inches of snow!

Here is some information about winter weather and the storms that can affect Colorado. I hope that you find it interesting!

Of all the seasons of the year, winter is the one that most people around the country would identify with Colorado. Our high elevation means that it is winter for most of the year on our mountain peaks, with the snow-free time lasting only a few weeks.

The winter season often begins well ahead of the calendar, as the first snows in Denver often come as early as mid-September. Newscasts from around the nation show us digging out of the deep drifts well before Halloween. Yet the big secret that we keep is that we are not left snowbound for months by these big storms. Instead, the rest of the country stops paying attention to us while the snow quickly melts, and we are out playing golf again by the end of the week!

Colorado serves as a breeding ground for some of the most powerful storm systems that bomb the rest of the central and eastern United States with heavy snow. Low pressure storm systems frequently develop just to the east or to the "lee" of the Rockies. Strong winds aloft at the jet stream level (about 25,000 to 30,000 feet) squeeze over the mountain peaks and then out over the plains. As the air moves out away from the mountains, the pressure falls, creating a low pressure area. If conditions are right, this low will begin to swirl moisture into the region from the Gulf of Mexico. This moisture spins into clouds, rain and snow, and before long, a storm is born. These storm systems first dump heavy snow over eastern Colorado and then churn toward the northeast. Heavy, wet snow, sleet, freezing rain and even thunderstorms mark the path of these storms as they roll on toward the Great Lakes. The soggy systems that spin out of eastern Colorado can give a false impression to the rest of the nation. Those eastern storms often dump heavy snow on Denver, Colorado Springs, Limon or other cities on the plains, but leave the mountains with a much smaller accumulation. Television weather reports in other cities show the snow in Denver and the folks in those cities think, "Wow, if Denver got a foot of snow, the mountains must have really been hammered!" In fact, eastern storms tend to be blocked off by the mountains of the Front Range, and the ski areas may have only scattered clouds while Denver gets the big snow. This was the case back in October 1984, when the Broncos played the Packers in a blizzard at Mile High Stadium. The national television audience saw all that snow in Denver and inundated the ski resorts with calls for reservations. Being good businesspeople, ski resort personnel didn't mention the fact that at the time, skies were clear in Vail and Aspen!

An old saying about Colorado weather is very helpful when trying to figure out if a storm system will bring heavy snow. Remember - Pacific front, mountains bear the brunt; southeast low, Denver gets the snow. As I mentioned earlier, storm systems on the eastern plains spin their heavy snow over the Front Range and adjacent plains, but may miss the mountains. In contrast, a moist storm front from the Pacific Coast may dump very heavy snow on the mountains, but have very little moisture remaining by the time the storm slips down into Denver. Those Pacific fronts have a rough time staying intact as they first hit the Cascades and Sierra Nevada Mountains, then have to cross the Wasatch Range in Utah and finally struggle over the Continental Divide. When the once mighty storm finally tops the last of three or four mountain ranges, moisture is so depleted that Denver doesn't even get a flake of snow! Sometimes these storms regenerate into a major system again once they get farther to the east of Colorado. The leftover circulation of the storm will start to tap into the rich supply of moisture available from the Gulf of Mexico and the storm will once again become a powerful precipitation producer! Our neighbors in Oklahoma, Kansas, Nebraska and points farther east get a messy winter storm - but Denver and adjacent cities in the I-25 corridor have just some clouds and gusty north winds on the backside of the storm. These storms are frustrating to forecast as they usually feature headline making weather from the West Coast, with huge waves, heavy rain, wind and mountain snow. The storm takes a couple of days to get here, so there is plenty of build up and anticipation. First our mountains get heavy snow, but then the storm seems to jump right over Denver, before bringing nasty winter weather to extreme eastern Colorado and most of the Midwest. Such a storm scenario makes it seem like Denver has a big clear plastic dome over us!

The recent weathercasts were filled with the term "blizzard", which is a derivative of the German word for lightning. Early European settlers on the high plains were astounded by the lightning-fast changes in the weather, especially when a powerful cold front roared through.

The true definition of a blizzard has more to do with wind and visibility than with snow - a storm earns blizzard status when winds of thirty-five miles per hour or higher are accompanied by falling or blowing snow that drops visibility to a quarter mile or less. The sky above can be clear, but blowing snow at ground level can produce blizzard conditions. A severe blizzard is bad - winds over fifty miles per hour with visibility near zero due to blowing or falling snow. Certainly our pioneer ancestors saw some very scary winter storms, but even today strong winter storms can bring peril to travelers. The wide-open eastern plains of Colorado offer no resistance to the wind and can allow howling storms to swirl snow into drifts five to ten feet high during the worst blizzards.

The passage of a strong Canadian cold front is a major weather story on the eastern plains, but often not for the mountains. The wall of granite to the west of Denver serves as a pretty good barrier for the cold, dense air. Quite literally, the cold air is just too heavy to climb up and over the mountains. Many times a surge of Arctic air will blanket the eastern plains of the state and put us in the deep freeze, while in the mountains, temperatures stay much milder as the cold front stalls in the foothills west of Denver.

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