Mike's Blog Archive: July 2009

We might be taking a break from all the severe weather, but the unusual weather pattern will hardly subside. This summer has been a wild one, from late night tornadoes to hail drifts several feet deep. Another change is expected to move through the area in the coming days.

Through Friday the weather will be very warm in the Denver Metro area with highs climbing into the lower 90s. A high pressure building over southwestern Colorado will allow for a dry and hot weather trend over the next couple of days. However, back to back cold fronts are going to make their presence by Saturday with a cool down on the way.

The first cold front will dive across the state early Saturday bringing a chance for storms, and slightly cooler temperatures. On Monday afternoon, a stronger cold front will make its way across the region dropping temperatures some 10-15 degrees by Wednesday. Some wet weather will accmpany these cooler temperatures as showers and thunderstorms will develop as the cooler air moves into the region.

The coolest weather will settle into the Front Range on Tuesday and Wednesday. A cool Canadian airmass will sweep across the northern plains and will bring a brief taste of fall to Colorado for the middle of next week. Highs will stay in the 60s and 70s over the northern mountains and adjacent plains, with nighttime lows dipping into the 40s to low 50s.

By the end of next week, the weather will return to typical late July - early August conditions with temperatures bouncing back to near 90. We are still watching for the advent of the "monsoon season". So far, that flow of tropical moisture is not yet apparent from Mexico across the desert southwest.

The wild weather that roared through Denver and vicinity late last night was caused by a rotating thunderstorm called a "supercell". We get this type of storm along the eastern plains every summer, but they are rare near the Denver area, due to our proximity to the mountains. It was especially rare to have a supercell form after 10 PM. A passing cold front, combined with an upper air disturbance helped to power laast night's storm.

Most tornadoes form from these large rotating thunderstorms. These monster storms tend to develop ahead of cold fronts that push southward from Canada across the central U.S. As the fronts sag into the warm and humid air that covers the southern plains, the colder air wedges under the warm air, creating lift. The lifted air rises up to form thunderstorms that can rapidly grow to heights of 40 to 50 thousand feet above the ground.

The storm pushes high into the sky, reaching into the jet stream - the band or river of fast moving air that flows around the world. It is the increase in wind speed with height that causes the thunderstorm to begin a large, slow counterclockwise rotation. This rotating thunderstorm is what is classified as a "supercell".

Once the supercell storm develops, the best analogy for thinking about how the tornado forms is to think of a figure skater doing a spin. The skater starts with their arms out, and is rotating rather slowly. As the skater brings their arms in, the rotation begins to speed up. In physics, this is called "the conservation of angular momentum". The rotation gets faster and faster as the size of the rotating column grows narrower. This is a very simplistic description, but eventually this narrow rapidly rotating column of air will drop to the ground as a tornado.

Tornadoes are classified by the wind damage that they cause. The original scale was developed by Dr. Ted Fujita from the University of Chicago. Dr. Fujita based his "F scale" on a 0 to 5 basis for tornadoes. The F0 is a weak tornado, while the F5 storms are the most powerful winds ever observed on Earth.

F0 - up to 72 mph - light damage F1 - 73 to 112 mph - moderate damage F2 - 113 to 157 mph - considerable damage F3 - 158 to 206 mph - severe damage F4 - 207 to 260 mph - devastating damage F5 - above 261 mph - incredible damage

About 70 % of the annual average of 1000 tornadoes nationwide are classified as F0 or F1.

About 28 % of all tornadoes fall into the F2 or F3 category.

Only about 2% of all tornadoes are classified as F4 or F5.

Often a severe weather season will come and go without a single F5 tornado reported.

However, about 80% of all tornadoes deaths are the result of the F3, F4 and F5 tornadoes. These storms are much less common, but much more dangerous.

As of 2007 this scale was replaced by the enhanced Fujita (EF) scale. The EF attempts to rate tornadoes more accurately, taking into account that it often requires much lower wind speeds to create F5-like damage. The new EF scale is now the official standard to measure the strength of tornadoes.

EF 0 - 65 to 85 mph

EF 1 - 86 to 110 mph

EF 2 - 111 to 135 mph

EF 3 - 135 to 165 mph

EF 4 - 166 to 200 mph

EF 5 - Over 200 mph

Most tornadoes in the United States occur in the central plains, with the greatest likelihood of twisters in the southern plains around Kansas, Texas and Oklahoma. Colorado lies of the western fringe of "Tornado Alley", but our state still averages between 40 and 60 tornadoes per year.

The peak season for tornadoes is in the spring and early summer. From March through June, about 70% of all the tornadoes in a year will occur. This is due to the fact that the weather patterns that are needed for tornado development are most common in the spring and early summer.

Tornadoes are not named like hurricanes are, but the strong or deadly tornadoes are usually remembered for the town or location that they affected. For instance, the infamous "Xenia Ohio Tornado" of April 1974, or in Colorado, the "Limon Tornado" in June of 1990 and now the "Windsor Tornado" in 2008 and now "Denver's Late Night Tornado" in 2009!

Perhaps the single worst tornado on record was the great "Tri-State Tornado" of March 1925. This huge tornado started in southeastern Missouri and tore a path of destruction all across Illinois, before ending in western Indiana. The twister covered a distance of 219 miles and was on the ground for over 3 hours. In the days before adequate warnings, the storm caught everyone off guard. The Tri-State Tornado killed 689 people, injured over 2,000 and caused 17 million dollars in damage - a very large figure in 1925!

In Colorado, the peak season for tornadoes is in early June. At that time, the almost daily dose of thunderstorms can easily rise up to the jet stream level and begin to rotate into a super cell. These storms tend to form along the Front Range, roll over the Denver metro area and then really get severe over the eastern plains of the state. About 90% of all Colorado tornadoes occur east of I-25. Although tornadoes can form in the high country, the rough terrain tends to disrupt the rotation needed to form a supercell.

In my 30 years in meteorology, I have been through many tornado watch and warning situations. I have walked through a small town in southern Wisconsin, named Barneveld, just hours after it was ripped apart by an F5 tornado. Only one time have I seen a tornado, as most of the time I am right here in the 24/7 Weather Center issuing warnings and weather updates, so I do not get much of a chance to chase these storms. However, we do have a crew of storm chasers that go out in search of these deadly weather events and you can follow along with them as the blog for us here at Channel 7.

Tornadoes have done some very unusual things. The powerful winds can pick up a railroad locomotive, lift a water tower off the ground, and drive blades of grass into walls just like a hammer and a nail. At the same time, there have been reports where tornadoes have picked a refrigerator off the ground, tossed it several hundreds of yards, dropped it back on the ground and not even broken an egg inside the refrigerator!

Tornadoes usually form on the back edge of the thunderstorm cloud, meaning that most of the storm has already passed overhead. Often the rain, hail, thunder and lightning have mostly gone by and then the tornado occurs. That is why you will often see the sky looking very bright behind the tornado - a dramatic contrast to the very dark funnel. After the tornado, the sky often quickly clears as the storm moves away. There are, however, no hard and fast rules for tornados, so sometimes the twister occurs in the midst of a large area of thunderstorms, so after the tornado occurs, it just rains and rains.

I thought since we are on the subject of storms, you might like to have a little information about rain and hail, some things that usually accompany a tornado...

Here are a few facts about rain and hail. Most of the rain that we get in Colorado forms from a mix of water droplets and ice crystals in the clouds. Under certain conditions, water will remain in liquid form even with temperatures that are well below freezing. This type of water is called "super-cooled". In most of our summertime clouds, we have a mix of super-cooled water and ice crystals floating around high above us. The ice crystals rapidly grow as they "feed" off of the super-cooled water and they basically form big fat snowflakes. These snowflakes fall slowly to Earth and begin to melt as they reach warmer air closer to the ground. The resultant raindrops will fall to Earth at about 15-20 mph.

In stronger thunderstorms, the tiny ice crystal gets bombarded by the super-cooled water thanks to the extreme turbulence in the storm cloud. The ice crystal forms a small stone of ice which is the beginning of a hailstone. If the storm is quite strong, there are intense updrafts of wind that can keep the growing hailstone suspended in the cloud for a long time. A hailstone that is the size of a golf ball needs an updraft of nearly 60 mph to stay aloft. A baseball sized stone requires a 100 mph updraft to keep it "afloat".

When the hail falls to Earth, they come zipping down at 70 to 100 mph. That is why it is a good idea to stay indoors during a major hailstorm!

You can still go to King Soopers and get a special WEATHER ALERT RADIO. The radio will signal when there is a storm nearby. If the radio stays quiet, there is nothing to worry about. Most of the local stores will still have these radios in stock, but if not, you can get the radios from our website.


I also have a book in the bookstores that will provide even more information about tornadoes. The book is titled THE COLORADO WEATHER ALMANAC and it is available in most bookstores or from Amazon.com

Our recent hot weather is going to change for a few days. A cold front passed through northern Colorado overnight. This front will drop temperatures across much of Colorado for the next two days; we should see close to a ten degree temperature difference between Monday and Tuesday.

There is a chance for scattered thunderstorms in the foothills and mountains due to the northeasterly winds behind this front. This upslope condition will be the main cause for any storms late in the day. On the eastern plains, the weather will be much calmer, a nice break from the stormy pattern that has dominated the area for most of the summer.

So far, July has been significantly drier than the month of June. We have seen 1.24 inches of precipitation so far this month. The average is about 1.37 inches, meaning Denver is 0.13 inches below normal. Despite the lack of moisture this month we are still 2.16 inches above average for the year. Typically Denver has seen 9.46 inches of precipitation by this point in the year. For 2009, we have seen 11.62 inches of precipitation leaving us well above average for the year.

How does the rest of July look? The Climate Prediction Center's forecast for the rest of the month is expecting very seasonal temperatures. This means Denver will see mid to upper 80s for high temperatures, and mid to upper 50s for overnight lows. For moisture, we can anticipate above average precipitation for the Denver metro area and the eastern plains for the rest of July. Extra moisture should make up for the 0.13 we are behind.

This year Denver’s July looks to become a very average month. Not contributing to the extra precipitation we have seen this year, but not negating it either.

With El Niño building in the Pacific the extra moisture could continue into the Fall and potentially bring lots of snow for this winter.

Mid July is typically the warmest time of the year in Colorado. Our average high temperatures are in the high 80s to low 90s in the Denver area, with upper 70s to mid 80s in the high country. The next few days will fill that billing, warm and comfortable conditions under mostly sunny skies. There will be some widely scattered thunderstorms in the forecast each day through the weekend. The storms will generally develop in the mid to late afternoon and should be diminishing after sunset. Rain chances will not be very high - only in the 20-30% range and most of the storms will be moderate - not severe.

The jet stream winds are blowing down from the northwest across the central Rockies, so most of our thunderstorms will drift from the northwest to the southeast at about 20 mph. The airmass over Colorado is pretty warm at higher levels, so the risk of large hail from the storms will be low, but some storms will produce some brief very heavy rain. Probably the biggest weather threat will be from the lightning, so be careful when the dark skies are near.

Despite the recent wet weather, the fire danger has been slowly increasing over Colorado. The western half of the state is the driest and there have been a couple of wildfires reported west of the Divide. All of the rain during the past several weeks has lead to a lush growth of grass and underbrush. As these plants mature, they will dry out and become much more likely to burn. Do not be lulled into thinking that the risk of wildfire will stay low for the rest of this summer. Despite the storms, be very careful with any outdoor burning, especially in western Colorado.

Our annual "monsoon pattern" should be developing by the end of the month. Just about every summer we get a rich flow of moisture moving north from the Pacific Ocean across Mexico and into the southwestern states. This pattern is what we call our "monsoon" and it often brings some very heavy rainfall to Colorado by the end of July and through much of August. The current northwest flow aloft is blocking this monsoon pattern, so our thunderstorms over the next 5 to 7 days will be smaller and will not produce widespread heavy rainfall.

Typical of summertime in Colorado, we will enjoy the mild and sunny mornings, dodge the drops in the afternoon, watch out for lightning, and worry about both fire and rain!

After a very stormy start to July, things will be drying out during the next week. A cold front moved in late Tuesday, making conditions less moist and decreasing the chance for thunderstorms. Temperatures will remain pleasant, in the upper 80s and low 90s throughout the rest of the work week.

Skies are a little hazy in the wake of the cold front, the airmass that has moved into Colorado may have brought just a little dust and smoke from southern Canada - perhaps from some distant forest fires. Nonetheless, it will be a pretty day today and very comfortable with highs in the 80s.

The weekend looks nice and sunny. Although there will be a slight chance of thunderstorms Saturday afternoon, conditions will still be much drier than they have been recently, so it is a great time to get outside and enjoy the nice July weather. The next week is looking just as pleasant, with temperatures remaining in the upper 80s to low 90s and more sunny skies!

As discussed in the former entry, El Niño has returned and will be affecting our weather patterns as we head toward August. With temperatures in the Pacific Ocean rising, the monsoon season will be strenghthened, bringing wet weather back to Colorado soon enough. Enjoy the drier days while you can, because thunderstorms will be frequenting your forecast again come August.

After three years, El Niño has returned and is expected to last through this winter. The last El Niño was back in 2006 and while it brought drenching storms to the West Coast, Colorado didn't feel as many effects as it normally would.

The term El Niño became familiar to the public's ear back in the early 1980s when a very strong El Niño event during the winter of 1982-1983 hit California with heavy, wet storms causing flooding and mudslides. El Niño typically occurs every two to five years and usually lasts for 12 months.

El Niño's impacts depend on a variety of factors such as the time of the year and how much the ocean temperature increases. This change in ocean temperature can cause a variety of effects. El Niño can suppress Atlantic hurricane activity, bring much needed winter precipitation to the arid Southwestern states, more mild winters in the North, and can decrease the risk of wildfires in Florida.

In Colorado, we typically see the most significant changes in the Southwest and Central Mountains where they tend to receive increased amounts of winter precipitation. On the Eastern Plains, the winters are usually more mild and the Spring has an increase in precipitation. In late summer, we often see an increase in monsoonal moisture giving us more storms through those dry months. Although in Autumn, El Niño is not usually very noticeable, although the big snowstorm in October of 1997 was during an El Niño year!

In the monthly El Niño diagnostic discussion released today by the NOAA National Weather Service Climate Prediction Center, scientists noted that eastern equatorial Pacific sea surface temperatures were at least 1.0 degree C above average at the end of June. From this, NOAA expects this El Niño event to continue developing over the next few months, with further strengthening possible.

According to Klaus Wolter of the University of Colorado- CIRES Climate Diagnostics Center NOAA-ESRL Physical Science Division, we have been under weak-to-moderate La Niña conditions since fall of 2008. These conditions continued through the winter keeping Colorado dry and mild. Over the past four weeks, however, conditions have changed toward an El Niño pattern in the Pacific. The current status of the El Niño / La Niña pattern could be considered "neutral", as the transition takes place. Many Colorado observations from the past few decades indicate that a neutral condition brings more moisture to Colorado.

If you would like to see more information from Klaus Wolter and his Executive Summary check out the Earth System Research Laboratory Web page.

For more information on El Niño, you can check out NOAA's El Niño site at http://www.elnino.noaa.gov or you can pick up my book, "Colorado Weather Almanac" at any bookstore or on Amazon.com.

Temperatures reached the 90 degree mark on Wednesday, a rare feat for this summer season. In an average year, the Denver area will hit ninety degrees 33 times. The record for the most days of 90 degrees or higher was in 2000, when we soared to the nineties 61 times! Not much chance of reaching that level in 2009, as the stormy weather pattern has kept the temperatures cooler than average.

The weather will be a bit cooler over the next 24 to 48 hours, thanks to a weak cold front that has slipped into the northern half of Colorado. This front will stall over the area and help to stir up afternoon showers and thunderstorms through Friday. With the increased cloud cover, the readings will stay in the 80s to near 90 degrees.

By the weekend, the front will weaken even further and move to the east of Colorado. Temperatures will rebound a bit as highs return to the low 90s Saturday through early next week. Rain chances will be low this weekend, unlike the soggy scenario we had over the Fourth of July Holiday.

Despite the warmer and drier pattern, we have had plenty of moisture and the landscape is lush and green along the Front Range. In that light, be careful to not get carried away with watering your lawn or garden. The folks at Denver Water want to remind you to "only use what you need". Water your lawn only 2-3 times per week and try to cut back on the watering time for each zone. Watering each zone for two minutes less can save an amazing amount of water - a billion gallons if every Denver Water customer followed those guidelines!

This work week brings pleasant temperatures and drier weather back to your forecast. Temperatures will be in the upper 80s and 90s throughout most of the week. Chances of thunderstorms will remain slight through Wednesday evening. Those storms will probably return for a bit on Thursday and Friday but with temperatures remaining warm, conditions will dry back out just in time for a beautiful weekend!

The storms that do fire on Thursday and Friday will be due to a weak cold front rolling through the area Thursday morning. The front will bring along more moisture and lift, which will help to generate the storms. The temperatures will see a change to the cooler side with highs on Thursday reaching into the upper 80s. Friday will see a return to the 90s with a slight chance of storms in the afternoon. The weekend will see a warm and dry reunion tour with highs topping out in the lower 90s.

After a soggy June and a similar start to July, our weather pattern will be drying out, at least for the next few days. The rains washed out a few Independence Day celebrations around the state, but things are set for a change toward sun and heat. A large ridge of high pressure will be influencing our weather for the upcoming week. There was another round of severe thunderstorms on Monday, but the trend will be a return to the usually toasty early July weather. Why have we had such a cool, wet Spring and early Summer? Perhaps we can attribute it to the apparent transition currently taking place from La Niña to El Niño conditions.

La Niña is the periodic cooling of the sea surface temperatures in the eastern Pacific Ocean, (the opposite of El Niño), related to changes in trade wind patterns. According to Klaus Wolter of the University of Colorado- CIRES Climate Diagnostics Center NOAA-ESRL Physical Science Division, we have been under weak-to-moderate La Niña conditions since fall of 2008. These conditions continued through the winter keeping Colorado dry and mild. Over the past four weeks, however, conditions have changed toward an El Niño pattern in the Pacific. The current status of the El Niño / La Niña pattern could be considered "neutral", as the transition takes place. Many Colorado observations from the past few decades indicate that a neutral condition brings more moisture to Colorado.

Keep in mind that El Niño / La Niña is suspected to influence the overall climate and should not be used to predict short-term local weather.

Wolter says that our recent weather has many features of an early monsoon - the seasonal flow of moist air from Arizona, New Mexico and Mexico. It appears that similar conditions will continue for our area into July. Wolter believes that eastern New Mexico to eastern Colorado will experience a slightly enhanced amount of precipitation for the next couple weeks. For the coming week though, strong high pressure will be in charge.

It’s anyone’s guess if El Niño will become the dominant force on our weather for the next few months into Fall. However if El Niño is here to stay through the rest of the year we can expect a much wetter more snow-filled winter for 2009-10. This would be especially true for the central and southwestern mountain areas.

If you would like to see more information from Klaus Wolter and his Executive Summary check out the Earth System Research Laboratory Web page.

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