The Weather Wire
May 2006 Volume 14 Number 5
Avg High 68.1
Avg Low 34.9
Snow - 0.3"
Season Snow - 30.2"
Precipitation - 0.67"
Avg High 70.5
Avg Low 43.8
Avg Snow - 1.3"
Avg Precip - 2.32"
Spring for many parts of the country means tornado season. Looking at the map below you can see where tornadoes occur most frequently. You will notice that eastern Colorado is included in “Tornado Alley”. Here along the Front Range we also see tornadoes, but due to our elevation they tend to be the weaker variety, instead of some of the monsters that the Plains states see.
Although tornadoes occur throughout the world, including India and Bangladesh, they are most intense and devastating in the United States. Tornadoes can strike at any time of day, but are much more frequent in the afternoon and evening, after the heat of the day has produced the hot air that powers a "tornadic thunderstorm" -- a thunderstorm that produces a tornado.
Tornadoes are common in Tornado Alley because of the Rocky Mountains to the west and the Gulf of Mexico to the south. In spring, a strong westerly jet stream flows across the Alley, creating instability and a trough of low pressure that draws warm, moist air in from the Gulf. "Conditions for the supercells [large, powerful thunderstorms] that spawn tornadoes require strong vertical wind shear [changes in wind speed and direction with height] and lots of instability. And that's exactly what happens in Tornado Alley.
The fact that supercells occur where warm, moist air meets cold, dry air suggests the source of energy for both t-storm and tornado: latent heat in the warm, moist air. Latent heat is heat you can't detect with a thermometer. An air-conditioner struggles to remove humidity from Houston's soggy July air because the air holds so much latent heat. Latent heat is the heat that is released to the atmosphere when water vapor condenses into liquid water. Heat is added to liquid water to turn it into a vapor. Thunderstorms release massive amounts of this latent heat as large amounts of air are lifted, cooled, forming clouds, rain and hail.
A tornadic thunderstorm can form where moist, warm air gets trapped beneath warm, dry air under a stable layer of cold, dry air. This air sandwich is called an inversion.
If the cap is disturbed by a front or movement in the upper atmosphere, the warm, moist air can punch through the stable air above it. The condensing of water vapor releases latent heat, and the warm air starts to spiral upward. Aided by different winds at different levels of the atmosphere, the rotating updraft gains velocity.
We look for several ingredients when looking for potential tornado formation:
1. A large, layered sandwich develops in the atmosphere. Hot, humid air is trapped beneath cold, dry air.
2. The "cap" (a layer of stable air between the hot and cold air) is disturbed by winds in the upper atmosphere or by the arrival of a weather front.
3. Lower-level air rises and expands in the reduced air pressure aloft.
4. As the air cools, moisture condenses, releasing latent heat which warms the air, making it buoyant, and causing it to rise at speeds up to 150 mph. By now, the cloud is a thunderstorm. Upper-level winds tilt the thunderhead, creating the anvil shape.
5. The thunderstorm may die out in intense rain and/or hail. Or it may spawn a tornado.
6. Interactions between air at various altitudes, humidities and temperatures cause rain, lightning, air circulation and strengthening of the rotating updraft, now called a "mesocyclone." Low-level wind helps cause this rotation, which is almost always counter-clockwise (seen from above) in the Northern Hemisphere.
7. A tornado may form below the mesocyclone. As the spinning air column narrows, it rotates faster and extends higher into the storm.
That, much simplified, is one theory of the origin of tornadoes. Scientists do not fully understand the first stage of formation, however. Some suspect that wind shear -- differences in wind speed at different altitudes - may start the vortex. Just as, when you roll a wad of clay in your hands, it eventually becomes a rotating column of clay, when the different bodies of air move against each other, they create wind shear and then a tornado.
In the picture to the right, stronger west winds aloft start a rotating column of air. As strong updrafts develop in a thunderstorm this column is brought vertical and we have the start of a tornado.
Much more common along the Front Range are funnel clouds. Many people see these and think that they are tornadoes. A funnel clouds never touches the ground, while a tornado stretches from the cloud base all the way to the ground. This is not to say that if you see a funnel cloud it is safe, most tornadoes start out as funnel clouds.
Although we do not see the big tornadoes that the eastern plains see, they certainly can cause damage. Even the weaker tornados have winds speeds in excess of 100 mph. By comparison the strongest tornados on the plains have winds in excess of 200mph, some approach 300mph.
Skyview Offers Safety Classes
As we discussed in last months newsletter, risk management is a critical component of each organization. One component is education, and Skyview Weather is proud to announce educational classes intended to promote weather awareness and increased safety. At this time, the following classes are available. We highly recommend these safety classes for your facility, please contact us for more information!
Basic Weather Safety - This 1 hour safety oriented class reviews basic weather safety, including flash flood safety, lightning safety, and tornado safety as well as reviewing NWS products, with specific emphasis on watches and warnings.
Basic Weather Safety and Severe Weather Spotting - This 3 hour class begins with basic weather safety, including flash flood safety, lightning safety, and tornado safety as well as reviewing NWS products, with specific emphasis on watches and warnings. Basic weather concepts and basic weather spotting is included, with emphasis on thunderstorm features.
Basic Weather Safety for Coaches - This 60-90 minute safety oriented class is similar to Basic Weather Safety, but designed for coaches and other administrators that must determine whether to suspend or cancel outdoor sports events. Heavy emphasis on lightning safety, as well as severe thunderstorm safety is presented. A review of NWS publicly disseminated products, including watches and warnings, is included. Basic weather safety, including flash flood safety, lightning safety, and tornado safety is discussed.
Extended Weather Safety and Spotting - As with the above classes, this all day class begins with basic weather safety, including flash flood safety, lightning safety, and tornado safety as well as reviewing NWS products, with specific emphasis on watches and warnings. Basic weather concepts and definitions are discussed, with extensive time on the role of spotters, and key spotting features of developing severe storms. Video presentations are included as well.
Radar 101 - Everyone utilizes radar on the internet, but just what exactly are you looking at? This 2 hour class answers that question, providing an introduction to radar, a review of publicly available radar products on the internet, and how to interpret what the radar. This class is critical for those organizations attempting to use publicly available internet weather for client and employee safety.
Getting the most from Skyview Weather ... Client-Skyview interaction - This 2 hour class focuses on how existing clients of Skyview Weather can get the most from Skyview Weathers extensive services. A review of what Skyview offers, product delivery, and Skyview Weather terms is presented.
All classes are presented using PowerPoint, and include handouts. Questions are always welcome, and encourage.. Selected classes include video as well. Please contact Skyview Weather for scheduling and pricing information. As always, you can reach us at 303-688-9175 or via email at email@example.com .
Printing Newsletters Now Possible
We have received many requests over the years of how to print this newsletter. Unfortunately, until now, the newsletter has been formatted for online and email viewing, and did not format properly for printing. However, with the release of the new Microsoft Internet Explorer Internet 7 Beta version 2, this is now possible! The new release of Microsoft Internet Explorer reformats the fonts to allow documents to be printed. The latest beta release of Internet Explorer can be found at http://www.microsoft.com/downloads/Browse.aspx?displaylang=en&categoryid=6 . Although a Beta version, we have not had any problems using the new software. Also, remember that older newsletters can be found on the www.skyview-wx.com website under newsletters, so you may print older newsletters!
More increases in drought conditions across Colorado was noted during April 2006, primarily over eastern and southern portions of the state.
The map below shows forecasted temperature deviances for May 2006. As can be seen, normal temperatures are expected for the Eastern 2/3s of Colorado with above normal temperatures anticipated for the Western 1/3 Colorado for May 2006.
The map below shows forecasted precipitation deviances for May 2006. Normal or near normal precipitation is expected for all of Colorado for May 2006.
As can be seen in the below map, much of the state remains out of widespread drought conditions, though the southern and eastern portions of the state are seeing increased areas of drought.
Another drier than normal month goes into the Denver weather statistics book. April 2006 finished with 0.67 inch or total moisture which is 1.26 inches below the normal of 1.93 inches. This marks the 6th month in a row with below normal precipitation. There were only 3 days that measured 0.10 inch or more and 6 days with 0.01 inch or more. The greatest 24 hour total was 0.21 inch on the 21st.
April is Denver’s 3rd snowiest month, but not this year. Only 0.3 inch of snowfall was recorded at the weather site near the former Stapleton airport. This occurred on the 25th. It should be noted that many areas along the Front Range received more moisture and more snowfall that the official amount, but yes overall it was drier than normal for just about everyone. The 0.3 inch recorded at the old Stapleton ties as the 4th least snowiest April with 1965.
It was a warmer than normal month with an average temperature of 51.5 degrees. This is 3.9 degrees above the normal 47.6 degrees. Even though it was well above normal, it was not warm enough to get into the top 10 warmest. The warmest April took place in 1981 and 1946 with an average of 56.4 degrees. For April 2006 temperatures ranged from a record setting 85 degrees on the 13th down to a low of 23 degrees on the 8th and 20th.
We did kick off severe and thunderstorm season as small hail was reported at DIA and the seasons first thunderstorm occurred on the 23rd.
The weather in Denver is quite changeable throughout the year. However, the month of May seems to feature the most changes from year to year, day to day or even hour to hour. Denver’s May weather can include almost any type of weather.
Late season frosts can nip plant growth because the average date of the last freeze is May 5th, but the latest date of a Denver freeze is June 2. On the other hand, afternoon temperatures in the 80s are common and the mercury occasionally climbs into the 90s.
May normally marks the beginning of the severe weather season. Violent thunderstorms with large hail and tornadoes are possible. These severe storms occur when moisture from the Gulf of Mexico moves into Colorado and collides with cold fronts still moving in from Canada or from the Pacific Northwest.
May is also the wettest month of the year and on average, precipitation normally occurs once every three days. May is tied with November as having the lowest percentage of possible sunshine.
Snowfall in Denver during May is rare but does occur as we saw last May. Denver’s high elevation intensifies late season cold air masses and on the average snow occurs in about two out of every five years.
Sunrise/Sunset (Denver area)
October 2005 to April 2006