The Weather Wire
March 2009 Volume 17 Number 03
Avg High - 53.0
Avg Low - 21.6
Snow - Trace
Season Snow - 16.9"
Precipitation - 0.04"
Avg High - 53.7
Avg Low - 25.4
Avg Snow - 11.7"
Avg Precip - 1.28"
Front Range Lightning
With all of the mild weather lately it is a reminder that spring is on the way and so is lightning season. As temperatures warm with increasing sunlight during the spring months the atmosphere becomes more unstable allowing for convective type precipitation or more intense precipitation and the threat of lightning. Even when temperatures are cold enough to support snow at the surface lightning can occur and often does a time or two each spring along the Front Range. This phenomenon is called “thundersnow” and can produce very heavy snowfall rates for a short time in a small geographical area. Thundersnow is rare, but can be dangerous because people often do not associate lightning with snowfall. Here in Colorado many people enjoy outdoor activities in the winter such as skiing or snowboarding and can accidently be in a bad spot at a bad time. Some precursors to thundersnow can be recognized with the type and intensity of precipitation. One form of precipitation called graupel or “soft hail” and can prelude a lightning strike as this type of precipitation forms in a convective atmosphere like hail in a summertime thunderstorm. Very heavy snow with literal “snow balls” or “clumps” of snow falling can also indicate a strike may occur as this is truly heavy snow with rates of greater than 3” per hour.
Lightning becomes increasingly more common in April and May with a maximum in August when the monsoon season is in full force. Looking forward to these months I thought it would be good to look back at 2008 and see how many lightning days we had starting April 1st and ending on October 31st. Below are graphs for number of thunderstorm days per month for a number of Front Range locations:
The thunderstorm season in 2008 was nothing special with below average precipitation for many areas. For a day to be considered a thunderstorm day there had to be a thunderstorm over the area that produced at least one strike. No precipitation has to fall for a storm to be called a thunderstorm, only lightning need be present as there are many days with “dry” thunderstorms. If we make the range even tighter from April 15th through September 15th, here is the % of days with thunderstorms:
Broomfield is surprisingly low in number of thunderstorm days compared to other Front Range locations and can be contributed to its small geographical area. Obviously the areas with the highest % of thunderstorm days are the foothills and Palmer Divide as these are “hot spots” for lightning and receive more strikes than almost anywhere else in the country except the extreme southeast US, but these areas have a much longer thunderstorm season, nearly year round. So, during the summer months parts of Colorado receive more lightning than anywhere else in America! Below is a strike density map for Colorado the brighter colors indicate areas with higher strike density:
Notice the bulls eye over the Palmer Divide and western foothills extending through the Pikes Peak region. This part of Colorado receives significantly more lightning than anywhere else in the state and this region is becoming more and more populated every year. A special offer from Skyview Weather this year: Half off our lightning services for the month of April!
Areas of drought are spreading across much of eastern Colorado as well as northwest Colorado as a result of our precipitation deficit year to date.
The map below shows forecasted temperature deviances for March 2009. As can be seen, it is expected that all of Colorado to have above normal temperatures for the month of March 2009.
The map below shows forecasted precipitation deviances for March 2009. Normal precipitation is anticipated for northern and western Colorado, with below normal precipitation expected for southeast Colorado for March 2009.
Areas of drought along the front range of Colorado as well as southeast Colorado are expected to develop as a result of the low levels of precipitation over the last several months.
February of 2009 is now the driest February ever! Since records have been taken all the way back in 1882 there has never been a trace of snowfall reported. This is due to the change of reporting locations from the old Stapleton site to DIA. The old site would have reported measurable snow and this would not have been the least snowiest February on record. Keep this in mind as we are now comparing apples to oranges in the climate department now that we are comparing the climate from one location to a new one. Lower than average snowfall will likely be measured at DIA for years to come even if the old site has "normal" snow. This is due to lower elevation and proximity to the foothills. As far as temperatures go we were incredibly warm not to set any temperature records. The average high for the month was 53 degrees, an astounding 5.8 degrees above normal, more like March. The average low was not as far off with a departure of 2.5 degrees on the warm side. February of 2009 can be remembered as warm, windy and very dry. The drought watch is now on!
We all know that March can bring some of the largest snow storms in Denver history and unfortunately we would need that and then some to get to near normal in snowfall this winter. For the month of March 11.7" of snow falls on average with 1.28" of total precipitation. Denver (DIA) is currently 22" behind in snowfall and with the nearly 12" we average this month almost 3 feet of snow is needed just to get back to normal. With the weather pattern we have experienced so far this winter with fast moving storms and unfavorable upper level flow I do not believe that we have much of a chance to even get to around normal in snowfall for the month, let alone make up some ground in the snowfall deficit. Temperatures really begin to turn around in March as overnight lows average the mid 20s instead of the teens in January and December. Referring to normal in March may be a little misleading as we are typically much warmer or colder with fast changing weather conditions.
Sunrise/Sunset (Jul - December Denver area)
October 2008 to April 2009