The Weather Wire
September 2010 Volume 17 Number 09
Avg High - 88.6
Avg Low - 59.0
Snow - 0.0"
Season Snow - 0.0"
Precipitation - 1.05"
Avg High - 77.4
Avg Low - 47.3
Avg Snow - 2.1"
Avg Precip - 1.14"
Believe it or not it is time to start talking about snow again now that we have entered the month of September. September is our transition month into fall as we typically experience our first freeze of the year and occasionally the season’s first snowfall as the Denver Metro area averages 2.1” of snow for the month. Since climate record keeping began back in 1872 there are 54 years with snow reported in September but there are 84 years without any snow reported at all. The chances are better for a snow free September than one with snow and we have not had any measurable snowfall during the month of September in the Denver Metro area since 2000. This year looks a little different though. The monsoon moisture associated with the summer thunderstorm season has been pushed southward a few weeks earlier than on typical years. This has allowed for cold fronts to begin entering the state from the north and west which will become stronger and colder later this month. With the fall like weather pattern progressing so rapidly Skyview Weather believes that there will be a good chance for measurable snowfall by the end of the month.
Just because there will likely be an early start to the winter does not necessarily mean that this year will be a snowy winter. All forecasts are pointing to a strengthening La Nina cycle this winter as the Climate Prediction Center notes:
“During July 2010 La Niña conditions developed, as negative sea surface temperature (SST) anomalies strengthened across the central and eastern equatorial Pacific Ocean” and “Nearly all models predict La Niña to continue through early 2011. However, there is disagreement among the models over the eventual strength of La Niña. Most dynamical models generally predict a moderate-to-strong La Niña, while the majority of the statistical model forecasts indicate a weaker episode. Given the strong cooling observed over the last several months and the apparent ocean-atmosphere coupling (positive feedback), the dynamical model outcome of a moderate-to-strong episode is favored at this time. Therefore, La Niña conditions are expected to strengthen and last through Northern Hemisphere Winter 2010-11.”(1)
A moderate to strong La Nina typically results in below normal snowfall and precipitation along the I-25 corridor. Temperatures are also typically below average. This happens because the northern branch of the jet stream becomes dominant bringing in colder air from the north which is a drier weather pattern for most of Colorado from the Continental Divide eastward. Storm systems that travel along the northern jet stream are typically faster moving which does not allow for Gulf of Mexico moisture to be drawn into the storm from the south and then wrapped around into the Front Range foothills on northeasterly upslope flow, thus there are typically less “big” snow storms or early and late season blizzards. Below is a comparison of jet stream patterns for “typical” El Nino years and La Nina years(2):
The major difference to note here is the weakened or complete lack of the southern jet stream as a blocking high pressure in the Pacific pushes storms into Canada which affects the Pacific Northwest leaving the southern tier of states on the drier side. The Front Range of Colorado extending from Fort Collins through Colorado Springs is somewhat stuck in the middle of both of these patterns and may not be affected very much in weak La Nina or El Nino years, but strong El Nino or La Nina cycles will generally leave their footprint. Since this year is forecast to be a moderate to strong La Nina the likelihood of a drier and colder winter appears to be greater and Skyview believes this will end up being the case.
For comparison last winter there were nearly 50 snowfall events that produced at least a trace of snow. This frequency is on the high side and this year Skyview expects between 36-42 events with snowfall amounts of around 15-20% below average. Precipitation will likely be even further below average (25-35%) mostly due to the fact that powdery or “cold snows” accumulate more snowfall with less moisture than wet or “warm snows”. For example Denver (DIA)* averages 61.7” of snow but this year between 48-54” of snow is expected. The Boulder area northward through Fort Collins may be an exception to this as these areas can sometimes benefit from a northwest flow pattern. There will likely still be 1 or 2 significant early and late season snowstorms that produce 12” or more of snow in and around Denver but the mid winter season from late December through February will likely feature infrequent and light snows with bouts of arctic air masses.
In summary Skyview Weather expects a drier than average winter in 2010-2011 with below average snowfall and near normal to colder than normal temperatures from November 2010 through May 2011.
(1)La Nina Forecasts http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf
(2)Jet Stream http://science.nasa.gov/media/medialibrary/2001/06/27/ast28jun_1_resources/jetstream.gif
With the widespread precipitation of the last several months few areas of drought remain over Colorado, though some dry areas have developed in western Colorado.
The map below shows forecasted temperature deviances for September 2010. As can be seen, normal temperatures are expected the eastern 2/3s of Colorado with above average temperatures expected in the southwestern areas of the state.
The map below shows forecasted precipitation deviances for September 2010. Below normal precipitation is expected across most of Colorado.
Little in the way of drought remains in Colorado.
August began wet with over an inch of rainfall through the first week of the month but the remaining weeks proved to be very dry only picking up an additional 0.02". As a result the month totaled 1.05" which is 0.77" below the normal of 1.82". The precipitation deficit for August now brings the yearly precipitation below normal as well with 10.55" so far this year and 11.07" on average which is 0.52" below normal. Temperatures were above normal but there were not any high temperature records tied or broken. There was a record high minimum temperature though of 67 degrees on the 18th. The average high was 88.6 degrees which is 2.6 degrees above normal and the average low was 59.0, 1.6 degrees above normal. These two combined resulted in a monthly mean temperatures of 73.8 degrees. 2.1 degrees above normal which is quite significant and nearly reached top ten warmest August status by only 0.3 degrees.
Fall is arriving early this year as the summer monsoon pattern broke down around mid August allowing progressively colder and colder weather systems to move into the northwest that will eventually bring the seasons first snowfall to the state. Temperatures really begin to drop off in September with the seasons first freeze occurring on many years. Snow is certainly possible and appears to be likely this month with 2.1" on average but there are also many years with no snowfall reported at all. The last September snowfall in Denver was in 2000. Average high and low temperatures drop more than 10 degrees from August with average an high of 77.4 and an average low of 47.3. Precipitation drops off to an average of 1.14" and we will likely come in lower than average this month as monsoon moisture has already been suppressed southward weeks ago. In general September is a very nice month to be outdoors as the leaves change there is more % of available sunshine than any other calendar month.
Sunrise/Sunset (July - Dec Denver area)
May 2010 to Sept 2010