The Weather Wire

September 2009                                                                                                            Volume 17 Number 09

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August

Avg High -85.6

Avg Low - 55.1

Snow - 0.0"

Season Snow - 38.1"

Precipitation - 1.14"

September

Avg High - 77.4

Avg Low - 47.3

Avg Snow -  2.1"

Avg Precip - 1.14"

2009-2010 Winter Outlook

The summer thunderstorm season is winding down now that we have reached September.  The spring and summer months have provided much needed moisture for the Front Range of Colorado which has resulted in above normal precipitation for many locations.  Could this be a sign of a cold and snowy winter to come or will the upcoming months feature warmer and drier than normal conditions? 

As far as this winter season is concerned the major player will be the onset of an “El Niño” cycle and the potential for above normal precipitation and snowfall for the 2009-2010 winter season.  Here’s what the NWS climate prediction center in Camp Springs, MD has to say about this upcoming El Niño:
 
CURRENT EL NINO CONDITIONS ARE EXPECTED TO STRENGTHEN AND PERSIST INTO THE BOREAL WINTER AND 2010. THE INCREASING LIKELIHOOD OF A CONTINUING EL NINO EVENT INFLUENCES THE TEMPERATURE AND PRECIPITATION OUTLOOKS FOR OND 2009 THROUGH MAM 2010. THE IMPACT OF EL NINO ON THE CLIMATE OVER NORTH AMERICA IS EXPECTED TO INCREASE FROM HAVING LITTLE OR NO IMPACT FOR SON 2009 TO THE GREATEST IMPACT DURING THE WINTER SEASONS. A WEAK TO MODERATE EL NINO EVENT IS EXPECTED DURING WINTER, THOUGH A STRONGER EVENT IS POSSIBLE…

With an El Niño expected this winter there will be a lot of hype about the potential for heavy snow and numerous large snow storms or blizzards.  Past El Niño years have certainly proved to be wet and snowy but there are also El Niño years that do not produce above normal precipitation.  Why does this happen?  It happens because of Colorado’s central location in the middle of a continent and the blocking effect of the Continental Divide for areas east of the Rockies.  In summary El Niño typically energizes the southern jet stream leaving the northern tier of states relatively dry and mild while the southern states often experience above normal precipitation with normal to slightly below normal temperatures.  Conversely, La Niña winters typically produce a more active northern jet stream with colder and wetter conditions in northern states with Colorado located on the line where some winters are cold and snowy while others may be drier, but close to normal or above normal in temperature.  Colorado’s location east of the Rockies requires the need for gulf moisture to energize our snow storms.  Since El Niño develops in response to rising Pacific Ocean temperatures and additional moisture is provided by the warmer water, but most of this moisture is actually “wrung” out over the Rocky Mountains west of the Continental Divide.  The active southern jet stream will provide the disturbances necessary for producing snow east of the Rockies, but the moisture for big snow storms still has to come from the Gulf of Mexico.  The track and speed of these storm systems determine how much gulf moisture and for long of a period of time it will affect the eastern parts of Colorado.  This is why “upslope” easterly/northeasterly winds are important as this wind direction at the surface imports moisture from the east that has originated from the gulf and pushes it up against the Front Range. The storm track can also drop too far south to provide Colorado with necessary “upslope” to generate snowfall thus drier than normal conditions may develop even in an El Niño year. 

As is always the case in Colorado predicting exactly what to expect this upcoming winter is a very difficult if not impossible task, but the trend of active weather over the past few months and forecasts of a strengthening El Niño cycle it is Skyview Weather’s belief is that this winter will feature above normal snowfall and above normal precipitation with near normal to slightly above normal temperatures.  There will likely be a few larger snow storms one of which is expected early in the winter in late October or November with one or two famous Colorado blizzards in the late winter or spring.  Snowfall frequency will likely be near normal to slightly above normal with the above normal precipitation expected to come from high moisture content snowfall as arctic air masses may not be as frequent this winter.  Typical snowfall for the Denver area is around 60” at old Stapleton Airport with around 70-75” of snow expected this upcoming winter with much higher amounts in the surrounding foothills and Palmer Divide areas. 

Drought Update

With the widespread precipitation of the last several months, little areas of drought remain in Colorado, though some dry areas have developed in Western Colorado.

The map below shows forecasted temperature deviances for September 2009. As can be seen, it is expected that much of Colorado to have  normal temperatures for the month of September 2009, with far southwestern Colorado expected to have above normal temperatures. 

The map below shows forecasted precipitation deviances for September 2009.  All of Colorado is expected to have normal precipitation for September 2009.

Little in the way of drought remains in Colorado, with no changes expected.

August Summary

August of 2009 was fairly benign compared to the previous months of spring and summer as precipitation was below average for the first time since March.  There was 1.14" of precipitation measured at DIA compared to 1.82" on average which was 0.68" below normal.  For the year there is still a surplus of 4.03" with 15.08" total so far this year.  Making the top ten in yearly precipitation is still not out of the question, but the upcoming fall and winter months will have to produce above normal precipitation to get close.  Temperatures continued to be below normal which has been consistent  this summer with the monthly mean temperature 1.3 degrees below normal.  Sunshine has been at a premium this this summer and in August the possible percent sunshine was 66% compared to 71% normally.