Outlook for Winter 2010 and a Storm (probably rain) on Sunday

Welcome back to the blog!  As crazy as it is during the Holidays, I'll try to start posting regularly again.  Hopefully there will be something to post about!

Everybody wants to know the same thing:  will this be a winter like 2009?  Will it even snow?  How much?

Those things are difficult to predict and long-range forecasting is really only slightly more accurate than throwing darts at index cards on the wall with snowfall totals and dates.  But it is possible to make an educated guess and large-scale climate conditions make it a bit easier.

Almost everyone has heard of El Niño and has heard it blamed for everything from huge storms in the western United States to a string of car thefts in Kansas City.  In reality, El Niño is defined as "warmer-than-normal sea surface temperatures in the western Pacific Ocean".  This warm water causes a succession of atmospheric responses that affect the weather in the United States.

The exact opposite condition is known as La Niña.  The cooler-than-normal sea surface temperatures in the western Pacific Ocean induce a low pressure zone there.  Since everything in the atmosphere is interconnected, this unusually low pressure has side effects throughout North and South America.

Currently, there is a moderate La Niña occurring and it's forecast to strengthen a bit (which is an odd turn of phrase since it actually means that sea surface temperatures in the western Pacific will get a little colder) during the months of November-January.

So, what is the typical winter experience for the United States during a La Niña of the strength we're expecting?

In terms of precipitation:

• Above-average precipitation in the Pacific Northwest and Northern Rockies
• Above-average precipitation in the Ohio Valley
• Below-average precipitation in the south-central and southeastern U.S.
• "Normal" everywhere else

In terms of temperature:

• Below-average temperatures in the coastal sections of the western states
• Above-average temperatures in the central and southern states
• "Normal" everywhere else

Here are two long-range outlook maps for the U.S. this winter.  They are issued by the Climate Prediction Center (CPC) of the National Oceanic and Atmospheric Administration (NOAA).  Since this is a long-term average map, it's less important to look at the precise location of, say, Washington D.C. than it is to consider what trends the maps suggest.

First precipitation:

Green regions labeled "A" indicate above-average precipitation.  The gradient lines are percentage probability of occurrence.  Orange regions labeled "B" indicate below-average precipitation.

Again, keeping in mind this is an average, this forecast map suggests that more storms will pass to our west than to our east.  If you like snow, you want coastal storms that pass offshore or move up the coast.  Storms that pass to our west do two things in the winter:  they bring rain or they bring ice.  Both of those suck. 

Now temperature:

Again, look at the trends.  Washington isn't quite in the "above normal" 30% area but it's close.  Generally, the forecast is for normal to slightly-above normal temperatures.  That, in combination with the first map, suggests a winter where snow events are less frequent than last year and where many storms are deflected to the west of us.

This all adds up to the likelihood of a winter with normal to slightly below-normal snowfall in the DC area and temperatures that are normal to just above-normal.

About Sunday.  A fairly large storm is expected to develop but most models are following the large-scale trends and suggest that the storm will pass to our west.  This will probably lead to a very rainy and cold Sunday.  It bears watching.  If the track is further to the east, it could get much more interesting.

If this were 2009, I'd be excited about the possibility but, for now, it seems like this one is going to be wet and not frozen.

We'll see how the models trend as the week goes on.