Wednesday, January 16, 2013

How cold next week?

Depending on what part of the country you live in, one may be wondering what's up with winter and how much cold weather is left.  At least for a good portion of next week, it does look like a significant cold outbreak will occur as an Arctic cold front plunges out of Canada.
By late Sunday, colder than normal air covers most of Canada, with temperatures more than 30 degrees below normal along the US border as forecast by the Global Forecast System (GFS) weather model.  This graphic is from, as below unless otherwise noted. 
Note the large area of high pressure over southern Saskatchewan as depicted by the GFS at the same time.  It has a central pressure of 1050 millibars.  Cold air outbreaks in the mid-latitudes are typically associated with a strong high pressure regions originating in the high latitudes.  The streamlines indicate the direction of the surface wind, which flows from high pressure to low pressure. 
By Tuesday afternoon the high pressure has continued a trek southeastward and is now centered in the Mississippi Valley, although it has weakened somewhat by this time with a pressure of 1031 millibars.  North winds continue across the eastern third of the United States.
The surface temperature anomaly map for the same time shows colder than normal temperatures across the eastern US.  Temperatures in the Mid-Atlantic are running about 15 degrees F below normal, which would translate to afternoon high temperatures struggling to reach 30 F in the Northern Virginia area. 
Other weather models corroborate the GFS story.  The European weather model, ECMWF (European Centre for Medium-Range Weather Forecasts) model shows 5,000 foot temperatures more than 15 degrees Celsius (27 degrees F) below normal across a large portion of the eastern US by Tuesday morning.  In fact, the ECMWF is more bullish with the cold outbreak than the GFS model.
The ECMWF model forecast temperatures for Manassas, Virginia for the next 10 days are shown above.  The top half shows the maximum temperature in six-hour time blocks, and the bottom half shows the minimum temperature in the same same time blocks.  The black line indicates official ECMWF model forecast, sometimes called the 'deterministic' model run, while the error bars show the ECMWF ensemble forecast range.  The size of the error bares is an indication of the uncertainty of the forecast.  More on ensembles in a future post : ).  Note how in the first few days the ensembles are fairly close to the deterministic run but further out in time they diverge.  By the 22nd through the 24th of January the ECMWF deterministic run indicates temperatures below freezing even during the day, although some uncertainty is indicated by a few of the blue error bars (ensemble members) extending above freezing.  Needless to say some cold weather is certainly on tap! 

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