Thursday, November 18, 2010

A tour of GFS temperatures for the next week

Today I'm going to do a relatively simple, mostly untechnical post just talking about what the GFS model is showing for th next week.  Yes, this is a model, and we know models don't verify.  But I looking at model forecasts simply because I think our models, since they were written with rather stringently-specified physical equations, do an excellent job of really manifesting a lot of basic weather concepts.  So, even though this isn't necessarily what's happening, it's still fun to look at what physically could happen.

So it's now this Thursday--where is the cold air that was forecast to be blasting into the central US today?  It's still building across Canada:
Fig 1 -- Northern Hemispheric plot of 500 mb height (shaded) and sea-level pressure (contoured) for 12 Z, Nov. 18, 2010.  From the HOOT website.
But our once-bullish models from last week have definitely backed off with the speed at which they are advecting the colder air into the central US.  Notice on the figure above how we have a trough off the Pacific Northwest, a trough over the Canadian Maritimes, and a very broad, low-amplitude ridge between the two.  This general pattern is being forecast to hold on by both the GFS and ECMWF models until the middle of next week when the persistant troughing over the northwest finally becomes progressive and gets across the country (with a fun, deep surface cyclone according to the GFS).  As long as the central US stays under that ridging, the cold air will be held back.

The model temperature forecasts still have the central US getting colder, though the consensus is now that this really won't happen until the middle of next week.  Here's the GFS forecast for the lows this Saturday morning.
Fig 2 -- GFS forecast surface temperatures for 12Z Saturday, Nov. 20, 2010--48 hour forecast.
We can see the really cold temperatures are there, with lows in the -5 to -20 degree Fahrenheit range across much of the Canadian Prairies. Even reflected at the surface you can still see that general pattern of trough to the west, trough to the east, and ridging in between. However, note that there appears to be some convergence in the winds along a line through the Oklahoma Panhandle then stretching along the I-44 corridor through central Missouri and southern Illinois. With cold air to the north and warmer air to the south and converging winds, this hints at frontogenesis in this region.  We can even see the pressure troughing starting to occur in the pressure contours along that line.  (Remember from the last post that cold fronts tend to be associated with pressure falls...).

Sure enough, by Monday morning, we can see that there is a loosely-defined front in this region.
Fig 3 -- GFS forecast surface temperatures for 12Z Monday, Nov. 22, 2010--96 hour forecast.
We can see a nice swath of warm Gulf of Mexico air that has advected into much of the lower Mississippi River valley and into parts of the midwest, contrasting with the colder air to the north.  Very frontogenic.  However, the surface pressure field remains rather ill-defined in this area.  What's the dominant low pressure center?  Is it over western Ontario?  Or near Kansas City?  The winds to the north of this front remain relatively light, partially because of the unorganized pressure field.  As such, no strong cold air advection is occurring.  However, you can see that the cold air was begun to expand in western Canada.  It has also gotten much colder in Minnesota, the Dakotas, and even down into Colorado.  Why can't the surface pressure field organize?  Probably because there's not much support from the structure aloft:
Fig 4 -- GFS forecast for 500 mb heights and winds for 12Z Monday, Nov. 22, 2010--96 hour forecast.
At 500 mb, we can see how there is a broad jet streak along that same line where the weak frontal boundary is at the surface.  This is no coincidence--the increase in winds aloft in a direction parallel to the temperature contours is a thermal wind response (there's that term again...).  Remember from before--temperature gradients below cause winds to change aloft.  (Though, admittedly, when I've been talking about thermal wind before, I've talked about it in the opposite way--if we see winds changing with height this means a temperature gradient below...but it works both ways...) We see that thermal wind connection beautifully shown here as winds aloft increase in response to the sharpening temperature gradient at the surface.  But otherwise--there's no well-defined shortwave trough anywhere along that frontal boundary to provide the right divergence aloft to spin something up at the surface.  Thus things remain poorly defined.

By Thursday of next week, things have become much sharper.
Fig 4 -- GFS forecast surface temperatures for 12Z Thursday, Nov. 25, 2010--168 hour forecast.
Over the first half of the week, a shortwave trough became much better defined along the periphery of that broader trough and that was enough to cause surface cyclogenesis. You can see now that we now have a relatively deep surface cyclone and the front has become anything but stationary.  Lows in the 20s have pushed all the way into Texas and Louisiana with very strong 20+ knot winds immediately behind the cold frontal boundary.  New England would be seeing a whole lot of snow if this cyclone were to organize in this way.  The coldest air has also moved eastward, now centered over western Ontario and northern Minnesota.  Temperatures are cold, for sure, but this cold isn't nearly as widespread as we may have originally been thinking.

One fun feature of the above map--note how there is a corridor of relatively warmer air stretching north along the high plains just east of the Rockies.  I'm talking about the swath of slightly warmer temperatures from western Kansas and Nebraska up through eastern Montana and into Alberta and Saskatchewan.  Why is it oddly warmer there when they should be in the middle of this frigid air mass?  Take a look at the wind field.  In that area, there are very strong westerly winds being forecast.  This represents downslope flow along the eastern slopes of the Rockies.  As air runs down the slopes of the mountains, it moves from lower pressures up at the mountain tops to higher pressures down near the surface.  This means the air compresses as it sinks and when air compresses, it warms.  We typically see this kind of warming associated with downslope winds.  If you've ever heard of the warm "chinook winds" along the Colorado front range or the Alberta Rockies, this is exactly what's happening.

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