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.|
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.|
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.|
|Fig 4 -- GFS forecast for 500 mb heights and winds for 12Z Monday, Nov. 22, 2010--96 hour forecast.|
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.|
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.