Autumn starts crashing in

It's that time of year--time to start looking ahead to the cooler temperatures that are inevitably on their way.  A taste of more fall-like temperatures is due this week for much of the central part of the country.

Right now, we still have the same general upper-air pattern that I talked about a week ago--a ridge over the west with troughing in the east:

GFS analysis of 500mb heights and winds, 12Z, Sept. 12, 2011.

However, by mid-week, a relatively strong upper-level trough is forecast to dig down across western Ontario and into the upper Great Lakes.

GFS 54-hour forecast of 500mb heights and winds valid 18Z, Wed, Sept. 14, 2011.

Notice that for the first time in a while we have a really significant jet streak visible (the strong winds indicated by the bright colors) over the upper midwest.  This jet streak represents a particularly strong portion of the larger jet stream which circles the globe over the boundary between cold polar air to the north and warmer, subtropical-like air to the south. During the autumn in the US as the northern hemisphere begins to be pointed more away from the sun, cold polar air creeps slowly southward.  We see this manifested in the upper-air pattern as the polar jet stream also creeping slowly southward.  When we start seeing jet streaks (remember--these are embedded in the larger jet stream) over the northern US again, it's evidence that the jet stream is on its way south and cold polar air is soon to follow.

Sure enough, the GFS is forecasting a large high-pressure center to move down from Canada and settle over the middle of the country.  The air mass accompanying this high pressure center is continental and polar in origin, bringing with it cold air and clear skies.  Here are the forecast low temperatures on Thursday morning:

GFS 72-hour forecast of 2-meter temperature (colors), mean sea-level pressure (contours) and winds (barbs) valid 12Z, Thursday, Sept. 15, 2011.

This forecast shows low temperatures in the low 20s in parts of Minnesota--with lows in the 30s and low 40s also predicted for much of the midwest and northern plains.  You might also notice how the leading edge of the cold air has a rather distinct boundary--there is a pronounced wind shift along the leading edge as well as a strong temperature gradient.  These are the classic marks of a cold front, and we really haven't had a decently strong cold front move through the country in quite some time.  As this front moves south and encounters rich moisture, storms and rain along the front will probably be possible on Wednesday and Thursday.  The model forecast for Wednesday night would seem to confirm this:

GFS 60-hour forecast of 6-hour accumulated surface precipitation valid 00Z, Thursday, Sept. 15, 2011.

There's a fair amount of precipitation forecast right along that frontal boundary.

Is this unusually cold weather for this time of year?  Indeed it is.  Here's a plot of the climatological records and normals so far this year for Chicago:

2011 observed, normal, and record values so far as of Sept. 11, 2011.  From the NWS WFO Chicago.

In the top panel of the graph, the bottoms of the pale blue bars indicate the record low temperatures.  For mid-September, the record lows are around 40 degrees, and based on the forecast above it looks like we may get close to that.  So, an unusually chilly few nights are coming up this week.