Sunday, September 4, 2011

cold frontal passage

A much welcome cold front, complete with high dew point and high humidity passed through Oklahoma last night. If it ain't the heat its the humidity. It did bring 0.2" of precipitation with it, and yet another band of very light precipitation will hit us today. (update: no extra rain.)

It appears that the continental modified air mass made its way down as expected albeit shallow. It looked more like an outflow boundary passage than a cold front (see Norman mesonet time series below). The next shot of "cooler" air comes later this morning with the drop in humidity but as the sun comes out it won't actually be cool. The baroclinic zone associated with a reinforcing shot of actual cold air is still in KS. Lets look at the sequence of potential temperature gradient maps from the University at Albany (http://www.atmos.albany.edu/gopher-local/surface/theta/).

The sequence is from 1200 UTC 9/3/2011 through 1200 UTC 9/4/2011 (I left 0900 UTC on 9/4 for brevity). You can see all kinds of baroclinic (yellow to red shading implies stronger potential temperature gradient) zones from around tropical storm Lee, to the cold front in KS, to the dryline in TX, to the hints of the modified cold front approaching OK but already into CO and the TX panhandle, and the back door cold front into AZ. 

From this perspective, the real cold air will actually arrive later today and tonight. Last nights 2" soil temperature under bare soil was 87F and this morning it was 74, while our 12" soil temperature under sod is still 82. It will be interesting to see what our actual versus forecasted high temperatures will be given this initial condition change. Currently our forecast high for today is around 83 and tomorrow it is 76.