THE BIG PICTURE

Casper, Wyoming, the next city in the WxChallenge, presented a new set of challenges for me, as the wind greatly influenced the temperature here.  Depending on the exact position of high and low pressure systems, the wind can blow strong all night or go completely calm.  Because I never forecasted for a place close to the Rocky Mountains, like Casper, I was unfamiliar with the local climate conditions that influence their weather and my temperature forecasts really suffered this first week.

The morning of October 17 was fairly typical for me, as my low temperature forecast was off by five degrees.  I predicted 37 degrees and the low was 32 degrees.  The surface map on this day, below, shows high pressure centered just to the west of Casper, with a weak stationary front to the north over Montana.  Normally, winds go relatively calm at night when high pressure is near; however, due to Casper's close proximity to the Rockies, this often isn't the case which I'll explain shortly.  These MOS tables from NOAA's Meteorological Development Lab agree in keeping the wind speed up on the early morning of October 17 which I thought might keep the temperature a little above these MOS values.


This is a surface map from 12Z on October 17 that shows high pressure centered just west of Casper, Wyoming, courtesy of the
Hydrometeorlogial Prediction Center.

Moving on to the upper-air pattern, you can see on the map of 500-mb heights and vorticity valid at 12Z October 17, below, that a ridge of high pressure is west of Casper, with a series of weak disturbances over the central plains.  Even with no strong storm system in the area, I still thought by looking at this forecast surface map from NCAR there would be enough of a pressure gradient between the high and the low over Canada to keep the winds blowing on this night, but as was usual during this forecasting week, I was wrong. Next, I'll tell you why and the valuable lessons I learned to improve my forecasts.


This is an image of 500-mb heights, vorticity and wind valid at 12Z October 17 that shows a ridge of high pressure to the west of Casper and a weak trough to the
east, courtesy of National Center for Environmental Prediction.


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