Sunday, October 26, 2014

cold front

source: weather.utah.edu
The above model image is a little complicated because you have to read it backwards--Today (Sunday) is on the right, tomorrow is in the middle, and Tuesday is on the left. Pay attention to the thick blue line. This is the freezing level and it is decreasing with time. This is the cold air behind the cold front that is passing through Utah right now.

Looking at Mesowest observations we can see the timing of this front. It appears to have passed Salt Lake City around 4:00 this morning. Frontal passages are evident from a change in rapid changes in temperature and humidity or rate of change, changes in pressure rate of change, and/or a change in wind direction. Looking at the station at the Salt Lake airport we can see all of these signals:

Temperature doesn't seem to drop suddenly, be can can see evidence of a change in air mass with the rapid drop in dew point temperatures. As the day progresses the temperature doesn't rise as rapidly even though it is sunny outside. This slow heating is due to the cold air that came behind the front.
source: mesowest.utah.edu

Changes in pressure also indicate frontal passages. Here we see the pressure holding steady until 4:00 this morning (the same time the front is identified in the temperature/humidity plot shown above). This is known as a pressure check, where fronts are identified as a time when there is an increase in the rate of pressure rise. Fronts can also be accompanied by a decrease in the rate of falling pressures.
source: mesowest.utah.edu

And finally, we frontal passages are also distinguished by a shift in wind direction.
source: mesowest.utah.edu

Other stations seem to agree with the timing of the front and show these same indicators of the frontal passage. Utah County began feeling the affects of the front between 6:00 and 7:00 this morning. BYU's weather station measured a wind shift at 7:00 AM.

 Finally, it's starting to feel like Fall. Time for some hot chocolate!

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