Friday, October 30, 2009

Analysis of surface conditions during a frontal passage

The cold frontal passage on October 30 brought more than just a torrent of rain to the Mid-South. While the flooding conditions and widespread 3-6" rainfall totals over two days were certainly impressive, the cold front itself also was easily captured by watching weather parameters before and after the front crossed the region. Take the graphs below from WXLIVE! in Bartlett:

The front crossed the weather sensors at about 7:45am (or just before the second number 8 on the lower scale, x-axis, and marked by the red dotted lines).

In the upper-left, the temperature (white line) stayed above 70F all night long (it was a mild night!), peaked just prior to the frontal passage, then began a nose-dive until about 2pm when it leveled off. The dewpoint (yellow line) did nearly the same thing. Rapidly falling temperature and dewpoint (a measure of moisture in the air) are good indicators of a cold front.

In the upper-right, the barometric pressure fell prior to the frontal passage, then immediately rose a LOT (0.20" Hg in 4 hours). The pressure bottoms out as the front crosses the sensor.

In the lower-left panel, the sudden change in wind direction is also easily seen. Prior to the front, the wind was consistently out of the south to southeast. When the front hit, it switched to the west, then stayed out of the west to southwest, ushering in the cooler and drier air noted in the upper-left panel. Wind speed (lower-right) also indicates the peak wind of nearly 30 mph just ahead of the front, then a relative lull for a little more than an hour following the front before picking up again during the mid-day hours.

All of these conditions make the timing of the front pretty easy to figure out at this location. Certainly looking out the window in this case would have made the exact timing a no-brainer as well, but it's interesting to see the effects of a strong front like this on measurable parameters. In all, 3.68" of rain was recorded at WXLIVE! Thursday and Friday.

For realtime conditions from WXLIVE!, check out Rapid Fire, or for current graphs like the one above, see this page.

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