Cold fronts are well-known for their transitions from one air mass (warm and humid) to another (cold and dry). In addition to a drop in temperature and dew point with frontal passage, winds shift from southwest before the front to northwest after frontal passage, showers and thunderstorms (sometimes severe) accompany the front and atmospheric pressure falls as the front approaches and rises following frontal passage. When an arctic cold front plows into an unseasonably warm and humid air mass, the transition is often dramatic.
Take a look at this image (Fig. 1) and you can see the spatial variations associated with the arctic front that is currently moving across the central U.S. Further north and west (where the air mass ahead of the front was not as warm and humid), and during nighttime hours, the transition was significant, but not dramatic. Consider Oklahoma City, OK where the front passed late on Friday night into early Saturday morning. The temperature just fell steadily with frontal passage; however, the dew point tumbled about 17 degrees in an hour (Fig. 2).
At Memphis, TN, where the front passed through shortly after midnight Sunday, the pre-frontal air mass was much more tropical. Thus, the temperature and dew point tumbled 18 degrees (70 degrees to 52 degrees) from around 1:30 a.m. C.S.T. to 2:45 a.m. C.S.T. (Fig. 3). The biggest drop occurred from 1:31 a.m. C.S.T. to 1:39 a.m. C.S.T. when the mercury fell 9 degrees. That translates into a computed rate of minus 67.5 degrees per hour.
Houston, TX saw a 16-degree tumble between 2:27am C.S.T. and 3:53 a.m. C.S.T. on Sunday morning (Fig. 4). During midday Sunday, Baton Rouge, LA (Fig. 5) saw the temperature drop from 75 degrees at 12:53 p.m. C.S.T. to 63 degrees a scant 29 minutes later. Between 2:22 p.m. C.S.T. and 2:24 p.m. C.S.T. the temperature dropped another 6 degrees. That secondary temperature tumble translates into a drop of 180 degrees per hour.
There is no way that temperatures would fall like this for an entire hour (thank goodness!). I merely determined the computational hourly rates to highlight just how dramatic the temperature tumble really was.
Dr. Ken Dewey, a meteorology faculty member at the University of Nebraska – Lincoln, provided another look at the front. Driving north from Austin (site of last week’s Annual American Meteorological Society conference), Dewey passed through the front and shared the following observation. “It was 71 degrees F last evening as we drove north of Dallas…at 75 mph it only took 3 exits to hit the snow squalls. I love these sharp temperature gradients.”
I have to agree with Dewey. When frontal transitions are this dramatic, it makes explaining cold fronts a lot easier.
© 2013 H. Michael Mogil