Monday, June 27, 2011

Cold front poised to move through the Mid-South

Sometimes in the summertime, it is hard to get a cold front to move far enough through the Mid-South to reap the benefits of drier air behind it, let alone cooler temperatures. We'll get our chance this week!  A cold front will sink south through the region on Tuesday, arriving in the metro area in the early afternoon hours. Ahead of and along the front, rain chances will increase to probably no worse than 50/50, so many folks have a chance of getting some rain, starting very early Tuesday morning.  By late afternoon, I expect to see storms, some of which will be severe, over north MS and moving south.

The biggest benefit of the front will be a temporary reprieve from very humid conditions we have had for several days.  If you follow the MWN Blog much, you know that we talk a fair amount about dewpoint. Rather than speak of relative humidity, which only really has meaning when pairing it with temperature, the dewpoint is a great measure of the amount of moisture in the air. Lately, our dewpoints have been in the lower 70s, which is very moist (and sticky) air. Dewpoints in the 60s, especially mid 60s, is much more comfortable.  Dewpoints in the 50s is indeed an unusual event during a Memphis summer.

The graph below takes a little explaining, but it is a time cross-section at Memphis taken from the morning GFS computer model showing plots of temperature (red line) and dewpoint (green line), relative humidity (red, green, blue background), and rain amounts (green vertical bars).  It is read backwards with the earliest time on the right and going into the future by reading right to left ( <--- ).  The far left is Saturday morning.  Altitude increases as you go up the graph.
6/27/11 (7am) GFS computer model output through Sat AM (click for larger image)
Anyway, a couple of things to point out.  Notice the high temperatures (peaks in the red line) are lower in the middle of the week behind the front (then start climbing again my late week), but dewpoints also fall mid-week. It is indicating off and on rain from early Tuesday morning through Tuesday evening (which may be a little late as an end time).  Finally, all the of the "blue air" starting Wednesday evening depicts a dry airmass, so there should be lots of sun/clear skies and the lower dewpoints during that time mean more comfortable air, even though temperatures will still be in the 90 degree range.

Bottom line: drier and cooler air will exist after we get through tomorrow's thunderstorm chances and the front passes through. By the way, I didn't show it, but the heat and humidity return for the holiday weekend!  Details in the MWN Forecast.

(We're also on the edge of a Slight Risk for severe weather with the frontal passage tomorrow. I expect to see the most organized severe weather threat over north Mississippi during the afternoon hours. An isolated high wind gust or hail report are not unlikely though over the metro area. More in the MWN Storm Center.)

For weather information for Memphis and the Mid-South, where and when you need it, visit on the web, on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.

No comments: