Sunday, August 11, 2013

A few unsettled days then a taste of early autumn, plus a note on the Perseids

For well over a week, NOAA's Climate Prediction Center, which issues medium range forecasts that go out up to 14 days, has been advertising a cool spell for mid August.  I tried to imagine that as I sweat through my clothes Friday morning while mowing the yard in upper 70s dewpoints and 80% humidity! Turns out, it looks like they were right... but more on that in a minute.

For the first half of this week, we get to deal with a rainy, humid pattern that results from a frontal system that has taken up residence over the region.  When the front was north of us, it was the railroad track that allowed training storm systems to flood southern Missouri and portions of northern AR. The past day or so, it slipped south and brought much of the metro a couple of rounds of thunderstorms (nearly 2" fell today at MWN headquarters in Bartlett).  As of this evening, it has again retreated just to our north, but will remain in the area until it gets a good shove from a stronger system and change in the upper level wind pattern.

So, for the next couple of days (at least into Wednesday morning), the front and tropical moisture from the Gulf will mean continued humid conditions and chances of t'storms. In fact, as the front draws closer again on Tuesday (this time pushed by another cold front that will finally "clear the air" of all this humidity and rain chances), thunderstorm chances will increase - with a good chance everyone sees decent rainfall by Wednesday morning. Any storms will be capable of very heavy rain thanks to the moisture-laden atmosphere overhead, as well as copious lightning and a few strong wind gusts, especially from any afternoon storms in near-90 degree air.

On Wednesday, everything changes.

Early taste of autumn

A trough (basically a "valley" in the upper-level weather pattern which typically hosts cooler air) will set up over the northern half of the eastern U.S.  This trough will finally push a humidity-busting cold front through the Mid-South. With cooler high pressure at the surface building in over the Ohio Valley, we'll finally see an end (even if temporarily) to the oppressive humidity and rainy pattern. In addition, temperatures will be below normal to end the week and start the weekend! Recall that it will still be August though, which means summer is not over, but hopefully everyone will appreciate the reprieve and early taste of autumn!  A couple of graphics below demonstrate the change in airmass and it's results for Mid-Southerners.

(By the way, many have asked, and there is no correlation between a mild summer - one in which temperatures have not threatened 100 degrees even once locally - and the following winter.)

On Friday morning, the GFS model shows a significant trough across the Mississippi Valley at the 30,000' (jetstream) level. Upper-level troughs indicate an intrusion of cooler air into the region under the trough.
GFS model temperature (red), dewpoint (blue) and relative humidity (green) for this week.
Note in particular the dewpoint drop beginning Wednesday the 14th , with some 50s early Friday and Saturday mornings. Also, note temps that peak in the lower 80s late this week and fall to near 60 a couple of mornings. This is an early guess and is likely overstating cool air, but it's a hint of early fall weather that is likely.
The Climate Prediction Center gives Memphis a 65% chance of below normal temperatures for the period August 17-21 and a 32% chance of near normal temperatures (which basically means almost no chance of above normal temps!)

Perseid Meteor Shower

Last note: tonight and tomorrow night are the peak viewing nights for the annual Perseid meteor shower. It's a shame that the weather pattern is promoting clouds across the area, because this meteor shower is typically one of the best of the year. Perseids can be visible dozens of times per hour given the right conditions.  If you want to try and see a few, the best viewing is done well away from city and artificial lights, looking straight up, from midnight until just before dawn.  If you just want to learn more, here's a great article from Perseid Meteor Shower is Peaking Now: How to Watch and here's a site where you can watch the meteors live online!

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