Friday, May 16, 2014

Huge trough provides a pleasant weekend, then summer arrives

An unusually cool few days culminated in a record low this morning and it appears the unseasonably cool weather will stick with us through the weekend before our first good spell of summer weather arrives next week. Unfortunately, cool weather also means unsettled weather and a few rain chances for Memphis in May BBQ Fest.

First, I'd like to share a few pics I received last night as showers passed over the area around 5-6pm.  The high-based showers (meaning the base of the clouds was fairly high) with sun peeking through the clouds and a stable layer of air beneath the clouds made for some interesting sky-watching and even a double rainbow!  The clouds with round "nobules" or "sacks" below them were mammatus clouds, which are more commonly associated with severe weather, but which also form when falling precipitation encounters a stable layer of air, thus making the cool formations on the base of the clouds.

Mike H sent us these pics of the mammatus clouds on Twitter.

Joey Sulipeck took this great pic of the rainbows over Barbecue Fest
Posted by the Memphis Redbirds Twitter account right before the first pitch last night!
Looking ahead to the next 72 hours, the huge trough of low pressure at the mid and upper levels of the atmosphere will  slowly break down and move east, but until it does, it means continued cool air over the area. Clear skies and light wind this morning allowed temps to fall into the mid 40s across the area. Officially, the airport dropped to 46, which breaks the record low for the date of 47 set last in 1967.

Besides cool temps, the trough also has upper-level disturbances rotating around it. As we are in the base of that trough, the disturbances pass over and bring us rounds of precip, one of which occurred last evening. The next fairly strong one is due in tonight and will bring a better chance of showers overnight into the early morning Saturday. Though a shower is possible this evening, most rain will hold off until after midnight and end by mid-morning Saturday if not a bit quicker. one more, slightly weaker, disturbance arrives Saturday night, bringing another chance of rain late Saturday night into early Sunday.

That should be the last chance of rain for some time as the trough moves east and a summer-like ridge of high pressure build in behind it for next week. The transition from trough to ridge is shown in the animation below, which depicts forecast atmospheric pressure and wind at about 18,000' up every 24 hours from this morning through next Wednesday. The yellows/red over the center of the country are this morning and that is the huge trough (it looks like a dip, or trough, in the pressure pattern). You can see that that area of yellows/reds weakens and shifts north, while a hill, or "ridge," or light browns moves east across the southern U.S. The south-central and southeast U.S. is under that ridge by Tuesday and Wednesday as it strengthens (light brown becomes pink). As that happens, the ridge warms temperatures and suppresses precipitation. In other words, a hot and dry week is ahead!

Loop of 500mb (18,000') pressure pattern and wind, showing a significant trough being replaced by a large ridge.
You can find our complete MWN Forecast, including temps that bottom out in the 50s this weekend and rise to near 90 by week's end, on our mobile apps and website. (A side note about the apps - new features are nearly complete and ready to release, so if you don't have the app yet, now is a great time to get it!)  Enjoy this fine weather this weekend, keep an umbrella handy in the mornings especially, then get ready for an early taste of summer as schools let out and we head towards Memorial Day weekend!

Erik Proseus
MWN Meteorologist

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