Thursday, August 11, 2011

Rounds of MCSs bring cooler weather

As reports from people I know in Oklahoma keep streaming in on Facebook, it looks like today is going to be one of the cooler days in the past several weeks in that area.  For a place that has seen daily highs above 100 for much of the past month, the fact that it's only 70 degrees in Norman, OK, around noon is quite spectacular.  That's more Seattle-like in terms of the weather...

But we can see in looking at a meteogram of weather variables since the beginning of the month that the high temperatures have been getting lower over the past few days:
Meteogram from the KOUN station in Norman, Oklahoma since August 1, 2011.  Temperature is in the top panel.  From the Weather Underground site.
In fact, the highs over the past few days have actually been pretty close to normal.

So what's helping promote this cooldown?  A roughly zonal pattern aloft has storms firing on the smallest shortwave perturbations.  Here's a mosaic radar image from 17Z today:
NEXRAD radar mosaic at 17Z, Aug. 11, 2011.
You can see a nice line of storms (an MCS) that pushed through central Oklahoma today, bringing lots of rain and cooler temperatures in its wake.  However, as with most of our summer MCSs, there wasn't "strong" upper-air support.  Here's the 500mb analysis from 12Z this morning.
GFS analysis of 500mb heights and winds for 12Z, August 11, 2011.
No startling features stand out over the southern plains.  There does look to be a little perturbation in the wind field through northeastern Oklahoma, but at this resolution, it's hard to tell.  Elsewhere, a shortwave trough that moved through the Pacific Northwest has led to a cool start out here this morning.  The deeper trough over western Quebec has also brought slightly cooler weather to the Great Lakes region.

As I mentioned in an earlier post, MCSs (particularly MCCs) don't need strong upper-air support--just enough to get them going.  However, their motion is still directed by the upper-level winds.  In the pattern this morning, you can see that the flow at 500mb is generally southeastward across the southern plains.  It's no coincidence that that's the direction the storms are moving.

However, while there's a relatively flat pattern with maybe some broad troughing aloft over the southeast, at the surface it's a slightly different story. Here's the surface analysis for 16Z this morning:
RUC 16Z surface analysis of temperature (colors), winds (barbs) and mean sea-level pressure (contours) for August 11, 2011.
Notice that there's still a big ridge of high pressure over much of the Mississippi River valley.  High pressure at the surface tends to indicate low-level subsidence, which means downward air motion near the surface.  This works against thunderstorm formation by suppressing the updrafts.  As such, as this MCS moves eastward out of Oklahoma, it's probably going to encounter a slightly more hostile environment to its continued maintenance or growth.  With little upper-air support (no potent shortwave troughs or jet streaks to speak of), the MCS will probably weaken and die off before too long--just like the last one did.  The SPC has continued a slight risk down into Louisiana and Mississippi for this MCS, as it may take a little while to completely die out.
SPC day one convective outlook for August 11, 2011 as of 1630Z.
Further north, a slight risk area in the northern plains doesn't extend into the surface high pressure region at all.  This slight risk area is in response to the potential for lift and storm development as that trough from the Pacific Northwest moves in.  The persistence of the surface high pressure over the mid-Mississippi valley will tend to keep storms away from the upper midwest and Great Lakes area over the next few days.

So...enjoy the cool weather while it lasts.  The CPC still has long-term outlooks indicating above-normal temperatures in the southern plains for the next  month or so.  I guess you just have to take what breaks you can...

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