Tuesday, April 19, 2011

Quick glance at severe weather this afternoon

As many people know by now, the SPC has issued a moderate risk of severe storms this afternoon for parts of the midwest from Arkansas up through the lower Ohio River valley.
Fig 1 -- SPC Day 1 convective outlook for April 19, 2011.
SPC surface mesoanalysis at 1PM CDT (18Z) shows a low pressure centered over central Missouri with a large warm, moist sector extending to the east.  This warm sector is bounded by a warm front to the north across southern Illinois and southern Indiana and also by a cold front to the west from central Missouri down through eastern Oklahoma.
Fig 2 -- SPC Mesoanalysis of surface mean sea-level pressure (black contours), temperature (solid red and brown contours), dewpoint (colored shadings) and winds barbs) for 18Z (1PM CDT), April 19, 2011.
If we take a look at the current visible satellite image, we can already see lots of small cumulus clouds popping up throughout the warm sector.  There's overcast conditions with thick low cloud banks north of warm front and even west of the cold front, to an extent.
Fig 3 -- Visible Satellite image from GOES-E at 1815Z (115 PM CDT), April 19, 2011.  From the College of DuPage.
Why are we seeing all of these little cumulus clouds pop up but no towering cumulonimbus clouds?  The answer is the capping inversion.  Here's this morning's 12Z sounding from Springfield, MO:
Fig 4 -- 12Z Sounding from KSGF on April 19, 2011.  From the SPC.
Notice how the temperature warms abruptly with height right at around 850mb in the upper left panel of the diagram.  Below that, the air is saturated--the dewpoint and the temperature are practically the same.  With the sun out, it has warmed up a bit since early this morning--the current temperature in Springfield is 80 degrees.  So, that lowest level beneath the capping inversion has now become more unstable--if the surface temperature has risen up to 80 degrees, the temperature probably decreases rather strongly with height up to the base of the capping inversion now.  With a lot of moisture near the surface as well, small clouds are able to start convecting because of the steep low-level temperature drop off (making it unstable) and all the moisture.  However, because of the capping inversion, the clouds can't grow above that strong warming with height.  This is why we see a whole lot of small, low-level cumulus clouds, but no big thunderstorms yet--the clouds are being "capped" by the capping inversion.  Of course, as the cold front moves through, things will start getting lifted.  The capping inversion layer here is actually potentially unstable--if lifted, it will immediately start weakening.  But potential instability is the subject for another blog...

In contrast to Springfield, MO's sounding, Lincoln, Illinois's 12Z sounding shows a slightly different picture:
Fig 5 -- 12Z sounding from KILX on April 19, 2011.  From the SPC.

Lincoln, Illinois was north of the warm front this morning, underneath that area of thick low-level clouds we saw on the visible satellite image.  You can see this thick cloudiness on the sounding above--the air is saturated all the way up to around 700mb--the dewpoint and temperature are the same through that entire layer.  There is also a rather strong inversion present from around 850-950mb.  Because that cloud deck has been around all day, there hasn't been much sunlight getting through and, consequently, very little warming.  In fact, at 2PM CDT, Lincoln's reported surface temperature was only 50 degrees--not the best for strong convection.  Furthermore, that inversion layer is NOT potential unstable--lifting will not quickly erode through that layer.  Granted the warm front will move north--and that will bring much warmer air behind it.  However, because Lincoln has already spent much of the day under clouds and is still very chilly, the odds of maintaining strong thunderstorms here and further north will be somewhat lower.  This is part of the reason why the risk drops off north of the warm front.

That was just a quick look at some of the setup for today.  The SPC is calling for large hail and strong winds with a few isolated strong torandoes in today's event.  Much of Missouri and southern Illinois is under a tornado watch as those cumulus clouds keep building under the cap, waiting for the cap to erode or for the cold front to come marching on through.  If you live in a threatened area, please stay alert this afternoon.  The past few weeks have shown us just how fast and destructive these storms can be.

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