Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 270242

942 PM CDT Sat Jul 26 2014

Issued at 935 PM CDT Sat Jul 26 2014

Trying to pin down convection threat for the remainder of tonight
remains somewhat of a head scratcher.  However, as we dig into the
details of the 00z UA data becoming doubtful that the the storm-
free conditions we are currently experiencing will continue
throughout the entire night.

Although all of the 00z raobs in the area are indicating that the
strong cap is persisting throughout the region early this evening, water
vapor imagery indicates that strong mid level WNW flow is
beginning to dig south as trof rotates around the unseasonably
strong mid level low near nw MN, and this should drag some cooler
mid level air into northern sections of the CWA during the
predawn hours aiding in the erosion of this wedge of warm mid
level air. In addition to this cooling and dynamics with the
shortwave, low level forcing should also be increasing as cold
front works its way into the mid-Mississippi Valley. And...
while the "cooled" air from the rain associated with this
afternoon`s MCV is currently parked over the CWA, airmass west of
our area (that missed out on the rain) remains very unstable at
02z, with MLCAPES of 4000-5000 J/KG, and this air will be
advected back into our area due to weak but persistent westerly
low level flow.

So...whether the storms form upstream and advect into the area,
or form in situ near the Mississippi River as suggested by the
latest HRRR, 18z NAM, and latest RUC runs, thunderstorms will
remain in the forecast overnight. Overall, think current trends of
ramping PoPs up after 05z is very reasonable, with the greatest
PoPs over northern and central sections of the CWA, where forcing
should be the greatest. A conditional threat of severe
thunderstorm is still in the cards for the region as noted in the
SWODY1, as large scale shear will be increasing as the westerlies
dip into the mid- Mississippi Valley and interacts with the
residually unstable ams.



.SHORT TERM:  (Through Late Tonight)
Issued at 324 PM CDT Sat Jul 26 2014

Primary concern continues to be convective trends for this evening.
Current trends are pointing more and more toward there being less
severe weather in our CWFA than previously expected, with the
primary threat for severe storms further north/northeast in
southeast IA and central IL.  A broad area of showers and stratiform
rain associated with an MCV which is spinning its way across the
state is putting the kibosh on much of the instability which was
forecast for late this afternoon/evening.  A limited ribbon of very
high CAPE (MLCAPE in excess of 3500-4000 J/Kg) remains over extreme
northeast MO into southern IA and a much larger area of high CAPE
over central and southern IL.  Storms are already rumbling just
north of our CWFA in southern IA and west central IL and I would
expect these areas of storms to continue to develop and strengthen.
Still cannot rule out severe storms from Kirksville to Quincy and
southeast to Litchfield and possibly Salem...but the probability
seems to be much lower than previously expected.

After the evening severe weather threat has passed, there is still
the possibility that there could be additional development
overnight.  RAP and NAM both show moderate moisture convergence at
850mb ahead of the front and generate QPF in response.  Have left
chance/slight chance PoPs going for most of the area through the
night.  The wind shift associated with the approaching cold front
should be sliding into the northern 1/3 of our CWFA between 09-12Z
with drier low level air trying to nose in.  This should effectively
cut off the chance for additional precip in those areas in the
pre-dawn hours.


.LONG TERM:  (Sunday through Next Saturday)
Issued at 340 PM CDT Sat Jul 26 2014

The cold front is expected to push through early Sunday morning.  A
minimal rain threat will exist during the morning near it, with a
dry afternoon on tap behind it.  The cooling effects will not be
felt too much early on, with another warm day expected:  max temps
still look to top 90 degrees for most areas near and south of I-70.
The big difference will be lower humidity from today, however.

After Sunday, an extended dry period is forecast as a deep longwave
upper TROF sets up over eastern North America with a stout NW flow
aloft for our region.  This usually means either dry, or surprise
hit-and-run systems this time of year, but almost always means below
average temps.  For now, am going with dry until Thursday, with low
PoPs thereafter as a system works its way down into our region and
stays for a bit.



.AVIATION:  (For the 00z TAFs through 00z Sunday Evening)
Issued at 655 PM CDT Sat Jul 26 2014

Mesoscale Convective Vortex (MCV) moved across north central
Missouri through southwest Illinois. Scattered light showers was
associated with the MVC while strong thunderstorms first formed
along the northeast quadrant of the MVC and eventually build ahead
of this featuer over central and parts of southeast Illinois.
Scattered light showers will continue over STL terminarls but will
end over UIN and eventually end over COU after 0200 UTC. Cold
front will move through UIN and COU between 0800 and 1000 UTC and
STL area after 1200 UTC. Scattered showers and thunderstorms still
possible with the passage of the cold front.

Specifics for KSTL:
Scattered showers will continue over area through 0200 UTC then
next chance for scattered showers and thunderstorms may occur with
the passage of the cold front before 1200 UTC. Southwest winds
will become northwest 10 to 15 mph after the passage of the cold





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