Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231752
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1252 PM CDT TUE AUG 23 2016

.SHORT TERM...  (Through Late Tonight)
Issued at 255 AM CDT Tue Aug 23 2016

Scattered showers and thunderstorms will move into central and
southeast Missouri this morning as low level moisture convergence
increases on the nose of a 30-40kt low level jet.  This will occur
ahead of a weak shortwave trough currently moving northeast across
the Central Plains in the west southwesterly mid level flow. The
chance of showers and thunderstorms will spread to northeast into
eastern Missouri and southwest Illinois by this afternoon as the
atmosphere destabilizes.  Thunderstorms will become likely tonight
over northeast Missouri and west central Illinois as an area of more
concentrated low level moisture convergence sets up ahead of another
shortwave trough.

Highs today will remain below normal as mixing will only be up to
900mb and clouds will be spilling into the area ahead of the
shortwave trough.  Lows tonight will be significantly warmer with
the higher dewpoints advecting into the area.

Britt

.LONG TERM...  (Wednesday through Monday)
Issued at 255 AM CDT Tue Aug 23 2016

Scattered convection should be ongoing north of I-70 Wednesday
morning due to good isentropic lift on the 30-40kt low level jet.
Lift weakens by 15-18Z and shortwave ridging builds overhead which
should bring a relative minimum to PoPs during the late morning
and early afternoon. Guidance indicates at least partial clearing
which given the strong late August sun angle should provide ample
heating to redevelop thunderstorms in the afternoon. NAM prints
QPF as far south as the I-64 in Illinois by 00Z Thursday while the
GFS has light QPF even further south across the Ozarks. I can`t
rule out some isolated or widely scattered thunderstorms that far
south given the aforementioned heating (GFS is developing 2000-
2500 J/Kg MLCAPE along and south of I-70) and the likelihood of
lingering outflow boundaries...but I think the main focus for
convection should be further north across northern Missouri and
southern Iowa in the vicinity of the synoptic cold front. This is
where we should have the greatest coverage and therefore highest
PoPs late Wednesday afternoon and Wednesday night. MLCAPE in
excess of 2500 J/Kg in the vicinity of the front combined with
0-6km shear of 30-35kts along the front should yield strong to
possibly severe thunderstorms. Think most likely convective mode
will be multi-cell clusters perhaps organizing into a strong line.
Forward propagating Corfidi vectors actually point northeast
through the night so not sure how far south the convection will
get. Gut feeling is that convection won`t make it as far south as
the I-70 corridor, but will likely keep some low PoP that far
south in case convection develops further south than anticipated.

The front creeps southward in the wake of Wednesday/Wednesday
night`s convection. However, lacking a strong shortwave to really
drive it south and break the back of the large upper level ridge
over the southeast, the front will have a hard time pressing too far
south.  Latest guidance has the front hanging up around I-70
Thursday night with a general repeat of afternoon and evening
convection much like Wednesday.  Overnight storms on Thursday night
should drive the front a little further south down across the
Ozarks, by Friday morning...but increasing low level southerly flow
Friday afternoon and Friday night will push the front back
northward.

Deep south-southwest flow will keep warm and humid weather
locked in over the region through the weekend into early next week.
Looks like seasonable temperatures in the mid and upper 80s with
lows in the 60s.  GFS almost continuously prints out QPF over the
area in the warm airmass.  This is likely overbaked given the lack
of forcing except on Saturday night when a weak wave moves across
the upper Midwest.  30kt low level jet veers to the west-southeast
across much of Missouri producing some decent isentropic lift and
moisture convergence across northern Missouri into central Illinois.
Think late Saturday night and early Sunday morning will be the best
shot for precip over the weekend, otherwise will handle with
slight/low chance.

Carney
&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1231 PM CDT Tue Aug 23 2016

A mix of VFR/MVFR cigs to persist through first half of forecast
period. With scattered showers and thunderstorms, hard to pin
down timing and coverage, so kept vcsh/vcts for metro area tafs.
KCOU to see better chances of storms this afternoon, so have tempo
conditions from 20z to 24z. By tonight, could see complex develop
over northern portions of forecast area, sliding through KUIN
after 03z, diminishing towards daybreak. Just have vcts mention
once again for taf sites along I-70 corridor with this complex. As
for winds, to persist from the southeast to south.

SPECIFICS FOR KSTL:
A mix of VFR/MVFR cigs to persist through first half of forecast
period. With scattered showers and thunderstorms expected, hard
to pin down timing and coverage, so kept vcsh/vcts for metro area
tafs through 11z Wednesday. As for winds, to persist from the
southeast to south before veering to the southwest after 18z
Wednesday.

Byrd
&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$
WFO LSX



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