Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 241802
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
102 PM CDT Tue Mar 24 2015

.SHORT TERM:  (Through Late This Afternoon)
Issued at 354 AM CDT Tue Mar 24 2015

Strong shortwave now moving across the eastern Rockies is forcing
lee-side cyclogenesis over the Oklahoma pan handle.  The surface
cold front looks to be nearly stationary now over extreme southwest
Missouri at this time, but the low level jet is pouring warmer and
more moist air up over the colder air at the surface.  Warm
advection precip has spread across much of northern Missouri and
into west central Illinois at this time, as well as parts of
southwest Missouri.  Short range guidance is in good agreement that
this rain with embedded thunderstorms will continue to expand and
overspread much of the area along and north of I-44/I-70 this
morning.  The rain cooled air as well as cool easterly flow should
help to keep the front south of I-70 in Missouri and I-64 in
Illinois today even though the warm advection will continue to be
quite strong.  The front will likely lift northward across some
portion of the area though.  South of the front temperatures will
likely get into the upper 60s to low and mid 70s, while north of the
front we`ll be hard pressed to break into the mid 50s.

Severe thunderstorms still a good possibility across a portion of
the area this evening.  Latest short range guidance shows a broken
line of thunderstorms moving into the area between 23-02Z this
evening as the surface low moves into central Missouri.  There will
certainly be enough deep layer shear for severe weather across the
area with values in excess of 50kts.  Models are showing 0-1km
helicity of 300-500 m2/s2 as well.  The limiting factor will be
instability.  Latest guidance has backed off from yesterday`s runs
on CAPE values with MLCAPE barely able to crack 500 J/kg and even
MUCAPE unable to get above 1000 J/kg.  This is most likely due to
the aforementioned surface front being further south than previously
expected.  The 4km NCEP WRF looks reasonable with the broken line
moving into central Missouri around 02Z and then making it about 1/2
way to St. Louis before weakening.  Still should get some strong
thunderstorms west of the Mississippi river, with locally damaging
winds being the primary threat due to the strong kinematics.  Can`t
rule out inch hail or larger if we get some rotating storms.  And of
course there continues to be a small tornado threat due to the low
level shear/helicity...especially in the vicinity of the warm front
as it tries to lift north ahead of the surface low.

Carney

.LONG TERM:  (Tonight through Monday)
Issued at 436 AM CDT Tue Mar 24 2015

A deep full-latitude trof will begin digging across the Great Plains
Wednesday forcing another surface low to develop over north Texas.
The surface front from tonight`s storms looks to be over the Ozarks,
and it is forecast to jump northward as the low spins up during the
morning.  Models are having some trouble deciding where the boundary
will set up, but the consensus is near the I-44 corridor in Missouri
and the I-70 corridor in Illinois.  A strong low level jet will be
pointed right at the low level baroclinic zone.  All models print
out QPF in excess of 2 inches along and south of the boundary.
Strong deep layer shear in excess of 50kts will again be over the
area, and the atmosphere will be more unstable with MLCAPE in excess
of 1000 J/kg and MUCAPE as high as 1500-2000 J/kg across our
southern zones according to the NAM.  GFS is a little more
conservative, but it should be plenty unstable.  Additionally, the
boundary does`t look to be moving very quickly, so with the low
level jet pumping moisture into the baroclinic zone for an extended
period, there will be the potential for flooding.  Will have to keep
an eye on the heavy rain potential.

Quieter and colder weather is still expected for Thursday through
Saturday as the longwave trof digs into the eastern CONUS allowing a
strong Canadian high pressure system to drop down across the Great
Plains and Midwest.  Temperatures well below normal are expected
Friday and Saturday as the high drops southeast.  Northwest flow
aloft will carry a couple of shortwaves down across the area.  The
GFS prints out light precip as these shortwaves pass, but have kept
mentionable precip out for now as the best forcing and highest QPF
look to be west of our CWFA.  Additionally the surface ridge to our
northeast/east will be pumping cold dry air in under the shortwaves,
so it might be difficult to get precip to reach the ground.

Medium range guidance diverges Sunday into Monday with the warmer
GFS showing a FROPA and light rain Sunday, and the cooler ECMWF
staying dry.  ECMWF brings the front through Monday with precip,
while the GFS has high pressure over us.  As there was no clear
preference in solutions, stuck close to the ensemble guidance for
Sunday and Monday.

Carney

&&

.AVIATION:  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1242 PM CDT Tue Mar 24 2015

Patchy light rain in the St Louis metro area should shift east of
this region later this afternoon. Only a slow rise in the IFR
cloud ceilings is expected this afternoon with a warm
front/stationary front south of the taf sites. A band or line of
convection is expected to move fairly quickly eastward through the
taf sites this evening as a strong upper level disturbance and
a surface low move through the area. The low level clouds should
finally clear out late tonight, but there may be at least light fog
late tonight and early Wednesday morning as the surface wind weakens.
The fog should dissipate by late Wednesday morning. E-sely surface
wind will veer around to a w-swly direction late this evening
after the passage of the surface low. The surface wind will back
around to a s-swly direction by late Wednesday morning.

Specifics for KSTL: There may be more light rain early this
afternoon. The fog should dissipate later this afternoon with the
ceiling rising into the MVFR catagory. Showers and thunderstorms
will move through STL between 02-06Z this evening. The low level
cloud ceiling should clear out late tonight with some fog
developing as the surface wind weakens. The fog should dissipate
by late Wednesday morning with just some mid-high level clouds
expected for Wednesday afternoon. E-sely surface wind will veer
around to a w-swly direction around 06z tonight, then back around
to a sely direction Wednesday afternoon.

GKS

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     52  46  71  41 /  50  90  20  90
Quincy          45  40  63  34 /  30 100  20  60
Columbia        56  43  69  38 /  80 100  30  80
Jefferson City  61  43  73  39 /  80 100  50  80
Salem           51  48  69  42 /  70  80  10 100
Farmington      61  48  72  42 /  80  80  50 100

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...FLASH FLOOD WATCH from Wednesday evening through late Wednesday
     night FOR Crawford MO-Iron MO-Jefferson MO-Madison MO-
     Reynolds MO-St. Francois MO-Ste. Genevieve MO-Washington MO.

IL...FLASH FLOOD WATCH from Wednesday evening through late Wednesday
     night FOR Clinton IL-Marion IL-Monroe IL-Randolph IL-St.
     Clair IL-Washington IL.

&&

$$

WFO LSX






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