Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 271745
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1245 PM CDT WED APR 27 2016

.SHORT TERM...  (Through Late This Afternoon)
Issued at 339 AM CDT Wed Apr 27 2016

A strong upper level storm system was centered over western KS early
this morning with a RIDGE axis over our region.  The surface
reflection was located over central KS, showing that it had not yet
become occluded, with a frontal boundary that has become nearly
stationary stretching eastward to near the I-70 corridor in MO and
IL.  Temps were in the 50s and low 60s across our region with what
was a band of severe TSRA but has since weakened moving towards our
region from western MO.

The storm system center is expected to track northeast to the mid-MO
river valley by late this afternoon, and with the RIDGE axis over
our way slow to yield ground, will result in the TROF associated
with the storm itself to become increasingly negatively-tilted, with
the TROF axis anticipated to be over our region during the afternoon
hours.  At the surface, the low center will become occluded by
this afternoon with a new triple point likely just NW of STL metro,
with the old east-west quasistationary boundary extending east from
this, and a cold front extending south from the new triple point.

With all of this in mind, it appears the leading more active
convective edge currently from central to south-central MO will
become increasingly detached from the enhanced stratiform pcpn
behind it early this morning as it gradually dissipates, but
the leading edge convective elements should still sweep thru much
of the forecast area in the process accompanied by gusts of 30-
40mph.  This round is expected to exit into eastern IL by late
morning.

A brief period of quiet wx for midday looks likely before the
approach of the upper TROF and increasing instability should set
off numerous TSRA by mid-afternoon areawide.  The synoptic setup
will be such that if things can de-stabilize enough for the
afternoon, there will be a tornado threat, namely near and just
northeast of the quasi-stationary east-west frontal boundary that
will probably be just north of STL by this time.  Otherwise, the
environmental shear will be sufficient for storms capable of large
hail and damaging winds, notably along the edge of the approaching
dry slot from the west.  Maintained likely and categorical PoPs for
this afternoon.

Temps are going to be tricky again, with a lot of conditions being
set on pcpn development and cloud cover, so stayed within the
boundaries of MOS, with 60s north of the front and 70s elsewhere.

TES

.LONG TERM...  (Tonight through Tuesday)
Issued at 339 AM CDT Wed Apr 27 2016

Focus for the short term will be convection and severe potential.

Forecast for tonight is conditional on how storms today unfold.
Believe there will be two main areas of storms at the beginning of
the period. One area shud be across ern portions of the CWA where
storms developing this afternoon near the triple point move ewd.
These shud exit the area mid to late evening. The second area of
convection is expected across nrn portions of the CWA under the
better upper level support. What severe threat develops this
afternoon, will linger into the evening, but out of the region by
Midnight.

Mdls are in fairly good agreement over the remainder of the short
term. The upper low lifts nwd on Thurs, leaving zonal flow over the
region. Can not rule out precip across far nrn portions of the CWA,
but believe the bulk of the precip will remain N of the fnt on
Thurs. With the sfc ridge building into the region, the remainder of
the week, and thru the weekend, is expected to be seasonably cool.
There is still some question of temps for Thurs as the 850mb fnt is
progd to be across srn MO/IL. The going forecast may still be too
warm, however, with deeper mixing expected and some uncertainty,
believe trend is in the right direction.

Looking into the extd, the deep trof over the Rockies is still
expected to eject into the Plains on Fri, with precip spreading into
the area Fri night and continuing thru much of the weekend. The
system this weekend has similarities to the current system impacting
the area. There is differences among guidance and any severe threat
will be conditional on location of sfc features and therefore a low
probability attm.

With a sfc ridge building into the area Mon and thru the remainder
of the forecast period, chances for precip are small. Temps are
expected to be aob seasonal average thru mid-week.

Tilly

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1235 PM CDT Wed Apr 27 2016

North-south oriented isolated-scattered showers and weak
thunderstorms is expected to move east of KUIN and metro terminals
by 1900 UTC. Another round of at least scattered thunderstorms is
expected to develop by late afternoon. Development may be east of
KCOU but is expected to be west of the metro terminals as well as
KUIN. Atmosphere should destabilize enough to cause some storms to
be strong/severe with large hail and damaging winds possible with
stronger thunderstorms in addition to heavy rain/IFR visbys. By
mid- late evening...thunderstorms are expected to move off to the
east of the terminals with VFR conditions expected into Thursday
afternoon. Only exception could be at KUIN as some high-end MVFR
ceilings are possible by mid morning on Thursday. Winds will
initially be southeast early this afternoon before turning out of
the west/southwest behind the frontal passage later on tonight.


Specifics for KSTL:

Isolated-scattered showers/weak storms expected to quickly move
to the east of the terminal early this afternoon. Partially
clearing sky to the west will advect in within the next 1-2 hours
and help destabilize the atmosphere. Scattered thunderstorms are
expected to develop to the west of the terminal by mid afternoon
with storms possibly affecting the terminal between late afternoon
and early evening. Stronger thunderstorms will be capable of
producing large hail and damaging winds as well as heavy rain/IFR
visbys. Threat of storms should exit by mid-late evening...with
VFR conditions in its wake. Winds will turn from the southeast
early this afternoon to the west/southwest behind a frontal
boundary late tonight/early Thursday morning.

Gosselin

&&

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

$$

WFO LSX


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