Area Forecast Discussion
Issued by NWS St. Louis, MO

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241
FXUS63 KLSX 271155
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
655 AM 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 12z TAFs through 12z Thursday Morning)
Issued at 645 AM CDT Wed Apr 27 2016

Rain will weaken or exit this morning as IFR CIGs slowly retreat
to the north. They should be exiting all STL metro sites in the
next hour or two, but may take much if not all of the morning
before lifting at UIN. Another round of storms will fire this
afternoon and handled with VCTS at most sites with targeting a
smaller range of time in the TAF with TEMPO groups until
confidence increases to slide to prevailing. Rain chances will
sharply decrease this evening and skies clear.

TES

&&

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

$$

WFO LSX



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