Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 261804
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
104 PM CDT Sat Jul 26 2014

.UPDATE:
Issued at 1241 PM CDT Sat Jul 26 2014

The going forecast for this afternoon still looks pretty good.
Will likely have to bump up PoPs to high chance/likely in an
afternoon update for central MO. MCS generated storms are
maintaining themselves, even strengthening. Current thinking is
that these storms may become marginally severe, but that the main
event will be later this afternoon/this evening developing on the
MO/IA border and moving east-southeast across the northern/eastern
1/3 of the CWFA. Current likely PoPs for late this
afternoon/evening still look pretty good for the moment. Will
discuss further in the full short-term update. Heat advisory looks
to be in good shape at this time so no changes expected.

Carney

&&

.SHORT TERM: (Today and Tonight)
Issued at 330 AM CDT Sat Jul 26 2014

Two major concerns over the next 24 hours are heat today and the
thunderstorm/severe threat this afternoon and tonight. The new
model runs continue to show a very favorable pattern for hot
temperatures today with a prominent low level thermal ridge in
place featuring H85 temperatures greater than +24 degC this
afternoon and veering southwest-westerly lower trop flow. This
should yield high temperatures well into the 90s with the I-70
corridor in line for 95-100. These values agree well with upstream
conditions yesterday. As dicussed in the previous AFD, mixing will
result in a dew point min from the eastern Ozarks into portions of
central MO, with the highest dew points displaced north and east of
the hottest ambient air temperatures. The best overlap and highest
heat indices near 105 degrees will be within metro St. Louis and
a few counties west and east, and I have expanded the heat
advisory this afternoon to account for the slight greater areal
extent.

Now for the thunderstorm and severe weather threat. There could be
a few spotty showers or thunderstorms through mid morning roughly
along an axis from Bourbon to St. Louis to Vandalia where the
westerly LLJ is producing some moisture convergence and there has
been an increase in cloud cover in the last hour or so. Otherwise
the main threat for storms and severe weather looks to be from
around mid-afternoon through this evening. I`m still not exactly
sure how this is going to unfold. The synoptic scale front should
be located across northern MO and central IL and the airmass along
and south of the front will be very unstable and initially capped
owing to warm mid level temperatures. The cap removal is the
largest unknown and could occur via a number of mechanisms - an
eastward moving MCV remnant from the weakening MCS over KS, the
leading edge of height falls/cooling aloft associated with the
southeast moving Saskatchewan upper low/trof, cooling associated
with mid level convection which could form above the EML over
northern MO. The concensus of most of the guidance is that robust
storm development will occur across northern MO into central IL
after 20-21z within the frontal zone. Unseasonably strong deep
layer shear and the high instability favor organized severe. The
overall pattern and shear/strenghening flow aloft are suggestive
of upscale growth from the late afternoon and evening into a forward
propagating MCS moving into the Ohio Valley with a high damaging
wind threat. The SPC moderate risk area has the zone of greatest
probability well delineated at this time.

Glass

.LONG TERM: (Sunday through Friday)
Issued at 330 AM CDT Sat Jul 26 2014

The cold front will extend from northeast Illinois to around St.
Louis into northern Oklahoma at 12z and should exit the CWA by
early afternoon as the upper low/trof moves southeastward into the
Great Lakes. The precipitation threat will be considerably lower
Sunday morning ahead of the front and I don`t expect to see much.
Increasing northwesterly winds will usher drier and cooler air
into the region during the afternoon in the wake of the front. The
large surface high will settle into the Nation`s midsection on
Monday and then become a dominant feature into midweek as a deep
upper trof persists over eastern NOAM. Below average temperatures
will prevail throughout next week.

Glass

&&

.AVIATION:  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1241 PM CDT Sat Jul 26 2014

Primary concern is thunderstorms this afternoon and evening.
Expecting the precipitation over west central/centra MO to
continue to move eastward. It is unclear yet how much these storms
will strengthen/expand, but some strong wind gusts and hail are
possible with this complex as it moves eastward. Another area of
concern will be in northeast MO/west central IL later this
afternoon/this evening where a complex of severe thunderstorms is
expected to develop and should move rapidly east-southeast. Wind
gusts in excess of 50kts and large hail will be possible with
this cluster. Still some uncertainty on the exact track of the
thunderstorm complex, though the most likely parts of the area to
be affected will be along and north of a line from KIRK to KSLO.

Afterthe storms move through this evening, expect VFR flight
conditions. A cold front will move through early Sunday morning
shifting the wind to the west-northwest.

Specifics for KSTL:

Primary concern at Lambert is the possibility of thunderstorms
early this evening. Current thinking is that the storms over
central/west central Missouri may not affect the terminal, but a
cluster of severe thunderstorms will develop over northern
MO/southern IA and race east-southeast. Think the best threat for
storms will be north of Lambert along and north of a line from
KIRK to KSLO. After storms clear the area this evening expect VFR
flight conditions to prevail. A cold front will pass through the
terminal early Sunday morning which will shift the wind to the
northwest.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...HEAT ADVISORY until 7 PM CDT this evening FOR Franklin MO-
     Jefferson MO-St. Charles MO-St. Louis City MO-St. Louis MO-
     Warren MO.

IL...HEAT ADVISORY until 7 PM CDT this evening FOR Clinton IL-Madison
     IL-Monroe IL-St. Clair IL-Washington IL.

&&

$$

WFO LSX






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