Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 260848
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
348 AM CDT Sat Jul 26 2014

.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 06z TAFs through 06z Saturday Night)
Issued at 1131 PM CDT Fri Jul 25 2014

VFR fcst with a couple chances of TSTMs. The first being a low
probability of precip around the metro area around sunrise. There
is a chance that the LLJ could aid in convection breaking out along
a warm front across sthrn IA and nthrn MO overnight. The bndry stretches
from S cntrl IA SE along the MS rvr and into the lower MS vly. Feel
chance of occurrence is too low to include mention attm. The next
chance is late Saturday aftn...thru the evng and into the
overnight hrs...esp for KUIN...as a cold front sags into the
region. With concerns over coverage and timing...have left the
fcst for now. Otherwise...expect winds aob 12kts with a srly component.

Specifics for KSTL:

VFR fcst with minimal chances for precip thru the prd. First
chance is around sunrise...but is too low to include attm. The
other chance is during the evng and into the overnight...but
coverage and timing issues preclude inclusion in the fcst attm.

2%

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...HEAT ADVISORY from 1 PM this afternoon to 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 from 1 PM this afternoon to 7 PM CDT this evening
     FOR Clinton IL-Madison IL-Monroe IL-St. Clair IL-Washington
     IL.

&&

$$

WFO LSX






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