Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 251033

533 AM CDT Fri Jul 25 2014

.SHORT TERM:  (Through Late This Afternoon)
Issued at 330 AM CDT Fri Jul 25 2014

Convective trends and the extent of pops are the biggest forecast
challenge today. Thunderstorms have blossomed within the last
several hours across central Iowa at the terminus of a stout
southwesterly LLJ, and along the eastern periphery of elevated
instability, deeper moisture and steep mid level lapse rates.
Short range guidance shows the LLJ slowly veering this morning,
providing favorable moisture transport and forcing as the flow
aloft and northwestely deep layer shear steer the activity east
and southeastward. The HRRR runs since 01Z have been very emphatic
supporting this scenario bringing showers and thunderstorms into
northeast Missouri and west central Illinois. I have increased
pops into the likely range in this area tapered to slight chances
just north of interstate 70. Believe that most of this activity
will be east of the MS river by early afternoon and exiting the
CWA shortly thereafter. This activity will likely delay/impeed the
northward retreat of the warm front, however in central MO there
should be no issue where highs will be around 90.


.LONG TERM:  (Tonight through Thursday)
Issued at 330 AM CDT Fri Jul 25 2014

The warm front should have exited the entire CWA by early evening
while the upstream cold frontal system will be slowly advancing
and stretching from the upper MS valley into the central Plains.
High heights aloft and warm mid level temperatures should suppress
any surface-based storm development within the large warm sector
spanning the CWA, however the area will remain subject to an
elevated shower and thunderstorm threat. While there is no defined
focus for thunderstorms, a westerly LLJ will be poised across the
area in conjunction with steep mid level lapse rates and good
elevated instability supporting a least a chance pop. These same
unfocused conditions will exist on Saturday morning. The set-up
should improve on Saturday afternoon. Strong surface heating
should yield temperatures well into the 90s with dew points in the
70s. This will result in afternoon heat indices in the 100-104 degree
range, just below advisory criteria. We will need to monitor this
closely as just a bit higher dew points and/or temperatures and a
heat advisory will be warranted. The aforementioned conditions along
with the presence of steep mid level lapse rates will yield very
unstable conditions during the afternoon/evening. There are
indications that the combo of heating and a disturbance in the
west northwest flow aloft should sufficiently weaken the cap
allowing for robust thunderstorm development along the west-east
oriented front stretching in the vicinity of far northern MO
through central IL into the OH valley. How far west development
occurs is uncertain but height falls/cooling aloft on Saturday
night could aid this process. Deep layer shear and instability is
quite favorable for organized severe with damaging winds the most
apparent severe threat at this time. I have some low pops on
Sunday ahead of the advancing front, but the overall threat of
precipitation will be lower as compared to the Saturday night.
Cooler and drier air will advect in earnest into the area on Sunday
night with a large surface high settling into the Nation`s
midsection on Monday. An unseasonably deep upper trof over eastern
NOAM through the first part of next week will keep the large
surface high in control and allow for another period of below
average temperatures as we exit July and head into August.



.AVIATION:  (For the 12z TAFs through 12z Saturday Morning)
Issued at 519 AM CDT Fri Jul 25 2014

Cluster of thunderstorms over southeastern Iowa will continue
tracking southeastward and will arrive at the KUIN TAF site by
around 12z. VSBYS may fall to high end of MVFR range as the
storms pass, but cigs will likely remain AOA 6K feet. Storms will
move into more stable atmosphere as they approach the St. Louis
metro TAF sites, and most model output suggests little or no
precip there. With that in mind, will keep all St. Louis area TAFS
dry. South winds will increase this afternoon.

Specifics for KSTL: Thunderstorms approaching from the northwest
should dissipate before moving into KSTL TAF site as they move
into more stable air. High clouds will remain through
at least midday...with south winds increasing to 10 to 14 mph by





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