Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 141141
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
641 AM CDT Mon Aug 14 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 332 AM CDT Mon Aug 14 2017

A weak disturbance was supporting isolated to scattered SHRA early
this morning over parts of eastern MO and southwestern IL, including
parts of the St. Louis metropolitan area. SHRA should move out of
the area during the morning hours.

At the surface, southwesterly winds ahead of a developing low
pressure system over CO/OK/KS will bring a warmer and more humid air
mass into MO/IL today. Highs today are expected to reach the mid to
upper 80s, and conditions will be noticeably more humid as dew
points rise into the mid/upper 60s. By tonight, shortwave ridging
aloft will start to build into the region behind the departing
shortwave while a new surface warm front develops over NEB/IA. H85
moisture convergence along the developing warm front may support
SHRA/TSRA tonight, but precipitation is expected to remain north of
the CWA based on the expected location of the front.

Kanofsky

.LONG TERM...  (Tuesday through Sunday)
Issued at 332 AM CDT Mon Aug 14 2017

The synoptic pattern at the start of the long term period will
feature weak southwest flow aloft over the Midwest. This southwest
flow will persist through the middle of the week, before yet another
trough builds into the Northeast this weekend veering the upper-
level pattern to the WNW.

Given the expected southwest flow this week, the pattern will turn
more active as weak PV anomalies traverse across the region. Tuesday
will feature a weak surface warm front lifting northward through the
area. The passage of this front will allow a more humid and unstable
airmass to invade the area from the south. Upper-level forcing for
ascent appears fairly weak on Tuesday, thus suspect most areas will
remain dry. But given the unstable airmass and the weak front
lifting through the area, can`t rule out a few showers/storms in the
area.

Better chances for showers and thunderstorms will come Tuesday
night, Wednesday, and especially Wednesday night as a cold front
approaches and eventually pushes through the region. Guidance has
come into better agreement that the front will push southeast of the
area by late Thursday through Friday as surface ridging builds in.
This should allow for mostly dry conditions through Friday
afternoon.

Guidance then differs substantially as we head into the upcoming
weekend. The GFS is the most aggressive in lifting the front back to
the northeast into the area, with the Euro keeping it well to the
south. Will go with an ensemble approach for now given the
uncertainty, but suspect the drier Euro solution for the upcoming
weekend is likely to be correct.

Temperatures will moderate to more seasonal values Tuesday into
Wednesday (and conditions will become more humid). However, given
the expected convection, there is certainly bust potential on these
values. In areas that don`t see significant convection, heat indices
approaching 100 degrees are likely. Temperatures will cool slightly
behind the passing cold front late this week into this weekend.

KD

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 626 AM CDT Mon Aug 14 2017

SPECIFICS FOR KCOU: An area of MVFR ceilings had moved into KCOU
during the few hours prior to TAF issuance. MVFR cigs are expected
to prevail for the first few hours of the TAF period until diurnal
mixing leads to rising cloud bases and a return to VFR conditions.
Fog will be possible after 15/06z.

SPECIFICS FOR KUIN: VFR conditions are expected for the majority
of the valid TAF period. Fog will be possible after 15/06z.

SPECIFICS FOR KSTL, KSUS, KCPS: An area of SHRA developed early
this morning near the St. Louis metro area TAF sites. This area of
rain will continue to affect the terminals during the first 0-4
hours of the TAF period. Once the showers have moved away from
the terminals, VFR conditions are expected for most of the TAF
period. Fog will be possible after 15/06z.

Kanofsky

&&

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

$$

WFO LSX


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