Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 270523
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1223 AM CDT Thu Jul 27 2017

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 323 PM CDT Wed Jul 26 2017

Primary concern will transition this evening from heat to localized
heavy rainfall. Will be trimming far northwestern portion of heat
advisory off as thicker midlevel clouds have kept temperatures in
the low 90s preventing heat index values from approaching 105F.
Rest of advisory still looks good and will continue until 8PM.

Still lots of uncertainty with respect to convective trends for
overnight tonight. Retreating warm front is currently across the
Mississippi River in Illinois and continues to push northeastward.
Believe heaviest axis of rainfall may set up roughly along that
boundary which is forecast to also be roughly collocated with
strong upper-level forcing for ascent via vorticity maxima and
low-level moisture convergence. This axis however will likely stay
east/northeast of the CWA. Showers and thunderstorms will likely
fill in behind that area along trailing cold front. Due to copious
amounts of cloud cover and more stable air across the lower-mid
Missouri Valley, convection will likely not be as rigorous as
thought 24 hours ago. Not seeing too much of a severe threat as
freezing levels are very high with only modest midlevel lapse
rates. Cannot rule out a wet microburst however in a stronger
core.

Main concern with convection tonight will continue to be on
localized heavy rainfall. There are quite a few mitigating factors
for widespread flash flooding tonight including dry antecedent
conditions (and low soil moisture percentiles), more stable air
upstream than forecast 12-24 hours ago mentioned above, and very
high flash flood guidance (2-3" in one hour). That being said,
will be leaving the flash flood watch alone for now as still
expecting a round of showers and thunderstorms within the watch
area in a very supportive environment (deep warm cloud depths and
very high PWAT values) for efficient precipitation production.
Training would need to occur though to produce problems in local
areas.

Scattered-numerous storms will continue along and ahead of the cold
front through the overnight hours into early Thursday morning
bringing for most areas some very welcome rainfall. Believe there
will be a lull in activity during the late morning hours before
mostly scattered storms fire along the cold front in the early
afternoon. Highest PoPs for Thursday afternoon look to be across
portions of southeast Missouri and southwest Illinois where cold
front is expected to be located. Localized heavy rainfall again may
be possible if storms train across the same areas.

High temperatures tomorrow will be quite a bit cooler than that of
today due to an increase in cloud cover and convection. Highs in the
mid 80s are likely for most, which is near to slightly below normal
for late July.


Gosselin

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 323 PM CDT Wed Jul 26 2017

High pressure will build over the region by Friday night. Northwest
flow will dominate the pattern throughout the forecast period.
The area will see temperatures below climatological normals for
both afternoon highs and morning lows along with dry weather. High
pressure aloft will move west in response to a series of short
waves diving south out of the high plains starting Monday morning.
A weak boundary will setup in southern Iowa ahead of a cold front
that will impact the area later in the week.



Kelly

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1156 PM CDT Wed Jul 26 2017

Thunderstorm trends continue to be the biggest forecast challenge.
Expect numerous to widespread showers and thunderstorms to affect
the forecast area...mainly north of I-44 in Missouri and I-64 in
Illinois. Thunderstorms will be capable of producing MVFR and IFR
conditions in heavy downpours as well as isolated wind gusts to
25-35kts. Also, expect ceilings across northern Missouri and west
central Illinois to drop to MVFR over the next few hours as low
pressure moves along a cold front which will drift southeast
through the period. MVFR ceilings will likely get as far south as
central Missouri and may get as far east as the St. Louis Metro
area into southwest Illinois by Thursday morning. Shower and
thunderstorm activity should diminish after 14-17Z Thursday, but
may increase again south of I-44 Thursday afternoon due to daytime
heating.

SPECIFICS FOR KSTL:

Occasional showers and thunderstorms will affect the terminal
through the overnight hours. VFR conditions will prevail, but any
storm that moves over the terminal will be capable of producing
intermittent IFR conditions in moderate to heavy rain. Wind
direction will be very variable due to storms in the area...though
the general direction should be from the south most of the time.
Expect thunderstorm activity to diminish after 12Z...though timing
is uncertain. Could see additional thunderstorm activity Thursday
afternoon, but current indications are that afternoon activity
will be south of the terminal.

Carney

&&

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

$$

WFO LSX



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