Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231731

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1231 PM CDT Wed May 23 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 316 AM CDT Wed May 23 2018

A surface ridge extended from a surface high over Lake Michigan
southwestward across across the CWA into south central Missouri
early this morning.  The only surface observation reporting fog at
this hour was Farmington, and surface observations indicate that
dewpoint depressions are close to zero in some areas. Will continue
to monitor for fog development through sunrise, particularly over
southeast Missouri and in river valleys where fog was the thickest
yesterday morning.

Still expect a mostly sunny sky across the area today as an upper
level ridge remains over the Midwest. Have added a slight chance of
thunderstorms over a small part of central Missouri this afternoon.
The HRRR and NAM forecast soundings at KCOU are showing the
atmosphere becoming uncapped with about 1000-1500 J/kg of CAPE. Any
storms that do develop will diminish with loss of daytime heating,
so expect tonight to be dry.

Expect temperatures to be above normal again today, and went at or
just above the warmest MOS guidance for highs.  Lows tonight will
once again only fall into the 60s.


.LONG TERM...  (Thursday through Tuesday)
Issued at 316 AM CDT Wed May 23 2018

High pressure centered over the Great Lakes and Ohio Valley on
Thursday morning will drift east off the Atlantic coast by Friday
afternoon.  The mid/upper level ridge over the Midwest will also be
flattening out and moving east during this time.  This will allow a
trof to dip into the Mississippi Valley which will help draw a Gulf
low pressure system up into the lower Mississippi Valley this
weekend.  Southerly flow on the western side of the retreating ridge
will bring increasing increasing instability to the Mid Mississippi
Valley on Thursday afternoon.  All model guidance is now printing
out light QPF Thursday afternoon over central Missouri.  This is
further east than yesterday`s model runs.  It doesn`t look like
there are any synoptic boundaries for convection to focus on, so
think only isolated showers/storms are possible.  The trend of PoPs
moving further east continues on Friday with chance/slight chance
PoPs covering most of the area Friday afternoon.  The convection on
Thursday and Friday look to be primarily diurnally driven...though
the GFS does develop a weak nocturnal low level jet on Friday night.
Not entirely sure the low level jet and subsequent nocturnal
convection isn`t caused by upstream grid-scale convective feedback
so have only kept chance/slight chance PoPs going for Friday night
rather than more widespread precip as the GFS suggests.

Medium range guidance handles the details of the aforementioned Gulf
low a bit differently this weekend into early next week...though the
general idea is essentially the same.  The low drifts up into the
southern U.S. Saturday through Tuesday spreading precipitation up
the Mississippi Valley toward the Midwest.  The ECMWF moves the low
up over Louisiana, directly over the lower Mississippi Valley through
Tuesday, while the GFS is further east and faster, moving the low up
over the Florida Panhandle through Georgia and up into
Tennessee/northern Alabama by Tuesday afternoon.  Both models
develop a ridge north of the low in a rex-block configuration
between Monday and Tuesday as well.  The GFS shows much more
widespread precip than the EC as is typical.  One way or another,
the chance for showers and storms will be increasing late this
weekend and early next week as the low brings deep Gulf moisture up
into the mid Mississippi Valley.  Little change in temperatures
through the period with highs in the mid to upper 80s and lows in
the mid 60s to near 70.



.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1225 PM CDT Wed May 23 2018

SPECIFICS FOR KUIN, KCOU: VFR for nearly the entire TAF period due
to the influence of a high pressure center over the Great Lakes.
Isolated SHRA/TSRA are possible near KCOU between 23/18z-24/01z,
but current thinking is that any isolated SHRA/TSRA will be too
widely scattered to include in the scheduled TAF. There is also a
slight chance that fog could temporarily reduce visibilities
between 24/09z-24/14z, but confidence is too low to include in the
TAFs. Winds will remain generally southeasterly through the TAF
period but could become light/variable at times.

SPECIFICS FOR KSTL, KSUS, KCPS: VFR for nearly the entire TAF
period. There is a slight chance that fog could temporarily
reduce visibilities between 24/09z-24/14z, but confidence is too
low to include in the TAFs attm. Winds will remain generally
southeasterly through the TAF period but could become
light/variable at times, especially after 24/00z.





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