Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210821

Area Forecast Discussion
National Weather Service Saint Louis MO
321 AM CDT Sat Sep 21 2019

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 318 AM CDT Sat Sep 21 2019

Main concern continues to be the potential for heavy rainfall
tonight through Sunday evening. Have added Moniteau county to the
Flash Flood Watch.

Latest surface analysis continued to show broad southerly flow
across Missouri and Illinois between a low over the central High
Plains and high pressure along the Carolina Coast. Showers have
already developed over central Missouri early this morning and
still expect showers and thunderstorms to become more numerous
across central and northeast Missouri into west central Illinois
through daybreak as the low level jet increases in strength which
will subsequently increase moisture convergence in the 925-850mb
layer. Will continue with likely/high chance PoPs across the
northwest half of the CWA today where the RAP/NAM maintains this
low level forcing. Farther southeast, will have lower or no PoPs
where showers and thunderstorm develop will be dependent on less
instability and weaker forcing.

Showers and thunderstorms will become heavier across the watch
area and more widespread across the CWA tonight and Sunday. There
continues to be relatively good agreement between the NAM/GFS
that an upper trough currently over the western CONUS will move
east across Missouri and Illinois on Sunday night. There will be
an increase in large scale ascent within the left entrance region
of an upper jet streak that will coincide with strong low level
forcing ahead of the cold front. Widespread 2-3"+ amounts are
likely across central and northeast Missouri and west central
Illinois Saturday night into Sunday night as the as the front will
be slow to move southeastward under the nearly parallel upper
flow. In addition, PWATS will be around 2" and warm cloud depths
will be around 12k feet suggesting flash flooding will be
possible, particularly with any training thunderstorms.

There will also be the potential for a few severe thunderstorms this
weekend.  The first will be late tonight across northeast Missouri
as severe thunderstorms that develop over western Missouri closer to
the cold front moves east into the CWA.  The main threat will be
damaging winds. On Sunday, a few strong to severe thunderstorms
will be possible ahead of the cold front as the atmosphere ahead
of the cold front becomes moderately unstable with deep layer
shear around 40kts.


.LONG TERM...  (Sunday Night through Friday)
Issued at 318 AM CDT Sat Sep 21 2019

Mainly dry weather is still expected Monday into Tuesday as an
upper ridge and surface high moves across Missouri and Illinois.
The global deterministic models and the GEFS still have some
differences with just how far south the front will move before it
stalls and then moves back north Wednesday into Friday. This will
have implications on showers and thunderstorms chances and when
they will occur. Have kept with a chance showers and thunderstorms
going mid-late next week.

Temperatures on Monday and Tuesday will be slightly cooler behind
the weekend front, but then warm back above normal ahead of the
midweek front on Wednesday



.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1136 PM CDT Fri Sep 20 2019

Isolated to scattered showers and storms are expected to develop
in central and NE MO early this morning, pushing east through the
morning hours. There is some uncertainty just how far south and
east these storms will reach, with UIN being the only terminal
where precip is likely this morning. The moisture that will help
develop these storms will also result in MVFR cigs in these
areas, perhaps falling to low end MVFR for areas directly impacted
by any storms.

Thinking storm activity will diminish somewhat in the afternoon,
though like the last few days, isolated storms will be possible.
Also like the last few days, it`s difficult to tell if any
terminals, will be directly impacted by these afternoon storms.
Moisture is a bit richer toward UIN and COU, so have opted to
keep a vicinity mention in those TAFs through the afternoon while
I`ve left the STL metro TAFs dry for much of the afternoon.



MO...Flash Flood Watch from this evening through late Sunday night
     for Audrain MO-Boone MO-Callaway MO-Knox MO-Lewis MO-Marion
     MO-Moniteau MO-Monroe MO-Montgomery MO-Pike MO-Ralls MO-
     Shelby MO.

IL...Flash Flood Watch from this evening through late Sunday night
     for Adams IL-Brown IL-Pike IL.



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