Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231745

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1245 PM CDT Fri Mar 23 2018

Issued at 1245 PM CDT Fri Mar 23 2018

Area of light rain that has developed over central MO will expand
eastward this afternoon and settle primarily over areas in IL and
near the MS river this evening before expanding westward back
again tonight. This should effectively halt much of the warming
today and look for near steady temps this afternoon with slight
cooling this evening.



.SHORT TERM...  (Through Late Tonight)
Issued at 414 AM CDT Fri Mar 23 2018

A shortwave will move across the Rocky Mountains today and into the
Great Plains tonight resulting in cyclogenesis over eastern Colorado.
Model guidance is in good agreement in handling this system as it
moves across the plains into the Mississippi Valley through tonight.
Increasing low level moisture convergence as the low level jet
strengthens ahead of the low this afternoon and tonight should
produce a wide swath of rain starting this afternoon and continuing
tonight.  The highest amounts of rain will likely be along and east
of the Mississippi River through tonight north and northeast of the
strengthening warm front.  This is coincident with eh strongest low
level baroclinicity and is on the leading edge of the low level jet
with +8 to +10 850mb dewpoints advecting into +3 to +6 air
temperatures.  There is some marginal instability advecting into
central Missouri during the evening, and further east overnight, so
so some thunderstorms embedded into the larger precipitation shield
are possible as well.  Temperatures today look to be near or a few
degrees below seasonal normals in the low to mid 50s due to cloud
cover and a persistent cool easterly wind.  Clouds and precip
tonight will likely keep any diurnal temperature fall to a minimum
so have kept lows mainly in the 40s across the forecast area.


.LONG TERM...  (Saturday through Thursday)
Issued at 414 AM CDT Fri Mar 23 2018

Mdls are in fairly good agreement thru the forecast period. The
forecast period begins with the upper ridge breaking down as a sfc
low tracks ESE thru the region. Given the 850mb low track, and the
location of the upper fnt, expect precip to remain over the ern half
to two-thirds of the CWA Sat morning. Showers will then pull SE out
of the area during the afternoon. While the bulk of the precip will
be out of the area by sunset Sat, have kept some low PoPs lingering
into the evening hours for mainly the ern third of the CWA as the
upper trof drops thru the area. If enuf moisture is available for
this trof, the potential exists for temps to be cold enuf for some
light snow or flurries to be possible. With the sfc ridge building
into the area for Sun, have trended twd the cooler guidance despite
ridging aloft.

Focus turns to Mon and the next system. Mdls are in fairly good
agreement with a wrmfnt lifting thru the area on Mon with showers
accompanying it. After a relative lull during the afternoon, expect
precip to become more widespread during the evening hours and thru
Tues as the cdfnt pushes thru the area. Given the prolonged period
of rain, possible convection and higher PWats, will need to monitor
this system for a potential of flooding headlines.

Cooling trend continues thru the end of the period with the sfc
ridge building into the area.



.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1245 PM CDT Fri Mar 23 2018

Rain has already moved into COU and will affect the STL metro
sites and UIN shortly. Look for conditions to deteriorate from VFR
currently to MVFR this evening and into IFR overnight as a surface
area of low pressure tracks just south of the I-70 corridor.
Easterly winds, gusty at times, will continue thru Saturday at
dawn. IFR conditions will likely continue for much of the day on
Saturday as winds back northerly and this shift in low level flow
should help to maintain. Rain is expected to diminish heading
into Saturday, but will probably briefly expand in coverage again
just beyond the valid period.

SPECIFICS FOR KSTL: VFR conditions will deteriorate to IFR
overnight and continue thru Saturday. Easterly winds, gusty at
times, will back northerly on Saturday with passage of surface low
pressure to the south.





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