Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 232008

Area Forecast Discussion
National Weather Service Saint Louis MO
308 PM CDT Fri Mar 23 2018

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 308 PM CDT Fri Mar 23 2018

A surface frontal boundary was draped across central Kansas and
southern Missouri and far southern Illinois this afternoon.  A solid
area of light rain has developed in response primarily to WAA aloft
and frontal lift.  Temperatures ranged from the mid 40s in areas of
steady rain to the mid 50s elsewhere.  Skies were cloudy across the
entire region withy easterly low level flow.  An upper level
disturbance was over the central High Plains and heading east
towards our region.

The upper level disturbance to our west will continue to approach
our region while the surface front remains quasi-stationary and
maintains the WAA aloft.  This should continue the light rain for
most of the area, with some intensity enhancement expected on the
approach and direct influence of the upper level disturbance
overnight and Saturday morning.  This will also be the best chance
at an isolated thunderstorm as well.  The precipitation will then
gradually diminish and retreat to the east and southeast Saturday
afternoon and evening.  Enough cold air may spill in late for areas
in west-central and central IL to briefly turn the pcpn over to snow
before ending, with minimal or no snow accum expected given the warm
surfaces and anticipated light intensities.

Temperatures tonight will not drop much with clouds, but the rain
will help, and conditional climo supports a 5-10 degree fall.  This
should lead to most sites bottoming out in the 40s.  A large temp
gradient is expected on Saturday with the surface wave of low
pressure tracking just south of I-70, separating low clouds and slow
to exit pcpn from potential for some sun and a more southerly low
level flow.  Max temps will range from near 40 in central IL to near
70 in parts of southeast MO with the tightest temp gradient expected
near the LO track just south of I-70.


.LONG TERM...  (Saturday Night through Next Friday)
Issued at 308 PM CDT Fri Mar 23 2018

The upper level disturbance giving us our sensible weather for
late tonight and Saturday will continue to affect our region for
Saturday evening, slowly exiting, and persisting longest over areas
in Illinois.  As mentioned in the SHORT-TERM discussion, there is
some potential for the pcpn to briefly end as snow, but should not
amount to much if any impact, with relatively warm ground and
anticipated light intensities.  Surface high pressure from the Great
Lakes will then extend its influence into our region late Saturday
night and Sunday giving us a brief respite from the rain.

Otherwise, much of next week will feature a classic wet pattern as a
strong Pacific storm comes onshore Saturday night and settles into
the far southwestern CONUS Sunday night.  The flow aloft will back
southwesterly over our region by Monday as an area of low pressure
at the surface develops and strengthens to our west with the main
Pacific storm then ejecting disturbances in this SW flow towards our
region.  This should result in a deep moist flow by late Monday and
this is currently expected to maintain fairly intact into Tuesday
night.  Models are forecasting PWs of 1.25" for much of our region,
and climatologically is near the possible max for this time of year.
Looks like a good categorical PoP situation.  Instability will be
limited but continues to indicate some potential for isolated
thunder and have maintained this for now.  That said, there is
decent model consensus on a rainfall event from late Sunday night
into Tuesday night which could result in 2-4" rainfall totals and,
if realized, could lead to some minor river flooding on area

Daytime max temps will look to bounce to above average during early
next week but high rain chances will make it tough.

The really good deep moist setup will take a hit with passage of a
significant northern stream shortwave late Tuesday night and should
see a more significant pattern shift by Thursday with passage of the
longitudinal upper TROF.  Rain chances look to linger for the middle
of next week but the threat of significant QPF looks severely
diminished.  Dry weather should then setup for late next week.



.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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