Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 180458

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1158 PM CDT Sat Mar 17 2018

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 415 PM CDT Sat Mar 17 2018

Low pressure moving into the Tennessee Valley will continue to drag
stratus southeast in its wake tonight. Meanwhile, guidance is
showing ridging building aloft and forecast soundings indicate a
strong subsidence inversion due to this ridge.  This indicates a
strongly stratified lower atmosphere.  This stratification will
likely allow little chance for the low level stratus deck to mix
out, especially given that our forecast area will remain in weak
cold advection at least into early Sunday morning.  Think the
blanket of clouds will limit cooling somewhat tonight...although the
northwest flow will make it a little chillier than this morning.
Stuck very close to the model consensus for lows for tonight.
However, with the thinking that those clouds will stick around for
much of the morning and perhaps into early afternoon, think the
colder side of guidance in the low to mid 50s for highs Sunday is


.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 415 PM CDT Sat Mar 17 2018

Weather for the remainder of the forecast will be dominated by two
systems.  The first is a shortwave that will eject eastward from the
large upper trof currently over the western U.S.  Models are showing
strong lee-side cyclogenesis late Sunday afternoon and Sunday night
as the wave moves off the Rockies into the Plains.  The surface
system is currently forecast to move through southern Kansas Sunday
night and east along the Missouri/Arkansas border on Monday.  All
guidance members show moderate to strong low level moisture
convergence ahead of the low along and south of I-70, wrapping back
around the low Monday afternoon into the early evening.  Likely to
categorical PoPs look appropriate fading to chance up across
northeast Missouri and west central Illinois.  Rain should end
Monday night as the system exits to the east.

Northwest flow aloft will persist over the Mississippi Valley for
the remainder of the week.  A couple of weak shortwaves move quickly
across the area in the northwest flow, which could produce light
rain first on Tuesday night and again on Thursday night into Friday.
Medium range guidance is disagreeing on timing and position of the
second strong system to affect the Midwest next weekend.  Current
thinking is that showers and thunderstorms will develop ahead of the
low either Friday or Friday night, but the timing differences
between the EC and GFS make this a low confidence forecast.



.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1152 PM CDT Sat Mar 17 2018

Still believe clouds will largely remain in place with cigs in the
low MVFR cat. While COU is currently in the higher MVFR, lower
cigs may be moving south twd the terminal. UIN is on the edge of
the cloud deck and may actually go VFR. However, believe the swwd
push will slow before clouds pivot and actually move back east.
That said, if clouds to clear out of UIN, fog is expected to
develop, although, do not currently expect visbys would drop below
MVFR cat.

SPECIFICS FOR KSTL/KSUS/KCPS: Expect low MVFR cigs to remain in
place thru Sun morning. Cigs will improve during the afternoon
hours. Conditions may actually improve to VFR during the late
afternoon hours. Expect lower cigs to move back into the area late
in the period with SHRA beginning around 09z.





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