Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 172354

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
654 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 00z TAFs through 00z Sunday Evening)
Issued at 604 PM CDT Sat Mar 17 2018

A sfc low continues to pull esewd away from the region as an upper
level s/w moves out of the region. Guidance suggests the area of
clouds over the region will largely stay in place as the sfc ridge
builds ewd. These clouds may begin to break up Sun afternoon. The
main question will be what cigs will accompany these clouds.
Patches of IFR remain across the area. While UIN has lifted to low
MVFR, believe lower cigs will move back into the area later
tonight. Likewise, these lower cigs may continue to drift/spread
south, eventually impacting COU/SUS/CPS. However, confidence is
lower on how far south these lower cigs will spread. Have kept out
of the TAF for now. Also, some patches of fog will be possible as
well, but with winds continuing aoa 5 kts, do not anticipate it
being widespread. Regardless, do not believe visbys would be lower
conditions than the prevailing cigs. Expect cigs to improve late

SPECIFICS FOR KSTL: Some question regarding IFR possibly impacting
the terminal tonight. Have held off adding to TAF for now.
Otherwise, expect cigs to remain low MVFR thru Sun morning and
lift during the early afternoon hours. Conditions may become VFR
during the late afternoon hours as either cigs lift or cloud deck
begins to become more scattered.



Saint Louis     37  56  42  51 /   0   5  60  80
Quincy          32  52  37  50 /   0   5  10  40
Columbia        36  55  41  51 /   0  20  60  80
Jefferson City  36  56  42  52 /   0  20  70  90
Salem           35  55  40  52 /   0   5  50  80
Farmington      36  56  42  54 /   0  10  80  90




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