Area Forecast Discussion
Issued by NWS St. Louis, MO

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297
FXUS63 KLSX 211804
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1204 PM CST Wed Feb 21 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 352 AM CST Wed Feb 21 2018

A very complicated forecast thru tonight is in store with a fair
amount of uncertainty. Focus thru tonight will be precip chances
as well as sfc temps that will determine ice accums or just a cold
rain.

Have generally trended twd the cooler NAM for temps which is also
slightly more aggressive with QPF. The NAM did a fairly good job
on the ongoing rain/temp setup and this soln seems reasonable.

Have kept a Winter Adv going for srn portions of the CWA. Expect
temps to drop to the freezing mark by sunrise and shud hover near
the freezing mark thru today. This shud be especially true for
the valleys with some cold air drainage. Believe these counties
will see waves of precip thru today, tho a brief lull is possible
early this afternoon before another s/w lifts newd and the WAA
increases again. This brief lull may allow for some warming, but
if the temp does rise above freezing, it shud drop quickly once
the precip begins again.

Some uncertainty remains regarding precip coverage for tonight.
While the bulk of the precip shud be confined to the sern and wrn
portions of the CWA, given the widespread lift over the area as well
as the strong WAA, believe there is enuf of a threat for some light
FZRA between these two areas. The FZRA may start out as some sleet
with wetbulb effects and perhaps even some snow across nrn portions
of the CWA if precip can begin ahead of the warm nose, but shud
quick change to freezing rain.

Going ice accumulations may be on the high side given the
uncertainty of the sfc temps forecast. Much of the precip is
expected to fall with temps hovering around freezing. That said,
some of this may be offset by lighter precip allowing faster
accretion.

Tilly

.LONG TERM...  (Thursday through Tuesday)
Issued at 352 AM CST Wed Feb 21 2018

Freezing rain will end from south to north on Thu due to a
combination of rising temperatures (FZRA changing to RA) and pcpn
tapering off. Most areas should be dry by afternoon, thus the
Winter Weather Advisories end at 18z Thu.

The warm front which will have lifted slightly northwards on Thu
will waver across the region on Fri before decidedly lifting
northwards on Sat in response to a developing wave on the
boundary. Temperatures should be quite warm on Sat, although there
may be a sharp temperature gradient between northeastern MO/west
central IL and southeastern MO on Saturday depending on the
northward progress of the boundary. Models are in broad agreement
that the cold front associated with the lifting wave will have
moved through the CWA by 12z Sun, but there is considerable
variation between models in how they get there.

Several disturbances will move across the area through the weekend
within an active southwest flow pattern aloft. This will bring
multiple opportunities for widespread SHRA/TSRA through Sat
night. PW values are forecast to be near the max values for Feb by
Saturday. Given the recent rainfall and saturated soils, a Flood
Watch or Flash Flood Watch will likely be needed at some point
after Thu`s pcpn ends and before the next round of pcpn begins. A
brief period of upper ridging is possible early/mid next week
during which little pcpn would be expected.

Kanofsky

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1144 AM CST Wed Feb 21 2018

Stubborn MVFR cigs in COU should move out this afternoon, but
likely remain in the St Louis metro area. MVFR cigs should advect
back into COU this evening and into UIN by early Thursday
morning. Light precipitation should move back into the taf sites
later tonight and early Thursday morning. Precipitation type
should be mainly freezing rain in UIN and COU, and rain or
freezing rain in the St Louis metro area. North surface wind will
veer around to a northeast direction tonight.

SPECIFICS FOR KSTL: MVFR cigs will likely remain in the STL
area through the period, possibly dropping into the IFR category
early Thursday morning. Light precipitation should move back into
STL later tonight. Precipitation type should be rain or freezing
rain/drizzle. North surface wind will veer around to a northeast
direction tonight.

GKS

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory until noon CST Thursday for Crawford MO-
     Iron MO-Madison MO-Reynolds MO-Saint Francois MO-Sainte
     Genevieve MO-Washington MO.

     Winter Weather Advisory from 6 PM this evening to noon CST
     Thursday for Audrain MO-Boone MO-Callaway MO-Cole MO-
     Franklin MO-Gasconade MO-Jefferson MO-Lincoln MO-Moniteau
     MO-Montgomery MO-Osage MO-Saint Charles MO-Saint Louis City
     MO-Saint Louis MO-Warren MO.

     Winter Weather Advisory from midnight tonight to noon CST
     Thursday for Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-
     Ralls MO-Shelby MO.

IL...Winter Weather Advisory until noon CST Thursday for Randolph IL.

     Winter Weather Advisory from 6 PM this evening to noon CST
     Thursday for Calhoun IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Monroe IL-Saint Clair IL.

     Winter Weather Advisory from midnight tonight to noon CST
     Thursday for Adams IL-Brown IL-Pike IL.

&&

$$

WFO LSX



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