Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 210008
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
608 PM CST Tue Feb 20 2018

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 317 PM CST Tue Feb 20 2018

Have two main concerns through tomorrow afternoon...ice accretion
and possible flooding. Have issued a winter weather advisory for
the entire CWA for the possibility for up to around a tenth of an
inch of ice of tonight into tomorrow morning. Have also issued an
areal flood watch for the possibility of localized flood given
that we have had some areas with up to 2 inches of rainfall so
far, and that we will continue to have widespread rain through
this evening with another 1 to 2 inches of rain.

Cold front is currently from near Hillsboro, IL to Belleville to
Steelville, MO. This front will move south of the CWA by mid
evening. Regional radars continue to show large band of
precipitation north of the front extending from the Great Lakes
into the Southern Plains. This band will continue to move
northeast across the CWA through at least this evening. It will
then begin to shift southeast overnight into Wednesday morning as
shortwave trough over central High Plains lifts northeast pushing
the precipitation band southward. Temperatures will gradually
drop tonight, gradually falling below freezing from northwest to
southeast with time. Forecast soundings favor freezing rain as a
precipitation type with shallow cold air undercutting the warmer
air aloft. Ground and road temperatures are relatively warm from
recent days which will act to lessen ice accumulation somewhat.
There should be enough ice accretion for some areas to see a tenth
of an inch of a ice...possibly more. Temperatures should rise
above freezing by late morning on Wednesday, causing any lingering
precipitation to change over to rain. Rain will linger over parts
of southeast Missouri into southern Illinois into Wednesday
afternoon.

There is some potential for flooding tonight as there continues to
be steady rain over the area, and there was up to 2 inches of rain
in some places over the past 48 hours. Another 1 to 2 inches of
rain is possible in the flood watch tonight which could cause
localized flooding.

Britt

.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 317 PM CST Tue Feb 20 2018

(Wednesday Night)

Still unfortunately do not have a lot of confidence in specifics on
Wednesday night. Biggest uncertainties will be how freezing line
evolves and how much precipitation falls where air temperatures are
below freezing. Believe there will be two main areas of
precipitation Wednesday night. One will be focused to the southeast
of the CWA across the lower Ohio and Mississippi River Valleys where
just to the north of the surface front and where strong upper-level
divergence is forecast beneath the RER of a jet streak aloft. The
northwestern extent of this precipitation will likely clip portions
of southeast Missouri and southwest Illinois but fall in the form of
plain rain as surface temperatures are expected to be in the mid
30s. A second area of precipitation in the form of freezing rain and
sleet is expected across the middle Missouri River Valley. This area
should by and large miss the CWA to the northwest. In between,
models show some spotty light precipitation which should be in the
liquid form. However, with parts of the area slightly below freezing
parts of the overnight hours, a light glazing is expected. Impacts
may be minimal across much of that area however as temperatures stay
very near the freezing mark. Area of more concern would be across
northeastern Missouri where temperatures in the upper 20s are
forecast.

(Thursday - Saturday Night)

Active weather pattern will continue through early this weekend.
Expecting 2-3 round of rainfall Thursday night through Saturday
night with the heavier rain focusing across portions of southeast
Missouri and southwest Illinois. First round Thursday night and
early Friday looks to be fairly benign with low rainfall rates but
rounds Friday night through Saturday night have the potential for
heavier rainfall rates with possible convection. Antecedant
conditions by that time also will have also changed fairly
dramatically between now and then. More concern in this time period
but we do have plenty of time to see how much rain falls with each
preceding round. Total (additional) rainfall amounts generally in
the 2-4 inch range appear likely from Thursday night through
Saturday night at this early juncture, with locally higher amounts
possible. While it is still too early to get too specific, it
certainly appears that the potential for river and flash flooding
will increase toward the end of this wet pattern we are entering.
Be sure to stay tuned for the latest forecast information throughout
the week.

(Sunday - Next Tuesday)

Seasonably mild and dry weather still appears likely to end the
weekend and early next week as surface high pressure slides eastward
across the mid-Mississippi Valley. Look for highs in the 40s and 50s
with lows vastly in the 30s.

Gosselin

&&

.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 549 PM CST Tue Feb 20 2018

Steady rain is falling across the forecast area, with widespread
IFR ceilings across much of central and eastern Missouri as well
as west central into parts of south central Illinois. ceilings are
creeping up to MVFR over parts of northern Missouri, and this
improving trend is expected to continue moving slowly southeast
through tonight. Sub freezing temperatures are undercutting the
rain this evening, so a period of freezing rain is expected across
the entire area from northwest to southeast through Wednesday
morning across the entire area. MVFR ceilings will also rise to
VFR from northwest to southeast across much of the area but
MVFR/IFR ceilings will likely persist over the eastern Ozarks and
southern Illinois through Wednesday

SPECIFICS FOR KSTL:

Expect IFR conditions to prevail at Lambert through most of
tonight. There may be occasional breaks in the ceilings, but I
think they`ll be brief and transient. Cold air is undercutting the
rain and temperatures are expected to drop to freezing between
08Z-10Z and there should be a few hours of freezing rain after
this happens. Timing is uncertain, and it could be as early as
06Z-07Z, and there could be some sleet mixed in with the freezing
rain at times, but confidence in mixed precipitation is not very
high at this time. Expect precipitation to end by 12Z-13Z or
shortly thereafter. Flight conditions should improve to VFR during
the morning.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch through late tonight for Cole MO-Crawford MO-
     Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Lincoln MO-
     Madison MO-Moniteau MO-Montgomery MO-Osage MO-Reynolds MO-
     Saint Charles MO-Saint Francois MO-Saint Louis City MO-
     Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO.

     Winter Weather Advisory from midnight tonight to noon CST
     Wednesday for Crawford MO-Iron MO-Jefferson MO-Madison MO-
     Reynolds MO-Saint Francois MO-Saint Louis City MO-Saint
     Louis MO-Sainte Genevieve MO-Washington MO.

     Winter Weather Advisory from midnight tonight to 6 AM CST
     Wednesday for Franklin MO-Gasconade MO-Lincoln MO-Saint
     Charles MO-Warren MO.

     Winter Weather Advisory until 6 AM CST Wednesday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Knox MO-Lewis MO-Marion MO-
     Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls
     MO-Shelby MO.

IL...Flood Watch through late tonight for Bond IL-Calhoun IL-Clinton
     IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-
     Marion IL-Monroe IL-Montgomery IL-Randolph IL-Saint Clair
     IL-Washington IL.

     Winter Weather Advisory from midnight tonight to noon CST
     Wednesday for Bond IL-Clinton IL-Fayette IL-Madison IL-
     Marion IL-Monroe IL-Montgomery IL-Randolph IL-Saint Clair
     IL-Washington IL.

     Winter Weather Advisory from midnight tonight to 6 AM CST
     Wednesday for Calhoun IL-Greene IL-Jersey IL-Macoupin IL.

     Winter Weather Advisory until 6 AM CST Wednesday for Adams IL-
     Brown IL-Pike IL.

&&

$$

WFO LSX



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