Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 111753
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1153 AM CST Thu Jan 11 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 346 AM CST Thu Jan 11 2018

Have issued a winter weather advisory of the entire CWA.  Have
staggered beginning times of the advisory from northwest to
southeast based on when the freezing precipitation is expected to
move across Missouri and Illinois.

Not much has changed since the previous forecast.  Strong low level
moisture advection and mid-level ascent ahead of the upper trough
will bring rain ahead of the cold front today. Did keep the chance
of thunderstorms over south central Illinois as the RAP forecast
soundings are showing some weak elevated CAPE this afternoon.

The cold front which currently extends from western Iowa into far
northwest Missouri and eastern Kansas is still expected to move
quickly from west to east across the CWA late this morning and this
afternoon. There will be a quick temperature drop behind it, with
the freezing line just a couple hours behind the front. Forecast
soundings still show that there will be a few hours of freezing rain
followed by a brief burst of sleet and light snow. Still only looks
like there will be a few hundredths of ice accumulation, but have
concerns about how quickly the freezing temperatures will move in
behind the front today.  A light glazing is likely on any untreated
surface. Snow amounts also look like little if any from across
central and northeast Missouri, to around inch over south central
Illinois where there will be slightly move lift tonight as the
system is exits the region.

Temperatures currently across the area are in the middle to upper
50s.  Temperatures will likely remain steady or rise a few more
degrees ahead of the cold front before they fall 20-30 degrees
behind it.  Lows tonight will be in the single digits and the teens,
causing any surface moisture to freeze quickly.

Britt

.LONG TERM...  (Friday through Wednesday)
Issued at 346 AM CST Thu Jan 11 2018

Upper level shortwave to exit region by midday on Friday. Could
still see some light snow over far eastern portions of forecast area
through Noon with little or no accumulation expected. Otherwise,
skies to clear out from west to east with decent CAA in store. Highs
will struggle to rise much from overnight lows. Highs should be in
the upper teens to upper 20s.

Surface ridge to build in Friday night. However, a weak upper level
short wave to slide southeast through region, so will see an
increase in clouds once again. Little moisture for it to work with.
So should move through dry at this time. Lows to dip down into the
single digits to mid teens. Highs on Saturday will be in the upper
teens to mid 20s.

By Saturday night, lows to dip down even more as clear skies and
light winds prevail. Lows will be in the zero to 10 above range.

Models still showing several clipper systems sliding through region
between Sunday and Monday. Guidance is having difficulties with
timing and placement of the precip, which is typical with these fast
moving shortwaves, but most models show a few hundredths to around a
tenth of an inch of liquid QPF for our area as the waves move
through. For now best chances of light snow Sunday will be over the
western half of Missouri while the second wave Sunday night through
Monday will be a bit further to the northeast. Will keep eye on
models as we get closer to this time period.

Otherwise, rest of extended looks dry and cold. The coldest night
will be Monday night with lows in the minus 5 to 10 degrees above
zero range and wind chills approaching advisory criteria over
portions of northeast Missouri and west central Illinois.

Byrd

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1135 AM CST Thu Jan 11 2018

A strong cold front is moving across the area this afternoon. The
front should be moving through KCOU and KUIN by 18Z, and into the
STL Metro area by 21-22Z, and through the forecast area into far
southeast Missouri and south central Illinois by 00-02Z. An area
of low MVFR and IFR stratus and some fog in heavier showers is
following the front...and these lower flight conditions are
expected to continue to move east southeast as the front moves
through the region. Precipitation types will change from rain, to
freezing rain, to sleet and finally a little bit of snow as cold
air pours into the area behind the front this afternoon and
evening. Current indications are that the best chance for wintry
precipitation will be in eastern Missouri into southern Illinois
this evening and overnight. Precipitation should end from
northwest to southeast through early Friday morning and lower
ceilings will give way to VFR flight conditions. Gusty southerly
flow will prevail ahead of the cold front, then the winds will
shift to the northwest after FROPA and gust to around 30kts.

SPECIFICS FOR KSTL:

VFR flight conditions currently at Lambert are expected to drop
shortly as lower ceilings move up from the south and southwest
ahead of the cold front. The front should pass through KSTL
sometime between 21-22Z with showers. Expect ceilings to drop to
low MVFR and possibly IFR behind the front. Will monitor trends in
central MO and amend as necessary. Should see temperatures drop
very quickly behind the front and light freezing rain/sleet
develop between 00-01Z. Light wintry precipitation will persist
for a few hours this evening...ending before 06Z. MVFR ceilings
and gusty northwest flow will prevail at Lambert until early
Friday morning. May see flight conditions improve faster than in
the TAF, but wanted to be conservative for this forecast period.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory from 3 PM this afternoon to 6 AM CST
     Friday for Crawford MO-Franklin MO-Jefferson MO-Lincoln MO-
     Saint Charles MO-Saint Louis City MO-Saint Louis MO-Warren
     MO-Washington MO.

     Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Friday for Iron MO-Madison MO-Reynolds MO-Saint Francois MO-
     Sainte Genevieve MO.

     Winter Weather Advisory until midnight CST tonight for Audrain
     MO-Boone MO-Callaway MO-Cole MO-Gasconade MO-Knox MO-Lewis
     MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-
     Pike MO-Ralls MO-Shelby MO.

IL...Winter Weather Advisory from 3 PM this afternoon to 6 AM CST
     Friday for Calhoun IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Monroe IL-Saint Clair IL.

     Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Friday for Bond IL-Clinton IL-Fayette IL-Marion IL-
     Montgomery IL-Randolph IL-Washington IL.

     Winter Weather Advisory until midnight CST tonight for Adams IL-
     Brown IL-Pike IL.

&&

$$

WFO LSX



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