Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 121744

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1144 AM CST Thu Jan 12 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 413 AM CST Thu Jan 12 2017

Cold front has reach reached a line just south of I-70 in Illinois
and just south of I-44 in Missouri.  This front will continue to
move south this morning as a weak shortwave trough moves quickly
east across Iowa and the surface high builds south into the area.
The light freezing rain which is currently over northeast Missouri
and west central Illinois will move out of the area in the next hour
or two as low level moisture convergence shifts east of the area.
Showers and and isolated thunderstorms may still be found ahead of
the front until it moves out of area later this morning.  Expect
mainly dry conditions this afternoon into this evening.

Then the main focus will switch to the potential for freezing rain
and ice accumulation tonight.  Will continue with the winter storm
watch starting at Midnight for counties along and south of I-70.
Appears that the chance for light freezing rain will begin to
increase over the eastern Ozarks late this evening as low level
moisture convergence begins to increase underneath large scale
ascent.  The freezing rain will spread north during the night. While
GFS/NAM/ECMWF are all showing the best chances for ice accretion
along and south of a Cuba to Sparta line before 12Z, cannot rule out
that freezing rain won`t be as far north as I-70 by late tonight so
will keep the watch as is.

Expect temperatures to fall behind where the front moves through
today.  Went close to MOS highs in most locations.  Also stayed
close to SREF 2 meter temperatures tonight which were close to
previous forecast.


.LONG TERM...  (Friday through Wednesday)
Issued at 413 AM CST Thu Jan 12 2017

Confidence is increasing in ice accumulations between .25 and .6
inch across most of the area.  Have expanded the winter storm watch
to include the entire area, though the new segments start at 18Z
Friday and 00Z Saturday to try to time the northward progress of the
precip.  Temperatures Friday and Friday night look plenty cold
enough for ice accumulation across the area with strong high
pressure over the Upper Midwest and cold east-northeast flow at the
surface to keep temperatures below freezing.  Think the majority of
ice accumulation along and south of I-70 will occur during this

Conditions change somewhat on Saturday as the high moves east and
weak wave moves along the front.  All models move the front north
through Arkansas to a greater or lesser extent.  Additionally, cold
advection over most of the area shuts down at least temporarily.
Periods of light rain will be continuing...and I think the release
of latent heat from freezing will warm temperatures above freezing
for about the southern 1/2 to 2/3 of the area.  Most MOS guidance
pushes temperatures up into the mid 30s to mid 40s for Saturday`s
highs.  I think this is too warm given the airmass that will be in
place, and most raw 2m temperatures from the models are not that
warm.  Ended up warming temps slowly through the day...with slowly
falling temperatures again Saturday night.

A shortwave moving northeast from the southern Plains will finally
push the front up through the area Sunday and Sunday night which
will finally bring an end to the freezing rain from south to north.
Relatively mild but wet weather is expected for the rest of the week
as a series of waves moves across the Midwest.  Followed ensemble
guidance temperatures closely for Monday through Wednesday.



.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1133 AM CST Thu Jan 12 2017

IFR conditions are expected to lift this afternoon with clouds
becoming sct by mid to late afternoon. Winds will veer to become
nely tonight and eventually ely tomorrow. Clouds will move back
into the area tomorrow and gradually lower late morning into the
afternoon. An area of FZRA is expected to develop across srn MO
late tonight into Fri morning and move nwd. Current approximate
timing thinking is onset of FZRA at KCOU/KSTL/KSUS/KCPS is around
18z Fri.



MO...Winter Storm Watch from late tonight through Saturday afternoon
     for Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson 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 Storm Watch from Friday afternoon through Sunday
     afternoon for Audrain MO-Lincoln MO-Monroe MO-Pike MO-Ralls

     Winter Storm Watch from Friday evening through Sunday afternoon
     for Knox MO-Lewis MO-Marion MO-Shelby MO.

IL...Winter Storm Watch from late tonight through Saturday afternoon
     for Bond IL-Clinton IL-Madison IL-Marion IL-Monroe IL-
     Randolph IL-Saint Clair IL-Washington IL.

     Winter Storm Watch from Friday afternoon through Sunday
     afternoon for Calhoun IL-Fayette IL-Greene IL-Jersey IL-
     Macoupin IL-Montgomery IL-Pike IL.

     Winter Storm Watch from Friday evening through Sunday afternoon
     for Adams IL-Brown IL.



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