Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170530

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1130 PM CST Sat Dec 16 2017

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 241 PM CST Sat Dec 16 2017

A vigorous shortwave is ejecting northeast through west Texas and
New Mexico this afternoon. Water vapor imagery is showing a good
moisture plume ahead of the wave moving out of the western Gulf of
Mexico and and tropical Pacific south of Mexico. Rain associated
with this moisture plume is already spreading northeast across
Texas. The northern edge of the clouds associated with the plume
are also spreading out ahead of the precip into Arkansas and
southern Missouri. Expect clouds to lower and thicken tonight over
our area as moisture deepens. Southerly flow and cloud cover will
produce a relatively mild night with lows bottoming out during
the late evening or just after midnight then remaining steady in
the upper 30s to mid 40s through daybreak.

Guidance has been very consistent over the past few days and
remains consistent in bringing precipitation up through the Ozarks
late tonight...reaching our forecast area by 09-11Z, continuing
to spread northeast to near the I-70 corridor by 14-16Z, and up
into northeast Missouri and west central Illinois by around 18Z.
Rain should end from southwest to northeast during the late
morning and early afternoon. QPF continues to be very light in
all guidance with highest amounts totaling up to 1/4 inch or
less. Clouds and precipitation will suppress temperatures
tomorrow so only expect highs to reach the mid and upper 40s.


.LONG TERM...  (Sunday Night through Next Saturday)
Issued at 241 PM CST Sat Dec 16 2017

After the PV anomaly responsible for Sunday`s precipitation has
moved out of the area, there appears to be little opportunity for
additional widespread precipitation through at least the middle
of the upcoming week. The pattern aloft will remain weakly
northwesterly to nearly quasizonal over most of MO/IL while a
strong PV anomaly near Baja California becomes cut off from the
prevailing flow. This feature will gradually weaken and shift
eastward, bringing a chance of rain to the southern CWA on Tue
night and Wed when it passes through AR. Model solutions then
diverge significantly by the end of the week/early next weekend,
leading to low confidence in any given model`s depiction of the
state of the atmosphere heading into the busy travel period ahead
of next weekend. There is general agreement that a positively-
tilted trough will reach the PacNW coast on Wed and move over the
Four Corners region on Thu, but there is no consensus on how the
upper air pattern evolves thereafter.

Temperatures will be as much as 10-20 degrees above average for
most of next week. A weak cold front will move through the area on
Tuesday night, but overnight lows on Tue night and daytime highs
on Wed will still be around 10 degrees above average even within
the cooler post-frontal air mass. The current forecast shows
cooler highs for next Fri/Sat after another cold front moves
across the area, but there is considerable uncertainty due to the
poor model agreement discussed above.



.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1130 PM CST Sat Dec 16 2017

VFR conditions and dry weather will prevail at the TAF sites thru
much of tonight, yielding to deteriorating conditions after
12z/Sun, with a transition to at least MVFR category expected. The
best potential for IFR category conditions will be in COU mainly
Sunday morning, with a lessening potential for UIN and STL metro
sites primarily due to time of onset. A quick shot of light rain
is still expected for all TAF sites during Sunday morning for a
few hours. Light S-SW surface winds will prevail.

SPECIFICS FOR KSTL: VFR and dry thru about 12/13z, with light rain
first around 13z and then falling ceilings to MVFR during late
morning and continue thru the afternoon and evening. IFR will get
close but at this time have kept it out due to onset timing being
more in the afternoon.





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