Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 160943

Area Forecast Discussion
National Weather Service Saint Louis MO
343 AM CST Sat Dec 16 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 337 AM CST Sat Dec 16 2017

Skies were clear across all of the CWA early this morning.  Surface
winds were staying out of the south to southwest between 5-15 mph
allowing temperature to stay at or above normal for the middle of
December. The upper ridge axis that is currently over the central
Plains will move east toward Missouri and Illinois today.  This will
act to keep the skies sunny throughout most of the day.  The GFS/NAM
is showing a fairly tight pressure gradient today between high
pressure centered over the southeast CONUS and a frontal trough
extending from low pressure over New Mexico northeastward into Iowa.
This should act to increase winds out of the southwest into the 10-
20 mph range during the day with higher gusts.  The full sun and the
favorable wind direction should allow for warming above MOS and SREF
mean temperatures today, so kept with going forecast temperatures.

Still looks like this evening will be dry, but there will be an
increasing chance of rain overnight over parts of central and
southeast Missouri as the upper low that is opening up approaches
the area.  Both the GFS/NAM show a marked increase in low level
moisture convergence beneath mid level ascent associated with the
trough across these areas, most notably between 09-12Z tonight. Have
gone with mainly chance PoPs except in a small part of the eastern
Ozarks where I have likely/categorical PoPs.  With the strong warm
air advection tonight, temperatures will stay above freezing across
the entire area.


.LONG TERM...  (Sunday through Friday)
Issued at 337 AM CST Sat Dec 16 2017

The upper air pattern will evolve considerably through next weekend
consisting of a split flow while trending more amplified as well.

Perhaps the best threat of precipitation and with greatest
confidence will be early in the period on Sunday. The models are
very similar showing a short wave trof within the southern stream,
weakening as it ejects northeast across the area Sunday in response
to an upstream kicker. Rain associated with the northeast edge of
a large precipitation shield will likely be ongoing across
southern MO at 12Z, impacting the southwest portion of the CWA.
This shield of rain will quickly overspread the area on Sunday
morning as large scale ascent shifts northeast with the advancing
short wave trof. Present indications are that most of the rain
should be over by midday as the trof axis stretches thru central
IL at 18Z with large scale subsidence by afternoon. Low clouds
should linger through Sunday night and into Monday morning and
there could be some drizzle or sprinkles across portions of
central and northeast MO on Sunday night. While Sunday will be
cooler due to clouds and morning precipitation as compared to
today, Monday looks be a return to above average temps, perhaps
well above average depending on clouds. There are some indications
a weak frontal boundary may be present at the start of the day
across northern MO and then the aforementioned stratus. However
southwest-west low level flow becomes prevalent with good WAA and
this should result in eroding clouds and frontolysis of any
thermal boundary should one be present.

The models begin to show there first sign of diverging with the next
cold front in the late Monday night-Tuesday time frame. The GFS is
basically about 6 or so hours faster bringing a cold front through
the area by Tuesday morning, a reflection of sharper, more
amplified northern stream troffing extending southward from a
Hudson Bay upper low. The GFS/ECWMF then diverge again with a
southern stream upper trof advancing eastward from the southern
Plains and across the lower MS Valley on Wednesday. The ECMWF is a
good deal stronger and further north and suggests roughly the
southern 1/4 to 1/3 of the CWA could see light rain associated
with the northern part of the deformation zone precipitation
shield. Model agreement continues to go downhill during the later
part of the week with big differences between the GFS and ECMWF
handling the split flow, dominant stream, and location and
evolution of a southern stream upper low/trof. In general the
ECMWF has a more dominant northern stream and points to the
evolution of a broad deepening upper trof centered through the
Nation`s midsection by next weekend. The GFS on the other hand has
a less amplified northern stream with an upper low still back in
the southwest. Sometime in the Thursday/Thursday night time frame
there should be a rather strong cold front move across the region
and it could be accompanied by a post-frontal band of wintry
precipitation. Confidence is very low in any details at this
point, but at face value it would appear that northern parts of
the CWA would currently have the best chance of measurable snow on
Thursday night/early Friday morning.



.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1118 PM CST Fri Dec 15 2017

VFR conditions, dry weather, and S-SW surface winds will prevail
at the TAF sites thru the valid period. A marginal LLWS event is
either ongoing for UIN and COU or still anticipated to begin
shortly for the STL metro sites and to continue until diurnal
mixing occurs around 15z as the low level jet translates into
gusty winds at the surface of up to 20kts. Winds will then
diminish around sunset (23z) and continue into Saturday evening.



Issued at 334 AM CST Fri Dec 15 2017

Record high temperatures for Saturday December 16, 2017

St. Louis  70 in 1889
Columbia   70 in 1889
Quincy     65 in 2006


Saint Louis     62  40  49  41 /   0  10  80  10
Quincy          57  35  46  37 /   0   5  80  10
Columbia        63  38  48  39 /   0  20  80  10
Jefferson City  65  39  49  40 /   0  30  80  10
Salem           55  37  47  40 /   0  10  70  10
Farmington      58  38  48  40 /   0  40  80  10




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