Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 091739

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1139 AM CST Sun Dec 9 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 305 AM CST Sun Dec 9 2018

The mid-Mississippi Valley remains between high pressure across the
Great Lakes and low pressure moving across the southeast. In the
upper levels, we find ourselves between two shortwaves in the base
of a broad trough. One wave is to our southeast, associated with
the previously mentioned surface low. The other wave is currently
over the northern Plains and will drop into the Midwest today. With
the southern stream wave and low expected to push off to the east
today, the surface high and northern stream wave will be the
dominant factors for the short term forecast.

In the near term, attention is focused on an area of fog developing
between roughly Salem and Farmington. This fog is easily seen on
satellite imagery, but unfortunately we have no observations
between those points and therefore cannot be sure how bad the
visibilities are within the fog. None the less, expected reduced
visibilities if traveling in that area.

Beyond that, the main forecast questions are how will the high
clouds evolve through the day and how will that impact temperatures.
The high clouds currently over the area are associated with the
southern stream system, and should move out fairly quickly this
morning. There are additional scattered high clouds upstream
associated with the northern stream wave, and these will likely move
through our area today. Despite these scattered clouds, we will
likely see plenty of sun today, which should allow us to get a bit
warmer than yesterday. However, weak northeast flow at the surface
will temper the warming, keeping highs in the 30s across most of the
area. Temperatures tonight will be quite cool again thanks to high
pressure, weak winds, and mostly clear skies.


.LONG TERM...  (Monday through Saturday)
Issued at 305 AM CST Sun Dec 9 2018

Below normal temperatures and dry conditions continue to be expected
through Monday with temperatures moderating and a more active
weather pattern beginning on Tuesday and persisting into next

The cold and dry pattern (longwave upper TROF over the eastern CONUS
and strong influence from an area of cold surface high pressure)
currently in place will persist into Monday.  The atmospheric column
will also be very dry, so despite another storm system tracking
slightly west and south of our region during this period, no
precipitation is expected to be generated by the system.
Temperatures will not be as cold on Monday as this past weekend, but
maxes between 35 and 40 are likely, which is about 5 to 10 degrees
below normal.

A shift in the pattern is then on tap for the middle and later
portions of the week, to one that is more progressive and active.
Instead of Canadian airmasses dominating our area, however, they
will be Pacific in origin, and result in a temperature regime that
is much more seasonable, if not slightly above normal.

Regarding precipitation chances, there is good agreement amongst the
operational models of an uptick in PoPs for much of the period from
Wednesday on, but there remains a large spread in specifics, such as
storm structure, timing, and track and this will affect in turn,
items such as precipitation-type.  The operational EC the last
couple of runs has been the biggest outlier and mainly discounted
for now.  The best model consensus has an initial shortwave
disturbance to track into our area on Wednesday, but moisture will
still be limited from the bone dry column of early week, with enough
moisture to generate precipitation probably not available until
Wednesday night.  If the timing of this system is early on
Wednesday, it will probably swing through dry.  If it leans towards
Wednesday night, the precipitation chances will/should be
mentionable.  Either way, precipitation-type in this case should
lean towards a cold rain with warm air aloft and temps above
freezing.  The main storm system we have been advertising the past
couple of nights is still on track for a late Thursday and Friday
timeframe, but the storm structure and timing is all over here.
Some models show a closed LO, others an open wave, and the EC has
the more extreme closed system and tracks it well south of the area.
 Most model solutions save the EC still favor the best precipitation
chances here, with a mostly rain event for much of the lifespan of
it with some snow still possible on the backend/northwest flank of
the storm.  If a closed LO is realized, snow potential will increase
substantially, but an open wave or a track further south will
decrease this to near zero given the lack of decent cold air
presence at the low levels.  Uncertainty remains high with several
items, resulting in continued low confidence on wintry precipitation
occurrence, amounts, and locations.

Prior to precipitation chances ramping up, look for Tuesday and
Wednesday to be the mild days of the week, relatively speaking, with
max temps poking above 50 degrees in areas.  Temperatures will edge
back to normal after that, with assumed increased cloud cover and
much higher precipitation chances.



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1128 AM CST Sun Dec 9 2018

Quiet/VFR conditions are likely through Monday morning across the
forecast area. A clear or mostly clear sky is expected with
perhaps a few high cirrus clouds as a surface ridge of high
pressure moves slowly across the mid-Mississippi Valley.


Surface ridge of high pressure is forecast to move across the area
late tonight. Northerly winds will become light/variable overnight
as a result along with just a few cirrus clouds.





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