Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 052158

Area Forecast Discussion
National Weather Service Saint Louis MO
358 PM CST Tue Dec 5 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 318 PM CST Tue Dec 5 2017

The upper air pattern continues to transition such that a deep
trough will be located over the eastern CONUS. This pattern favors
occasional intrusions of colder air at the surface which will
keep temperatures near or below normal through at least the first
half of the weekend.

Gusty winds will diminish tonight after the pressure gradient
relaxes. Winds will turn southwesterly overnight, then become
northwesterly tomorrow. Wind gusts will increase again tomorrow
afternoon, but gusts should be much lower compared to what was
observed today and yesterday.

The first of several embedded disturbances will rotate across the
area tonight within the developing longwave trough over the
eastern half of the CONUS. Although a sprinkle or flurry cannot be
ruled out late tonight, widespread precipitation does not appear
likely at this time.


.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 318 PM CST Tue Dec 5 2017

Additional upper disturbances will move across the area at times
from Wed night through Sunday. The models normally struggle with
these types of embedded disturbances, and it is common to see
significant temporal and spatial variations with each new set of
model runs. For example, most model runs from 24-36 hours ago
depicted a potent PV anomaly which was expected to bring light
accumulating snowfall to parts of MO/IL on Fri/Fri night, but the
latest model runs are about a day slower and slightly farther east
with this feature. Ultimately, there will be low confidence in
any particular model solution until these embedded PV anomalies
become apparent on satellite imagery and have been sampled by the
upper air network. The take-home message is that this upper air
pattern could produce a light snowfall event (less than an inch of
accumulation) sometime late this week or early this weekend, but
there is very low confidence in the timing. It`s entirely possible
that confidence will remain quite low until perhaps as little as
12-24 hours before precipitation begins.

Temperatures will remain near or below normal through at least the
first half of the weekend. At least one cold front is expected to
reinforce the cold air mass in place across the central CONUS late
this week, but there is low confidence in the timing of the front.
Additional fronts are possible late this weekend and early next



.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1140 AM CST Tue Dec 5 2017

VFR conditions are expected through the valid TAF period. Gusty
winds will diminish this evening and then back slightly, becoming
southwesterly overnight. Winds will then veer after 12z and
become northwesterly after 17z.





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