Scientific Forecaster Discussion
fxus63 klot 140456
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
1056 PM CST Fri Dec 13 2019
1210 PM CST
Through Saturday night...
A pocket of lower clouds is in place across far northwest Illinois near
a stationary/cold front, but this area will not advance quickly
as the low level flow is still weak and out of the southwest.
Therefore the remainder of the afternoon will be mild with a
minimum in cloud cover. The surface trough axis will approach
this evening and tonight and will make for light northwest winds
or just light and variable winds. There is an area of light
precipitation across Iowa with one of a number of weak upper
level systems that will pass through our area this evening into
the early overnight hours. Dry low level air and weak low level
forcing would preclude much more than flurries.
Late tonight into early Saturday the front gets a bit more
movement and will be able to bring in a deepening low level cloud
field. Forecast soundings would suggest that ice production would
be lacking once this cloud shield arrives. Forcing aloft is fairly
weak over NE Illinois and northwest in given that the more organized upper
level energy will pass through WI and toward the Ohio Valley.
There is some weak energy aloft with the cold front which will
get some more movement overnight. During this time the cloud
layer could get deep enough for some drizzle or even freezing
drizzle. Given the low/patchy coverage of drizzle, we have fairly
low precipitation chances in the forecast. Surface temps look
like they may be a bit more marginal and currently look to remain
at freezing or slightly above closer to Chicago, but outlying
areas and toward north central Illinois will be below freezing
during the period of potential precipitation. This would be most
favored with the cold front late tonight toward daybreak in the
Rockford area, and slightly later for the remainder of the area.
Drizzle/freezing drizzle forecasting is usually of lower
confidence but can be fairly impactful should coverage be a bit
higher than expected. Otherwise, the front will come through
Saturday morning and we will still have some patchy light
precipitation. We will be cold advecting tomorrow, so expect
temperatures to remain in the low to mid 30s with abundant cloud
cover in the morning. We could see some afternoon clearing if we
are able to mix out the lower clouds, but with a Stout low level
inversion this is not the most probable scenario. Weak high
pressure Saturday night will make for a cool but quiet night.
332 PM CST
Sunday through Friday...
Main concern in the long term continues to be the potential for
accumulating snow Sunday night into Monday as well as the
potential for some freezing rain in our far southeastern counties.
Model solutions are coming into better agreement that
accumulating snowfall is possible across the area Sunday night and
into Monday morning. An east-west oriented 850mb frontogenesis
band sets up Sunday evening across across Kansas/NE border through
northern MO and central Illinois. This should gradually weaken and lift
off to the northeast into our area by Sunday night. With surface
temps mostly in the 20s, any snow that does fall will stick to the
ground. Significant snow remains a lower probability outcome. We
could also see some icing potential in our far southeastern
counties where forecast soundings have temps aloft above -10c
which should limit ice nucleation. If this were to occur it would
result in more of a freezing rain scenario given that surface
temps will still be below freezing or near freezing at that time.
While there is increasing confidence in accumulating snow across
the area, there is still the possibility that some areas do not
see any snow, especially across our northern counties. Will have
to continue monitoring model trends as the event gets closer.
Looking beyond Monday, mostly quiet weather is expected as we as
the trough axis shifts off to the east and ridging builds in
across the area. Temps will be fairly cold behind the Monday
system with highs only in the 20s both Tuesday and Wednesday.
Temps gradually return to near normal by the end of the week with
temps in the mid 30s.
for the 06z tafs...
Weak cold front will move across the terminals early Saturday
morning resulting in a gradual increase in northwest winds and the
arrival of MVFR and possible IFR ceilings. Best chances for IFR ceilings
would appear to be at rfd with all ceilings likely to begin to slowly
lift late morning into the afternoon, though probably remaining
MVFR much of the afternoon before possibly lifting to VFR late in
the day. By afternoon, northwest winds will freshen up a bit and
will probably gust upward toward 20kt at times. Still some
potential for -dz/-fzdz but most likely scenario would seem to be
little/no precip most locations. By the time any dz would fall at
Ord/mdw/gyy, expect temps to be above freezing. Best chances of
-fzdz looks to be at rfd, though even there confidence is low.
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