Scientific Forecaster Discussion
fxus63 klot 051147
Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
547 am CST Thu Dec 5 2019
329 am CST
Aloft, an impressive upper low continues its spin over the
western Continental U.S. With northwest flow still in place across the Great Lakes
At the surface, winds have become light and mostly variable early
this morning. This combined with clearing skies, some areas could
see temps drop into the low to mid 20s.
Winds turn southerly by mid morning ahead of an advancing cold
front. Although this will provide some limited warm air advection
into the region, mid to high level clouds streaming out ahead of
the front move in by noon, which could limit our warming somewhat.
Still expect highs to reach into the mid to upper 40s to near 50.
The cold front then moves across the area Thursday night. Winds
become gusty along and behind the front out of the north,
especially over the lake and areas downwind. As has been the case
the past few days, a dry layer at the low-to-mid levels should
limit any precip chances associated with this system. Temps on
Friday will be noticeably cooler, with highs mainly in the 30s as
high pressure settles in and winds become light and variable.
329 am CST
Friday night through Wednesday...
In the wake of Friday morning's cold front, winds will finally
begin to fall off Friday evening as the core of a 1030+ mb high
begins to settle in (briefly) overhead. With the injection of
drier air into the region, temperatures should fall off pretty
quickly during the evening hours. Have nudged low temperatures
down a bit under the blended guidance, although it looks like lows
may actually be hit towards midnight before southwesterly flow
returns overnight as the high pressure axis shifts off towards the
Ohio River valley.
Saturday is looking like a pretty nice day as breezy southwest
winds drive the lingering cool airmass northward which should
allow high temperatures to rise back into the lower 40s for most.
This pleasant weather will be short-lived, however, as additional
warm advection will likely deliver a round of increasing low
stratus to the area Saturday night into Sunday as a lead shortwave
drifts across the area. Even with the increased cloud cover on
Sunday, however, persistent south to southwesterly flow ahead of
the next weather disturbance will continue to pump warm air
poleward. This should allow high temperatures to rise solidly into
the 40s if not lower 50s across parts of the region. The mid-
levels continue to look a bit too dry to support much in the way
of precipitation development during the day on Sunday, although
can't entirely rule out perhaps a few stray showers developing
under the lingering subsidence inversion. Have capped pops below
15% as a result, but do ramp them up pretty quickly on Sunday
evening as better upper jet dynamics move in ahead of the next
incoming cold front. Thermal profiles look to solidly support a
plain rain through Sunday night, even as surface temperatures fall
into the upper 30s near the I-39 corridor towards daybreak on
Overnight guidance has come into a bit better agreement regarding
the large-scale evolution of the early-mid week weather system
with the trend in both deterministic and ensemble guidance towards
a lower potential for phasing between a large/potent northern
stream wave and a faster-moving southern stream system.
Previously, the GFS/gefs guidance was showing an increased
potential for a slower/deeper/more phased solutions overall, but
has notably trended towards the rest of the faster global model
suite. The net result is generally lighter precipitation amounts
and less potential for snow before dry air shuts off precipitation
production on Tuesday. Still obviously some potential for subtle
changes in timing and amplitude of the various pieces here, but
latest indications are for just a brief hit of light snow on the
backside of the departing low Monday evening and into Tuesday.
The main story into the middle of next week continues to be on
the blast of very cold air in the wake of this departing system.
Global guidance is all in solid agreement for a day 5+ forecast
driving a core of anomalously cold 850 mb air across the upper
Midwest (likely into the -15 to -20 c below normal range) by
Tuesday night and Wednesday. Have continued the previous trend of
undercutting the blended guidance at this point, especially for
lows Tuesday night and highs on Wednesday with the latest National
blend of models indicating temperatures that look a bit too warm
towards the 75th percentile of the guidance envelope. Near zero or
even some sub-zero air temperature readings certainly appear to
be in play by Tuesday night, especially across our northwest zones
away from The Heart of Chicago. Below zero wind chill values look
like a good bet for many by Wednesday morning as brisk northwest
winds may persist for a bit before the core of the Canadian high
builds overhead. Longer range guidance does, however, indicate
that this cold intrusion will be short-lived, with a pretty quick
moderation taking place by the end of next week.
for the 12z tafs...
VFR conditions in place this morning, and are expected to continue
today into this evening. Light and variable winds will pick up
out of the south later today, with a trend towards the south
southwest this evening. Do expect a cold front to swing across the
terminals later tonight into early Friday morning, ushering in a
wind shift to the northwest and MVFR ceilings. Conditions look to
remain dry with this front though. Kept these ceilings in place
for the remainder of the period, however, it's possible that some
clearing may occur slightly earlier than currently forecast.
Lm...Small Craft Advisory...in nearshore waters until 9 am Thursday.
Visit US at http://weather.Gov/Chicago (all lowercase)