Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 11/28/19

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 842 PM CST Wed Nov 27 2019

.UPDATE...
842 PM CST

Evening Update...

Winds continue to gradually diminish across the area this evening.
Overall, going forecast appears in good shape and other than a few
adjustments for current observational trends in temps and wind
gusts, no changes made.

Deep low pressure continues to pull away from the region this
evening, with a center now filling over southwestern Ontario.
Impressive isallobaric gradient noted across the western Great
Lakes region, where 13 mb pressure rises were analyzed in the past
3 hours. While still blustery, winds have eased across the
forecast area since this afternoon, with gusts generally in the
25-35 mph range as of 8 pm. Further diminishing will continue
overnight as the gradient weakens more rapidly with the departure
of the surface low. GOES-16 nighttime microphysics channel
continues to indicate extensive stratus lingering in the cyclonic
low-level flow across the upper Midwest, though strong height
rises indicative of deep subsidence will likely erode some holes
in the overcast later tonight and early Thursday. Already noting
some large holes across eastern Wisconsin, where some of the
largest pressure rises have occurred recently. Persistent cool
advection on northwest low level winds should produce overnight
low temps in the upper 20s in most spots, though may be a few mid-
20s across north central IL if some clearing can occur and winds
will eventually be lightest by sunrise. Additional mid/high level
cloud cover will augment whatever lower clouds remain Thursday,
making for a mostly cloudy Thanksgiving Day, though surface high
pressure ridge will spread into the area producing light winds
making below-average daytime temps in the mid-30s feel better than
the strong gusty winds of today.

Updated digital and text forecast products available.

Ratzer

&&

.SHORT TERM...
127 PM CST

Through Thursday night...

No changes planned to high wind warning at this time. Recent ACARs
soundings out of ORD and MDW are showing 50-55kt winds within 3kft
of the surface. Cloud cover is likely somewhat limiting the
effectiveness of these winds being transferred to the sfc, but
still seeing some sporadic 50kt+ gusts. LOT and ILX WSR-88d VWPs
both show winds in the lowest few thousand ft starting to decrease
and this trend is likely to continue through the afternoon with a
gradual decrease in winds/gusts expected the remainder of the
afternoon with an even quick decrease this evening.

Subsidence in the wake of the departing cyclone is resulting in
some erosion of the stratus from central IL west into southern IA.
Most guidance is pretty aggressive in this trend continuing
tonight and while confidence is low, have shown some decrease in
sky cover tonight, though with continued low level cyclonic flow
and loss of sunshine, confidence in sky trends tonight is low. The
extent of clearing will have an impact on temps as wind rapidly
decrease this evening. For now, have shaded temps a bit toward the
warmer side of guidance, though if skies clear out temps could be
a bit cooler than what is in the grids.

Any clearing we see tonight into Thursday morning will give with
mid-high level overcast overspreading the area Thanksgiving
afternoon into Thursday night in advance of the next system...

- Izzi

&&

.LONG TERM...
252 PM CST

Friday through Wednesday...

The large upper trough now digging over the far western CONUS will
be a major player in the sensible weather across our area late
Friday through Sunday. Overall, this large upper trough is
expected to eject out a significant (negatively tilted) upper
level impulse out across the Colorado Rockies late Friday and
Friday night, before developing into a closed upper low over the
Plains on Saturday. This upper low, and an associated area of low
pressure is then expected to gradually shift eastward towards our
area by Sunday.

Locally it appears that precipitation chances will ramp up late
Friday into Friday night. This looks to occur as better moisture
feeds into the area on an increasing southerly low-level jet
downstream of the aforementioned upper level impulse. Because the
atmosphere looks to initially be dry and cool over the area, it
appears that the precipitation could onset as a period of sleet,
snow and rain, especially over far northern IL Friday evening.
This possibility continue to be reflected in the latest forecast.
While some minor sleet accumulation is possible, the mixed
precipitation is expected to change over to all rain area-wide
overnight Friday night and continue as rain on Saturday. This as
strong warm air advection should continue to warm the near surface
cold layer well above freezing into Saturday.

Some rain looks to also continue into Saturday night, as the main
storm system begins to approach the area. The center of this
storm system then is forecast to move overhead sometime on
Sunday. As this occurs, the primary concern shifts to the
possibility for rain changing to snow showers during the day as
the atmospheric column cools considerable under this upper low.
Considering this is still 4 days away, there are uncertainties on
any potentials for snow accumulations under this low. Current
model guidance suggests that surface temperatures could be
marginal to support much in the way of accumulation, but this will
have to be watched. For now, I have opted to just mention a rain
and snow mix during the day Sunday into Sunday night.

The weather should quiet down a bit early next week, with cool,
but seasonal type temperatures expected.

KJB

&&

.AVIATION...
For the 00Z TAFs...

Gradual decrease in winds this evening and MVFR ceiling trends
through the period are the primary aviation forecast concerns for
the 00Z TAFs.

Deep surface low pressure was pulling away from the region across
Georgian Bay and into Ontario early this evening. Strong pressure
gradient is beginning to slowly ease, with winds gradually
diminishing. Regional 23Z observations depict west winds are
mainly in the 20-25 kt sustained range with gusts around 35 kts,
and these should continue to slowly diminish overnight, while also
turning more northwesterly with time. Winds are expected to
eventually turn northerly Thursday morning as surface high
pressure ridge approaches from the west, and a few models
indicate a 010-020 direction at times during the day. Greater
confidence of a light north-northeast later in the day/evening.

GOES IR satellite imagery shows an extensive area of MVFR stratus
and stratocu in cyclonic flow trailing the low across the region.
Strong subsidence in the wake of the mid-level disturbance may
lower inversion heights enough to develop holes/erode some of this
and scatter out the MVFR cigs after midnight into Thursday
morning, though would like to see trends develop in satellite
imagery to add confidence in this occurring on a large enough
scale to remove ceilings from the forecast just yet. May see
redevelopment of slightly lower lake-induced MVFR cigs into
ORD/MDW late Thursday/Thursday evening as low level flow becomes
light east- northeast.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Lakeshore Flood Advisory...INZ002 until 6 AM Thursday.

LM...Gale Warning...IL nearshore waters until midnight Thursday.

     Gale Warning...IN nearshore waters until 3 AM Thursday.

&&

$$

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