Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 01/12/22

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 527 PM CST Tue Jan 11 2022

.SHORT TERM...
Issued at 300 PM CST Tue Jan 11 2022

Through Wednesday night...

The main winter weather concern, albeit a low confidence one, is
the potential for a narrow area of heavier precip rates mid
Wednesday morning through the early afternoon, primarily for far
northern Illinois. If the precip rates are heavy enough, the
concern is that this would be a burst of snow causing at least
some travel impacts.

In the near term, our windy moderation coming off a few very cold
days is well under way, with south-southwesterly wind gusts up to
30-35 mph. These gusty winds will continue through this evening
while veering to more southwesterly despite the lingering
glaciated snow cover for most of the area as a very impressive
west- southwesterly low level jet of 50-60 kt between about 2kft
and 3kft AGL. Expect temperatures to continue to climb past sunset
and then probably level out by the mid to late evening.

Weak cold front or moreso surface trough passage and departure of
the low level jet off to the east after midnight will cause winds
to quickly ease and temps to drop back to the mid to upper 20s.
Our lower amplitude short-wave of interest for tomorrow is
currently over the Canadian Rockies and will quickly cruise in
tomorrow morning and interact with a fairly tight baroclinic zone
over the western Great Lakes and upper MS Valley. Even at this
near term lead time, there is plenty of uncertainty on exact
trajectory of the wave and overlap of forcing and sufficient
saturation for precip. Note that the NAM and GFS have been
generally north of the IL/WI state line, while the ECMWF (and its
ensemble/showing likely probabilities of measurable precip) and
the Canadian favor the state line area and points just south.

While confidence is low due to lingering uncertainty in the
guidance, concern is elevated for a 2-4 hour burst of heavier
precip in form of snow due to the presence of transient strong
f-gen as well as steep mid-level lapse rates near/over 7C/km.
These can be "gotcha" set-ups that can put down a quick inch or
two of snow and cause travel impacts. Even though deterministic
forecast is only coating to several tenths of snow, will message a
limited snow risk in the HWO. The thermal profile is also in
question, if precip occurs, as initial warm nose may pose some
threat for brief freezing rain at onset, but will let later shifts
assess this. A trailing area of light precip could sag south
during the afternoon, though confidence also low on this aspect.

Aside from the low-confidence but potentially impactful period
mid-late AM through early afternoon for portions of far northern
Illinois, tomorrow will feature above normal temps ranging from
mid 30s I-80 and north to upper 30s to around 40 south. Another
weak cold front will move across the area on Wednesday night ahead
of next period of interest Thursday morning, covered in the long
term AFD below. Low temps will be above normal in 20s to around 30.

Castro

&&

.LONG TERM...
Issued at 237 PM CST Tue Jan 11 2022

Thursday through Tuesday...

The forecast points of interest for the latter half of the week
into the weekend continue to center on the series of disturbances
(i.e. wave train) that bring snow chances. Predictability with
these drops off quickly, due in part to their low amplitude nature
as well as the recent trend in guidance (particularly with
Friday-early Saturday).

The first of these disturbances in this period will be dropping
southeastward over the region on Thursday morning. This will be
traversing on the periphery of the polar lobe extending over
Hudson Bay, riding the left exit region of a 140+ kt jet. Guidance
is in decent agreement of the 500 mb vorticity center tracking
over/very close to northeast Illinois and northwest Indiana, and
with a focus for ascent from the pre-dawn hours through the
morning. The elevated lapse rates are noteworthy with this wave,
being around 7.5 C/km from 800 to 600 mb. The saturation is a
little less impressive as is the depth of the dendritic growth
zone. The extended portions of the RAP and HRRR show a few
hundredths to even a tenth of an inch of QPF for parts of the CWA,
and a fairly good signal that some part of the CWA should see at
least brief light snow. Have nudged up PoPs above a blend of the
latest guidance, and will be a period to watch with its
intersection of the Thursday morning commute. Temperatures look to
be in the mid-upper 20s during the possible brief period of snow.
In the wake of the stronger forcing, there is a marginal signal
for some drizzle after, but that would be late morning into
afternoon, with temperatures nudging up to or a little above
freezing.

For the rest of Thursday into early Friday, north-northwest
surface winds will veer to northeast. Cold advection will be
limited for that wind direction this time of year, but may still
be enough for lake effect clouds and some low quality light snow
into northeast Illinois/far northwest Indiana. The forecast
inversion heights are only 3,500-5,000 ft AGL, so not notably high
for anything robust if it were to materialize.

The next digging system for Friday afternoon and night into early
Saturday has been one that has consistently been shown to be a
potentially stronger one. The 12Z deterministic guidance suite
largely showed a westward shift with this, and to a lesser degree
the GEFS and ECE runs. This wave is still traversing the Gulf of
Alaska region and is low amplitude. The hard right turn that it is
forecast to take in northwest Canada is key, as just slight
variance in that will dictate its track into the U.S., the
eventual phasing time with another wave into the Plains, and
overall where an accumulating snow track is. The WPC Cluster
Analysis, while based on 00Z ensemble data, did show about 30% of
the ensemble members still having a solution that brings QPF
fairly far east from the mean and smears the gradient into and
over our area. For now, the guidance provided blend of PoPs...from
basically 30 percent east to 50 percent west...provides a
meaningful representation of our lower-end certainty. So from the
previous forecast, this is only a slight edge downward --
basically a lesser shift than the 12Z guidance did from the 00Z.

It should be noted too, that through Friday into early Saturday
there continues to be some non-heavy lake effect snow possibility
into northeast Illinois as the east-northeast winds steadily
increase around that digging system. Inversion heights never
really come up much, but the temperature profile is forecast to
cool later Friday night into Saturday morning.

No changes made beyond that in the forecast. Looks like another
couple waves Sunday through Tuesday that could introduce a hearty
shot of cold air by the mid to latter part of next week.

MTF

&&

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

South to southwest winds will likely remain gusty well into the
evening hours, particularly at ORD/MDW. Recent ACARS flights out
of ORD show about 45kt at 2000ft AGL with some increase in winds
just off the deck expected this evening before winds through the
low level shift more westerly later this evening and subside. VFR
CIGS expected through Wednesday, likely lowering some to FL080-100
by midday Wednesday. Some potential for lower CIGS with stratus
toward the end of the 30 hour MDW/ORD TAFS, but confidence isn`t
high whether it arrives within the 30 hour TAF or more likely just
beyond, so kept it out for now.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning...nearshore waters until midnight Wednesday.

&&

$$

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