Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/20/22
Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
942 PM CST Mon Dec 19 2022
.UPDATE...
Issued at 940 PM CST Mon Dec 19 2022
The going forecast tonight into the day Tuesday is in solid
shape. Flurries have been heavily struggling to materialize out
of mid-level virga. The 00Z DVN sounding observed a not-too-shabby
85% relative humidity from 850-500 mb, but much drier at ILX
(more upstream of much of our area in this flow pattern) and also
observed on Chicago aircraft soundings. Cloud bases have almost
entirely been above 7,000 ft and the radar imagery "hole" has not
been closing -- i.e. the low-levels are stubbornly dry. The
northwestern forecast area remains where patchy flurries might be
seen the rest of tonight. While the weak 700 mb trough and even
weaker surface trough inch eastward through Tuesday morning, the
saturation depth begins to diminish despite the low-level moisture
increasing. Still see the chance of any flurries or "snizzle"
being on the low end of the spectrum based on some of the latest
00Z guidance forecast soundings.
Temperatures will remain steady through the rest of the night
under the canopy of mid-level clouds and a light south wind.
Also no changes made to the evening update of the Winter Storm
Watch for late this week.
MTF
&&
.SHORT TERM...
Issued at 305 PM CST Mon Dec 19 2022
Through Tuesday night...
Unlike the latter half of the week, things are expected to remain
fairly humdrum here through the remainder of the first half of the
week. There are still a few things forecast-wise to discuss during
this near term time frame though, including the potential for
flurries tonight, a low (but non-zero) chance of freezing drizzle
across portions of northern Illinois tomorrow, and another bout of
chilly overnight temperatures Tuesday night into Wednesday.
High pressure remains over the region this afternoon, though
regional radar mosaics do show some relatively light precipitation
echoes to our south and to our west gradually approaching our
forecast area within a zone of relatively subtle warm air advection/
isentropic ascent. The expectation is for these echoes to become
increasingly more prevalent over our CWA as we head into this
evening as the mid-levels (including the dendritic growth zone)
become increasingly saturated with time. Dry sub-cloud air will keep
all hydrometeors relegated to virga initially, but with time, do
anticipate that this drier low-level air will be eroded enough for
snowflakes to start reaching the ground. However, with poor lapse
rates within the DGZ and the overall signal for ascent remaining
quite muted throughout tonight, this should not amount to anything
more than just a period of flurries for most locales across our CWA.
There is a chance that our northwestern most counties see snow
come down at a steadier rate than elsewhere in our CWA late
tonight, but almost all forecast guidance has trended towards
keeping all QPF fenced off to the north and west of our forecast
area, so seeing anything more than a dusting here tonight is
looking like an increasingly bleak prospect.
A cold front will traverse through the area from the northwest
tomorrow with a narrow strip of stratus/stratocumulus expected to
filter in immediately on its coattails. There have been some
indications that this low cloud deck could be deep enough to support
drizzle for up to a few hours across the northwest half or so of our
CWA, with much of any drizzle that does occur likely ending up as
freezing drizzle with temperatures expected to remain at or below
freezing across this portion of our forecast area throughout the day
tomorrow. However, most 12Z guidance did seem to trend a little
drier/towards a not sufficiently deep low cloud layer, so have
peeled PoPs back a bit in response to this trend. While the
overall potential for freezing drizzle seems fairly low at this
point, any potential threat for icing always bears watching, so
will continue to keep a close on eye on whether these trends
uphold in forthcoming model runs.
Final thing to note is that Tuesday night looks like it will be
another chilly one -- at least across the northern half or so of our
forecast area. With northerly winds being light enough for only
minimal cold air advection to occur, low temperatures Tuesday night
into Wednesday morning will heavily depend upon how much the
aforementioned low clouds clear out overnight. Current indications
are that these clouds will loom over the southeastern third or so of
our CWA throughout the night, stunting radiational cooling efforts
and suspending temperatures in the 20s there. Farther northwest,
there is a signal that there should be a decent amount of clearing
that occurs, and as a result, temperatures should fall into the
teens and single digits here with the coldest readings expected to
be found along/near the Illinois-Wisconsin state line. The
relatively light winds should prevent wind chills from being too
much colder than the actual air temperatures, however.
Ogorek
&&
.LONG TERM...
Issued at 305 PM CST Mon Dec 19 2022
Wednesday through Monday...
* Travel impacts expected to begin Thursday, mainly western CWA
* Rapidly deteriorating conditions Thursday evening as a ferocious
arctic front rips across the area resulting in temperatures
rapidly falling through the teens into the single digits
* Blizzard conditions will develop Thursday night, especially
late, and continue through the day Friday
* Bitterly cold wind chills expected during the height of the
storm, posing a significant threat to life for anyone who
becomes stranded in the storm
After extensive collaboration with WPC and neighboring offices,
have opted to hoist a long lead time winter storm watch for
blizzard conditions Thursday night through Friday night.
Models and their respective ensembles have remained locked into
solutions depicting rapid cyclogenesis Thursday into Friday as
the cyclone tracks from southern IL to lower Michigan. Various
models depict explosive deepening of this low with central
pressure dropping 25-35mb in 24 hours. This type of explosive
intensification is quite rare in this region and is expected to
result in extremely powerful and potentially damaging winds late
Thursday night and especially Friday.
The deformation band with this cyclone is expected to be quite
intense owing to the extremely strong omega and weak static
stability/slantwise instability. Very steep mid-upper level lapse
rates do raise concerns about the potential for thundersnow even
later Thursday night into Friday morning. Expecting snow to liquid
ratios to average around 15:1, possibly 20:1 western CWA where DGZ
will be deeper. The one factor that *should* keep the snow:liquid
ratios somewhat in check is the very strong winds which would tend
to rip about the larger/fluffier dendrites.
Have seen some general downward trend in QPF values over the past
24 hours in guidance, but QPF generally still supports widespread
6" totals with even 1-2 foot totals possible associated with any
more intense banding/possible thundersnow. We are still very far
out and not planning to advertise snowfall amounts in public
products at this time, as much could change between now and then.
It is also worth stressing that the amount of snow that falls is
a secondary or tertiary concern among the hazards. The expected
temperatures will leave this snow extremely susceptible to blowing
around and even if only a couple inches of snow falls, a blizzard
would still be likely given the nature of the snow and magnitude
of the strong winds. This is one of the reasons for the early
winter storm watch.
Impactful winter weather, in the form of accumulating snow is
expected to begin in the western portions of the CWA during the
day Thursday. However, opted not to include this portion of the
storm in the winter storm watch as the threat level of this snow
will dwarf what will be expected during the blizzard portion of
the storm.
If current guidance is close to verifying, then conditions Friday
could rival the 2011 Groundhog Day blizzard, particularly in open
areas. Travel would become extremely dangerous and life
threatening, particularly in light of the bitterly cold
temperatures during the height of the storm.
Falling snow is expected to end Friday afternoon or evening, but
strong winds and blizzard conditions are expected to continue
Friday evening and only slowly abate through the night. Some
blowing snow will likely continue into Saturday with bitterly cold
temperatures through the holiday weekend.
- Izzi
&&
.AVIATION...
For the 00Z TAFs...
Forecast concerns for the terminals...
*Chance for flurries this evening
*Winds become northwesterly behind a weak front Tuesday afternoon
*Period of MVFR ceilings along the front
Radar continues to show an area of light snow and flurries
stretching from southwestern Wisconsin into central Illinois with
another area over central Iowa. At this time most of the
observations across northwestern and central Illinois have not
been reporting any snow yet likely due to the pronounced area of
drier air in the subcloud layer. However, with time this layer is
expected to saturate some as the flurries move eastward into
northeastern Illinois later this evening, especially for areas in
areas in northwestern Illinois. No major changes were made to the
going forecast with this update with a TEMPO for flurries at the
Chicago terminals and the prevailing flurries with a TEMPO for
light snow at RFD which continues to look reasonable. Otherwise
VFR conditions will prevail through tonight with light south-southeast
winds becoming southwesterly around midnight.
A weak frontal boundary is expected to move across the area late
Tuesday morning into the afternoon which will turn winds
northwesterly behind the front and increase speeds slightly around
10 kts. Guidance continues to show that the frontal passage should
be dry as forecast soundings indicate ample dry air in the mid-
levels with only a shallow area of moisture in the low-levels.
While a brief period of drizzle still cannot be completely ruled
out, confidence continues to remain low and therefore no formal
mention in the TAFs at this time. However, the low-level moisture
is expected to develop an area of MVFR ceilings for a brief period along
and just behind the front with ceilings expected to return to VFR
towards the end of the TAF period.
Yack
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-
ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108...6
PM Thursday to 6 AM Saturday.
IN...Winter Storm Watch...INZ001-INZ002-INZ010-INZ011-INZ019...6 PM
Thursday to 6 AM Saturday.
LM...None.
&&
$$
Visit us at weather.gov/chicago
Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/NWSChicago
www.twitter.com/NWSChicago
www.youtube.com/NWSChicago