Area Forecast Discussion
Issued by NWS Chicago, IL

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000
FXUS63 KLOT 301020
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
420 AM CST Wed Nov 30 2022

.SHORT TERM...
Issued at 255 AM CST Wed Nov 30 2022

Through Thursday...

The seesawing of temperatures in this late autumn and La Nina-
like pattern continues today, as northern Illinois and northwest
Indiana will be on the cold side of the seesaw. Temperatures are
falling through the 20s early this morning, which by value is
near normal for late November. However, that is about 30 degrees
colder than early last evening, and wind chills are around 45
degrees below what temperatures were at 6 P.M. last evening!
Temperatures will not rise much today with the main story once
again being winds.

The low pressure driving this cold advection is now at 982 mb low
over central Ontario and that`s four standard deviations below
normal for that area this time of year. The tight pressure
gradient in its wake will continue moving over the region, with
pressure rises through mid-afternoon of 4-5 mb per 3 hours. The
cold air advection (CAA) profiles behind strong systems often
support mixing at least sporadic gusts to the speed at the top of
the mixed layer, and this has been seen early this morning (39
mph at both ORD and MDW). While some cyclonic flow stratocumulus
is likely along and north of I-80 today, model sounding boundary
layer profiles support gusts to around 40 mph. A few models show
some higher hours, namely the GFS, where gusts might inch into the
41-45 mph range, but that should be sporadic. As big of story
will probably be the sustained winds as they regularly should be
near 25 mph today. Also to note that winds look to be slow to
subside this evening with a continued gradient and an advertised
slight uptick in the 925-950 mb wind speeds.

Cold air advection profiles such as this usually support some
stratocumulus in the absence of strong height rises and/or an
incoming strengthening subsidence inversion. This is the case
today, especially for the northern half of the area (north of
I-80), where mostly cloudy conditions are probable, at least at
times. The cloud layer does look to be around -12C and this can
support flurries in such a pattern. This cloud layer though is
very shallow (<700 ft on guidance soundings and a recent upstream
MSP sounding). So flurries are not anticipated at this time.

Temperatures look to "rebound" to around 30F today with afternoon
wind chills only recovering a few degrees from morning values.
This evening`s wind chills will fall into the single digits, and
actual low temperatures will drop into the teens area-wide with
high confidence.

Consistent with the progressive pattern, a high pressure ridge
will quickly scoot over the area early Thursday morning. South
winds resume on Thursday afternoon under some thin high clouds,
with highs forecast to increase 5 to 8 degrees from today.

MTF

&&

.LONG TERM...
Issued at 330 AM CST Wed Nov 30 2022

Thursday night through Tuesday...

Key Messages:

* Strong winds Friday and especially Friday night into Saturday
  morning (45+ mph gust potential) with another sharp cold front
  approach and passage

* Potential for light precipitation early next week but latest
  guidance trended away from a higher impact event

The pattern will remain very progressive through the long term
period, which is leaning away from being conducive to any moisture
laden system and higher precipitation amounts. It will also result
in a continuation of the windy stretch we`ve had the past few days.

A stout low level jet of 40-45 kt at 950 mb will develop overhead
Thursday night and result in a breezy night with slowly rising
temperatures after any brief dip in the evening. The setup wind
wise Friday through Saturday is quite reminiscent of that happened
Tuesday and today. Strong warm advection and at least patchy lower
clouds Friday again adds some uncertainty to top end wind gusts
from the south, though 35-40 mph should be attainable with upside
(near advisory) potential per BUFKIT soundings, especially with
southward extent. Stout pressure falls on Friday afternoon of near
10 mb/6-hour and 3-4 mb/3-hour should help tap into the base of
the low level jet as deepening low pressure tracks to western Lake
Superior.

A more limited period of moisture return offers less of a threat
for any "frisky" convection Friday evening, with a chance for
isolated to perhaps widely scattered lighter showers immediately
ahead of the powerful cold front. Temperatures that will have
risen into the low-mid 50s later Friday afternoon will remain in
the 50s and possibly even warm some ahead of the front. This will
also be conducive to the low levels remaining more mixed and
support continued 35-40 mph gusts out of the south-southwest.

The difference noted for post-frontal winds Friday night into
Saturday vs. this morning is an increasing threat for a period of
solid advisory criteria winds. 00z model cycle continued to ramp
up the lower-level wind fields with the system and 6-hour pressure
rises in the 14-18 mb range. Strong low level cold advection
immediately behind the front and the aforementioned sharp pressure
rises will support a rapid wind shift and surge of gusts up to
40-45+ mph through Saturday morning and then easing late in the
day. Temperatures will climb little if any from wherever they
bottom out Saturday morning, with upper 20s to around 30F in grids
Saturday afternoon keeping wind chills in the teens for most.

1035 mb high pressure will pass just to our south Saturday night,
but close enough for the boundary layer to decouple in the very
dry air mass with dew points as low as the single digits on some
of the models. This will result in lows in the teens for most and
a decent chance for colder than in the official forecast, especially
in favored spots. The entrenched very dry air mass on Sunday will
keep mean a dry and seasonable day with highs in the upper 30s as
warm advection recommences.

The big change from recent days comes Sunday night and onward,
with a decided trend away from a deep mid latitude cyclone
affecting the region, likely due to the fast northern stream
dominant flow regime and lack of phasing in a growing % of
guidance members. Trended the forecast dry Sunday night, with
chance PoPs reasonable Monday-Monday night, though uncertainty
with type. A below to well below normal air mass should arrive on
Tuesday, with the rest of next week currently favored to be colder
than normal. Signs again leaning away from any big precip
producers, though might have some opportunity for light snow mid
next week just beyond day 7.

Castro

&&

.AVIATION...
For the 06Z TAFs...

Aviation Forecast Concerns:

* Blustery west-northwest winds with gusts around 30 kts through
  much of the forecast period before gradually diminishing
  Wednesday night.

* Low-MVFR initially may scatter out to VFR for a time early this
  morning, then redeveloping thin MVFR/VFR layer (~3000-3500 ft)
  Wednesday morning.

Sharp surface cold front and associated showers have cleared the
forecast area at late evening. Blustery west-northwest winds with
gusts around 30 kts have spread in behind the front, and will
persist though most of the forecast period before gradually
diminishing Wednesday night.

Solid MVFR ceilings around 1500 feet have spread across the
terminals, though GOES IR satellite imagery does depict a ragged
back edge of this deck crossing the Mississippi River and thus
MVFR deck may scatter out around 06Z for KRFD and 08-09Z for the
Chicago metro terminals.

Forecast model guidance shows some spread with extent of
redevelopment of shallow stratocu across the area Wednesday, with
some of the drier solutions not depicting much in the way of cloud
cover and others suggesting a broken but thin 3000-3500 foot
layer filling in across far northern IL/northwest IN Wednesday
morning. Have indicated this through mid-afternoon or so, with
persistent dry low level advection and strengthening deep
subsidence behind the departing upper trough eventually likely to
scatter things out. Confidence is only medium in these ceiling
trends however. Clear skies are then expected Wednesday night.

Ratzer

&&

.MARINE...Issued at 255 AM CST Tue Nov 29 2022

A 982 mb surface low across Ontario will continue to shift away
from the area today but its pressure gradient in its wake in
tandem with cold advection will keep the westerly winds strong.
In addition, the thermodynamic profiles are now unstable over the
water with the colder air, as opposed to yesterday when it was
more stable. Sustained winds over the southern nearshore waters
are forecast to be 25 kt with gusts to 35 kt and even probably a
few to 40 kt. Confidence is on the higher side in these
expectations. Gales will be slow to ease this evening and then
wind speeds will drop markedly overnight.

Another sharp cold frontal approach and passage Friday and Friday
night presents the next opportunity for gales. Depending on mixing
depths on land, south to south-southwest gales are possible Friday
afternoon and evening within a couple miles of shore. Behind the
cold front, westerly gales are likely late Friday night through
the early afternoon on Saturday, with 40 kt gales forecast and
potential for up to 45 kt gales.

MTF/Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning...nearshore waters until 10 PM Wednesday.

&&

$$

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