Chicago, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

425
FXUS63 KLOT 211158
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
558 AM CST Thu Nov 21 2024

.KEY MESSAGES...

- Winter Weather Advisory and Wind Advisory headlines in effect
  today for portions of the forecast area.

- Quick burst of snowfall rates approaching 1 inch per hour is
  expected this morning into midday/early afternoon for the
  Chicago metro area.

- Total snowfall amounts expected from 2-4" in the metro, to
  1-3" farther west/south.

- West-northwest winds become strong/gusty later this morning
  and eventually turn north. Gusts as high as 50 mph at times
  before easing late today/this evening.

&&

.DISCUSSION...
Issued at 352 AM CST Thu Nov 21 2024

Through Friday:

Have elected to issue a Winter Weather Advisory for the Chicago
metro from 7 AM to 1 PM today for a quick burst of 2-4"
snowfall, and a Wind Advisory for all IL counties and
Lake/Porter counties in IN from 10 AM to 8 PM for strong winds
gusting as high as 50 mph.

Early morning GOES vapor imagery indicates a deep trough and
closed low centered over the western Great Lakes region. A
potent mid-level short wave was noted wrapping cyclonically
west-southwest around the northern periphery of the main
circulation with a 994 mb surface low over the eastern U.P. of
Michigan as of 3 AM. The strong vort and associated surface low
are progged to dig south down Lake Michigan today on the back
side of the upper level low, with attendant strong forcing
expect to produce a period of moderate to heavy snowfall rates
across northeast IL from just after sunrise through early this
afternoon. Snowfall amounts are expected to range from 3-4"
across a good portion of the Chicago metro area, to 1-3" farther
west and south. While these amounts are not exceptional, it
appears as if the bulk of the accumulation will occur over just
a 3 to 4 hour period in which accumulations on roads is likely,
given temps around 30F this morning. This rapid accumulation,
and being the first snow of the season tips the scales toward
issuing a Winter Weather Advisory. Snow is expected to decrease
in intensity this afternoon, and mix with and eventually change
over to mostly rain as surface temperatures rise into the upper
30s in the wake of the upper low/cold pool. Rain and rain
showers will then slowly diminish through this evening, except
for some lake-effect showers which will continue to affect parts
of northwest IN through Friday.

In addition to the wintry weather, very strong and gusty west-
northwest to north winds are expected due to the fairly quick
southward progression of the surface low and tightening surface
pressure gradient. Strongest winds, 25 to 35 mph with gusts as
high as 50 mph at times, are expected from mid-late morning
through early this evening beneath a 50+ kt low level jet on the
western periphery of the low. The axis of strongest winds looks
to favor the IL counties of our cwa, Lake Michigan and portions
of Lake and Porter counties near the Lake Michigan shore in
northwest IN. Wind gusts will ease to 25-30 mph tonight, but
will remain blustery into Friday. Temperatures will reach the
40s area-wide Friday, though the blustery north winds should
keep wind chills in the mid-upper 30s for much of the day.

Ratzer


Friday Night through Wednesday:

A lowering subsidence inversion and westward veering of the
low- level flow will gradually end lake effect rain showers in
northwest Indiana Friday night.

Ridging should then bring quiet conditions Saturday into
Sunday, but with plenty mid-level cloud cover. A longwave trough
over the northern Great Plains will then interact with a pocket
of Pacific moisture to produce an area of rain somewhere over
the western Great Lakes Sunday night into Monday, with ensemble
guidance still favoring the main area of precip to remain well
north of the forecast area. Another quick-moving wave within
broader WSW flow aloft will approach the mid-Mississippi River
Valley late Sunday as a cold front drifts southeastward across
the forecast area. Currently, the front is expected to clear the
CWA before the wave arrives and low-level Gulf moisture advects
northward, keeping rain chances mostly south of the area.
However, with decent spread in forecast time of the features
mentioned above, will maintain NBM chance PoPs through much of
the day Monday.

General Pacific flow with small chances of rain is then
expected Tuesday into Wednesday before the CONUS-scale pattern
amplifies with deeper troughing across the Great Lakes into the
northeast around Thanksgiving.

Kluber

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 557 AM CST Thu Nov 21 2024

Key Aviation Messages:

- Quick period of moderate to briefly heavy snow roughly 8am to
  noon before precip begins to change to rain (ORD/MDW
  accumulations 2-4", highest on grassy and elevated surfaces)

- IFR ceilings and LIFR visibility with the snow at times this
  morning, then IFR ceilings early to mid-afternoon as precip
  changes to rain.

- WNW winds quickly increasing to over 30 knots by mid-morning,
  then becoming NW/NNW up to 35-40 knots during the afternoon.

Scattered -SHSN with perhaps brief bouts of MVFR visibility
will persist for a couple hours before a shield of SN quickly
moves over the Chicago terminals in the 14-15Z hour.
Visibilities of 1SM will be common for several hours this
morning, with periods of 1/2SM also expected. Cannot rule out a
couple occurrences of 1/4SM as has been observed upstream across
central Wisconsin. Accumulations at the Chicago terminals (RFD)
are expected to be in the 2-4" (1-2") range, with accumulations
likely on all surfaces given snowfall rates up to 1" per hour.

After 18Z, the snow is expected to quickly transition to rain
while ceilings lower into IFR levels for a few hours. Periods of
DZ will follow during the rest of the afternoon before
scattered lake-enhanced -SHRA shift east by mid-evening.

With the strong low pressure system tracking southward across
lake Michigan this afternoon, strong WNW winds to over 30 knots
will quickly develop during the snow this morning. The winds
will continue to strengthen with gusts to 35 knots or higher
while veering NW as the snow transitions to rain. NW/NNW gusts
of 35-40 knots will persist through the afternoon then steadily
diminish to gusts around 25 knots by mid-evening. There is a 30%
chances that the wind direction veers just east of north for a
couple hours early this evening.

Kluber

&&

.MARINE...
Issued at 640 PM CST Wed Nov 20 2024

- Gale watch upgraded to a Gale warning Thursday morning into
  Thursday evening. Small Craft Advisory remains in effect
  tonight.

Strong and gusty westerly winds will continue this evening and
overnight. While some intermittent gale force gusts to 35 knots
are likely tonight (mainly on elevated platforms), a stronger
push of northwesterly winds will arrive through Thursday
morning. Frequent 40 to 45 knot gales are expected, especially
Thursday afternoon. Winds will turn north and eventually
northeasterly Thursday evening.

Once gales subside Thursday night, Small Craft Advisories will
be needed into Friday evening for the Illinois nearshore and
into Saturday morning for the Indiana nearshore waters.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Advisory from 10 AM this morning to 8 PM CST this evening
     for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
     ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-
     ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

     Winter Weather Advisory until 1 PM CST this afternoon for
     ILZ005-ILZ006-ILZ012-ILZ013-ILZ020-ILZ103-ILZ104-ILZ105-
     ILZ106-ILZ107-ILZ108.

IN...Wind Advisory from 10 AM this morning to 8 PM CST this evening
     for INZ001-INZ002.

LM...Small Craft Advisory until 7 AM CST this morning for the IL
     nearshore waters.

     Gale Warning until midnight CST tonight for the IL nearshore
     waters.

     Small Craft Advisory until 10 AM CST this morning for the IN
     nearshore waters.

     Gale Warning from 10 AM this morning to midnight CST tonight
     for the IN nearshore waters.

&&

$$

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