Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/21/22

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 634 PM CDT Tue Sep 20 2022

.MESOSCALE DISCUSSION...
Issued at 437 PM CDT Tue Sep 20 2022

Thunderstorms continue along a narrow axis now extending from just
north of Rockford northeastward to central Lower Michigan. One storm
in particular over Poplar Grove in Boone County acquired
supercellular characteristics with a large and deep core and mid-
level rotation, and the cell over Rockford appears to be following
the same evolution. The storms are moving into an environment
characterized by MLCAPE >2000 J/kg and effective deep- layer shear
>40kt, which will remain supportive of supercell structures.

As the storms move E/SE over the next few hours into McHenry,
northern DeKalb, northern Kane, and potentially even northern Cook
counties, they appear likely to remain strong to severe with
damaging winds and hail as the main threats.

Borchardt

&&

.SHORT TERM...
Issued at 225 PM CDT Tue Sep 20 2022

Through Wednesday night...

It`s a deceptively busy day in the Great Lakes with several areas of
thunderstorms, choppy waves on Lake Michigan, a lake-modified warm
front draped across the Chicago area, and unseasonably (to
dangerous) heat.

After convection tracked down the lower spine of Lake Michigan this
morning, a hybrid outflow boundary/lake breeze pushed into
northeastern IL and southeastern WI leading to falling temperatures
and choppy waves. The boundary now serves as a composite warm front,
which extends into southwestern Wisconsin. As the boundary came
ashore, waves jumped up to 3-5 feet in the span of an hour from
Wilmette Harbor to Racine, and will likely climb more over the next
few hours owing to persistent southeasterly winds sustained near
20kt. After collaboration with NWS Milwaukee, we issued a short-
fused Beach Hazards Statement for northern Cook and Lake counties in
Illinois in anticipation for dangerous rip currents to continue
through this evening. Southwest of the front, it`s an unseasonably
warm and steamy day with temperatures in the lower 90s and dew
points in the lower 70s. Heat index values range from 95 to 105F
-- a rare feat for this time of year! If outdoors, hydrate often
and take frequent breaks in the shade.

Meanwhile, a narrow axis of thunderstorms continues to fester from
Ludington, MI to Dodgeville, WI and appears to be associated with
ever-so-modest warm-air advection atop the aforementioned warm front
draped from southwestern Wisconsin to northeastern Illinois. Thus
far, convection has struggled to build south of the Illinois state
line owing to unseasonably warm 850mb temperatures of +22C (recently
sampled by aircraft ascending and descending into MDW) acting as a
low-level cap. However, recent 1-minute imagery from GOES-16 has
shown a localized but persistent area of cumulus growth as far
southwest as Cedar Rapids, IA. With continued moistening and
cooling of the base of the cap, thunderstorms may very may well
become established near (or survive to) the IL/WI state over the
next few hours. With the hot and humid airmass in place beneath
steep mid level lapse rates of nearly 9 K/km and the increasing
proximity of an upper-level jetstreak over Minnesota providing
some 35-40kt of effective deep-layer shear, any thunderstorm that
can become established may be strong with damaging winds and hail
as the main threats. For now, have introduced 15-20% chances for
thunderstorms this evening north of I-88, though trends will need
to be monitored closely over the next few hours.

Tonight, the low-level jet will reintensify and take aim at Lower
Michigan, focusing remaining thunderstorms to our northeast (that`s
the expectation, as least). Unseasonably moist boundary layer
moisture will hold overnight lows in the lower 70s in spite of
mostly clear skies.

Tomorrow will be "transition" day as we go from summer to fall. As
the upper-level jet streak propagates eastward over Michigan, a cold
front will surge south across the Great Lakes leading to falling
temperatures and plummeting humidity levels. Chances for
precipitation along the front look slim at least through the early
afternoon owing to dry mid-level air. However, increasing low-level
moisture convergence into the afternoon in tandem with increasing
upper-level diffluence in the broad right entrance region of the
jetlet will likely encourage the development of scattered showers
and a few thunderstorms by tomorrow evening mainly focused south of
Interstate 80. Based on the forecasted timing and location of the
front in relation to the arrival of the synoptic-scale forcing, most
precipitation may end up anafrontal (e.g. behind the front) thus
limiting the intensity of any thunderstorm. However, at least a
localized damaging wind threat may materialize if convection is able
to develop on the front itself. Such a threat appears highest along
and south of a line from Pontiac to Rensselaer during the
afternoon. All showers and thunderstorms will drift south after
sunset giving way to a seasonably chilly night with overnight
lows in the upper 40s near the WI state line to lower 50s to the
south.

Borchardt

&&

.LONG TERM...
Issued at 231 PM CDT Tue Sep 20 2022

Thursday through Tuesday...

Main highlights:

* Fall like pattern setting in on time for the start of fall with
  lows in the 40s area wide to start Friday

* Dangerous conditions on the lake on Thursday

Fall begins at 8:03 PM CDT on Thursday with the Autumn Equinox.
Fittingly enough, the summer-like pattern we`ve been in will
quickly transition to a faster west-northwest flow aloft pattern
common in fall. The first amplified trough passage on Thursday
will bring highs only in the low-mid 60s with dew points in the
40s, quite the contrast from the heat and humidity away from the
lake this (Tuesday) afternoon! Expansive near 1030 mb high pressure
will spread from the northern Plains on Thursday morning to Lake
Michigan on Friday morning.

A sharp pressure gradient over the region will yield brisk northerly
winds gusting up to 25 mph, and over the lake contribution of
unstable conditions (cool air mass bottoming out at +2 to +3C at
850 mb over warm lake waters) will yield gusts up to 30 mph to
briefly 35 mph along the shore. A Beach Hazards Statement will be
needed, with the main message to stay away from the immediate
shore due to large breaking waves capable of sweeping unsuspecting
onlookers into the lake.

Winds will diminish Thursday night as the high builds overhead,
and mostly clear skies initially amidst the very dry air mass will
support strong radiational cooling and lows in the low-mid 40s for
many locations. Exception to 40s lows could be downtown Chicago
with a forecast of around 50F there. Expect mid and high clouds to
increase toward daybreak Friday. Friday will be a cool day after
the chilly start. An elongated short-wave extending from trough
over the Canadian prairies will pivot east and with decent upper
jet support, should overcome the antecedent dry column for
scattered showers late day and especially into the evening, higher
coverage north.

A warm front will shift north behind the short-wave for a nice and
mostly sunny warm up back into the 70s on Saturday. Weak low
pressure near Lake Superior on Sunday will deepen in response to a
sharp short-wave trough and strong upper jet. Despite limited
moisture with dew points in the 50s, Sunday afternoon`s cold front
passage could have isolated to widely scattered showers and a few
thunderstorms as cold 500 mb temps steepen lapse rates some to
yield up to 500 J/kg of MUCAPE amidst temps topping out in the low 70s.

While the medium range guidance varies on how long the sharpening
long wave trough sets up over the Great Lakes and eastern Canada
next week, Monday will be another cool day with highs in the mid
60s following lows in the upper 40s (locally mid 40s) to mid 50s
Sunday night. Confidence decreases on Tuesday due to the aforementioned
uncertainty regarding how progressive the trough is. The high
temperature outcome on Tuesday ranges from near seasonable 60s to
around 70, to downright chilly mid 50s-low 60s on the ECMWF.
Official forecast of 63-68F range is between the NBM 25th and 50th
percentiles, so a slight lean toward the cooler side of the
spectrum.

Castro

&&

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

Aviation Forecast Concerns:

* Strong/severe Thunderstorms moving over Chicago terminals very
  early this evening.

* Winds convectively disturbed at issuance, should gradually veer
  from southeast to southwest early this evening.

* LLWS developing later tonight with 40 kt southwest low level
  jet.

* Cold frontal passage shifts winds northwest Wednesday morning,
  with gusts near 20 kt persisting much of the day.

Cluster of strong/severe thunderstorms moving over Chicago
terminals will continue to move east-southeast of KORD/KMDW early
this evening. Maintained VC/tempo mention through 01Z, though may
end a bit earlier than that. Thunderstorms will have convectively
disturbed the wind field, with some east/southeasterly component
likely early before veering back to the southwest at around 10 kts
for the rest of the evening/overnight hours. Low level jet of 40
knots will develop from the southwest later tonight, with a period
of associated LLWS conditions after midnight until about sunrise.

Surface cold front pushes south across the area Wednesday morning,
with a wind shift to the northwest. Winds will remain north-
northwest through the day, with gusts near 20 kt. While some
patchy light rain can`t be ruled out, the greatest chance of rain
and a few isolated thunderstorms looks to develop south of the
terminals.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006-ILZ103 until 7 PM Tuesday.

IN...None.
LM...Small Craft Advisory...nearshore waters until 9 AM Wednesday.

&&

$$

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