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000
FXUS63 KLOT 201734
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1234 PM CDT Tue Apr 20 2021

.UPDATE...
1041 AM CDT

An impressive band of snow has developed near the I-80 corridor
in association with the upward branch of a strengthening low to
mid-level frontogenetic circulation along the baroclinic zone.
Snow in this band has been falling at a decent rate, with
visibilities falling under a mile. This has lead to some minor
accumulations up to around a half inch on grassy and elevated
surfaces, but with warm pavement temperatures roads largely
remain wet.

This band of heavier snow will gradually sag to the southeast
into this afternoon, likely impacting areas south of I-80 over
parts of east central IL and northwestern IN. To the north of this
band, the activity will become more showery in nature into this
afternoon as the main upper impulse approaches. While snow rates
will general be light with these showers, a few localized bursts
of briefly more moderate snow will be possible due to their
convective nature. However, the overall low residence time of any
heavier showers will limit additional accumulations, especially
as surface temperatures remain just above freezing. For this
reason, the main potential for accumulations into this afternoon
will primarily be tied to heavier rates occurring with the better
organized band of snow. Some localized areas could experience an
inch or two of snow with this band, mainly on grassy surfaces.

Expect the main band of snow to end across my southeastern areas
by early this evening, though some lingering scattered snow
showers do look to continue during the evening.

KJB

&&

.SHORT TERM...
324 AM CDT

Through Wednesday...

No substantial changes in the forecast today, with slushy snow
accumulations still expected for portions of the area, primarily
east of I-55 and well south of I-80.

A mid-level wave currently exiting the central Rockies will cross
northern Illinois late this afternoon under the right entrance of
a 110kt upper jet streak. Widespread precip currently blanketing
a larger portion of Kansas will spread ENE along an existing low-
level baroclinic zone over and just south of the far southern CWA.
Based on the current latitude of precip develop and recent trends
in model guidance, have made a slight southward shift in the main
precip axis today. This favors the best chance for at least 0.25"
of precip to the south of a line from Peoria to Morris to
Valparaiso. Encroaching low-level dry air from the north will also
make a dent in precip north of I-80 this morning and likely keep
precip totals there at or under 0.10". Where precip is lighter
across northern Illinois, low- level diurnal warming under
increasingly colder temps aloft will support a transition from
broad light synoptic precip to more showery activity this
afternoon.

The greatest snow accumulations are still expected across the
southern CWA where precip rates will be low enough to keep temps
in the mid 30s for most of the day. Slushy accumulations of 1-2"
almost entirely on non-paved surfaces are possible south of the
aforementioned line, with a narrow corridor of >2" not out of the
question across the far southern CWA.

Even with lower precip rates farther north, the continued influx
of drier low-level air will maintain low-level wet bulb temps
close to freezing, allowing for snow as the dominate precip type.
While snow will struggle to accumulate this morning, the more
convective nature of the afternoon showers may result in heavier
bursts of snow capable of producing quick localized accumulations
of a few tenths of an inch.

Note that the record April 20th snowfall for Chicago is 0.2"
(1943) and for Rockford is a Trace (five times, last in 1959).

Tonight: Broader synoptic-scale precip will end by mid-evening
with the loss of mid-level moisture. However, steepening low to
mid-level lapse rates will support some continued snow shower
chances into the overnight hours, during which a couple additional
tenths of accumulation will be possible.

With a trend to more cloud cover under the approaching upper low,
the risk a temps falling well below freezing may be decreasing.
However, there should be enough breaks in the clouds in
conjunctions with continued low-level CAA to allow temps to fall
into the upper 20s for most of the area, with mid 30s in the core
of the Chicago metro.

Wednesday: Unseasonably cold air aloft highlighted by a core of
-35C 500 hPa temps with a -3SD anomaly will cross northern
Illinois midday. This will support a well-mixed PBL below steep
lapse rates of >8C/km up through at least 700 hPa. Meanwhile, mid-
level vort max will cross the CWA during the late morning through
mid-afternoon. Scattered showers are expected to develop across
the area, but especially over northeast Illinois where lapse rates
are steepest. Precip type possibly favors more snow than rain due
to low-level wet- bulb temps only slightly above freezing.
Additionally, MLCAPE values surpassing 200J/kg combined with EL
values nearing 12kft/-25C could pose the risk of isolated
lightning strikes. Given the convective nature of the precip,
graupel will likely occur with any deeper cells. Low-level
inverted-V soundings also suggest the potential for gusty winds
with any showers.

Kluber

&&

.LONG TERM...
324 AM CDT

Wednesday night through Monday...

Large scale upper trough is progged to be moving off to the east of
the area Wednesday night, though with one last smaller scale short
wave rotating east-southeast into the departing trough axis. Cold
mid-level temps associated with this wave/departing upper trough
(around -22C depicted at 700 mb) combined with a deeply mixed
boundary layer (adiabatic lapse rates from the surface up to 700 mb)
will likely continue to support scattered convective rain/snow
showers early Wednesday evening. Loss of diurnal low level
instability and passage of the short wave trough axis should allow
these showers to dissipate by late evening, with subsidence
developing across the region after midnight and inducing clearing
skies. Clearing within the cold airmass is expected to aid in
producing widespread freezing temperatures across the forecast area,
with lows in the upper 20s outside of the core of the Chicago metro
area by sunrise Thursday. We`ll likely need freeze warning headlines
again for Wednesday night, but for simplicity at this time we`re
continuing to mention the Wednesday night impacts in the additional
information section of the current headline in effect for Tuesday
night.

Mid-level short wave ridging then develops across the area Thursday
into Thursday night in the wake of the departing upper trough.
Strong 500 mb height rises in excess of 80 meters is progged across
the area initially Thursday, indicative of deep large-scale
subsidence. Along with very dry thermodynamic profiles, subsidence
should promote plenty of sunshine across the area and allow a quick
rebound of temperatures back into the mid to upper 50s, and somewhat
breezy west winds limiting lake breeze potential. Guidance remains
in general agreement in depicting a low-amplitude mid-level short
wave approaching the mid/upper Mississippi River valley later
Thursday night, which would likely bring increasing high cloud cover
into the region after midnight. Lows will likely dip into the 30s in
the dry air mass prior to the arrival of high clouds however,
possibly producing some patchy frost.

This mid-level wave is then progged to dampen across the region
Friday, as it propagates into the upper ridge position. Low level
return flow develops across the Plains in association with this
feature, but becomes weaker and less-focused as the amplitude of the
disturbance decreases across our area. Lower and mid level southwest
flow does provide warm/moist advection into the region by Friday
afternoon however, with most guidance depicting the potential for
some showers. Global models continue to show some differences with a
couple of additional short waves within developing northern and
southern streams Friday night into Saturday, though the general
consensus remains for shower chances to persist into the day
Saturday as these waves move east and a cold front associated with
the northern stream wave pushes through the area. Temperatures do
continue to moderate Friday into Saturday with southwest low level
flow, though this will likely be modulated by increased cloud cover
and a potentially stronger southern stream low as noted in the ECMWF
and ECS ensemble runs. Highs both days should be near 60, and
possibly into the mid-60s if cloud/shower coverage is lower. A
stronger southern stream low per the ECMWF would also result in a
greater potential for northeast surface winds and cooler temps off
of Lake Michigan however on Saturday.

Both of these short waves and their developing larger scale upper
trough are progged to move off to the east of the area Saturday
night with a broad, flat upper ridge depicted across much of the
central CONUS by Sunday. Associated surface high pressure moves
across the forecast area during the day, allowing dry weather and
likely some lake cooling with temps in the 50s to near 60 expected
to round out the weekend. Upper ridge is then progged to amplify
across the central CONUS/Midwest early next week, in response to an
upstream long wave trough moving into the West Coast. Developing
southerly flow into the region should support warmer temps (perhaps
70s) by Monday, with the best potential for warm advection
showers/storms currently looking to remain off to our northwest.

Ratzer

&&

.AVIATION...
For the 18Z TAFs...

As "system snow" departs to the south, scattered snow showers will
increase in coverage this afternoon across northern Illinois. We
currently believe the greatest coverage should be south and west of
the Chicago terminals due to low-level dry air drifting in from the
east, though a flurry or two cannot be ruled out through sunset.
At RFD, occasional drops in visibility to as low as a mile will
be in play through early evening. Winds will remain light and
variable through the overnight hours.

As a potent upper-level waves passes through the western Great Lakes
tomorrow, numerous showers are expected to develop across the area
by mid-morning and last through the evening hours. Based on
forecast instability parameters, the synoptic regime, and overall
similarities to previous events (4/2/2016 and 10/20/2018), we
believe the showers will have thunderstorms-like characteristics
with cumulus towers to 20 kft, gusty winds and related turbulence,
visibility drops to a mile or lower, a mix of rain, snow, and
graupel (small hail), and occasional lightning strikes. With
collaboration with CWSU ZAU, opted to aggressively and offer
SNRAGS as early as 15Z and vicinity thunder as early as 17Z, with
the highest coverage expected between 18-22Z. Confidence is high
that the showers will occur and medium that they will impact the
Chicago terminals.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Freeze Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-
     ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-
     ILZ108...midnight Wednesday to 9 AM Wednesday.

IN...Freeze Warning...INZ001-INZ002-INZ010-INZ011-INZ019...midnight
     Wednesday to 9 AM Wednesday.

LM...None.
&&

$$

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