Hail in Illinois, April 1, 2024

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1254 PM CDT Mon Apr 1 2024

.KEY MESSAGES...

- Continued waves of showers and even some thunderstorms are
  expected through Tuesday, with the strongest storms may
  produce damaging hail south of US-24.

- Continued heavy rainfall over areas south of I-80, where
  additional waves of showers and thunderstorms are expected,
  which may lead to flooding of low-lying fields and ditches.

- Periods of wintry weather, strong northwesterly winds, and
  even lakeshore flooding may occur Tuesday into early Thursday.

- However, forecast confidence remains lower than average
  Tuesday into early Thursday, and will depend on the track and
  depth of a surface low pressure system lifting into the
  region. Stay up to date on the forecast!

&&

.DISCUSSION...
Issued at 309 AM CDT Mon Apr 1 2024

Through Tuesday morning:

The synoptic set up for Monday into Tuesday has not wavered too
much in latest hi-res guidance. An upper level low in the
southwest will move northeastward and phase with another upper
level low dropping south out of Canada later tonight which will
dig down and deepen into Tuesday morning providing showers and
the risk for thunderstorms in the forecast through the period.

A stationary front extending eastward from the surface low in
southeastern Colorado through Central Illinois will prevail,
with maybe a slight drift northward through the day as the upper
trough phases. Isentropic lift over the front is keeping
continued showers and thunderstorms north of the front for areas
south of I-88 this morning. However, drier air aloft is
expected to move in by around 5 AM which should help suppress
the rain activity. While a stray lingering sprinkle cannot be
completely ruled out, models have trended drier this morning
setting up a dreary, cloudy day.

While there is some uncertainty on the exact timing of the
movement of the surface low, it is projected to move eastward
into the Plains through the day bringing a renewed effort for
showers and thunderstorms to the area with some of the strongest
storms potentially spitting out some small hail. With the
greatest instability and steeper lapse rates south of I-80,
there remains a level 1 out of 5 for storms to become severe and
develop damaging hail up to an inch in diameter; however,
models have trended the threat area even further south of
US-24.

Additionally, dew points 50 degrees are expected this afternoon
south of I-80 (pushing 60 degrees in southern Ford County) as
well as anomalously high precipitable water values up to 1.5
inches closer to the front. With warm rain processes making for
efficient rainfall rates and antecedent wet conditions from the
last two waves of showers and storms creating already damp
soils, the risk for flooding of low-lying fields and ditches is
expected through Tuesday morning. The main locations with the
risk for flooding are for areas south of I-80; however, there is
a signal that locally highest totals may be south and east of
I-55 where some locations might see around two inches of rain
total.

The good news is, that for the next 24 hours, temperatures
through the atmosphere should allow for only rain to be the
precipitation type. However, as the low phase east, more cold
air advection will provide the risk for more frozen precip types
to move in. For more information for Tuesday afternoon and
beyond, see the discussion below.

DK

Tuesday Night through Sunday:

Tuesday into Wednesday, the parent trough responsible for the recent
waves of showers and thunderstorms across the region will finally
eject northeastward into the Lower Great Lakes and merge with a
secondary upper-level trough diving south out of Canada. The net
result will be a deepening surface low tracking roughly from St.
Louis, MO to Indianapolis, IN on Tuesday, and then a rapid jog
northward into the Lower Great Lakes on Wednesday. As would be
expected, ensemble model guidance and deterministic counterparts
offer differences in the exact track and depth of the low, leading
to lower than average forecast confidence (particularly on
Wednesday).

Tuesday: As the surface low develops across southern Illinois and
central Indiana, a deformation shield of precipitation will develop
across eastern Iowa, northern Illinois, and southern Wisconsin.
Precipitation rates will peak during the afternoon hours as north to
northwesterly wind gusts increase to 20 to 25 mph, leading to a
wet and blustery evening. Forecast thermal soundings continue
to walk the fine line between rain and a heavy wet snow,
particularly across northwestern Illinois where cold air from
the merging upper-level trough from Canada will arrive fastest.
A reasonable worst case scenario is that a narrow band of 2-4"
of slushy snow accumulations materializes somewhere northwest of
a line from Rochelle to Evanston, while an equally plausible
"best" case scenario is that snowflakes simply mix with rain. At
this point, our forecast will continue to feature a mention of
snow along and northwest of I-55 Tuesday afternoon.

Tuesday night into Wednesday: The surface low is expected to rapidly
deepen while lifting north-northeastward into the Lower Great Lakes.
Now, the greatest spread in ensemble guidance is in the track and
depth of the low Tuesday night into Wednesday. One ensemble cluster
shows the surface low lifting into the thumb of Michigan before
retrograding westward toward Grand Rapids, and is depicted largely
by the GEFS. Another ensemble cluster depicts the low lifting
further toward the northwest toward Lake Michigan with a westward
jog into southern Wisconsin and northern Illinois, and is largely
depicted by the EPS. The majority of CMCE clusters shows something
inbetween with the low lifting toward Detroit, MI and then
retrograding westward into southern Wisconsin and Lake Michigan.
Now, it sure seems like more and more individual ensemble members
across successive model runs are showing the scenario where the
surface low retrogrades into southern Wisconsin and northern
Illinois. So, we'll see if the GEFS falls in line going forward. All
the while, the depth of the low varies within each cluster and
between ensemble systems from as low as 975mb to as high as 995 mb.
Needless to say, the forecast from Tuesday night into Wednesday
depends on the eventual depth and track of the low leading to
continued lower than average forecast confidence.

Regardless of the track of the low, the general idea is that a
shield of snow will continue along western flank of the surface low
Tuesday night into Wednesday as north to northwesterly winds rapidly
increase in magnitude. In the scenario where the low lifts into and
stays in Lower Michigan, much of our area would be in line for
continued snow and north to northwesterly wind gusts of 45 mph
Tuesday night into Wednesday. Even with steady snow throughout the
day, accumulations may be limited due to relatively marginal surface
temperatures and the strong April sun angle. However, a slushy inch
or two cannot be ruled out where snow rates end up heaviest. In
addition, persistent winds down the spine of Lake Michgian would
lead to waves big enough to flood parks and trails along the
northwestern Indiana shore of Lake Michigan particularly Wednesday
into Thursday morning. In fact, pattern recognition is raising
concern that waves could easily grow to 20 feet tall, which would
lead to a pronounced threat for shore erosion, as well. However, the
scenario where the low lifts toward Lake Michgian and retrogrades
westward into northern Illinois and southern Wisconsin would suggest
much of our area gets a "break" in precipitation and winds on
Wednesday (as they would be shunted westward toward the Mississippi
River), wave heights would be substantially lower, and there would
only be a quick hit of snow and northwesterly winds along the
backside Wednesday afternoon and evening. So, the range in forecast
outcomes Tuesday night into Wednesday ranges from pretty unpleasant
to easily tolerable. Since our gridded database is deterministic in
nature, we can't show two forecasts at once. So, we'll continue to
take the "middle of the road" approach and advertise snow and gusty
northwest winds across our entire area Tuesday night into Wednesday
for the time being. That is all to say, stay up to date on the
forecast.

Thursday into Friday: the parent surface low is expected to pinwheel
northeastward away from our area. However, the residual upper-level
trough and associated pool of cool low-level air will take time to
follow suit. As a result, temperatures will be slow to warm with
highs only in the 40s on Thursday and perhaps the low to mid 50s on
Friday, warmest away from Lake Michigan. Thereafter, ensemble model
guidance continues to signal a warm-up from the weekend and into
next week during which highs should return back to the 60s.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1253 PM CDT Mon Apr 1 2024

Key Aviation Messages:
- Isolated TS mid to late afternoon, with a (30%) chance a
  couple TS could produce small hail or gusty winds
- Extended period of IFR ceilings tonight and Tuesday
- SN potentially mixing with RA late Tuesday afternoon

A cluster of isolated elevated TS developing along the
Mississippi River from far northwest IL into west-central IL
will lift ENE through this afternoon. While TS is not expected
to be continuous during this time, at least a couple TS should
affect the general Chicago metro airspace roughly in the 20-23Z
window. There remains a chance (30%) that small hail or gusty
winds over 30 knots accompany the storms.

Otherwise, the forecast window from this evening into Tuesday
afternoon will consist of periods of -SHRA with IFR ceilings and
NE winds with gusts to 25 knots. At the very end of the period
(around or after 22Z Tuesday), colder air will filter into the
area with a NNW wind shift. The -RA may begin to mix with -SN by
00Z, so have included -RASN in the TAF during this time.

Kluber

&&

.MARINE...
Issued at 309 AM CDT Mon Apr 1 2024

Tuesday night through Wednesday, a surface low pressure system
is expected to rapidly deepen while lifting into the Lower Great
Lakes. As the low deepens, north to northwesterly winds will
increase with along the western flank of the low. As would be
expected several days in advance of a storm system, available
model guidance depicts several forecast tracks and depths of the
surface low, ranging from lifting into central Lower Michigan
to lifting directly over Lake Michigan, and depths ranging from
28.80 to 29.10 inches, respectively. If the low were to lift
into Lower Michigan, gale force winds to 45 kt and even locally
storm-force winds to 50kt would be a pronounced threat across
southern Lake Michigan from Tuesday night to Thursday morning.
In addition, waves would grow dangerously large to over 15 feet
east of Gary. However, if the low lifts directly over Lake
Michgian, winds may end up considerably weaker with perhaps only
a brief period of gale force winds to 35 kt as the low pulls
away Wednesday night into Thursday morning. Even with forecast
uncertainty and in collaboration with NWS Milwaukee, felt the
prudent approach was to issue a Gale Watch from Tuesday night
through early Thursday morning. Mariners are encouraged to stay
abreast on the forecast as confidence increases in the path and
strength of the low pressure system.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 1 AM CDT Wednesday for the IL
     nearshore waters.

     Gale Watch from late Tuesday night through Thursday morning
     for the IL and IN nearshore waters.

     Small Craft Advisory from 7 PM this evening to 1 AM CDT
     Wednesday for the IN nearshore waters.

&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
338 AM CDT Mon Apr 1 2024

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-020845-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
338 AM CDT Mon Apr 1 2024 /438 AM EDT Mon Apr 1 2024/

This Hazardous Weather Outlook is for portions of north central
Illinois...northeast Illinois and northwest Indiana.

.DAY ONE...Today and Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
    Limited Hail Risk...up to 1 inch in diameter south of US-24
    Limited Damaging Wind Risk...up to 60 mph south of Interstate 80.
    Limited Flooding Risk south of Interstate 80.

DISCUSSION...

After a brief break from the rain Monday morning, chances for showers
and scattered thunderstorms increase this the afternoon and will last
through Tuesday morning. The highest chances for thunderstorms
will be near and south of Interstate 80, where some of the storms could
produce some hail, mostly pea to penny sized, but potentially up
to a quarter south of US Highway 24. Heavy rain could result in localized
flooding in low-lying fields and ditches.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Tuesday...
  Limited Thunderstorm Risk.
  Limited Snow Risk north of Interstate 55.
Wednesday...
  Elevated Non Thunderstorm Wind Risk.
  Limited Snow Risk.
  Elevated Lakeshore Flood Risk in northwestern Indiana.
Sunday...
  Limited Thunderstorm Risk.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed south of US-24 Tuesday afternoon.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east to northeast at 40 mph.

LMZ740>745-020845-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
338 AM CDT Mon Apr 1 2024

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Today and Tonight.

Weather hazards expected...

  Hazardous conditions for small craft.
  Limited Thunderstorm Risk.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Tuesday...
  Hazardous conditions for small craft are likely.
Wednesday and Thursday...
  Gale force winds and or extremely hazardous waves are possible.
Friday...
  Hazardous conditions for small craft are likely.
Sunday...
  Limited Thunderstorm Risk.

DK

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