Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250003
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
703 PM CDT Sun Mar 24 2024

.KEY MESSAGES...

* Three periods during which wind gusts may approach or exceed
  45 mph are expected over the next 48 hours. The first is
  Monday afternoon east of I-55 and south of I-80, second is
  early Tuesday morning east of I-55, and third is Tuesday
  afternoon areawide.

* Monday is looking primarily dry east of I-39, before widespread
  rain moves across the area Monday night into early Tuesday
  morning. A break in rain is expected toward daybreak Tuesday.

* A threat for severe thunderstorms continues to increase
  Tuesday morning, especially east of I-55.

&&

.DISCUSSION...
Issued at 214 PM CDT Sun Mar 24 2024

Through Tuesday:

National satellite imagery augmented by RAP analysis and radar data
depict a sprawling low pressure system centered in far southeastern
Colorado well to the southwest of a surface high pressure system
centered in far southwestern Quebec. Waves of showers and a few
thunderstorms are parading within the warm conveyer belt of the
cyclone from central Oklahoma into central Iowa, southeast of a
burgeoning area of snow across the northern Plains. With a steady
feed of dry low-level air via breezy southwesterly winds on the
flank of the aforementioned high pressure system, our area is quiet
at this hour. Over the next 24 to 36 hours, the surface low pressure
system is expected to move through the Southern Plains and lift into
Upper Great Lakes leading to periods of inclement weather across
much of the central and northern United States. The latest and
greatest model guidance continues to advertise a later arrival of
the system locally, and is leading to modest changes in our
forecast.

First, as the low-level pressure gradient tightens overnight and
tomorrow, southeasterly low-level flow will gradually increase and
provide a reinforcing feed of dry air into the region. As a result,
mixing heights may be able to more readily build into the base of a
low-level jet after daybreak tomorrow, and lead to southeasterly
wind gusts of 35 to 45 mph especially east of I-55 and south of I-
80. At this point, the signal for frequent gusts over 45 mph during
daylight hours on Monday doesn`t currently look high enough to
justify a Wind Advisory (grand ensemble probabilities for gusts >45
mph are only 20-30% tomorrow afternoon), though our midnight shift
will be able to take another look. In addition, the feed of dry low-
level air may act as a wall to the eastward progression of the warm
conveyer belt precipitation tomorrow, effectively stalling rain from
moving east of I-39 through much of the day. For now, opted to
tighten up the gradient in PoPs across our forecast area to
advertise predominantly dry conditions east of I-39 and especially I-
55 through sunset.

Tomorrow evening, the surface low pressure system will begin lifting
northward into the upper Mississippi River Valley, pulling a plume
of 1"+ PWATs into northern Illinois and northwestern Indiana. An
ensemble of CAMs support the warm-conveyer belt of parading showers
finally shifting eastward, leading to high confidence (100% PoPs) in
rain for our entire area (especially from sunset to midnight Monday
evening). With cool-season forcing atop warm-season quality
moisture, efficient rain rates will lead to a swath of 1-2" of rain
somewhere across the region, seemingly favoring areas west of I-55.
In fact, any embedded convective element may allow for streaks of 2-
4" of rain, as suggested by 24-hour HREF LPMM. If such rainfall
amounts materialize, ponding in fields and typical low-lying spots
may occur. In addition, contingency forecasts that use the 95th
percentile GEFS QPF suggest several rivers may rise to bankfull where
rain is heaviest. For now, opted to issue a Hydrologic Outlook (ESF)
to highlight the threat for ponding in farm fields, low-lying spots,
and rises in river levels for all Illinois counties to account for
uncertainty in the exact placement of the band.

Early Tuesday morning (after midnight), ensemble model guidance has
started to hone into a threat for efficient mixing into the
strengthening low-level jet along the backside of the departing axis
of warm conveyer belt rain and as the surface low makes its closest
approach. Forecast BUFKIT profiles and HREF ensemble minimum and
mean wind gusts support gusts in the 45 to 55 mph range east of I-55
from around 3 AM to 10 AM Tuesday, and explicit HRRR/ARW output
suggests gusts may even approach 60 mph. Forecasting surface
nocturnal warm-advection wind magnitudes is always a challenge,
though conceptually this looks to be the type of scenario where
strong to locally damaging non-thunderstorm winds can occur. For
now, opted to only increase our gusts to around 45 mph in our
official forecast in favor of letting our midnight shift take
another look to consider a Wind Advisory for at least part of the
area.

After daybreak Tuesday, attention will turn to an apparently growing
threat for thunderstorms along the eastward-sweeping cold front of
the system lifting into Wisconsin. The most recently available high
resolution model guidance continues to advertise the northward
advection of a narrow tongue of low-level moisture ahead of the
front and a secondary upper-level shortwave early Tuesday morning.
Forecast soundings taken ahead of the front depict low-centroid
instability within a strongly sheared effective kinematic profile,
which are the hallmarks of low-topped supercell environments.
Perhaps most interestingly, 12Z CAM guidance is depicting a subtle
signal for a secondary surface low to develop along the cold front
in northern IL, leading to locally backed and augmented low-level
storm relative inflow. Taken together, the environment Tuesday
morning appears supportive of a band of low-topped supercells with a
threat for locally damaging winds and tornadoes. Based on the
current timing of the system and arrival/trajectory of the tongue of
low-level moisture, the highest threat for such a scenario appears
to be along and east of I-55 and further eastward into Indiana and
Michigan, though any continued slowing trend in the arrival of the
system would no doubt pull the threat for severe weather further
west. These types of severe weather scenarios are basically all or
nothing, and are entirely contingent on the quality of low-level
moisture. At this point, the SPC D3 Level 1/5 threat level outlook
remains appropriate though adjustments may be needed based on
observational trends going forward.

One final thing - Southwest winds will increase markedly behind the
cold front Tuesday afternoon thanks to deep mixing afforded by CAA
and stout pressure rises (on the order of 6-10mb/6 hours). As a
result, Tuesday afternoon will be yet another period to watch for
wind gusts approaching 45 mph across parts if not all of the
area.

Borchardt

Tuesday Night through Sunday:

High pressure will build over the area late Tuesday behind the
departing storm system which should clear thing up nicely for the
middle of the week. A tight pressure gradient on the backside of the
storm will keep conditions breezy for Wednesday, though not nearly
as windy as we`re expecting for Monday and Tuesday. Efficient cold
advection will keep highs stuck in the 40s area-wide on Wednesday. A
second center of high pressure focused to our southwest will take
precedence come Thursday and pump some milder air into the region.
The warm advection in conjunction with what should be a sunny day
will allow highs to return to the 50s on Thursday.

We`ll see steady height rises aloft Thursday night into Friday as an
upper ridge moves over the area. This will help pull us back into
the 60s for both Friday and Saturday. A mid level shortwave is
progged to spin up a surface low over the northern Plains on Friday.
The system`s warm front is slated to set up somewhere in the
vicinity of the CWA and most guidance is throwing up at least some
spotty showers in the area. This potential will carry on through the
weekend as the low pressure center and it`s trailing cold front work
across the region.

Doom

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 639 PM CDT Sun Mar 24 2024

- LLWS expected tonight
- Strong south winds expected to develop by mid morning Monday,
  occasional gusts to 40kt possible mid morning into the
  afternoon
- SHRA will develop at RFD during the morning, likely building
  to a steady rain in the afternoon
- SCTD SHRA are possible at the Chicago terminals Monday
  afternoon

Moderately strong and gusty east-southeast winds expected to
continue this evening. Frequency and magnitude of gustiness may
vary at times this evening with a tendency for frequency and
magnitude of gustiness to wane overnight. While OCNL gusts will
likely persist after 06Z, the frequency should decrease.
Meanwhile, winds will increase to 45-55kt around FL020 this
evening and continue overnight resulting in the potential for
LLWS.

After sunrise Monday, we should begin to tap into the slightly
weakening winds a couple thousand feet off the deck. As this
occurs, look for winds to turn more southerly (160-180) and
increase with frequent gusts 30-35kt by mid morning Monday and
continuing into the afternoon. There is a around a 40% chance
for some occasional south (160-180) wind gusts to 40kt by 15Z
Monday and potentially at times into the afternoon.

Rain is expected to mostly stay west of the immediate Chicago
terminals Monday, though likely impacting RFD. The rain should
eventually result in lower CIGS/VSBY at RFD by mid to late
afternoon. Immediate Chicago terminals (ORD, MDW, GYY, DPA) may
see some spotty showers Monday afternoon into the early evening.
More persistent rain should begin to impact immediate Chicago
terminals Monday evening.

- Izzi

&&

.MARINE...
Issued at 214 PM CDT Sun Mar 24 2024

Several periods of gale force winds may occur over the next 48
hours. Easterly winds will turn southeasterly and build this
evening to feature regular gusts to 25 to 30 kt. Southeast gusts
to 30 kt with a few gale force gusts to 35 kt can be expected
throughout the day on Monday. Tuesday, there is a signal that
more regular gale-force gusts to 35 kt may occur. For this
reason, a Gale Watch is in effect from early Tuesday morning
through Tuesday evening. Winds are expected to subside Tuesday
night. LMZ740 should see significant waves get up to around 10
ft tonight and again Monday night, and closer 6 to 8 ft across
LMZs 741 and 742 due to the southeasterly wind direction.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 AM CDT Tuesday for Winthrop
     Harbor to Northerly Is. IL.

     Gale Watch from late Monday night through Tuesday evening for
     the IL and IN nearshore waters.

     Small Craft Advisory until 3 AM CDT Tuesday for Northerly Is.
     IL to Michigan City IN.

&&

$$

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