Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241120
AFDLOT

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

.KEY MESSAGES...

- Scattered snow/graupel showers this morning across portions
  of northern IL

- Potent storm system to impact the area Monday through Tuesday
  bringing the threat for soaking rainfall, gusty winds, and
  perhaps even embedded thunderstorms

- Strong non-thunderstorm winds on Tuesday gusting at times up to
  40 to 45 mph.

- Low but increasing potential for a few strong to severe
  thunderstorms late Tuesday morning and early afternoon with
  mainly a damaging wind threat east of I-55/57.

&&

.DISCUSSION...
Issued at 356 AM CDT Sun Mar 24 2024

Through Monday Night:

The main weather focus in the immediate near term is the area
of scattered snow showers that is pivoting through western and
northern Illinois this morning. While these snow showers have
thus far struggled to reach the surface in our forecast area,
forecast soundings do show the low-level dry air eroding over
the next few hours which should allow that to change especially
for locales near I-39 and north of I-88 where the combination of
forcing and instability look to be a bit better. Given that
temperatures across the area are sitting near, if not just
below, freezing I would not be surprised to see at least a light
dusting (less than 0.2 inches) occur primarily on grassy and
elevated surfaces through daybreak. Additionally, some instances
of graupel (squishy ice pellets) may occur with more robust
showers as forecast soundings do show at least modestly steep
lapse rates within the DGZ.

The snow/graupel showers are expected to wane by mid-morning as
mid- level drying takes over. However, the continued warm
advection (forcing) ahead of our next system will continue to
maintain partly to mostly cloudy skies through this afternoon.
But otherwise dry conditions are forecast for today with
temperatures warming into the mid-40s to lower 50s this
afternoon along with breezy southeast winds gusting 20 to 25
mph. Though areas along the Illinois lake shore do look to
remain in the lower 40s due to onshore winds.

After today our attention turns to the broad upper trough
currently sprawled across the western CONUS. As this trough
continues to push eastward today, a surface cyclone is expected
to develop along the lee-side of the Rockies and begin to track
northeast towards the northern Great Lakes. Guidance continues
to be a good agreement with the track of the low and the
resultant area of precipitation that is expected to develop
along the eastern and southern flanks. It is these areas of
precipitation that will spread into northeast Illinois and
northwest Indiana during the day on Monday and persist through
Monday night. Thankfully, the forecast track does keep our area
on the warm side of the system with rain as the expected
precipitation type.

While guidance continues to indicate that the true warm sector
should establish west of the Mississippi River, impressive deep
layer moisture (PWATs of 0.9 to 1.0 inches) is expected to
advect into our area courtesy of a stout low-level jet. This
moisture in combination with the broad forcing will support
soaking to potentially heavy rainfall across a good swath of
northeast Illinois and northwest Indiana during the later
portion of Monday into Tuesday morning. In fact, ensemble
guidance shows a 50 to 60 percent chance for rainfall totals
around an inch with a 10 to 15 percent chance for localized
totals nearing 2.0 inches. However, despite the good agreement
there is still some uncertainty as to where exactly the heaviest
swath of rainfall will occur which is something we will have to
hone in on with subsequent forecast cycles.

In addition to the soaking rainfall, the aforementioned stout
low- level jet will also aid in generating gusty southeast winds
Monday and Monday night. While the strongest gusts will be
dictated on how deeply the atmosphere will mix, there is a
signal that gusts in the 30 to 40 mph range are a safe bet with
the potential for occasional gusts to exceed 40 mph especially
Monday evening.


Yack

Tuesday through Saturday:

There`s been a shift towards a slightly slower progression with
the sprawling area of low pressure in guidance tonight as we head
into Tuesday. While the low-level moisture profile continues to
look fairly unimpressive with surface dewpoints only in the upper
40s and maybe lower 50s, the slower progression now allows this
modestly higher theta-e airmass to surge into parts of our area on
Tuesday morning prior to the arrival of the cold front. 0-3 km
lapse rates are actually progged to become fairly steep through
the late-morning hours (near 7-8 C/km in spots), although in
reality this will hinge on whether shower activity remains
widespread during this time.

The kinematic parameter space is forecast to remain very
impressive through midday, with 850 mb southwesterly flow near or
in excess of 50 knots. And while the mid-level jet will quickly be
scooting east of the region, effective deep layer shear values
(roughly 0-4 km given equilibrium levels under 8/9 km) are still
forecast to remain elevated near 50 knots, certainly sufficient to
facilitate storm organization and mid-level rotation given enough
instability. We`ll need to keep a close eye on this given the
noted slowing trend as fast-moving, low-topped bowing
segments/LEWPS would not be out of the question from mid-morning
Tuesday through early-afternoon before the cold front arrives.
The main area we`re monitoring is roughly along and east of
I-55 and I-57.

With the arrival of the cold front, temperatures will fall
through the 40s late in the afternoon. Strong southwesterly winds,
gusting up to 45 mph appear plausible during this time as well as
the boundary layer deepens and mixing into 40 knot 800 mb flow
occurs, along with a bit of an isallobaric component as 3-6 mb/6
hour pressure rises overspread the region.

The atmosphere will settle down on Wednesday as a mid 1020s mb
surface high builds across the Central Plains and into the Upper
Midwest. Highs will remain in the 40s, although this modest cool
down will be very short lived as model guidance supports a quick
rebound back into the 50s and 60s through the end of the week,
with lake cooling returning on Friday. The next precipitation
chances may arrive towards Friday and into next weekend as a warm
front sharpens up in the vicinity.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 620 AM CDT Sun Mar 24 2024

The main aviation weather concerns are:

- Breezy southeasterly winds developing today.

- LLWS tonight.

- Much stronger/gustier winds developing Monday morning.
  Potential for some crosswind impacts on E-W runways with wind
  directions from 160-180.

Southeasterly winds will become breezy/gusty with time this
morning and afternoon, with the strongest gusts (around 25
knots) occurring late in the afternoon. While there`s a small
chance for a flurry/sprinkle later today at RFD, chances and
impacts are too low to justify a mention in the TAF.

Gusts will become more intermittent this evening. At the same
time, a very strong low-level jet will intensify over the
region leading to a period of LLWS at all terminals, with winds
at 2 kft AGL peaking near 55 knots late tonight.

Surface southeasterly winds will then quickly increase on
Monday morning as mixing into the base of this low-level jet
occurs. It`s possible that gusts briefly approach 35 knots late
Monday morning which could result in modest crosswind impacts
at ORD. Chances for rain showers are expected to increase just
beyond the end of the current TAF window.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from noon today to 7 AM CDT Tuesday for
     Winthrop Harbor to Northerly Is. IL.

     Small Craft Advisory from 7 PM this evening to 7 AM CDT
     Tuesday for Northerly Is. IL to Michigan City IN.

&&

$$

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