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000
FXUS63 KLOT 180554
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1254 AM CDT Thu Apr 18 2024

.KEY MESSAGES...

- Periods of showers on Thursday with a few thunderstorms and
  instances of heavy downpours possible.

- Much cooler over the weekend with frost/freeze concerns,
  particularly on Saturday night.

&&

.DISCUSSION...
Issued at 228 PM CDT Wed Apr 17 2024

Through Thursday Night:

We find ourselves beneath mostly cloudy skies this afternoon
with just a few pockets of very light rain found north of
I-88. The clouds and rain are remnants of the same storm system
that brought yesterday`s and this morning`s thunderstorms to the
area, which is now centered up in central Wisconsin. As the low
continues to pull away from the region this afternoon,
subsidence behind the departing system will shave away what
little low level instability we have and the light rain should
call it quits after mid-afternoon. Cloud cover will clear up
nicely through the evening and overnight, but will fill back in
tomorrow morning ahead of our next rain system.

A center of low pressure will spin up in the western Plains
tonight and will quickly trek into the lower Midwest by midday
tomorrow. The behavior and track of the low is not well agreed
upon for tomorrow. What looks to be a small majority of guidance
suggests that the low will track south of the area. The NAM and
Euro are farthest north with the track and pull the center of
the low into the southern reaches of our CWA. A few
thunderstorms in our south look rather likely if the storm takes
such a track with ample instability and forcing near the center
of the low. Thunder parameters drop off pretty quickly as you
move north of the low pressure center. If the low tracks not too
far south of the CWA (a popular opinion among models), tall,
skinny CAPE profiles just north of the low could still promote a
few light thunderstorms in our south, but may favor a bit more
the potential for a period of healthy downpours.

Models are in good agreement that we`ll see intense deformation
of the low level wind field occur on the northwestern
periphery of the system. This will lead to a robust
frontogenetic circulation barreling across the region just
trailing the center of the low. It`s influence on the low itself
is uncertain and is a big contributor to the overall lack of
confidence with this storm. Several models, including a big
chunk of available high-res guidance, have the deformation
stretching and elongating the orientation of the low prior to
moving into the region. Such solutions spread out the coverage
of instability a bit, in some instances pulling it farther north
into our CWA, while also diffusing the magnitude of low level
forcing. The RAP is one such example of this.

The initial push of rain, which may miss parts of the CWA to the
south, will occur during the morning into the afternoon. It
looks like we`ll see a relative lull in activity during the
afternoon. This is before a second round of showers is expected
during the evening. This second round will be a direct result of
the trailing f-gen circulation and presents a concern for an
area of sizable rainfall amounts. A handful of models drop a
swath of 1"+ of QPF on parts of the area underneath this band of
potentially heavy showers. With so much yet to be resolved with
this system, we`ll have to keep a close eye on model trends
into tomorrow.

Doom


Friday through Wednesday:

After several days of unseasonable warmth, a dry and cool
airmass will settle across the region Friday into early next
week. Westerly winds gusting to 30 mph across portions of
northern Illinois combined with the dry airmass will support the
potential for elevated brush fire spread Friday afternoon.
Daytime highs in the 50s Friday through Sunday will moderate
closer to seasonal normals in the 60s by Monday. While a surface
ridge is expected to remain west of the forecast area Saturday
night, some decoupling will allow temps to fall into the 30s
nearly areawide. Will maintain frost mention in the grids for
areas away from the lake and the core of the Chicago metro.

While forecast guidance is in decent agreement with a compact
mid-level wave digging southeast across the Upper Great Lakes
Monday night into Tuesday, somewhat complicated phasing of a
western Canada trough and another longwave trough over Hudson
Bay suggests forecast guidance is subject to notable changes
this far out. At this time, will advertise high-end chance to
low- end likely PoPs with a few embedded thunderstorms.

Kluber

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1254 AM CDT Thu Apr 18 2024

Main Concerns:

- Rain and BR with 2-4SM VSBY this evening

- Prevailing lower MVFR CIGs this evening with at least
  temporary IFR CIGs

- Low chance for embedded TS this evening with a much higher TS
  chance well south of the terminals

Quiet VFR conditions are in store until later this morning.
Surface low pressure will track to the south of the terminals
through tonight. Lead -RA/-SHRA will likely move through from
the late morning through the mid afternoon, followed by a
probable gap or lull. Steady light rain with occasional
moderate rain is then expected for a period this evening, along
with BR. VSBY will drop as low as 2-3SM. In addition, CIGs will
deteriorate, including a signal for at least temporary IFR
during the highest rain rates. The rain will exit east by 07z or
so, though the timing of improvement back to VFR CIGs is a bit
unclear. While the chance for TS this evening is not zero at the
terminals, potential is too low to warrant any mention in the
TAFs.

Light northwest winds early this morning will shift over to
northeast in the mid morning. North-northeast wind of 10-15 kt
gusting up to 15-20 kt are expected this evening. Winds will
shift to northwest as the rain ends tonight with continued
gusts up into the 15-20 kt range or so.

Castro

&&

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

&&

$$

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