553
FXUS63 KLOT 051950
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
250 PM CDT Thu Sep 5 2024

.KEY MESSAGES...

- Period of scattered showers and a few thunderstorms expected
  tonight

- Lake effect showers expected across the lake effect belt of
  northwest Indiana Friday night into Saturday

- A big cooldown arrives heading into the weekend with highs in
  the 60s before warming back up early next week.

&&

.DISCUSSION...
Issued at 250 PM CDT Thu Sep 5 2024

Through Friday Night:

Water vapor imagery early this afternoon shows a gradually
amplifying trough over the northern Plains and upper Mississippi
Valley. Jet streak is also evident on the west side of the
trough, an indication that this trough is still poised to
amplify as it digs southeastward into the western Great Lakes
tonight. At the surface, a cold front extends from west of GRB
southwest across portions of eastern and southern Iowa. Band of
some widely scattered showers continues near this front at mid
afternoon.

This front should continue to move southeast across our CWA
tonight. An axis of some drier over much of our CWA is
sandwiched between some deeper moisture near/south of the
Kanakakee River Valley and an axis of moisture pooling near the
approaching front. Guidance suggests that this dry axis will be
pinched off by the moisture approaching from the northwest
along/ahead of the cold front. Some modest increase in synoptic
forcing associated with the aforementioned jet streak rounding
the base of the digging trough will probably lead to at least
some uptick in the coverage of showers, especially later this
evening, as it moves across our CWA tonight. Instability is
progged to be pretty meager, but with a few hundred J/kg of
MUCAPE progged by most guidance, opted to hang onto the slight
chance of thunderstorms.

Secondary cold front will move across the area late
tonight/early Friday morning with a more noticable drop in
temps/dewpoints expected behind this front. Deepening mid-upper
level trough is progged to close off into a mid-upper low over
the western Great Lakes Friday afternoon/evening. Associated
cold air aloft should lead to some modest diurnally driven
instability Friday afternoon across our CWA, which could be
enough to support some isolated to perhaps scattered showers.
Forecast soundings suggest depth of the convective layer should
extend up to around 10kft, which isn`t terribly deep, however an
80-90kt 250mb jet is progged to nose down the west side of the
trough placing northern IL in the favorable left exit region
Friday afternoon. Have opted to maintain slight chance pops for
showers for now, but wouldn`t surprise me if coverage justifies
slightly higher pops eventually despite the meager depth of the
convective layer.

First preview of early fall like temps expected Friday and
especially Friday night. This will help lake effect parameters
quickly become quite favoarable for showers Friday night. By the
time the lake effect really ramps up, it should be in the
typical northwest Indiana lake effect belt Friday night. Lake
induced CAPE values and equilibrium levels from the Bufkit
algorithm often times appear overdone, but decided to leave a
slight chance of lake effect thunderstorm in the forecast for NW
IN Friday night, especially after midnight.

- Izzi


Saturday through Thursday:

The core of an unseasonably cold airmass will be situated
firmly across the Great Lakes region on Saturday. 850 hPa temps
of 3-5C amid generally sunny skies away from the lake effect
belt of northwest Indiana should result in high temps in the mid
to locally upper 60s (about 10-13F below normal). High pressure
will drift southwest of the forecast area Saturday night. A
rather chilly night is in store given a cold and dry airmass in
place as well as generally clear skies and weakening gradient
flow with the high. Min temps in the lower 40s appear likely
away from the core of Chicago heat island, with MOS-based
guidance event suggesting a decent chance of the typically
coldest locations falling into the upper 30s.

Thermal profiles over Lake Michigan Saturday favor a solid band
of lake effect rain drifting east and weakening over northwest
Indiana east of I-65 through the day. Cannot rule out some
lightning as well, particularly in the morning. A deep lake-
induced convective layer with weak shear and the expectation of
a land-breeze convergent boundary will also favor waterspout
development.

The CONUS-scale pattern will gradually become more zonal Sunday
through much of next week as a series of low-amplitude ridges
shift eastward over the northern Great Plains and western Great
Lakes. Resultant surface high pressure sprawling across the
Appalachians along with an active sub-tropical jet confining
Gulf moisture to the far southern states supports an extended
period with dry and mostly clear skies Sunday through at least
Wednesday (and likely Thursday). Temperatures will quickly
moderate to near normal levels Sunday and Monday and above
normal levels (with highs well into the 80s) mid- week.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1256 PM CDT Thu Sep 5 2024

- Period of SCTD -SHRA expected mid evening into the early
  overnight hours
- MVFR CIGS expected late tonight into Friday morning
- Gusty north winds develop Friday morning

Cold front will move across the region this evening resulting in
a wind shift from west/southwest to northwest. Some scattered
showers will likely accompany the front as it moves across the
terminals, with about a 20 percent chance of a brief
thunderstorm at the terminals. Showers will push south of the
terminals late this evening/early overnight hours. MVFR CIGS are
expected to develop/push south into the terminal areas late
tonight and likely linger at least through mid-late morning
Friday. Isolate showers are possible Friday afternoon but
coverage is expected to be limited, so opted not to introduce
SHRA into the final 6 hours of ORD and MDW`s 30 hour TAF, but a
brief shower threat does exist Friday afternoon. Finally, north
winds will increase and become gusty Friday morning and remain
moderately strong and gusty into Friday afternoon.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from late tonight through late
     Saturday night for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from late tonight through late
     Saturday night for INZ001-INZ002.

LM...Small Craft Advisory from 4 AM Friday to 10 PM CDT Saturday
     for the IL nearshore waters.

     Small Craft Advisory from 4 AM Friday to 4 AM CDT Sunday for
     the IN nearshore waters.

&&

$$

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