441
FXUS63 KLOT 161544
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
Issued by National Weather Service Gaylord MI
1044 AM CDT Tue Jul 16 2024

.KEY MESSAGES...

- Still warm and humid today, though with lower peak heat indices of
  85-95.

- Chance of scattered showers and thunderstorms south of I80 this
  afternoon, though severe weather threat has ended.

- Period of mainly dry and seasonably cooler weather Wednesday
  through the weekend, with highs in the upper 70s/low 80s.

&&

.UPDATE...
Issued at 1043 AM CDT Tue Jul 16 2024

Synopsis: Large scale pattern this morning features an upper low
over northwest Ontario...northern Illinois just on the southern
periphery of stronger westerly flow around the south side of this
system. Cooler and somewhat drier northwesterly mid level flow into
the state as well...but still muggy at the surface with dew points
still in the upper 60s/lower 70s. 13z surface analysis shows a
cold front over southern Wisconsin/southeast Iowa extending
northeast to a 1002mb low along the Ontario/Quebec border. Mixed
skies across northeast Illinois at late morning...with a band
of low clouds along/behind the cold front to the north...and
across the south and west Chicago suburbs. Scattered showers
along the front across southern Lake Michigan...axis of
500-1000J/kg MLCAPE across southern Wisconsin. Dry across the
forecast area thus far.

Forecast Update:  Cold front will slowly slip south across northern
Illinois this afternoon...pretty evident on radar from roughly RAC-
RFD-WBC.  As mentioned above still pretty muggy out there...already
some 500-1000J/kg MLCAPE with not much CINH.  Not much in the way of
large scale support but the chances for some additional convection
popping this afternoon is non-zero either along the front or perhaps
along the lake breeze which should develop later this afternoon.

JPB

&&

.DISCUSSION...
Issued at 348 AM CDT Tue Jul 16 2024

Through Wednesday:

Impressive bowing MCS which brought extensive severe weather to
portions of the forecast area Monday evening as moved off to
the east and south of the forecast area early this morning.
Elevated convection continues just south of the cwa across
central IL and central IN, with our southern cwa border just
being brushed by lighter trailing stratiform precip and
occasional embedded thunder at this time. This activity should
continue to sag slowly off to the southeast this morning as a
parent mid-level short wave propagates off to the east, and the
low level jet gradually veers more west- northwesterly. A weak
surface cold front was analyzed from 1003 mb low pressure east
of the Straits of Mackinac (northern lower MI), through central
WI and southeast IA, and this front will push slowly southeast
across the forecast area today. While the front will transit the
southern half of our cwa during a diurnally favorable time this
afternoon, the strong outflow boundary from last evening`s
storms will shunt the more substantial low-level instability
well south of the area into downstate IL/IN. So while scattered
afternoon showers and thunderstorms are expected (mainly south
of the I-80 corridor), the threat of severe weather is expected
to be well south of our cwa. Can`t completely rule out a
sprinkle or two farther north in association with another mid-
level speed max rotating through the southern periphery of a
seasonably deep upper trough over Ontario and the upper Midwest
mid-late afternoon, though most guidance forecast soundings
appear too dry for any precipitation of consequence north of
I-80.

The aforementioned upper trough is progged to continue to
amplify and deepen across the region into Wednesday, with a
strong mid-level vort digging across the western Great Lakes
region Wednesday afternoon. This large scale ascent may keep
isolated to scattered shower chances in place along our southern
border with ILX/IND/IWX into Wednesday, closer to the low-level
baroclinic zone north of what will become our stalled
stationary front across the Ohio Valley.

Temperatures will be slightly cooler behind the front today,
with afternoon highs expected to range from the low-mid 80s far
north to the mid-upper 80s south. Dew points will be a little
slow to drop behind the weak, slow-moving front and will remain
largely in the 70s, making for afternoon heat indices in the
85-95 range from near the IL/WI border to south of I-80.
Noticeably more comfortable conditions can be expected
Wednesday, with highs around 80 and lower humidity.

Wednesday night through Monday:

Global guidance is in good agreement in amplifying an upper
ridge across the western CONUS/NOAM through the remainder of the
forecast, with an upper trough focused across the Northeast.
This will allow for expansive surface high pressure to build
across the Mississippi Valley, western Great Lakes and the
Midwest from Wednesday night through Friday. This looks to
support a well-deserved period of dry weather and a break from
the recent heat and humidity into the coming weekend. Daytime
highs from the mid-upper 70s into the lower 80s are expected,
with daily lake breezes near Lake Michigan. Overnight lows look
to range from the upper 50s to lower 60s. Models are also in
decent agreement in depicting an amplifying mid-level short wave
dropping across the region in the Sunday-Monday timeframe,
though with surface high pressure ridging blocking the return of
Gulf moisture any precip potential looks fairly marginal at
this distance. Blended NBM guidance brings some spotty slight
chance pops (<20%) by Monday, but all in all we appear to be in
store for a decent stretch of dry, pleasant weather.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 546 AM CDT Tue Jul 16 2024

Aviation Forecast Concerns:

- Period of MVFR ceilings and light fog early this morning.
  Patchy IFR mainly west of KORD/KMDW.

- Light WSW winds become NW later this morning, with a period of
  modest NE winds for KORD/KMDW/KGYY by this evening.

A weak surface cold front will push southeast across the
terminals through midday. Very moist low-level air mass and
light convergent wind field will support patchy IFR to MVFR
ceilings for a few hours early this morning, with lowest
conditions (IFR) generally expected west-northwest of KORD/KMDW.
Cloud bases should lift to VFR by midday. Winds will start out
light west-southwest, then shift northwest behind the front by
midday. KORD, KMDW and KGYY will likely see a brief period of
modest (less than 10 kt) northeast flow off the lake toward
evening, with winds then shifting back light north-northwest
before midnight. Northwest winds and VFR conditions then
expected through end of the current TAF period.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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