731
FXUS63 KLOT 160848
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
348 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.

&&

.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 /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1219 AM CDT Tue Jul 16 2024

Aviation Forecast Concerns:

- Isolated SHRA in the vicinity of Chicago terminals pre-dawn.

- Period of MVFR ceilings likely into mid-morning.

- Light SW-NW winds becoming more solidly NNW by afternoon.

Severe squall line has cleared the terminals as of midnight,
with a few scattered showers lingering across SE WI and LM which
may pass through the vicinity of the Chicago metro terminals
into the pre-dawn hours. Moist low-levels in the wake of the
storms and in advance of a weak cold front which will move
through the area by midday will likely support at least patchy
MVFR ceilings and visibility in stratus and light fog. Surface
cold front moves through by midday, with winds shifting
northwest and conditions improving to VFR. Can`t rule out some
spotty showers along the front midday/early afternoon, though
coverage/probability look too low for TAF mention at this time.
Scattered thunderstorm potential of better coverage looks to be
well south of the terminals Tuesday afternoon.

Winds remain northerly into tonight, and may briefly turn
slightly NNE at KORD/KMDW, though confidence is low in a
prolonged shift to NE.

Ratzer

&&

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

&&

$$

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