570
FXUS63 KLOT 191055
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
555 AM CDT Fri Jul 19 2024

.KEY MESSAGES...

- Generally quiet through the weekend.

- Periodic shower chances Monday-Thursday, with Wednesday having
  the highest chances of scattered showers and thunderstorms

- No typical July heat and humidity through next week.

&&

.DISCUSSION...
Issued at 330 AM CDT Fri Jul 19 2024

Through Saturday Night:

Broad high pressure analyzed at 1022-1023 mb draped across the
region will remain in control, while weakening slightly to
around 1020 mb. Some scattered diurnal Cu will develop today and
then get scoured out by the lake shadow as the lake breeze
pushes inland this afternoon. Mostly clear to partly cloudy
skies tonight will be followed by partly cloudy to partly sunny
skies on Saturday as a weak 700 mb disturbance probably causes
scattered to broken diurnal Cu. A less pronounced lake breeze
shadow is favored on Saturday.

Highs inland today will be in the upper 70s to around 80F and
then a degree or two warmer on Saturday, while mid-upper 70s are
forecast along the immediate lakeshore both days. Pleasant,
relatively cool low temperatures in the mid-upper 50s (low-mid
60s in/near Chicago) are in store tonight, with mostly cloudy
skies resulting in lows 3-5F milder on Saturday night.

Sunday through Thursday:

A broad trough axis will be over the region, though only
featuring modest forcing and minimal flow aloft until Wednesday.
The surface high pressure will maintain influence on Sunday.
Antecedent drier low levels will likely keep any spotty showers
confined to primarily north and northwest of the CWA. Can`t
completely rule out isolated high based showers/sprinkles over
parts of far northern Illinois in the afternoon.

Isolated showers will then be a possibility on Monday afternoon,
mainly inland of the lake (20-30% chance/coverage), with
perhaps a few thunderstorms as weak instability develops. A
modest short-wave pivoting across the mid-MS Valley may then
focus any isolated (~20% chance/coverage) convection (showers
and maybe a few t-storms) mainly south of I-80 Monday night.

Dew points and PWATs will creep up some on Tuesday, supporting
scattered shower development toward mid day and especially in
the afternoon. Extensive cloud cover and mid-level lapse rates
of 5.5 C/km or less will limit destabilization and keep any
thunderstorm coverage lower than shower coverage (slight/~20%
chance/isolated coverage). A bonafide mid-upper low forecast to
drop southeast toward the area on Wednesday will support the
highest shower chances of the week (40-60% PM hours), and
cooler mid-level temps marginally steepening lapse rates for a
better chance for scattered non-severe thunderstorms.

Weak surface low pressure associated with the upper low should
drag its cold front through the area by Thursday as the upper
low translates eastward to the central and eastern Great Lakes.
Shower chances are favored to trend downward to 20-30% as a
result, with only a slight chance of thunderstorms.

Forecast highs Sunday through Thursday are generally in the
upper 70s to lower 80s, coolest along the lakeshore as afternoon
lake breezes and/or prevailing onshore flow keep temps slightly
cooler. Overnight lows will be in the 60s, mildest in Chicago.

Castro

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 555 AM CDT Fri Jul 19 2024

VFR conditions are forecast through the TAF period with few to
scattered 5-6000 ft diurnal cumulus. Winds will be light westerly to
at times variable through midday before an expected inland push of
the lake breeze turns winds easterly at GYY/MDW/ORD. Guidance
continues to trend a bit faster with the lake breeze arrival and
have accordingly nudged the easterly wind shift to 1830Z at MDW and
1930Z at ORD. Winds then return to light and variable this evening
with increasing high cloud coverage expected Saturday.

Petr

&&

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

&&

$$

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