518
FXUS63 KLOT 060818
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
318 AM CDT Sat Jul 6 2024

.KEY MESSAGES...

- Mostly dry conditions are expected, but cannot completely
  rule out the chance for a light pop-up sprinkle/shower during
  the afternoon

- 25 to 45 percent chance for showers and storms Sunday
  morning/afternoon, mainly northwest of a La Salle to Lake
  County line

- Unsettled Sunday night-Tuesday afternoon with periodic showers
  and thunderstorms, highest chance/coverage on Monday.

- Seasonable temperatures will continue through next week.

&&

.DISCUSSION...
Issued at 318 AM CDT Sat Jul 6 2024

Through Sunday:

An upper-level trough continues to gradually move to the east
away from Chicago. Upper-level height rises in its wake
suggests that much of the area should stay dry through the day.
However, soundings have kept a swath of mid-level moisture
lingering around the area. This in combination with daytime
heating along a weak lake breeze could be enough to support the
development of a few isolated to widely scattered sprinkles or a
rogue shower this afternoon.

The next upper-level shortwave trough currently dropping
southward into the western Dakotas will grow into a much longer
wave through Saturday. As it moves into Minnesota Sunday
morning, a weak shortwave is expected to extend southeastward
over Iowa and move into northern Illinois. Winds on Sunday will
turn to the south-southwest and advect in higher moisture
content. Lapse rates aloft and wind shear through the afternoon
are not overly impressive, so the threat of severe weather will
be low. However, there should be enough instability as the
better area of vorticity moves in ahead of the wave to allow for
showers and thunderstorms to move into the area. The favored
track of the impulse toward southeastern Wisconsin will favor
the higher chances (higher PoPs) generally northwest of a La
Salle to Lake County line. The long wave trough continues to
move eastward for renewed chances for showers and storms Sunday
night, covered in the discussion below.

DK


Sunday Night through Friday:

The threat and coverage of showers and thunderstorms is expected
to increase from northwest to southeast across the area Sunday
night through at Monday night owing to the approach of a slow
eastward moving cold front. During this period, several waves of
scattered northeastward moving showers and thunderstorms are
favored along and ahead of the front, with each wave being driven
by a series of mid-level impulses tracking northeastward along
the surface frontal boundary. The highest chances for these showers
and storms are expected to remain primarily northwest of I-55
Sunday night into Monday morning, before the chances begin to
ramp up farther east and southeast later Monday into Monday
night. The threat of severe weather looks to remain rather low
with this activity. However, we will have to keep an eye on the
possibility for some locally heavy rainfall given the potential
for some northeastward training along the slow moving cold front
within an environment featuring high PWATs (up to 150% of
normal).

The surface cold front should begin to shift east of the area
early on Tuesday as the main mid-level trough axis makes steady
eastward progress into the Western Great Lakes. However, in spite
of the early frontal passage, expect the chances for scattered
showers and storms to continue into Tuesday as relatively cool
mid- level temps under the upper trough shift over a still warm
and fairly humid post frontal airmass. Accordingly, we continue to
carry 30-40% PoPs. The threat of these showers and storms should
come to a quick end later in the afternoon, however, as the trough
shifts to our east.

For the Wednesday-Friday period, the threat of organized
showers and thunderstorms looks to become rather low owing to
prevailing mid-upper level height rises, overall parched air
aloft, and surface high pressure centered over the northern
Great Lakes. However, we will be keeping a close eye on the
remnants of Beryl later in the week after it moves inland from
the TX Gulf Coast. While it appears the remnants may track
primarily south of our area, we cannot at this point rule out the
possibility of some showers and/or storms making it into parts of
the area for a period later in the week. Nevertheless, at this
time we will continue with a more optimistic and mainly dry
forecast for Thursday into Friday.

Temperatures through the period are expected to remain seasonably
mild, with daily highs generally in the 80s. However, onshore
flow Tuesday PM through Friday will result in locations near the
Lake Michigan shore remaining several degrees cooler.

KJB

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1227 AM CDT Sat Jul 6 2024

Key messages:

- A brief (1 to 3 hour) period of MVFR CIGs and north-
  northeasterly winds early this morning.

- Easterly wind shift with lake breeze expected at main Chicago
  terminals this afternoon.

Currently a weak back door front is shifting southward across
northeastern IL. While this is not expected to cause a major
change in the going forecast, it does appear that a wind shift
to a north-northeasterly direction will occur at the main
terminals for a brief hour or two period just after 06z early
this morning as the boundary passes. This wind shift is also
likely to be accompanied by a short period of MVFR CIGs
(2,000-3,000 ft AGL). Opted to message this wind shift within
the going tempo for MVFR CIGs from 06-09z.

Otherwise, prevailing winds will settle back into a light (8kt
or less) northwesterly direction by daybreak, and will remain
that way through at least early this afternoon. Thereafter, a
lake breeze is expected to shift inland across northeastern IL.
This will result in an easterly wind shift at ORD and MDW
sometime in the 20-21z timeframe. Winds are then expected to
become very light (under 5 kt) from the south-southeast
tonight.

KJB

&&

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

&&

$$

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