748
FXUS63 KLOT 102333
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
633 PM CDT Fri May 10 2024

.KEY MESSAGES...

- Quick (an hour or 2) hit of showers, and a few embedded
  thunderstorms, tonight.

- Drying out for Saturday with breezy northwesterly winds gusting
  up to 30 mph during the day.

- Warm with low humidity on Mother`s Day, mainly dry until the
  evening when a few showers will be possible.

- Widespread rain (potential for locally heavy totals) likely on
  Monday-early Tuesday...Coolest conditions next week on Tuesday.

&&

.DISCUSSION...
Issued at 300 PM CDT Fri May 10 2024

Through Saturday night:

A quick hitting band of showers will shift southeastward across
the area with a cold front later this evening into the
overnight hours as a compact and fast moving mid/upper level
impulse digs southeastward out of the Arrowhead of MN. The fast
movement of this feature will result in only an hour or two
period of precipitation at any given location tonight. The best
window for this period of rain looks to come in the 9pm to
midnight timeframe for northwestern portions of the area
(including the Rockford area), 10pm to 1 am timeframe for much
of the Chicago metro area, and in the 11pm to 3am time period
for southern/southeastern sections of the area. Steepening lapse
rates could support a few embedded gusty thunderstorms with
this activity tonight, though no severe weather is anticipated.

The cold front, and any showers and cloud cover associated with
it, will clear the area prior to daybreak Saturday morning.
In its wake, northwesterly winds will become gusty (up around 30
mph) for a period on Saturday, though speeds should abate through
the afternoon as an afternoon lake breeze push turns the winds
onshore across northeastern parts of IL. Partly cloudy conditions
during the day will allow temperatures to top out in the upper
60s to around 70, especially for inland areas. However, onshore
flow during the afternoon will result in cooler afternoon
conditions near Lake Michigan.

The forecast for Saturday largely looks to remain precipitation
free for much of the area. While this is the case, there is a
non-zero threat (up to about a 10% chance) for a few isolated
afternoon showers, primarily near the area of enhanced low-level
convergence along the lake breeze boundary in northeastern IL.
While I would not be surprised to see a couple of these, it
appears they would be short lived, and cloud depths would be far
to shallow to support charge separation and hence thunder. For
these reasons, I have opted to maintain a dry forecast for the
time being. Otherwise, expect quiet and mainly clear weather
Saturday night.

KJB

Sunday through Friday:

Breezy southwest winds, warm advection, dew points mixing out to
the upper 40s to around 50F, and plenty of sunshine will set the
stage for a warm Mother`s Day Sunday. Look for highs to reach the
upper 70s to lower 80s, albeit with low humidity levels. A weak
cold front will approach the area Sunday night. With neutral
height tendencies and diurnally unfavorable timing, a few showers
and maybe a weakening thunderstorm or two developing over
Wisconsin and northeast IA late in the day may be able to drift
into far northern Illinois in the evening. 30-40% PoPs are focused
north of I-80 in the evening and then generally in the 20-30%
range overnight when it may very well be primarily dry. It`s
unclear if MUCIN will be overcome to support any lightning, though
maintained some low thunder chances.

Monday into Tuesday, the upper-level wave currently in the process
of becoming "cut-off" in the southwestern United States is expected
to become "re-absorbed" into the upper-level flow while drifting
eastward toward the mid Mississippi River Valley. As the surface
low reflection associated with the upper wave drifts northeast,
the cold front approaching Sunday night will drift south and
take on back-door characteristics on Monday. With forcing from
the approaching wave and perhaps the backdoor front, showers and
a few thunderstorms may break out Monday morning through midday.

Over the last few model cycles, operational and ensemble
guidance has come into better agreement in a period of inclement
weather with widespread rain/thunderstorms Monday afternoon into
Monday evening and overnight. With this increased signal,
collaboratively adjusted PoPs into the 75-80% range centered on
Monday afternoon, though envision likely (60-70%+) PoPs Monday
evening will also trend higher. Seasonably high PWATs of up to
1.5" (150-200% of normal) and slow motion of showers and any
embedded thunderstorms may support some locally "hefty" rainfall
totals in the 1-2.5" range. The system TROWAL should gradually
pivot southeast into Tuesday, drying things out from north to
south. It appears the appreciable rain into the daytime hours
on Tuesday should primarily be south of I-88 or south of I-80
and likely exiting south by or during the afternoon.

High temperatures near normal (low 70s) on Monday, except
cooling off near the lake as winds flip onshore in the
afternoon, will be decidedly cooler on Tuesday. Looking at low-
mid 60s if that inland and likely only mid 50s lakeside with
breezy north-northeast winds gusting up to 25-30 mph, strongest
near the lake. Wednesday will trend back closer to normal inland
but stay cool near the lake with continued onshore flow. The
next (uncertain) shower and thunderstorm chances will return
Thursday-Friday amidst near seasonable temps (highs in 70s).

Castro/Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 632 PM CDT Fri May 10 2024

Key Aviation Impacts for the current TAF period:

- Line of showers expected this evening and early overnight with
  localized gusty winds accompanying the line

- Gusty northwest winds expected on Saturday

Partly cloudy skies expected for the next few hours with
southwest winds occasionally gusting up to 20 knots. A cold
front is set to move over northern Illinois this evening. With
it brings a line of showers and localized stronger wind gusts
out of the northwest. No changes with timing at Chicago
terminals, 04Z to 07Z. There may be a brief moment of MVFR cigs,
but most showers should have clouds that remain VFR.

Drier conditions behind the front will set up a VFR day on
Saturday. Main impact on Saturday will be driven by stronger
northwesterly winds, where gusts can be expected through 00Z
around 25 knots with isolated bursts up to 30 knots possible.
Lastly, there remains a weak signal in models for the potential
of a lake breeze developing late afternoon. Northeast winds were
added to KGYY`s TAF, but with lack of strong signal and
consistency from guidance, it was left out of the Chicago
terminals for this forecast package.

DK

&&

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

&&

$$

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