030
FXUS63 KLOT 122018
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
318 PM CDT Sun May 12 2024

.KEY MESSAGES...

- Small chance for a couple of thunderstorms near the WI state
  line this evening.

- Showers and thunderstorms expected (90%+ chance) Monday
  afternoon and evening, with locally heavy rainfall amounts.

- Showers continue into Tuesday before ending from northwest to
  southeast in the afternoon and evening.

- Seasonable temperatures and chances for showers and storms
  will return by the end of the week.

&&

.DISCUSSION...
Issued at 318 PM CDT Sun May 12 2024

Through Tuesday:

All is currently quiet across the area early this afternoon.
While this is expected to continue to be the case for a majority
of the area through tonight, we will be watching for the
possibility for a couple of thunderstorms to move into parts of
far northern IL (especially near the south central WI state line)
late this afternoon into the early evening. These thunderstorms
are already ongoing and developing across parts of IA in the
vicinity of a surface boundary. Given their expected eastward
movement it remains possible that some could shift into parts of
far northern IL near the WI state line early this evening before
weakening. Severe weather is not anticipated, but certainly cannot
rule out some instances of locally gusty winds with them. These
storms should weaken through the evening the boundary layer
stabilizes.

Our main weather maker for Monday afternoon through Tuesday
continues to be the the upper low currently traversing the central
and southern Rockies. This feature, along with an associated
surface low will track eastward across the Mid-Mississippi Valley
into the Ohio Valley late Monday through Tuesday. However, prior
to its arrival, a backdoor cold front (associated with low
pressure in Ontario) will drop southward into northern IL Monday
afternoon and evening. As it does so, increasingly southerly low-
level trajectories in advance of the approaching low will foster
a northward surge of a moist and unstable airmass right into
northern IL during the afternoon. Accordingly, numerous showers
and thunderstorms are expected to develop into the area Monday
afternoon and evening.

While poor deep layer wind shear should curtail the threat for a
more organized severe weather threat, some instances of locally
gusty winds may occur with the strongest storms during the
afternoon and evening. Of most concern with these showers and
storms is the threat for periods of heavy rainfall into Monday
evening along and north of I-80. Seasonably high PWATS of 1.4"+
combined with the propensity for individual cells to train in
northeastward oriented corridors across northern IL may result in
some localized rainfall amounts in excess of 2-3", and hence could
lead to hydro concerns across parts of the area.

The main threat of thunder will sag south of I-80 late Monday
night into Tuesday, though rain showers will linger areawide into
Tuesday. The rain threat will then come to an end from
northwest to southeast Tuesday afternoon and evening as the main
storm system exits to our east. Aside from the rain, breezy and
cool northeasterly low-level winds on Tuesday will result in a
seasonably cool day for the area as inland temperatures hold in
the low to mid 60s, and remain in the 50s near the lake.


Tuesday Night through Sunday:

A surface high and upper-level ridge will move over the region
following the eastward departure of this system Tuesday night
into Wednesday. This will support a period of quieter and warmer
weather for the mid-week period (Wednesday into at least the first
half of Thursday). Accordingly, inland temperatures both days
look to rebound back into the 70s, though onshore flow will keep
conditions several degrees cooler near the lake. Thereafter,
there are signs in the ensemble guidance that the weather pattern
could turn a bit more active again, potentially supporting some
additional periods of showers and thunderstorms, particularly
Thursday night into Friday.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1228 PM CDT Sun May 12 2024

Key Aviation Messages:

- Small chance of a SHRA or TS near RFD late today-this evening

- Widespread SHRA and scattered TS likely Monday afternoon

- Wind shift to northeast likely Monday PM with uncertain timing

Primarily quiet conditions are expected through Monday morning.
While we still can`t rule out a gusty SHRA or TS near RFD late
this afternoon through this evening, the chance is too low for
continued inclusion in the TAF. Breezy southwest winds gusting
in the 20-25 kt range are in store until near sunset, followed
by steady around 10 kt southwest to SSW winds tonight and Monday
morning. There may be sporadic gusts in the late evening and
overnight as a low level jet moves overhead, especially at ORD
and MDW.

An active period is then likely Monday afternoon and evening
from waves of showers and thunderstorms associated with an upper
level disturbance and a cold front shifting south. Forecast
soundings suggest a favorable setup for on station TS at times,
though there`s some timing uncertainty. In addition, the
anticipated wind shift to northeast behind the cold front could
occur as early as 19z and as late as around 00z Monday evening.
With the above in mind, indicated -SHRA VCTS in the ORD and MDW
TAFs and included a PROB30 for IFR TS impacts and the northeast
wind shift. An earlier wind shift could also support the
development of lower CIGs streaming in off the lake.

Castro

&&

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

&&

$$

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