265
FXUS63 KLOT 121728
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1228 PM CDT Sun May 12 2024

.KEY MESSAGES...

- Warm and breezy conditions today.

- Shower and storm chances return late tonight through Tuesday,
  with a threat for heavy rainfall Monday into Tuesday.

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

&&

.DISCUSSION...
Issued at 314 AM CDT Sun May 12 2024

Through Tuesday:

Mid-level ridging will continue to build into the area today
which will maintain mostly sunny skies and mostly dry
conditions. Winds will also become southwesterly this morning
which will advect in warmer air allowing highs to top out in the
upper 70s and lower 80s areawide this afternoon. The mostly
clear skies will also lead to strong diurnal mixing this
afternoon promoting breezy winds gusting generally in the 20 to
25 mph range, but localized gusts to 30 mph are possible.

Heading into tonight, the closed upper low spinning over the
Four Corners will begin to eject into the central plains as it
broadens into a shortwave trough. At the same time, a weaker
shortwave is expected to pivot into the upper Midwest from the
Canadian Prairie Provinces. While these waves are not forecasted
to phase with one another, their close proximity will generate a
broad area of confluence along the mid-Mississippi Valley which
may allow for the development of isolated showers and storms
late this afternoon into tonight. However, guidance continues to
vary on the degree of moisture return this afternoon which may
further limit the already spotty shower/storm coverage. Given
this uncertainty, I have decided to keep a slight chance (around
20%) for showers and storms for areas along and north of I-88
through tonight but I anticipate that most of will remain dry.

Better rain chances are expected for Monday through Tuesday as
the stronger southern shortwave treks into the Ohio River Valley
and tries to develop a weak surface low over central IL.
Guidance is in much better agreement that sufficient moisture
will advect into the area Monday morning which will generate a
broad area of showers and perhaps even some embedded
thunderstorms as the trough steepens lapse rates aloft Monday
afternoon. While the better jet core associated with the trough
will be focused into the deep south, the combination of broad
forcing, modest instability, and deep moisture (PWATs around 1.4
inches) will be sufficient to produce some instances of heavier
rainfall especially with any thunderstorms. Therefore, will
need to keep an eye on where the highest totals end up as
localized river and stream rises may occur given the rainfall
totals of 0.5 to 1.5 inches currently forecast.

While the rain and storm chances will continue through Tuesday,
the better coverage is expected to shift south of I-80 Tuesday
afternoon as the trough and weak surface low moves further into
the Ohio River Valley. That said, winds will become
northeasterly and increase in speed on Tuesday which will
greatly reduce temperatures as cooler air advects off of Lake
Michigan. Therefore, expect high temperatures to be notably
cooler on Tuesday than Monday with readings in the low to
mid-60s for most with 50s likely near the lake.

Yack

Tuesday night through Saturday:

Wednesday and at least the first half of Thursday, a surface ridge
will move through the Great Lakes allowing for a rebound in
temperatures into the low to mid 70s. With an onshore component of
the wind both days, temperatures along the Lake Michgian shoreline
will be stuck in the upper 50s to lower 60s.

Toward the end of the week, ensemble model guidance advertises a
return for chances for showers and thunderstorms across the middle
Mississippi River Valley as several upper-level shortwaves of
varying origins attempt to phase overhead. With each respective
shortwave pretty far away and prone to little wobbles in intensity
and track before they arrive (one is traversing through eastern
Russian, another is meandering through the Straights of Alaska, and
the third is meandering offshore of California), our forecast for
the end of next week may carry sizable adjustments going forward.
However, enough of an ensemble signal for precipitation remains to
continue mid-range chance (30-50%) PoPs to close the workweek.
Ensemble model guidance favors near average temperatures toward the
end of the week, as well (highs in the low to mid 70s).

Looking beyond this week, there is no signal that favors well above
or below normal temperatures, or well above or below normal
precipitation, through the remainder of May. In other words, May
2024 appears on track to wrap up in typical fashion.

Borchardt

&&

.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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