883
FXUS63 KLOT 121112
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
612 AM 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 /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 612 AM CDT Sun May 12 2024

Light southwesterly winds at press time will increase in magnitude
throughout the morning and regularly gust above 25kt this afternoon.
An addition, scattered cumulus clouds based near 8kft will develop
by mid-afternoon.

A shower or thunderstorm may develop this afternoon within an
modest but uncapped moisture axis near the Wisconsin state line.
For now, felt maintaining a PROB30 group at RFD was prudent,
especially since any thunderstorm that develops would be prone
to producing gusty westerly winds (up to 30kt). However,
confidence may not be high enough to justify converting to a
TEMPO group in later TAF packages. Meanwhile, at
DPA/ORD/MDW/GYY, the chance of a thunderstorm remains below 30%,
and accordingly, no formal mention will be made in the outgoing
TAF package.

Tonight, surface southwesterly winds will slacken in magnitude
to aob 10kt. At the same time, a 35 to 40kt southwesterly low-
level jet is expected to develop over the terminals between
03-09Z. Some consideration was made to introduce LLWS for the
overnight period, though felt there is not enough of a signal
in criteria being met more than on an isolated basis at least at
this time.

Borchardt

&&

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

&&

$$

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