Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 132035
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
335 PM CDT Wed Mar 13 2024

.KEY MESSAGES...

- Numerous showers and storms expected after midnight through
  Thursday morning. Confidence high with these.
    - A small hail and a wind threat also exists with some of the
      stronger storms tonight into Thursday morning, especially
      south of I-80.

    - Heavy rainfall will accompany the storms, and some risk of
      localized flooding exists into Thursday morning, especially
      along and south of I-80 where multiple rounds of storms may
      move over the same areas.

- Redevelopment of widely scattered storms possible Thursday
  afternoon and evening, mainly south of I-80. Confidence with
  this period of storms is low.

- Colder with below average temperatures later this weekend into
  early next week. A couple of periods of scattered snow showers
  possible.

&&

.DISCUSSION...
Issued at 335 PM CDT Wed Mar 13 2024

Through Thursday night:

A surface frontal boundary has sagged southward through the
morning and now resides near the I-80 corridor. Showers are
expected to continue developing north of this boundary late this
afternoon and evening as warm air advecton (isentropic upglide)
continues to overtop this boundary. However, more substantial
showers and thunderstorms will develop tonight, especially after
midnight south of I-80 in response to increasing theta-e advection
along the nose of a strengthening southwesterly low-level jet.
This activity will gradually spread northward into the Chicago
and Rockford metro areas in more of a scattered fashion after 3
AM. While some isolated hail threat exists overnight with some of
the stronger cores due to the presence of steep mid-level lapse
rates, we will also have to keep a close eye on hydro concerns,
especially south of I-80 due to the likelihood of multiple rounds
of convection overnight through Thursday morning.

In addition to the scattered storms developing overhead tonight,
we will be keeping an eye on the convection developing across
eastern KS and northwestern MO this afternoon. This activity is
expected to build upscale into an MCS this evening, then track
east-northeast near and along the west-east oriented
quasistationary surface boundary late tonight into Thursday
morning. The track of this MCS is favored to track across at least
the southern half of my area early Thursday morning, and may
result in a strong gusty wind threat south of the surface front
(areas across my far southern counties in IL) around daybreak
Thursday. Flooding concerns could develop south of I-80 into
Thursday morning as this MCS looks to move over the same areas
that experience showers and storms overnight. As a result, a
corridor of excessive rainfall amounts of 2 to 3"+ could occur
south of I-80 through Thursday morning. Confidence on the exact
placement of the heaviest rainfall remain low at this time, but we
will highlight this threat with an ESF.

This early morning MCS is likely to shift the effective surface
front farther south into parts of central IL and IN later in the
morning. This adds questions as to the extent of our severe
weather threat later in the day as it remains unclear as to how
well the area will recover following the morning activity.
Accordingly, confidence on any strong to severe thunderstorm
redevelopment over our area remains low at this time. If any
afternoon storms do develop, they would be most favored south of
I-80 along southward shifting secondary frontal boundary.

Cooler weather will filter in over the area in the wake of this
secondary frontal boundary into Thursday night. A period of very
low clouds and fog will also be possible across northern IL in the
vicinity and north of the surface frontal boundary Thursday
afternoon and evening.

KJB

Friday through Wednesday:

Other than the limited chance for light showers exiting the area as
the front moves east, Friday will begin with stronger northerly post
frontal winds over the area. As the main front tracks east of the
area, winds will gradually diminish into the afternoon remaining
out of the north. Cooler air will move southward with mostly low
to mid 50s over northeastern Illinois while temperatures in
northwest Indiana just south of Lake Michigan will likely remain
in the mid to upper 40s.

Between Saturday and Monday, a series of short wave troughs will
move down from Manitoba over the southwestern Great Lakes. The
first moves down over Saturday afternoon with a cold front moving
over the area. Winds out of ahead of the front will be out of the
southwest with the potential for gusts over 30 mph during the
afternoon. Warmer air will briefly advect into the region helping
bump temperatures up back into the upper 50s. Winds will diminish
and pivot to the northwest behind the front. Most profiles have
fairly dry conditions with the front, but there is a slim chance
for some spotty showers to kick up, particularly in northwest
Indiana downwind of the lake, with the frontal passage. Colder air
moves in to the area Sunday morning.

Late Saturday, the next shortwave starts to move south broadening
into a longer wave that will undergo some phasing through Monday
morning. As it moves down, a latitudinally oriented 300 mb jet
will strengthen. The area of enhanced lift associated with the jet
will pass over on Sunday. There are also some indications of weak
f-gen circulations which may add a a bit of extra forcing. With
850 mb temperatures dropping down to -8 to -15 deg Celsius and wet
bulb temperatures expected to remain below freezing, there is a
chance for precipitation changing over from rain to snow showers
Sunday evening into Monday morning.

Temperatures overnight Sunday into Monday are expected to be in the
20s, with the potential for wind chills in the teens as winds out
of northwest will remain 10 to 20 mph. As the long wave trough
slowly starts to move east, there is a chance for decreasing
clouds cover, but temperatures on Monday remain in the 30s. Some
models are suggesting another weak wave may pass over for another
chance for a few flakes Monday night into Tuesday, but confidence
remains low at this point. Beyond that potential wave, upper level
ridging over the northern places grows on Tuesday for a gradual
warming and drying trend at the end of the forecast period.

DK

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1253 PM CDT Wed Mar 13 2024

The primary aviation weather concerns through the 18Z TAF period are
as follows:

* Intermittent showers this evening through the day on Thursday

* Scattered thunderstorms expected late tonight into Thursday
  morning

* MVFR cigs late tonight expected to go IFR for Thursday

Isolated showers are anticipated to begin popping up around the area
by mid-afternoon. Coverage will build some through the evening with
widespread rain expected by the middle of the night which should
stick around through the better part of Thursday morning. Scattered
thunderstorms are forecast to develop among the rain, some of which
may feature some locally strong winds. The best potential for
thunder appears to be during the earlier half of the morning,
including during the predawn hours. Periods of MVFR vsbys are
anticipated while the rain persists. There is a signal for some
fog/mist to keep vsbys below 6SM into Thursday afternoon, although
confidence is higher in vsbys recovering once the widespread rain
moves out by mid-late morning. Light, isolated showers will continue
to float around for the rest of the TAF period.

VFR cigs will move overhead during the latter part of this
afternoon. They`ll gradually lower through the evening and into the
night as the bulk of the system approaches. MVFR cigs could develop
as early as the middle of the night and are expected by around 11 or
12Z. IFR cigs look to take hold shortly thereafter with a strong IFR
signal kicking in by around 13Z. Confidence is rather high in IFR
cigs hanging overhead through the day on Thursday. Meanwhile, winds
between 5 and 10 kt will be quasi-easterly for the better part of
the period, but direction could get pretty erratic, possibly going
around the compass, during the afternoon on Thursday.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 4 PM Thursday to 7 PM CDT Friday for
     the IL nearshore waters.

     Small Craft Advisory from 10 PM Thursday to 10 PM CDT Friday
     for the IN nearshore waters.

&&

$$

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