Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261148
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
648 AM CDT Tue Mar 26 2024

.KEY MESSAGES...

- Potential for a few strong-severe storms later this morning
  through midday. Locally greatest potential across NW Indiana
  and far NE Illinois. While a lower threat, a brief tornado
  cannot be ruled out.

- Non-thunderstorm winds gusting at times to 40 mph today.

- Return to seasonable temperatures by the end of the week.

- Another period of active weather likely late week into a least
  early next week.

&&

.DISCUSSION...
Issued at 258 AM CDT Tue Mar 26 2024

Through Wednesday:

The primary focus today continues to be on the potential for a
fast-moving line of low-topped convection later this morning, as
well as synoptic/gradient wind gusts into the afternoon.

The main surface feature of interest is a cold front which is
currently pressing across eastern Missouri. Unfortunately, there`s
nothing really of note to track with this boundary at this time,
with guidance indicating initial convective cells developing later
this morning as the front sweeps across central Illinois. There`s
been a trend towards perhaps just a bit less in the way of overall
instability and depth compared to the last few nights. That said,
guidance is in generally good agreement, indicating low-level
lapse rates increasing towards 7-8 C/km through the mid to late
morning hours immediately ahead of the cold front, with
commensurate increases in 0-3 km MLCAPE.

Very low-topped convective elements will likely develop in this
regime (tops may remain at or below 20 kft), pushing through our
forecast area between about 10 am and 1 pm (earliest SW, latest
NE). While the main core of strongest 500 mb winds will be
shifting east of the region ahead of the cold FROPA, 70-90 knots
of flow is still progged to be present overhead as lower-level
flow (850-700 mb) actually increases a bit. With such a strong
low-level wind field in place during this time, the potential for
a few strong-damaging wind gusts remains with any deeper/more
robust cores. While a lower potential, a brief tornado can`t be
totally discounted in such an environment, although we`ll note the
hodographs and low-level kinematic profiles don`t look quite as
concerning as they did the other day with generally more veered
surface flow. The primary strong/severe threat looks like it
should focus across far NE Illinois and NW Indiana, with the
threat increasing further through the day as activity exits our
area to the east.

As far as non-thunderstorm winds go: after a lull in gustiness
overnight, winds have started to tick back upwards as low-level
steepened a bit in the wake of the earlier stratiform rainfall.
Intermittent gusts to around 40 mph or so will continue through
the morning. Southwesterly gusts are then expected to ramp up this
afternoon with the passage of a cold front. Guidance suggests
that stratus will likely be fairly expansive in the post- frontal
airmass which may limit the overall depth of mixing into the
strongest winds aloft. As a result, it looks like gusts should end
up under Wind Advisory criteria this afternoon, occasionally
gusting to 40 mph through about sunset.

This evening, with steep low-level lapse rates and a lingering,
cooling stratus deck, maintained the mention of flurries.
Eventually, a subsidence inversion will build down below 850 mb,
reducing cloud tops sufficiently to end any precipitation chances.
Low temperatures tonight will hinge on whether we clear out, and
latest indications are that cloud cover may remain somewhat
persistent. With this in mind, nudged overnight temperatures up
just a bit. Clouds should gradually erode through the morning and
afternoon on Wednesday with just some lingering westerly breezes
and highs in the 40s.

Carlaw


Wednesday Night through Monday:

Mid-level ridging is expected to build into the area on
Thursday which will begin to advect warmer air into the area. As
a result temperatures are expected to warm toward more
seasonable readings in the 50s and 60s by Friday and remain as
such into at least the early part of next week. However, periods
of onshore winds do look to be possible Friday onward which may
keep temperatures cooler along the Illinois and/or Indiana
lakeshore.

In addition to the building warmth, there continues to be a
strong signal that a more active pattern will once again develop
late week. The first period of inclement weather is forecast to
get underway during the late Friday into Saturday timeframe as
a baroclinic zone stalls across portions of the mid-Mississippi
Valley and southern Great Lakes. Guidance continues to show a
shortwave disturbance passing over the boundary which looks to
develop showers and perhaps even a few thunderstorms in our
area. While there does appear to be good agreement on this
evolution, uncertainty remains in regards to the strength of the
shortwave, timing, and low-level moisture available. Therefore,
the coverage and intensity of showers and especially
thunderstorms remains in question.

As this initial disturbance exits, the aforementioned boundary
is forecast to get pushed south, likely due to storm outflow,
which may keep the bulk of any additional showers/storms on
Sunday south of our area. However, if the coverage of storms is
less than guidance depicts then this boundary could remain in
our forecast area and yield greater rain chances on Sunday. So
for now have maintained the 20 to 30 percent POPs offered by the
NBM.

Heading into the early part of next week a broad upper trough
is forecast to develop over the western CONUS and gradually
propagate east on Monday into Tuesday. However, each guidance
member has a different depiction as to how the larger trough
will interact with other waves within the pattern and therefore
confidence is low on which solution will become reality. Though,
it does appear that periods of precipitation will be in the
forecast through at least the early part of next week if not
longer.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 648 AM CDT Tue Mar 26 2024

Forecast concerns for the terminals...

- Brief period of thunderstorms later this morning into early
  afternoon

- Gusty southwest winds through this afternoon with gusts of 30
  to 35 kts expected

- MVFR ceilings expected to persist through Wednesday morning

The earlier rain has largely come to an end this morning except
for a few remnant showers currently moving through RFD. While
dry conditions will prevail for the next couple of hours, a thin
line of shallow convection (thunderstorms) is starting to
develop across central IL along a cold front and is expected to
move into the terminals after 15z. Once storms reach the
terminals they should move through in about an hour, but have
decided to keep a two hour window in the TEMPO groups to account
for any changes in speed as storms develop. Regardless, storms
will fully clear the area between 18z and 19z this afternoon
leaving only intermittent showers for the rest of the day.

Aside from the showers/storms, gusty southwest winds are
expected to persist through this afternoon with gusts topping
out in the 30 to 35 kt range. Winds will gradually diminish this
evening, but occasional gusts in the 20 to 25 kt range are
forecast to linger into Wednesday. Additionally, directions will
become more westerly this evening in the wake aforementioned
cold front.

Finally, ceilings are expected to remain MVFR through the
majority of the TAF period. However, IFR ceilings have developed
this morning and will briefly move over the terminals through
13z. While precipitation is expected to end this evening, the
combination of cooling temperatures aloft and the lingering
MVFR ceilings may allow periods of flurries to develop late
tonight into early Wednesday morning. Given the spotty nature of
the flurries, confidence is too low for a formal mention at
this time. Regardless, no accumulation or significant impacts
are expected with any flurries that do occur.

Yack

&&

.MARINE...
Issued at 258 AM CDT Tue Mar 26 2024

A few gale force gusts of 35 kt may occur through daybreak,
primarily on elevated platforms. Thereafter, a cold front will
sweep across the Lower Great Lakes and cause a period of
southwesterly gale force winds of 35 to 40 kt through this
evening. A Gale Warning remains in effect for the Illinois and
Indiana nearshore waters later this morning through 7 PM this
evening. A follow-up Small Craft Advisory will likely be needed
for a period this evening and overnight.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 AM CDT this morning for the IL
     and IN nearshore waters.

     Gale Warning from 10 AM this morning to 7 PM CDT this evening
     for the IL and IN nearshore waters.

&&

$$

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