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000
FXUS63 KLOT 151731
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1231 PM CDT Mon Apr 15 2024

.KEY MESSAGES...

- A couple waves of showers and thunderstorms expected Tuesday
  through Wednesday morning. Some may be severe and produce
  locally heavy rainfall late Tuesday afternoon through early
  Wednesday.

- Windy late Tuesday through Wednesday with peak gusts 40+ mph,
  highest on Wednesday.

&&

.DISCUSSION...
Issued at 348 AM CDT Mon Apr 15 2024

Through Sunday:

A surface ridge of high pressure shifting across the western Great
Lakes will result in another sunny and warm day. Inland
temperatures this afternoon will top out in the 70s, though
persistent onshore easterly winds through the day will foster
cooler conditions near Lake Michigan.

While quiet weather will persist locally today, our main focus
during the period continues to revolve around the potent spring
storm system expected to shift across the Midwest into the western
Great Lakes late tonight through Wednesday. This developing storm
system will strengthen today across the central High Plains as
strongly diffluent upper-level flow traverses the Rockies. This
storm system will be responsible for spawning severe thunderstorms
late this afternoon into tonight well west of our area across the
Plains. However, the severe weather threat will shift eastward
towards our area late Tuesday into Wednesday as the whole storm
system tracks eastward across the Corn Belt.

An arc or two of elevated scattered showers and thunderstorms
is likely to develop northeastward into the area Tuesday morning
as warm air advection begins to ramp-up over a surface warm
frontal boundary across central IL and IN. This initial round of
morning storms is expected to be elevated and largely non- severe.
However, Tuesday afternoon, warm sector convection is expected to
fire west of the area across parts of IA and MO in closer
proximately to the strong dynamics associated with the approaching
storm system. It will be this activity we will have to monitor as
it shifts east-northeastward into our area late Tuesday afternoon
and evening. The presence of strong low and deep layer shear will
certainly support organized severe convection, with an all
hazards severe threat (including tornadoes), most notably along
and west of the I-39 corridor late Tuesday afternoon into the
early evening. The severe threat may ultimately end up continuing
with the eastward shifting convection well into the evening, even
for areas east of I-39. However, it remains a bit unclear at this
time with the eastward extent of this severe threat, as the
storms may tend to diurnally weaken with time. Nevertheless, the
SPC has the all of the northern IL in a slight risk for severe
storms.

As the upper low merges into the upper trough it takes on more
of a negative tilt early Wednesday morning with the region still
situated under the left exit of the southern stream upper jet.
While there are some questions as to how much instability there
will be to work with into early Wednesday morning and the timing
of the various waves as showers and storms wrap around the
surface low. With the slightly slower timing, forecast soundings
would support surface based convection persisting (or redeveloping)
Wednesday morning into early afternoon, especially east into
northwest Indiana. Will hopefully get a better idea of how late
the severe threat persists as high-res guidance becomes
available. Winds turn westerly on Wednesday with the strong
winds persisting and likely peaking during the afternoon on
Wednesday with gusts up to 40+ mph possible.

Beyond Wednesday can`t rule out some rain on the north side of
a weakening surface late Thursday into Friday then before
surface high pressure builds southeast into the Upper Midwest
bringing a cooler airmass for the latter half of the week with
high temperatures in the 50s to lower 60s and lows in the 30s
and lower 40s.

KJB/Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1230 PM CDT Mon Apr 15 2024

Key Aviation Impacts for the current TAF period:

- Easterly wind gusts increase to around 25 knots after 15Z
  Tuesday

- A strong storm system approaches the Chicago Metro Area
  Tuesday night

There are no short term impacts to area terminals. Conditions
through 12Z tomorrow will be VFR with easterly winds around 10 knots
with occasional gusts up to 15 knots.

A strong low pressure system will move northeastward from the Great
Plains toward southern Wisconsin tomorrow. There is a chance for
some elevated showers to approach the area tomorrow morning.
Instability is not overly robust and forcing would have to be strong
enough to tap into it, but there is enough to keep a PROB30 group
for the potential for thunder to develop. Winds will remain out of
the east and increase with gusts to 25 knots (isolated up to 30
knots) by Tuesday afternoon.

The main wave of convection is expected after 21Z. Showers are
expected to move over the area from the southwest to northeast with
the potential for thunderstorms to develop with them. While timing
may be adjusted in later forecast packages, there was high enough
confidence to add it to the TAF as a TEMPO for ORD and MDW. Vis and
cigs may reduced down to MVFR levels during the strongest cells as
they pass over. Lastly, towards 00Z, a south southeasterly low
level jet will begin to ramp up at 925 mb. If winds continue to
mix down through the night, that will help inhibit any LLWS
threat, but a couple models have winds decoupling at the
surface. For now, it was left out of the 30 hour TAF due to
lower confidence at the end of the forecast period, but will be
monitored.

DK

&&

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

&&

$$

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