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000
FXUS63 KLOT 252332
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
632 PM CDT Thu Apr 25 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms will move over the area
  Friday afternoon and evening. A few may linger through the
  overnight hours into Saturday.

- Saturday and Sunday look warm and breezy with unseasonably
  warm highs in the lower 80s and overnight lows in the low to
  mid 60s.

- Waves of showers and thunderstorms appear poised to parade
  near and west of the Mississippi River Saturday and Sunday.
  Much of our area may end up on the outer edge of all activity
  until Sunday night.

- Lowering humidity levels will be realized Monday and Tuesday
  before the next system approaches toward the middle of next
  week.

&&

.DISCUSSION...
Issued at 216 PM CDT Thu Apr 25 2024

Through Friday Night:

Seasonable temperatures are expected tonight, with cloudiness
increasing in advance of approaching lead shortwave ejecting out
of the developing long wave trough over the western U.S. The first
wave of warm air advection-driven precipitation should begin
encroaching on the area Friday morning. Strengthening low level
easterly flow should also serve to advect in some drier air,
which should chip away at the leading edge of the precipitation
Friday. As the rain begins to run into drier air, its forward
progress should slow as it moves across the CWA. Rain should
begin during the morning toward the I-39 corridor, with rain
chances likely spreading east into the Chicago metro area early-
mid afternoon, then into northwest IN by mid afternoon.

While an isolated thunderstorm cannot be ruled out west toward the
I-39 corridor Friday afternoon, the better chances of some
scattered thunderstorms will hold off until Friday night as better
(elevated) instability surges into the CWA. Scattered showers and
thunderstorms are expected areawide Friday night in advance of
the northward surging warm front. The warm front is expected to
lift north and east of the area by late evening with decreasing
chances/coverage of showers and thunderstorms overnight.

The low temperatures Friday night will occur during the evening
hours. Strengthening southerly winds will usher in an
unseasonably warm and humid (by April standards) air mass. By
sunrise Saturday morning, temperatures should rise to the 60s
with dewpoints not far behind, readings far more typical of June
than April!

- Izzi

Saturday through Thursday:

Friday night into Saturday, a secondary upper-level trough will dig
into the southern United States, causing upper-level heights to rise
across the Great Lakes in the wake of a departing, and decaying,
lead wave. As a result, increasing subsidence should cause any
leftover showers and thunderstorms to decay after daybreak Saturday
morning. Even with a cloudy start, temperatures on Saturday are
poised to jump into the upper 70s to lower 80s as an axis of
850mb temperatures of +12 to +14C builds into the region.
However, if clouds thin quickly, temperatures may even jump into
the mid 80s. Meanwhile, aggregate troughing across the central
United States will keep the local low-level pressure gradient
tight, allowing for southwesterly winds to become gusty from
mid-morning onward. The strength of the winds will depend on how
quickly clouds erode, with more aggressive mixing supporting
gusts as high as 45 mph.

Even with a warm and moist airmass in place on Saturday, a lack
of robust forcing mechanisms and the presence of a capping
inversion support a dry forecast. However, if a thunderstorm
were to develop, marginal shear profiles (along the southeastern
flank of the jet stream positioned well to our west) may
support pulses to severe intensity. For now, opted to cap PoPs
at 20% areawide Saturday afternoon.

Saturday night, the secondary upper-level trough will shift
northeastward into the Plains and induce the development of a broad
low-level jet across the Mississippi River Valley. With the terminus
of the low-level jet focused near the Mississippi River, it appears
that a large chunk of our area should remain dry through daybreak
Sunday. However, parts of northwestern Illinois (along and west of a
line from Mendota to McHenry) may get grazed by waves of showers and
thunderstorms throughout the night and especially toward daybreak.
Saturday night will be unusually mild with lows in the low to
mid 60s. In fact, the record warm low temperatures may be
threatened for the April 27 calendar date at Rockford (61F,
1990) and Chicago (67F, 1915). In addition, the moist boundary
layer may prevent full decoupling and allow for southwesterly
winds to continue gusting near or above 30 mph.

Sunday remains a day with low forecast confidence owing to an
apparent dependence on the evolution of convection the night before,
and a continued modest slowing trend of the arrival of the secondary
upper-level trough. At this point, it appears prospects are
increasing for remnant convection Saturday night to decay as it
moves across northern Illinois after daybreak on Sunday, setting the
stage for another warm and breezy day. With the later arrival of the
secondary upper-level trough, the next round of showers and
thunderstorms during the afternoon and evening to be focused
near/west of the Mississippi River. For now, felt a middle of the
road approach was fair with highs near 80 degrees, breezy
southwesterly winds gusting near 40 mph, and low-end chances
for thunderstorms (40 to 60% chances, highest west of I-39)
throughout the day. Whatever develops to our west in the evening
may then move through overnight in a decaying form into Monday
with the supporting instability axis and cold front, though
again, confidence is low.

Taken altogether, our area may end up just on the outside of the
episodes of severe weather through the weekend (not that we`re
complaining).

Assuming the cold front does sneak through without much noise Sunday
night through early Monday, early next week looks decidedly
quiet. The cold front itself doesn`t look particularly strong,
so temperatures Monday and Tuesday should still be above average
and in the low to mid 70s. Humidity levels will also be
noticeably lower. Ensemble model guidance advertises the arrival
of a trough in the general region in the Wednesday to Friday
timeframe, which will present the next opportunity for showers
and thunderstorms.

Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 631 PM CDT Thu Apr 25 2024

Key Aviation Messages:
- SE gusts to around 25 knots Friday afternoon becoming SSE
  over 25 knots by late Friday evening.
- Low chance (<20%) of TS late Friday evening.

VFR conditions are expected through at least Friday afternoon.
E winds up to 10 knots this evening will veer SE with gusts to
20 knots Friday morning followed by gusts to 25 knots in the
afternoon and early evening.

An area of decaying -SHRA is expected to spread across the
terminals Friday afternoon. TS is not expected with during this
time. Scattered -SHRA with some MVFR ceilings should continue
through Friday evening, but higher coverage will favor the late
evening period in response to increasing warm air advection with
a 50-60 knot LLJ. TS is possible with these -SHRA, but
confidence is currently too low to include in the TAF this far
out. Meanwhile, winds will begin veering SSE/S by the end of the
period, with some gusts possibly nearing 30 knots.

Kluber

&&

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

&&

$$

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