892
FXUS63 KLOT 281739
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1239 PM CDT Sun Apr 28 2024

.KEY MESSAGES...

- Waves of showers and thunderstorms through Monday. Localized
  flooding remains possible early this morning, and there is a
  low (~15%) chance for an isolated severe thunderstorm or two
  to occur later this afternoon or evening.

- After a break on Tuesday, a stormy pattern returns midweek
  along with continued unseasonably warm temperatures.

&&

.UPDATE...
Issued at 1036 AM CDT Sun Apr 28 2024

An area of showers continues to lift across northern IL into
southern WI this morning along a warm front which will provide
a brief reprieve for most of the area. However, additional
showers and thunderstorms are expected to redevelop across
eastern IA and western IL this afternoon and evening as the
upper low pivots into the upper Midwest.

There continues to be some uncertainty as to how much
destabilization will occur before storms develop this afternoon
due to the expectation for mid-level stratus to persist
overhead which will play a role in storm coverage and intensity.
Though, recent satellite trends do show pockets of clearing
occurring across eastern MO and adjacent areas in IL which could
drift into our area and provide some sun to help destabilize
the environment. If sufficient instability can materialize as
guidance suggests, the presence of 30 to 40 kts of deep shear
should allow for some storms to organize and contain a threat
for severe weather primarily in the form of gusty winds and
hail. Additionally, hi-res forecast soundings do show some
modest low-level shear near the aforementioned warm front which
could support a low-end tornado threat across northern IL this
afternoon into early evening. Given that instability is expected
to wane quickly after sunset I suspect the main window for any
severe weather will be confined to the 3 to 8 PM timeframe and
largely be focused near I-39.

While the severe threat will conclude this evening, showers and
possibly a storm or two are expected to continue through tonight
into Monday as a cold front advances through the area.
Otherwise, seasonably warm and breezy conditions can be
expected with gusts of 30 to 35 mph this afternoon.
Temperatures will top out in the mid to upper 70s this afternoon
for most, but areas along the Lake County, IL lakeshore will
remain cooler due to onshore winds.

Yack

&&

.DISCUSSION...
Issued at 356 AM CDT Sun Apr 28 2024

Through Monday:

It`s been a rainy and stormy early morning for much of the area as a
low-level jet has been funneling a persistent stream of moisture our
way. MRMS and radar estimates indicate that, through 345 AM, between
about 1 and 2.5 inches of rain has fallen across a corridor from
the southwest Chicago suburbs towards Peoria, with lower totals
elsewhere. With the low-level jet still going strong, rainfall is
expected to continue over the next few hours and may ultimately
culminate in some minor flooding of fields, ditches, and areas
of poor drainage, mainly in the aforementioned corridor. Fortunately
though, radar trends suggest that the instability reservoir in the
region has gradually become depleted as a result of the widespread
convection, and rainfall rates should continue to taper off as we
approach daybreak.

The shower/storm activity is expected to diminish over the course of
this morning as the low-level jet weakens, paving the way for much
of our forecast area to see plenty of dry time during the daytime
hours today. However, with isentropic upglide continuing along the
eastern periphery of a low pressure system situated beneath the
upper-level trough over the Great Plains, additional rain showers
will be funneled northward along the Mississippi River Valley
throughout the day. Some of these showers will likely trickle into
western portions of our forecast area this afternoon, though
overall coverage is expected to be no more than scattered. The
northward transport of warmth and moisture will also likely
support MLCAPE recovering to around 500-1000 J/kg this afternoon
in the zone of shower activity, so it`s very much possible that a
few storms develop within this corridor as well. Shear profiles
actually look fairly decent given the overhead placement of both
the low- and upper-level jets, so if some deeper convection can
get going, it could take on supercellular characteristics and
become capable of producing damaging winds and/or hail. A brief
tornado also couldn`t be ruled out with any organized storm that
crosses the warm front near the Illinois/Wisconsin state line.
Confidence in this outcome playing out is relatively low, and even
if it came to fruition, the severe weather threat would remain
isolated, so the Storm Prediction Center`s Day 1 Marginal Risk
covers this possibility well.

This afternoon, several storms are also expected to develop along a
frontal zone to our west/southwest in Iowa and Missouri and will
probably consolidate into a more linear multicell cluster that will
propagate towards our forecast area this evening. Similar to
yesterday evening, these storms are likely to lose steam as they
encounter increasing quantities of convective inhibition with
time and eastward extent. Consequently, the likelihood of severe
weather occurring tonight as these decaying storms roll through is
quite low, but if they managed to maintain their intensity, then
some strong to marginally severe wind gusts couldn`t be ruled out.

Tonight`s band of showers/storms will likely depart our forecast
area early Monday morning as a mid-level dry slot barges into the
region. With low-level cold air advection yielding steepened
low-level lapse rates and the continued presence of the nearby
upper-level trough providing forcing support, isolated to
scattered low-topped convective showers may develop during the
daytime on Monday. However, the quality and depth of moisture
trapped beneath a low-level temperature inversion doesn`t look
great overall and could potentially prevent these showers from
developing altogether or, at the very least, limit their overall
coverage. Have slashed PoPs a good bit for Monday as a result,
keeping the highest probabilities focused in our southeastern
CWA, where the influence of the dry slot won`t be as great as
farther northwest and higher inversion heights will provide a
better chance for clouds to grow deep enough for showers to
redevelop. A cold front is then expected to pass through the
area during the latter half of the afternoon into the evening,
effectively shutting off shower chances behind it.

Ogorek


Monday Night through Saturday:

Heading into Monday night most of the area will be dry, though
a slower eastward push of the surface cold front would result in
showers and storms potentially lingering into early Monday
evening across portions of northwest Indiana. Modest upper-
level height rises are expected in the wake of the
aforementioned front which will promote a period of dry weather
through the daytime hours on Tuesday.

The reprieve looks to be short lived, however, as a series of
shortwave troughs are forecast to traverse across the north-
central CONUS. This will result in periods of showers and
thunderstorms beginning Tuesday night and persisting through at
least the end of the week. The details on timing and coverage of
showers and storms will continue to be refined with later
updates. Temperatures will remain unseasonably warm with
afternoon highs in the mid to upper 70s through Thursday then
turning a bit cooler and less humid heading into next weekend.

Yack/Petr

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1239 PM CDT Sun Apr 28 2024

Key Aviation Impacts for the current TAF period:

- Scattered showers northwest of Chicago with the potential for
  some thunderstorms around KRFD this afternoon

- Widespread showers and thunderstorms expected this evening
  and through Monday morning

After a brief window of drier, VFR conditions, the next round
of showers moves into the area this afternoon ahead of a front
tonight. The risk for thunder was low enough around Chicago
terminals to leave it out of the TAF currently; however, a TEMPO
for -TSRA was placed into the KRFD TAF given the higher
instability aloft expected to the west. Winds are slowly
becoming out of the southwest as the warm front drifts
northward. As cloud cover erodes, stronger gusts around 25 knots
are expected to return to area terminals, though the occasion
gust up to 30 knots is not out of the question.

After 00Z, a cold front will start its movement eastward over
Illinois through the overnight hours. This will bring a renewed
threat of showers and thunderstorms at terminals that will
provide another opportunity for MVFR/IFR conditions from lower
cigs and vis. Current expectation is for the cold front, and
associated heaviest rain, to be over the lake after 12Z Monday
morning. However, post frontal showers are expected to linger
through the morning hours. While there is lower confidence on
the exact timing for when rain cuts off, drier conditions are
expected Monday afternoon as southwest winds gust around 20
knots and gradually become slightly more west to southwest by
the late afternoon.

DK

&&

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

&&

$$

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