-- Discussion containing changed information from previous version are highlighted. --
867
FXUS63 KLOT 291735
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1235 PM CDT Mon Apr 29 2024

.KEY MESSAGES...

- Locally heavy rainfall may result in ponding in poor drainage
  areas through daybreak.

- There is a low chance (20%) for an isolated storm or two this
  afternoon, mainly east of I-55.

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

&&

.DISCUSSION...
Issued at 322 AM CDT Mon Apr 29 2024

Through Tuesday:

GOES water vapor imagery early this morning depicts a sprawling
low pressure system currently centered over southwest Minnesota
with showers extending southeast along the warm front into
southeast Wisconsin. Out ahead of the trailing cold front an
expansive area of showers and isolated embedded thunderstorms
continue in the warm sector extending from northern Illinois
down through southeast Missouri along the western and northern
edge of a 40-50kt low-level jet well ahead of the trailing cold
front. Current RAP mesoanalysis highlights that marginal mid-
level lapse rates have limited the amount of instability to work
with (MUCAPE 500 J/kg or less) though this combined with
effective shear of 35-45kt south of I-80 have allowed for a few
isolated elevated non-severe storms to develop early this
morning. If a more persistent updraft can become established
some small hail can`t be ruled out prior to daybreak east of
I-55 and south of I-80. The broader northeast motion of the
showers suggest rain will continue across much of the area
through daybreak and then gradually decrease in coverage from
west to east through mid-late morning.

Highest rainfall rates earlier this morning were confined to a
pencil thin line extending from western Livingston county,
through Joliet and toward O`Hare where ponding on roadways and
poor drainage areas may linger through daybreak due to how much
rain fell over the past 30 hours in the same areas. The
localized nature of the highest amounts will limit the potential
for broader flooding issues this morning.

A few spotty showers and maybe even an isolated storm or two
(20% chance) may develop later in the morning and into the
afternoon along the surface cold front as it moves through.
Strong to severe weather is not anticipated. Temperatures today
will warm into the upper 60s to around 70 with breezy southwest
winds. Heading into tonight surface high pressure settles over
the region ushering in a less humid day on Tuesday with clear
skies and warm temperatures in the 70s. An afternoon lake breeze
may limit warming and cool down temperatures near the
lakeshore.

Petr


Tuesday Night through Sunday:

Tuesday afternoon, an upper-level shortwave trough will trek into
the northern Plains/Upper Midwest and ignite convection in/near the
central Missouri River Valley. This convection is likely to
consolidate into a line or broken arc and track towards northern
Illinois Tuesday night, but will likely start to fall apart as it
approaches our forecast area as it loses some forcing support and
outruns the better instability to our west. Nevertheless, most
available guidance has this activity surviving long enough to
produce measurable rainfall and perhaps a few lightning strikes in
our CWA, so will continue to carry chance PoPs and slight chance
thunder probabilities that are focused primarily in our western
counties for the Tuesday night time frame. Assuming that it makes it
here, this activity should disperse no later than mid-morning on
Wednesday. Wednesday is otherwise shaping up to be a dry and mild
first day of May with highs currently forecasted to be in the mid-
upper 70s across most of the area. There are signs that a backdoor
cold front could make a push inland off of the lake late in the day,
but this may come too late to have an appreciable effect on the
day`s high temperatures near the lakeshore.

Additional periods of showers and thunderstorms appear likely in the
Wednesday night through Friday time frame as a deeper upper-level
trough swings into the Upper Midwest. An initial bout of warm air
advection-driven convection may be seen Wednesday night into early
Thursday. While some spread still remains in ensemble guidance, this
precipitation is favored to lift north of the area by Thursday
afternoon in tandem with a surface warm front, which would leave our
forecast area in the breezy warm sector of a 1000-1004 mb low
pressure system passing to our northwest. If this does indeed occur,
then much of our forecast area should see temperatures climb into
the upper 70s to mid 80s Thursday afternoon. However, if the warm
front doesn`t lift as far north as presently expected, then
locations that remain north of this frontal boundary on Thursday
will see cooler temperatures than presently advertised in our
gridded forecast database.

In either scenario, the low pressure system`s cold front will
eventually track through the area Thursday night into Friday,
bringing an additional wave of showers and storms with it. We will
need to monitor the potential for hydrologic concerns during this
time frame given the recent rainfall and that precipitable water
values near or in excess of 1.5" (near-record values for this time
of year here, per DVN and ILX sounding climatology) are progged to
be drawn northward out ahead of the cold front, but it`s still too
early to have much confidence in that with a few questions still
remaining about the track and overall evolution of the low pressure
system. In the wake of the cold frontal passage, temperatures are
favored to return closer to normal going into the weekend before
another warm-up appears to be on the horizon going into next
week.

Ogorek

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
-- Changed Discussion --
Issued at 1235 PM CDT Mon Apr 29 2024

Main Concerns:

- Timing improvement of MVFR CIGs this afternoon

- Widely scattered SHRA this afternoon

- Potential lake breeze wind shift to easterly late day Tuesday

A cold front will shift across the terminals this afternoon. Out
ahead of the cold front, MVFR CIGs, widely scattered SHRA, and
gusty southwest winds (gusts to ~25 kt) are the items of note.
With the main axis of SHRA back near RFD and the back edge of
the MVFR CIGs west of RFD as of this writing, pushed back timing
of CIG improvement and VCSH mention a bit in the TAFs for the
Chicago area terminals. Winds will shift westerly and quickly
diminish early this evening.

The only item of uncertainty on Tuesday is timing a
potential/probable lake breeze wind shift to easterly at ORD
and MDW. Opposing westerly winds should be strong enough to keep
the lake breeze at bay until toward or around 00z Wednesday
(tomorrow evening). Thus will likely address the wind shift
with the next full TAF issuance this evening.

Castro
-- End Changed Discussion --

&&

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

&&

$$

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