510
FXUS63 KLOT 290822
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
322 AM 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 /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 113 AM CDT Mon Apr 29 2024

Key Messages:

- Rain through the early to mid-morning with a few embedded
  storms. Spottier showers may then continue into this
  afternoon.

- MVFR ceilings possible at times through the early afternoon.

Rain coverage has begun to increase across the area at the start
of this TAF period, and widespread rainfall is expected to
affect the terminals through the early to mid-morning hours.
Limited instability should keep the threat for lightning fairly
isolated, but did recently note a few lightning strikes
southwest of the Chicago within the leading flank of shower
activity, so have maintained the inherited VCTS for the Chicago
metro TAF sites, but pushed the timing up a bit. Based on the
observed progress of an elongated outflow boundary stretching
from central Illinois down through the Ozarks, it`s possible
that the threat for lightning may end a little earlier than
presently advertised in the TAFs, but will err on the side of
caution and maintain a more pessimistic end time for the VCTS
mentions for now.

In addition to MVFR visibilities beneath the steadiest showers,
MVFR ceilings may be observed at times as well. This could
initially occur with the steadier shower activity and then
perhaps again later this morning into the early afternoon after
the bulk of the rain has moved out of the area, per the latest
ceiling guidance. However, as diurnal mixing commences, any MVFR
ceilings should gradually lift towards VFR levels with time.
Even after much of the rain has ended, some spottier showers may
continue near the terminals through the early afternoon or so,
so will continue to carry a VCSH mention in the TAFs to
highlight that.

Lastly, southwesterly winds will become breezy once again after
the rain departs. Gusts are not expected to be as strong as
they have been the past two days, with peak gusts this afternoon
likely to be somewhere around 25 kts. Winds will then turn
westerly and subside this evening behind the passage of a cold
front.

Ogorek

&&

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

&&

$$

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