806
FXUS63 KLOT 041848
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
148 PM CDT Tue Jun 4 2024

.KEY MESSAGES...

- Scattered showers and thunderstorms develop mid to late
  afternoon, and increase in coverage this evening. A few
  stronger storms may produce with gusty winds and small hail,
  in addition to heavy rainfall and a localized flooding
  threat.

- Following isolated to scattered showers Wednesday evening,
  mainly dry and comfortable (seasonably cool) through Sunday.

- Breezy to windy on Thursday with gusts up to 30-40 mph.

&&

.DISCUSSION...
Issued at 145 PM CDT Tue Jun 4 2024

Through Wednesday:

Before the thunder activity concerns, there is a lake breeze
from Chicago northward, keeping a lid on temperatures near the
lake (70s) whereas most inland areas remain on the warm and
muggy side as temperatures, even with the clouds, march on into
the mid 80s.

A mesoscale convective vortex (MCV) continues to shift northeast
into eastern Iowa. Ahead of the MCV There is a fair amount of
cloud cover across northern Illinois and northwest Indiana,
which is limiting instability. There are some weak waves in the
south-southwest flow and we are starting to see some of the
radar echoes on KLOT reaching in the ground in the form of
mostly light shower activity. Expect this to drift northward,
with some erosion on its northern edge. Thunder coverage will
low with this leading activity until the MCV drifts a bit
farther NE, given that MLCAPE will remain a bit more limited
closer to 500 J/Kg. Behind the opaque mid and upper level cloud
shield continued moist advection with dewpoints in the mid to
upper 60s will lead to increased instability approaching 1000
J/Kg or so of MLCAPE closer to the MCV across central and
western Illinois where capping is weaker. Therefore expect an
uptick in shower and thunderstorm coverage west of I-39 and to
the southwest, but also there is a narrow corridor across
northwest Indiana. Some of this activity will migrate closer to
the Chicago area toward early evening.

MLCAPE will not be quick to erode this evening as the MCV
advances into northern Illinois. This would likely be a more
favored period for a bit more storm coverage. Gusty winds
would be more favored severe hazard given some elevated low
level lapse rates through early evening. PWATs will also
increase ahead of the MCV as low level flow gets enhanced, which
suggests pockets of heavy rain this evening. While the flow
aloft is initially somewhat weak, enhanced of the low-mid level
southwesterly flow would suggest the individual storm elements
will be moving fast enough to preclude any significant flash
flooding in spite of the high PWAT airmass. However, increased
low level jet/storm regeneration is of some concern for
localized flooding/ponding and is probably the hazard of most
concern this evening into the early overnight hours. The main
area of concern will be west and south of the core Chicago metro
area closer the low/slightly better instability.

A fairly sharp upper trough, currently across the northern plains
will steer a cold front through the area in the area in the
morning. Ahead of the trough showers should expand in a
scattered to numerous nature again overnight, with storm
coverage gradually decreasing overnight into early Wednesday.
With a deep moist profile, pockets of heavy rainfall may lead to
ponding on area roadways into the morning commute.

The cold front will usher in cooler and drier air in the
afternoon in the Chicago area and into northwest Indiana, and
the front will also bring the end to any lingering showers, at
least for a brief time in the afternoon.

KMD

Wednesday Night through Tuesday (previous discussion)

A secondary cold front associated with the unusually deep (for
June) low pressure system and upper trough over north-central
North America will move across the area Wednesday night. A short
window of column moistening ahead of the front, but more
importantly, strong upper jet support, will result in a period
of isolated to scattered showers north of US-24. The highest
shower coverage should be near and north of I-88. Forecast
soundings depict fairly low freezing levels for this time of
year, though equilibrium levels will also be low (at or under
h5) with temps generally warmer than -10C, only yielding 100-200
J/kg of MUCAPE. Thus, while thunder chances with the evening
and early overnight shower activity aren`t zero, they`re fairly
slim and only warrant slight chance mention. Confidence is
higher in shower occurrence, hence coverage wording in the
official forecast. Skies will clear out in the pre-dawn hours of
Thursday, allowing temperatures to drop into the upper 50s to
lower 60s.

Deep mixing in a dry (dew points 45-50F) post-secondary frontal
air mass on Thursday along with a tight pressure gradient will
result in gusty west-northwest winds peaking during the afternoon.
Official forecast gusts are up to 35 mph and upside potential from
model soundings suggests at least sporadic 40 mph gust potential.
The strongest winds will be over our Illinois counties. High
temperatures will reach the mid to upper 70s thanks to the deep
mixing and dry air mass in place.

A blocking pattern will effectively trap anomalously deep upper
level low pressure (from the Wednesday and Wednesday night cold
front passages) over the central and eastern Great Lakes through
the weekend. Our area will be in dry northwest flow on the
southwest flank of the upper low, resulting in very little if any
rainfall during our what is typically our wettest time of year.
Atypically comfortable conditions can be expected Friday through
Sunday, with below normal high temperatures in the 70s away from
lake cooling, 40s-50s dew points, and overnight lows in the
50s. Low rain shower chances may return early next work week if
the upper low stays in close enough proximity to our east as it
finally shifts south from its weekend home. Below normal
temperatures will likely continue until the mid to latter part
of next week.

Castro


&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1149 AM CDT Tue Jun 4 2024

Key Aviation Impacts for the current TAF period:

- Scattered thunderstorms near the terminals in the late
  afternoon.

- Gusty southeast winds expected before the second round of
  storms

- A better chance (50 percent) for showers and storms at
  terminals this evening as showers continue overnight.

- Cigs are expected to lower overnight into Wednesday morning,
  low confidence in clearing time

VFR conditions prevail through this afternoon. Winds will be out
of the southeast and gusts are expected to strengthen through
the afternoon ahead of the showers. An isolated gust up to 30
knots cannot be ruled out completely, but better chances for
gusts to be 20 to 25 knots.

As a weak boundary moves northward in the late afternoon,
chances increase for showers and thunder to pass near the
terminals. Confidence in coverage for the first round remains
low. The chance for rain and thunder occurring at the terminals
is at about 25 percent or less before 00Z, but some thunder
will be around thus we will carry a VCTS mention. A southeast
lake breeze is expected to develop to help reinforce confidence
in southeasterly winds as the prevailing direction. However, as
the next round of storms move northward, it would not be
surprising if some occasional flipping to 190 happened between
00Z-02Z.

The next, more robust wave is expected to move over the
terminals after 00Z to bring the next round of showers and
thunderstorms over the region. Confidence increased enough to
convert the chances for thunder to a TEMPO group for TSRA at
area terminals, though timing may be adjusted in the next TAF
installment.

Showers will continue through the overnight as a cold front
moves over the area. Cigs are expected to lower tonight down to
at least low MVFR levels. There is a growing signal for IFR
conditions early Wednesday morning, but confidence was just low
enough to keep the SCT group in the TAFs for now. VFR conditions
are expected on Wednesday as conditions dry out and winds
become westerly with gusts around 20 knots.

DK

&&

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

&&

$$

Visit us at weather.gov/chicago