542
FXUS63 KLOT 120523
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1223 AM CDT Wed Jun 12 2024

.KEY MESSAGES...

- Following very warm conditions away from the immediate IL
  lakeshore on Wednesday, some decaying showers/t-storms may
  push into parts of mainly far northern IL Wednesday night.

- Chance for widely scattered thunderstorms on Thursday PM in
  parts of the area. Some storms may be strong to severe.

- Hot and humid conditions are expected early next week, though
  thunderstorms may occur at times and modulate the heat.

&&

.UPDATE...
Issued at 918 PM CDT Tue Jun 11 2024

The only item of note for tonight remains a small chance for a
few showers and perhaps even a few storms, although whether any
precipitation develops in our area in the first place remains
unclear. Most model guidance suggests a region of pretty steep
850-700 mb lapse rates will push across the area late tonight
into early Wednesday morning. Modest warm advection will also be
present in this layer as an attendant surface and low-level
trough axis swings through. Most evening guidance remains just a
bit too dry to "convect" within the zone of steepened lapse
rates, but if cooling and moistening within the 800-700 mb layer
is just a bit more significant than currently advertised, some
elevated showers and storms--rooted above 800 mb--would be
possible very late tonight and into early tomorrow morning.

Carlaw

&&

.DISCUSSION...
Issued at 310 PM CDT Tue Jun 11 2024

Through Wednesday Night:

A primarily quiet and much milder night is in store tonight,
with the occasional minor exception of a few isolated showers or
sprinkles at times. Lead ascent pushing across the far
northwest CWA and southern Wisconsin as of this writing may be
able to wring out spotty high-based sprinkles/brief light
showers near and north of I-88 over the next few hours. The
southern flank of a respectable short-wave trough will push
southeast across the area this evening and overnight.
Expectation is that we`ll stay capped to elevated convection
with dry low to mid levels persisting, so maintained only slight
chance showers/sprinkles mention. If any isolated/widely
scattered thunderstorms develop near the upper MS Valley this
evening, a few may survive into the northwest third or so of the
CWA. Will let the evening shift assess whether a slight chance
of thunderstorms will need to be re-inserted into the gridded
forecast. Most areas will remain dry tonight, with PoPs only in
the 10-20% range.

Wednesday will be the warmest day for the CWA as a whole since
back on May 21st, thanks to full sun, southwesterly warm air
advection, and afternoon dew points mixing out into the 55-60F
range. 850 mb temps rising to the upper teens Celsius and 925
mb temps into the mid 20s C supports highs in the upper 80s to
lower 90s away from the immediate Illinois shore. The magnitude
of low-level flow may be just weak enough to allow a lake breeze
hugging the shore to slip just inland along the immediate
Illinois shore Wednesday afternoon, though confidence is low.
Indicating highs in the mid 80s there, which will drop back to
the upper 70s if the lake breeze does push in.

Convection erupting over the upper MS Valley near a cold front
late Wednesday afternoon into the evening will push southeast in
the late evening and overnight. It will encounter a much more
hostile environment for convective maintenance locally with
antecedent capping and unfavorable timing diurnally. For this
reason, maintained general idea of chance (25-40%) PoPs over
the northwest third of the CWA and slight chances (~20%) down to
just northwest of the I-55 corridor. It`s certainly possible
that any thunderstorms will fall apart before reaching the CWA,
though for now official forecast shows broad-brushed
chance/slight chance of showers and thunderstorms late Wednesday
night. It will be the first warm summer night of this month with
lows in the upper 60s to lower 70s due to cloud cover and a
nocturnal low-level jet keeping moderate southwest winds going.

Castro

Thursday through Tuesday:

Any showers that make it to the area in the pre-dawn hours of
THursday may hang around early Thursday morning, but will
dissipate and move out by midday. Partly cloudy skies will allow
temperatures to climb into the low 90s.

Storm chances return Thursday afternoon and evening as a shortwave
moves north of the area. Southwesterly low-level flow will bring
moisture into the area, building instability in the afternoon. Model
guidance depicts CAPE values in the 1500-2500 J/kg range. On the
aggressive end, the NAM and GFS seem to overdo the moisture
return with dewpoints in the low-to-mid 70s (most models cap
dewpoints in the mid-to-high 60s) allowing CAPE values to
balloon over 3000 J/kg. Southwest winds ahead of the front will
also limit the amount of shear that is present, with values
capping out around 30-35 kts in the afternoon.

A surface cold front will advance across the area in
the late afternoon/early evening providing lift for storms to
initiate. The frontal gradient looks fairly broad early in the
afternoon, so convergence and lift will be on the weaker side. This
may limit convective coverage until later in the day. This isn`t to
say that storms won`t initiate earlier in the afternoon,
however, the more favorable time frame appears to be closer to
sunset as the frontal gradient tightens and low-level winds turn
more southerly ahead of the front, increasing convergence and
shear. This places the highest chance for storms across the
southern portion of the CWA closer to sunset. It is even
possible that the storms miss completely, initiating to the
south of the CWA. Any storms that are able to form have the
potential to become severe given the instability and shear
values that are expected.

Quieter weather returns Friday as a ridge builds into the central
US. A return to seasonable temperatures looks likely Friday and
Saturday with highs in the low-to-mid 80s. PoPs chances return on
Sunday as models are in pretty good agreement that a shortwave will
eject out of the Rockies and move across the central US. Most
guidance shows that this shortwave will move just north of the area.
The timing of this system is still uncertain, however. Left PoPs at
slight chance across the northern half of the CWA.

Moving into next week, the signal for hot and humid conditions
continues to persist, however, there is disagreement on the
magnitude of the heat. NBM guidance is aggressive, suggesting
record highs on Monday. GFS ensemble guidance keeps the ridge
much flatter than the ECMWF and Canadian ensembles resulting in
cooler temperatures Sunday through Tuesday. The GFS is also
much more convectively active, while the ECMWF remains quite
dry. Opted to go closer to the NBM 10th percentile for high
temperatures Sunday through Tuesday to tone things back a bit as
confidence is not high enough right now to forecast record
highs this far out with the potential for convection and a
flatter ridge. However, if the high temperatures of the NBM and
ECMWF do verify, heat indices will likely be over 100 degrees.

Carothers

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1223 AM CDT Wed Jun 12 2024

Only forecast concern is a chance of showers, possibly with
isolated thunder, early Thursday morning, at the end of the 30
hour ORD/MDW tafs.

A weak disturbance will move across the area early this morning
with a mid deck expected to develop across the area. There is a
small chance of sprinkles or a few showers toward daybreak this
morning but confidence is too low to include any mention with
this forecast and trends will need to be monitored over the
next few hours.

There is another chance for showers and possibly isolated
thunderstorms early Thursday morning. This activity will be
dependent on how widespread convection becomes northwest of the
area Wednesday night and how far it can persist before it
dissipates Thursday morning, with a better chance for RFD and
northwest IL. Confidence is too low to include anything for the
new ORD/MDW 30 hour tafs.

Light southerly winds to start will turn more southwest by
daybreak with speeds increasing to 10-12kt later this morning
with a few gusts possible. Winds are expected to turn more
westerly this afternoon then back southwesterly this evening.
Low level winds will increase tonight, though they appear to
remain below low level wind shear criteria. However, Its
possible some gusts into the 15-20kt range may develop tonight.
cms

&&

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

&&

$$

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