-- Discussion containing changed information from previous version are highlighted. --
900
FXUS63 KLOT 060154
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
854 PM CDT Wed Jun 5 2024

.KEY MESSAGES...

- Period of showers tonight, followed by mainly dry and windy
  weather on Thursday.

- Seasonable to seasonably cool conditions and breezy Friday
  through this weekend.

&&

.UPDATE...
-- Changed Discussion --
Issued at 854 PM CDT Wed Jun 5 2024

A vigorous sheared shortwave continues to make steady progress
across Wisconsin this evening, with two notable embedded vort
lobes clearly evident on moisture-channel loops. A mid and
upper-level thermal gradient is evident, with the Lincoln, IL
sounding sampling 500 mb temperatures near -5 C, while at
Minneapolis it`s nearly -20 C.

Intense DCVA/height falls interacting with a region of steep
mid-level lapse rates (thanks in part to the aforementioned cold
500 mb temperatures) is working to crank out high (ish) based
showers and thunderstorms. It`s no surprise the vast majority of
lightning activity has been confined north of the Wisconsin
state line under the colder temperatures aloft, and suspect that
will remain the case in our region tonight. Still, it`s just
cold enough that a few lightning strikes can`t be ruled out as
this activity sweeps across northern Illinois through the early
overnight hours.

With very steep 0-3 km lapse rates in place and a front that`s
racing eastward at nearly 40 mph, a brief period of gusty winds
may occur, either with the frontal passage itself and/or within
any more rambunctious shower activity (numerous sites in
southern Wisconsin and NE Iowa have reported gusts into the
35-45 mph range).

Only made some minor updates to refine PoP trends through 1-2
AM. Otherwise, the forecast is in good shape.

Carlaw
-- End Changed Discussion --

&&

.DISCUSSION...
Issued at 241 PM CDT Wed Jun 5 2024

Through Thursday Night:

Early this afternoon, we find a cold front making its way across
the CWA. The main body of precip pushing well out ahead of the
boundary has made it way off to our east. However, a second,
awfully narrow band of low-topped convective showers have popped
up just ahead of the front and is moving across our southern
and eastern CWA. A skinny CAPE profile and a large mid-level
hydrolapse have kept the thunder away and we expect that to
continue until this line moves away from the CWA late this
afternoon.

We`ve been quick to clear up behind the front with lots of
sunshine now gracing areas north and west of I-55. Steep low
level lapse rates are stirring up a 20-25 kt LLJ and bringing
breezy conditions to the area today. Meanwhile, a second, more
subtle cold front is currently throwing up some light showers in
far north-central Iowa. These showers are expected to maintain
themselves as they work into northern Illinois this evening.
Ahead of this wave, forecast soundings, for the most part, have
a very similar look. They feature steep lapse rates atop the
diurnally-driven, near-surface inversion and anywhere from 300
to, at most, 500 J/kg of MUCAPE tucked beneath a mid level
inversion. This mid level inversion will keep ELs capped at
between 600 and 500 mb with the coldest air through unstable
layer averaging at around -5C. Such a profile is certainly
capable of producing convective showers, although will likely
struggle to produce much in the way of lightning. Although since
thunder isn`t entirely out of the question, left a slight
chance for thunderstorms in the forecast. The highest shower
coverage and best chance at seeing embedded thunderstorms will
be north of I-80 where we find the more appreciable moisture and
just about all of the ample instability. Additionally, a newer
signal from the HRRR is for these showers to produce some gusty,
non-damaging winds. This seems possible if the rain can get
some of the stronger winds aloft to punch through the near-
surface stability. We should start to see showers move into our
northwest and approach the I-39 corridor sometime around mid-
evening and progress across the CWA through the middle of the
night.

A stout low level jet will drop over the area from the northwest
during the morning tomorrow. Steep lapse rates mixing through
the jet, as well as a tight surface pressure gradient behind the
departing system, will bring windy conditions to the CWA
tomorrow. While there is still discrepancy on the magnitude of
the jet, it looks likely that we should see regular gusts in the
25-35 mph range during the late morning and afternoon, however
gusts could very well exceed 40 mph at times. An area of
interest to potentially watch for higher gusts is north of I-80
and west of the Fox Valley during the latter part of the morning
where some guidance thinks we may be able to mix down a 40+ kt
LLJ before the jet diffuses some for the afternoon. Winds will
step down during the evening, but may remain a bit gusty through
the night.

Doom


Friday through Wednesday:

An upper-level low over Manitoba will elongate across Ontario
and Quebec while stalling into the upcoming weekend. This will
bring a period of seasonable to seasonably cool conditions with
daytime highs generally in the 70s. With shearing instability
within faster WNW flow aloft on the southern extent of the
upper-level low, overall guidance suggests that a developing
wave will interact with a pocket of Pacific moisture to produce
a low (10-30%) chance for light showers Saturday morning into
the evening. Guidance typically struggles to handle the specific
timing and strength of these waves this far out, so will
maintain only slight chance PoPs for now.

As the elongated wave begins to breakdown across the Upper
Mississippi Valley on Sunday, a trailing moisture-starved trough
axis brushing the CWA to the northeast may produce widely
isolated shallow showers during the afternoon. Otherwise,
ridging building in from the west should result in dry
conditions and gradually warming conditions Monday and Tuesday.

Kluber

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 620 PM CDT Wed Jun 5 2024

Aviation weather concerns are:

- Period of showers late this evening, some with gusty/erratic
  winds.

- Intermittent breezes/gusty westerly winds overnight before
  winds increase Thursday morning and afternoon. Brief window
  Thursday PM where winds could gust over 35 knots.

A region of high-based showers continues to track east across
Iowa and will arrive in the region later this evening. TEMPO
groups for -RA/-SHRA look good and have largely maintained the
inherited timing. Can`t entirely rule out a lightning strike
with this activity, although the potential is too low to justify
any mention in the TAFs. Any more robust showers will be capable
of producing erratic and gusty winds.

A reinforcing front will pass through, towards 03/04z INVOF RFD
and closer to 05z at the Chicago-area terminals. While not
in the outgoing 00z TAFs, there`s some potential winds could
gust briefly to 25 knots with this feature`s arrival.

Winds will increase through Thursday morning and afternoon,
generally from a 280-300 direction. There`s a brief window
during the early-mid afternoon where a few gusts near of just
above 35 kts could materialize, although this potential appears
too brief to warrant extra lines in the TAFs. Winds will remain
gusty into the evening but will gradually ease with time.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 10 AM to 10 PM CDT Thursday for
     Winthrop Harbor IL to Gary IN.

     Small Craft Advisory from 1 PM to 10 PM CDT Thursday for Gary
     to Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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