509
FXUS63 KLOT 010150
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
850 PM CDT Sun Jun 30 2024

.KEY MESSAGES...

- Dangerous swimming conditions at area beaches through tonight
  due to breezy onshore winds and high waves.

- Return to more typical summer temperatures and humidity
  Tuesday onward with periods of showers and thunderstorms at
  times too.

&&

.UPDATE...
Issued at 849 PM CDT Sun Jun 30 2024

No significant changes to going forecast tonight into Monday.

Sprawling, 1026 mb surface high pressure was centered over far
northern WI/upper MI this evening, and will continue to build
slowly southeast across Lake Michigan by morning and then into
the central Great Lakes region through the day Monday. Breezy
north-northeast flow along the southeastern periphery of the
high will linger across the area (especially over Lake Michigan
and into parts of the metro and northwest Indiana) through the
evening, before gradually diminishing overnight as the high
approaches. Clear skies, light winds and a cooler/drier air mass
will support overnight low temperatures around 50 (some upper
40s possible in the coolest spots) away from the core of the
Chicago metro, with upper 50s downtown and along the immediate
lake shore.

With the surface high off to the east of the area on Monday,
modest winds will turn east-southeasterly, with lake cooling
limiting temps to the lower 70s along the IL shore, while
warming into the mid-70s elsewhere. Mostly sunny skies are
expected, with only some increasing thin cirrus later in the
day.

Ratzer

&&

.DISCUSSION...
Issued at 300 PM CDT Sun Jun 30 2024

Through Monday Night:

Early this afternoon, a fair weather cumulus deck overhangs the CWA,
but drier air being fed into region is slowly eroding the cloud
cover. Cold advection driven by a prominent region of high pressure
centered up in Minnesota has kept temperatures in the middle 60s to
near 70 degrees, cooler than what you would typically expect on the
last day of June. Meanwhile, breezy northerly winds over the lake
since last evening have been consistently churning waves in the 5
to 7 foot range with several peak waves reported at just under 10
feet. Gusts are gradually waning this afternoon and that, for the
most part, should continue through this evening and overnight, and
waves will respond accordingly. However, several pieces of high res
guidance are resolving one final surge of wind, especially along the
far southern lakeshore, during the mid to late evening. Most models
suggest more 20 to 25 kt gusts are in store, although the HRRR has
been saying over numerous runs now that a period of near 30 kt
gusts could be on the table along the Indiana lakeshore. That
said, decided to extend the ongoing beach hazards statement and
small craft advisory through 12Z Monday across all zones.

The aforementioned high will propagate southeastward across the
Great Lakes region tonight and tomorrow. This will keep conditions
nice and quiet during the day and the milder return flow will bump
afternoon temperatures into the middle and upper 70s. Meanwhile, the
large upper high in the Plains responsible for the fair weather will
get shunted to the east Monday night by a digging trough. The
surface response will be a center of low pressure working across the
Plains during the night. The more appreciable rain chances arrive
during the day on Tuesday as the storm system approaches, but a
developing warm frontal boundary could extend eastward from the
storm and bring a few showers to our far northwestern CWA before
daybreak. The brunt of the storm is covered in the long term
discussion below.

Doom


Tuesday through Sunday:

Following a quiet and seasonably cool start to the week on Monday,
an active belt of west-southwesterly upper level flow is expected
to set up across much of the Midwest into the western Great Lakes
downstream of aggregate upper troughing settling over the
northern Plains. Accordingly, conditions will become supportive
for a few episodes of showers and thunderstorms through the to
later portion of the period as a warm and humid airmass works
back northward across the region.

The first period of showers and thunderstorms is expected to set
up mainly to our west-northwest across the upper Midwest Monday
night in response to a corridor of enhanced theta-e advection
along a strengthening southwesterly low-level jet axis (45-50 kt).
This activity is expected to track east-northeastward into WI and
perhaps as far south as parts of far northern IL (north of I-88)
late Monday night into Tuesday morning. Since most of this
activity is likely to outpace the instability axis, much of this
convective activity should be in a weakening state by early
Tuesday morning. Nevertheless, we continue to carry some chance
POPs into Tuesday morning, highest along the WI state line.

Following any lingering morning showers it appears much, if not
all of the afternoon may be precipitation free across the area as
the really humid and unstable axis remains to our west in closer
proximately to an approaching cold front. However, temperatures
will be on an uptick during the day on breezy southerly winds.
Accordingly, readings are likely to top out in the low to mid 80s
on Tuesday.

Afternoon convection is expected to develop to our west Tuesday
as the prefrontal airmass destabilizes in the vicinity of the
aforementioned approaching cold front. This front, as well as any
associated thunderstorms, will then shift eastward into our area
into Tuesday night. This is certainly not a favorable diurnal
timing for robust convection in our area. However, given
respectable deep layer bulk shear and a stout low level jet, it`s
possible scattered thunderstorms could persist well into the night
despite the stronger synoptic forcing remaining well north the
area. Nevertheless, it appears the higher threat area for any
strong to severe storms will largely remain west of the area on
Tuesday. Following the cold frontal passage late Tuesday night,
the front is expected to lay out in a west-east orientation
somewhere across central parts of IL and IN on Wednesday. If it
ends up setting far enough south, we could end up with a primarily
dry day Wednesday, albeit a very warm one, with highs well into
the 80s.

Heading into the July 4th holiday on Thursday, the front is
forecast to gradually lift back northward as a warm front
Wednesday night into the day on Thursday. Depending on how far
north the front gets will determine the extent of shower and storm
coverage during the Thursday into Friday timeframe as another
shortwave trough pivots into the Great Lakes. At this point,
guidance is favoring an afternoon and evening show for our area on
Thursday with the greatest coverage in our southern CWA. However,
this general timing is still low confidence given that we are
still several days out and existing storms on Wednesday will play
a role in frontal placement. Therefore, those with outdoor plans
for the 4th should keep an eye on this period.

Regardless of how Thursday plays out, it does appear that
another front should push through on Friday and finally bring
the periods of showers and storms to a close heading into next
weekend. Though the typical late June early July heat and
humidity is forecast to remain through the period with highs in
the mid to upper 80s and dew points in the 60s to around 70 each
day.

KJB/Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 611 PM CDT Sun Jun 30 2024

- Breezy NNE winds diminish through late evening.

- East winds Monday, becoming southeast Monday evening.

Fine VFR flying conditions will persist through the TAF period.
Diurnal cu will fade with sunset, with clear skies overnight and
then some gradually increasing high cirrus Monday afternoon into
Monday night

Surface high pressure over the northern Great Lakes will drift
southeast across Lake Michigan overnight and to the central
Lakes by Monday evening. Breezy north-northeast winds will
linger this evening, especially across the metro terminals and
particularly at KGYY right along the lake, but will diminish
late evening and overnight. With the high off to the east on
Monday, winds will start out easterly as we mix out in the
morning, turning east-southeast and eventually southeast in the
afternoon inland. Winds will remain more easterly into early
evening at KORD/KMDW however due to the lake breeze. Winds will
turn light southeast later Monday evening for KORD/KMDW.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement until 7 AM CDT Monday for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement until 7 AM CDT Monday for INZ001-
     INZ002.

LM...Small Craft Advisory until 7 AM CDT Monday for the IL and IN
     nearshore waters.

&&

$$

Visit us at weather.gov/chicago