071
FXUS63 KLOT 040602
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
102 AM CDT Thu Jul 4 2024

.KEY MESSAGES...

- Widely isolated thunderstorms possible south of I-80 Thursday
  morning and areawide by late afternoon. Conditions do not
  favor widespread thunderstorm potential.

- For prime July 4th fireworks display time Thursday evening
  (through ~10 PM CDT), thunderstorm chances appear low but not
  zero, with higher t-storm coverage expected overnight/early Fri.

- Next favorable window for scattered or higher thunderstorm
  coverage appears to be Sunday PM and on Monday.

- Temperatures averaging near seasonable through mid next week.

&&

.UPDATE...
Issued at 814 PM CDT Wed Jul 3 2024

No significant changes to going forecast for tonight. Main
focus continues to be on the potential for isolated (10-20%
coverage) showers and thunderstorms south of about a Pontiac IL
to Wheatfield IN line this evening, and along the WFO LOT/ILX
border region overnight.

Latest surface analysis depicts a weak, nearly-stationary cold
front along the above mentioned line. Surface temps in the
low-80s and dew points in the low-70s characterize the low-level
air mass ahead of the front, though RAP forecast soundings
indicate fairly poor lapse rates above a shallow boundary layer
maintaining some weak capping. Low-level winds are very light,
less than 10 kts on either side of the front, providing only
weak low-level convergence as well. Mid- level winds (and
resulting deep layer shear) have increased a bit since this
afternoon however, in association with a weak mid-level short
wave tracking east-northeast out of Missouri, which may yet aid
in developing a few isolated showers or thunderstorms across our
far southeast counties through mid-late evening and a couple of
shallow, isolated showers have developed over far southeast
Iroquois and Benton counties recently. The weak front is
expected to slowly sag toward a Bloomington IL to Rensselaer
line overnight, which along with the aforementioned mid-level
wave, may result in a few additional isolated showers or storms
across our far south cwa overnight. Going forecast has this
well-covered, with slight (20%) pops along the boundary through
the night.

Forecast into Thursday continues to look reasonable as well,
with a gradual increase of convective chances south of the
Illinois and Kankakee river valleys by mid-late morning as the
front begins to lift slowly back north in association with a
potential MCV developing from activity currently ongoing across
western KS.

Ratzer

&&

.DISCUSSION...
Issued at 300 PM CDT Wed Jul 3 2024

Through Thursday:

A cold front drifting SSE has continued to force isolated warm-
core showers within an axis of pooling low to mid 70s dew points
roughly south of a Pontiac to Valparaiso line. Modest mid-level
warming around 500-600 hPa should greatly mitigate thunderstorm
chances, but a few sporadic lightning strikes cannot be ruled
out with the strongest cores. This activity will continue to
shift SSE with the front into early evening.

The cold front will stall/wash out across the far southern CWA
and over central Illinois tonight. Isolated showers and perhaps
a few lightning strikes cannot be ruled out at anytime tonight
south of a Bloomington to Rensselaer line as the right entrance
of a 80 knot upper speed max remains generally stationary above
a weakly capped and higher theta-e airmass.

A mid-level wave crossing southern Colorado this afternoon will
force a cluster of convection along a boundary lifting
northward across southern Kansas/Missouri overnight. Guidance
depicts a decent amount of spread in the downstream trajectory
of the (now convectively enhanced) wave is it tracks into
central/southern Illinois by late Thursday morning. This is
likely the result of the MCV favoring a WNW to ESE MUCAPE
gradient toward far southern Illinois in opposition to the
stronger WSW steering flow aloft directed into central Illinois.
Overall expectations are for the better coverage of convection
to remain south of the forecast area, but for the footprint of
the wave to potentially force isolated to widely scattered
storms south of the IL/Kankakee River Valleys late morning into
the early afternoon.

Beyond early afternoon, diurnal heating and modest low-level
moistening will support a weakly capped airmass across much of
the CWA. Overall forcing by this time looks to be quite nebulous
with little mid to upper-level forcing in place. However, any
subtle low-level convergent boundaries like residual outflows,
an expected lake breeze, or axes of differential heating from
scattered mid/upper-level clouds could ultimately force isolated
showers or possibly a few storms mid-afternoon into early
evening. This does not appear to be a wash-out by any means, but
enough for those with holiday plans to keep mindful of forecast
updates through the day.

Kluber


Thursday Night through Wednesday:

Early to mid Thursday evening, surface low pressure will be
centered over southern Minnesota, with its trailing cold front
extending southward across central Iowa and northern Missouri. The
parent short-wave trough will also be centered well back to the
north and west, resulting in only minor mid-level height falls
into the western Great Lakes. A majority of forecast soundings
depict a largely capped environment, at least to any deeper
convection capable of producing lightning. With relatively dry
mid-level air being drawn northeastward and sunset, it`s
questionable if just enough moisture may materialize at the right
level to tap into 250-1000 J/kg of MUCAPE.

Given the above with frontal convergence/forcing well west but
modest large scale forcing and near climo PWATs, there may be
spotty isolated to perhaps widely scattered showers (highest
chance/30-40%) near and west of I-39 in the 7-10pm timeframe.
Current signs point toward the peak fireworks display time
probably being dry in much of the area, though as noted, can`t
rule out a few showers in the area. Confidence is lower in any
thunderstorms being able to develop, at roughly 15-30%, highest
near/west of I-39. This period will needless to say be monitored
closely. Shower chances may uptick some/ooze farther east in the
late evening, though thunder potential is still questionable.

As the cold front sweeps across the region after midnight,
stronger large forcing (increased height falls and frontal
convergence) and a bit better column moisture should more readily
erode MUCIN to offset the diurnally unfavorable timing for
numerous showers and scattered thunderstorms. A seasonably muggy
night can be expected out ahead of the cold front, with temperatures
in the 70s most of the night and dew points well into the 60s.

On Friday, following a quieter balance of the morning, the
seasonably stout mid- upper trough (500 mb heights in the mid 560s
DaM) will translate east from Wisconsin to northern Lake
Michigan. Steepened low-mid level lapse rates from cold air
advection and maximized mid-level height falls should result in
isolated to widely scattered showers near and north of I-80. Can`t
rule out an isolated rumble of thunder or two, though diminishing
lower level moisture (decreasing Td) looks to be a limiting
factor. A fairly tight pressure gradient over the region and
deeper mixing will result in westerly winds gusting up to 30-35
mph. Looking at highs in the mid 70s to lower 80s, coolest for
the northwest 1/3 or so of the CWA.

Friday night through Saturday evening will be quiet from weak
surface high pressure sliding east across the area, along with
seasonable temperatures, albeit with lower humidity levels.
Broad west-southwest to southwest flow aloft and seasonable to
above normal column moisture (PWATs 100-150% of normal) will
likely open the door for periodic scattered shower and
thunderstorm chances Sunday through Monday evening. While there
may be sufficient deep layer shear for some storm organization
during peak diurnal favorability, it doesn`t appear to be a
favorable synoptic pattern for noteworthy severe weather. In the
wake of the cold front passage Monday night, humidity levels may
come down some, with primarily dry conditions.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 102 AM CDT Thu Jul 4 2024

Forecast concerns for the terminals...

- Lake breeze turning winds easterly at Chicago area terminals
  Thursday afternoon

- Spotty showers and/or storms possible Thursday afternoon, but
  better chances after midnight Thursday night


Generally quiet weather conditions are expected for tonight into
Thursday morning with light west-northwest winds and BKN to OVC
VFR cirrus. A thunderstorm complex, currently over eastern
Kansas, is expected to move into central IL around daybreak
Thursday morning which may develop some isolated showers along
a stalled frontal boundary roughly from a KPNT to KRZL line.
While this should keep any notable rain south of the terminals,
some guidance is trying to bring sprinkles into at least the
Chicago sites but confidence on this occurring is low.
Furthermore, guidance also continues to hint that some spotty
showers and/or thunderstorms may attempt to develop across
northern IL near a lake breeze Thursday afternoon. However,
given the rather meager instability and lack of robust forcing
suspect the guidance is overdone and put the chance of any
showers/storms Thursday afternoon <20%. Therefore, have
maintained dry forecasts at the terminals during this timeframe.

Given that dry conditions should prevail through Thursday
afternoon, the main forecast concern is the aforementioned lake
breeze that is expected to develop and move through the Chicago
terminals during the late morning and early afternoon. As per
typical, guidance does vary a bit on exact timing but the
advertised easterly wind shifts at ORD, MDW, and GYY between 14z
and 16z seem reasonable. Elsewhere, winds will settle into a
southeasterly direction Thursday afternoon with speeds remaining
generally under 10 kts areawide.

Heading into Thursday evening, a more robust upper disturbance
is expected to pivot into the Upper Midwest and move a cold
front through northern IL and northwest IN late Thursday evening
and overnight. The warm and humid airmass ahead of the front
should allow showers and thunderstorms to develop along the
front and persist overnight as it tracks through the area.
However, timing on when exactly the front and showers/storms
will arrive remains uncertain especially near RFD. Therefore,
have decided to maintain a dry forecast at the 24-hour TAF sites
for now but did introduce a PROB30 at ORD and MDW where
confidence is slightly higher.

Yack

&&

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

&&

$$

Visit us at weather.gov/chicago