275
FXUS63 KLOT 031745
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1245 PM CDT Wed Jul 3 2024

.KEY MESSAGES...

- A few showers north of I-80 this morning, then some chance for
  showers/storms near/south of Hwy 24 late tonight Thursday,
  otherwise mostly dry today through the 4th

- Cold front brings showers and thunderstorms through the area late
  Thursday night into Friday.

- Mostly dry conditions for the weekend, but increasing shower and
  storm chances late Sunday into Monday.

- Seasonable temperatures into next week.

&&

.DISCUSSION...
Issued at 319 AM CDT Wed Jul 3 2024

Through Thursday:

All that`s left of the earlier MCS is a shrinking area of some
light rain falling from the remaining orphaned anvil shield.
Anticipate this area of rain to completely dissipate well before
sunrise.

The surface cold front is lurking upstream with a ribbon of
deeper moisture pooling along and just ahead of the front, which
stretches from near Green Bay to eastern Iowa as of 08z.
Objective analysis does show several hundred J/kg of weakly to
uncapped MLCAPE ahead of the front and coincident with the low
level moist axis. Over the past hour or so, there has been a
narrow band of relatively weak convection developing along the
front.

Short range guidance does show that narrow moist axis spreading
eastward ahead of the encroaching cold front this morning,
which should allow for a continuation of that band of scattered
showers and perhaps an isolated thunderstorm as the front moves
across the CWA. This activity should pass Rockford by sunrise
and through much of the Chicago metro area by mid-late morning.

The front is expected to continue settling southward today with
the best chance of renewed shower and thunderstorm development
likely setting up south of our CWA later this afternoon. Opted
to hang onto a small shower chance this afternoon for Benton
County and adjacent portions of nearby counties, but character
of the afternoon looks dry.

A modest low level jet is progged to develop over the eastern
Great Plains into the lower Missouri Valley tonight and could
support some elevated convection developing over Missouri. This
activity will probably remain south of our CWA, but could be
close enough to justify carry some small chance pops south of
Highway 24 overnight tonight.

Thursday, a vigorous shortwave trough is progged to dig
southeast into the central Plains. Guidance is in surprisingly
good agreement in really not showing much northward surge of the
deeper moisture and more unstable air in advance of this
shortwave. Ultimately, during the day Thursday, there appears to
be little to no synoptic forcing to support showers or storms.
Was very tempted to remove mentionable precip chances from the
forecast altogether, but the stalwart ECMWF continues break out
some convection in the afternoon, particularly south of I-80.
Opted to lower pops to slight chance over most of the CWA,
maintaining some low end chance pops near and south of the
Highway 24 corridor, in closer proximity to the deeper moisture
and greater instability.

Weak lake breeze this afternoon should provide a bit of cooling
right at the beaches this afternoon. Elsewhere, highs should
climb into the mid-upper 80s, with relatively comfortable
humidity levels (dewpoints mostly in the low-mid 60s). Light
synoptic easterly flow Thursday will allow for a more formidable
lake breeze to spread inland Thursday afternoon, with footprint
of lake cooling Thursday afternoon expected to be larger.
Outside of the lake cooling, highs will once again climb into
the mid 80s.

- Izzi

Thursday Night through Tuesday:

An upper trough is expected to be digging across the Upper
Mississippi Valley as a surface low develops over northern IA
and southern MN Thursday evening. As the trough and associated
low pivot into the northern Great Lakes Thursday night, a cold
front will get forced across northern IL and eventually
northwest IN late Thursday night into the day on Friday. Ahead
of this front a warm and humid air mass will be in place across
the region which should maintain at least some modest
instability overnight Thursday night. Therefore, showers and
thunderstorms should develop along the front in IA and spread
into our CWA as the boundary traverses our area. While the
threat for any severe weather with the front continues to look
low, guidance is depicting a 50-55 kt 500 mb wind max overhead
which should generate at least 30-40 kts of effective shear for
storms to work with. Given the aforementioned instability I
suspect most storms will struggle to grow upscale enough to be
strong to severe, but this may be something we have to monitor
as we get closer. That said, the continued signal for a late
arrival of the aforementioned front, likely after midnight for
many locations; should yield a mostly dry July 4th evening for
any outdoor activities and firework shows.

The cold front will continue to progress across the area Friday
morning with shower and thunderstorms gradually diminishing
from west to east Friday afternoon. However, guidance has
started to show the upper trough developing into an upper low
and stalling over the Great Lakes Friday evening. If this does
occur as forecast, then the residual moisture behind the front
may be sufficient to redevelop some showers and/or storms late
Friday afternoon and evening mainly near the IL-WI line. Despite
the lingering precipitation potential, temperatures should
still warm into the lower 80s for most of the area but some
slightly cooler readings are possible near the IL-WI line if
showers materialize. Additionally, winds on Friday look to be
rather breezy in the wake of the front with westerly gusts in
the 20-30 mph range possible.

Heading into the weekend, the upper trough/low is forecast to
move into the eastern Great Lakes Friday night which looks to
allow a surface high to build overhead on Saturday. Most
guidance members keep this high in place through the entirety of
the weekend which should promote rain-free conditions for any
outdoor plans. While some guidance does hint at a shortwave
trough digging into the Mississippi Valley on Sunday, it appears
that any resultant showers/storms in our area may hold off
until Sunday night. Given that this period is at the tail end of
day 5, no changes were made to the 20 to 40 percent POPs
offered by the NBM. However, I still think these are overdone
given the mid-level dry air forecast and may need to get lowered
with future forecasts. Otherwise, expect seasonable temperatures
to remain throughout the weekend with readings in the low to
mid-80s each afternoon.

Looking ahead into next week, guidance appears to get into
better agreement on a broad trough settling somewhere in the
vicinity of the Great Lakes which would support periodic shower
and storm chances. While it is obviously too soon to know
specifics on timing and coverage, the NBM did offer some 20 to
30 POPs which seem reasonable at this range. Furthermore, if
this pattern does materialize as guidance suggests then we may
also see temperatures favoring readings near or possibly just
below normal for this point in July.

Yack

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1243 PM CDT Wed Jul 3 2024

A weak cold front crossing the Chicago terminals at this time
will settle well south of the area tonight into Thursday, with
VFR conditions prevailing during this time. WNW winds around 10
knots will diminish under 5 knots tonight under a passing
surface ridge as directions gradually veer NE by around sunrise.
Winds will then settle E/ESE around 10 knots Thursday
afternoon.

While the current forecast decpicts a dry forecast, a small
(10%) chance exsists for widely isolated TS late Thursday
afternoon near and south of the Chicago metro as a weak mid-
level wave crosses central Illinois.

Kluber

&&

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

&&

$$

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