814
FXUS63 KLOT 041948
AFDLOT

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

.KEY MESSAGES...

- Lingering showers, possibly an isolated storm, this afternoon
  with diminishing coverage through this evening. Additional
  storm chances arrive after midnight tonight.

- Mostly dry conditions for the weekend, but there will be increasing
  rain chances late Sunday through Monday.

- Seasonable temperatures will continue through next week.

&&

.DISCUSSION...
Issued at 202 PM CDT Thu Jul 4 2024

Through Friday night:

The earlier cohesive/widespread light-moderate rain has
diminished. With several vorticity maxima and embedded jet
streaks still in the vicinity, however, additional isolated to
scattered showers will likely continue this afternoon. Locally
greater coverage appears plausible 1) near/north of the WI state
line in the vicinity of the lake breeze, 2) near the I-39
corridor and points west where a bit more instability resides,
and 3) well south of I-80 where broad warm advection remains
north of a modest 25 knot southwesterly 850 mb LLJ in the
vicinity of a subtle surface boundary. While we can`t rule out
a few lightning strikes, forecast soundings depict gradually-
warming and drying mid-levels through the rest of the
afternoon, likely in response to developing subsidence on the
backside of the departing parent shortwave. Localized downpours
will be possible owing to very slow cell motions. Finally, while
not an overly-conducive setup, we`ve received several funnel
cloud reports from central Illinois. Based on radar motions,
there`s clearly a subtle boundary that showers are tied to,
providing a little low-level vorticity/spin boost, so this
potential will continue until showers diminish.

Through the 5-7 PM timeframe, it looks like most if not all of
our forecast area will trend precipitation-free as subsidence
shifts across the region yielding favorable conditions for
firework displays.

Overnight (likely after midnight), shower and storm chances will
increase as the shortwave currently pinwheeling into western
Minnesota scoots east across Iowa. Have delayed the arrival
time of precip chances based on latest guidance which generally
matches current upstream conditions well. Suspect the greatest
storm coverage will focus north of I-88 in closer proximity to
large scale forcing, but sufficient MUCAPE exists for isolated
to widely scattered storms to "zipper" farther south ahead of
the incoming front/trough. While mid and upper winds are strong,
shallow effective inflow depths and equilibrium levels should
curtail storm organization. Can`t rule out a stronger cell with
some small hail/gusty winds, however.

Activity will push east of the region through Friday morning. By
midday and into the afternoon, scattered showers will develop in
the deeply mixed cold advection regime with coverage increasing
the farther north of I-80 you go. Continue to advertised isolated
thunder chances (20%), but guidance does depict a region of mid-
level warming which may tend to inhibit more widespread
electrification. Activity will tend to gradually diminish through
the evening hours.

Carlaw


Saturday through Thursday:

Following a likely quiet Saturday, a unsettled period is in
store Sunday through Tuesday. On Saturday, a weak trough axis
will push southward and weak high pressure over southern Lake
Michigan in the afternoon will enable a lake breeze to develop
and push inland. Despite relatively cool mid-level temps, height
rises and dry mid-level air should preclude any showers developing
out of deeper Cumulus. Highs will be a bit below normal, in the
upper 70s to lower 80s inland, and mid 70s lakeside. Dew points
in the lower 60s will make it feel seasonable.

On Saturday night, while it will quiet and seasonably mild
locally, attention will turn to the possible development of an
at least loosely organized convective complex west of the MS
River from northwest Missouri to southern Minnesota. This
activity will be associated with a short-wave pivoting toward
Lake Superior. It appears probable that the showers and
thunderstorms will be on a weakening trend as they outrun the
instability axis while tracking off to the east-northeast in the
morning. Highest PoPs in the 30-50% range for the northwest 1/4
to 1/3 of the CWA in the morning fit the above thinking reasonably
well.

There may be a window toward mid day into the early afternoon,
possibly aided by remnant outflow from the decaying morning
convection, for widely scattered showers and thunderstorms to
develop northwest of I-55 as MLCAPE in that area builds to up to
1000 J/kg with minimal capping. A sharp CAPE gradient to the
southeast should greatly limit any development for the southeast
1/2 to 2/3 of the CWA. More pronounced height rises in the mid-
late afternoon amidst modest mid-level flow and lapse rates
only around 6C/km should cause showers and storms to diminish.
Forecast highs on Sunday are generally well into the 80s, except
along the immediate Illinois shore, with a bit more sticky
feeling from dew points 2-5F higher than on Saturday.

The Sunday night into Monday timeframe is appearing a bit messy
as a slow moving cold front works its way eastward. Weak
perturbations streaming over the area in southwest flow aloft
should be able to generate another round of scattered showers
and embedded thunderstorms late in the evening and overnight,
with the highest PoPs/coverage (30-60%), near and northwest of
I-55. Following a possible lull in coverage Monday morning, a
slightly more robust impulse will encounter a minimally (if
that) capped and moist air mass with temps in the 80s and dew
points 65-70F. Scattered showers and non-severe thunderstorms
appear probable (40-60% PoPs) in the afternoon into the evening.
The trough axis to our west will finally push east Monday night,
which may keep convection going a bit longer into the night than
is typical diurnally.

A cold front associated with the short-wave and surface low
moving into the Great Lakes region Monday night will exit the
CWA Tuesday morning. Despite the frontal passage, relatively
cool mid-level temps associated with the short-wave trough and
dew points well into the 60s should yield modest destabilization
and the potential for isolated to widely scattered showers and
thunderstorms (20-30% PoPs) from the late morning through mid
afternoon.

Mid-level height rises and parched air aloft will likely
suppress thunderstorm chances on Wednesday and Thursday, and
possibly precip. chances altogether. We are officially showing
some low (20-30%) PoPs, however. Near seasonable temperatures
in the lower to mid 80s away from the lake will continue.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1247 PM CDT Thu Jul 4 2024

The primary aviation weather concerns for the 18Z TAF period
include:

* On and off pockets of non-impactful rain throughout the period

* The likelihood of MVFR cigs beginning early Friday morning

Intermittent periods of light rain or sprinkles will continue
through this afternoon and into this evening with no related impacts
expected. Additional pockets of light rain will be possible
overnight, although perhaps the most appreciable chance comes ahead
of an impending cold front in the roughly 08-12Z timeframe. At RFD,
this potential will arrive closer to 06Z and a light thunderstorm
cannot be ruled out up that way tonight either. Confidence is rather
high in a several hour period of MVFR cigs beginning predawn on
Friday and lasting through the bulk of the morning. Most models
suggest the MVFR should clear away by noon but will likely return
during the latter part of the afternoon as more pockets of rain look
to move in. There is also a signal that the cigs refuse to budge
during the afternoon and we remain MVFR throughout the day.

Meanwhile, light southeasterlies will persist into this evening.
Confidence on wind direction takes a tumble for the overnight
period, but best guess is SSW while magnitude should remain below
10 kt. It`s very possible winds even go variable for several hours
overnight. Winds will build through the morning on Friday as they
establish a WSW direction and get gusting to around 20 kt by late
morning.

Doom

&&

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

&&

$$

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