212
FXUS63 KLOT 050206
AFDLOT

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

.KEY MESSAGES...

- Shower and thunderstorm chances increase from the west mainly
  from midnight onward, though isolated showers or storms can`t
  be ruled out late this evening mainly west of Chicago.

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

- Seasonable temperatures will continue through next week.

&&

.UPDATE...
Issued at 906 PM CDT Thu Jul 4 2024

Skies have become partly cloudy across most of the area since
late this afternoon, in the wake of a mid-level short wave which
has pushed east of the forecast area. Despite some lingering
outflow boundaries from earlier showers and a weak lake breeze
which has pushed inland across parts of the Chicago metro, low-
levels have largely been stabilized by earlier cloud cover and
outflow as well as loss of diurnal heating. The exception has
been across the north suburbs of Chicago near the WI border,
where somewhat better low- level lapse rates were noted earlier.
A couple of weak showers had briefly developed there during the
late afternoon, though relatively warm mid-level temps (700-600
mb layer) in the wake of the departing wave helped to limit any
further development there.

For the remainder of the evening and overnight hours, attention
turns to an area of showers and thunderstorm from eastern IA
into western WI, which were occurring in the vicinity of an area
of surface low pressure associated with a seasonably-deep mid-
level short wave tracking eastward over the upper Midwest.
Extrapolating ongoing convection across IA ahead of the systems
cold front, it looks like our greater thunderstorm chances will
hold off until after 11 pm or so for our northwest cwa, good
news for Independence Day fireworks shows and festivals.
However, can`t rule out some isolated development in advance of
this activity mainly west of the Fox River Valley across
northern IL prior to this within an area of modest low-level
warm advection ahead of the upstream wave. Some earlier CAM runs
depicted some isolated cells trying develop (new 00Z NAMNest
even tries to develop showers along the weak lake breeze, though
forecast soundings suggest this overdone/unlikely). 00Z DVN
RAOB does indicate some steeper lapse rates above 700 mb ahead
of the approaching wave, with an area of mid-clouds noted in
satellite data approaching our western cwa. This might be a
slightly better area for isolated elevated convection. Nothing
appears imminent, but will continue to monitor these areas for
some potential isolated development over the next couple of
hours.

Otherwise, showers and thunderstorms should spread east across
the area after midnight, though most guidance suggests thunder
coverage will decrease with eastward extent overnight. Touched
up pops a bit through the evening hours mainly to bring better
storm chances in slightly later and tighten up pop gradients.
Otherwise, no significant changes made from late tonight into
Friday morning.

Ratzer

&&

.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 /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 640 PM CDT Thu Jul 4 2024

Forecast concerns include...

Chance for showers overnight/early Friday morning.
Chance for showers Friday afternoon.
Mvfr cigs Friday morning.
Gusty westerly winds Friday.

An isolated shower will be possible this evening near the IL/WI
state line. There will be a better chance for showers overnight
into the predawn hours. The current activity over northeast IA
is expected to move across the area in a weakening phase. There
may still be isolated thunderstorms at RFD and across northwest
IL but only showers are expected across the Chicago terminals
and its possible some areas will remain dry. There will be
another chance for showers Friday afternoon across the entire
area. An isolated thunderstorm is possible but expected activity
should remain mainly showers.

Mvfr cigs may develop with the showers overnight into the
predawn hours and then persist through mid morning Friday before
slowly lifting. There is a small chance for ifr cigs, especially
across northwest IL and at RFD.

Winds will likely be light and variable this evening, perhaps
calm at times. Wind directions, when they occur may favor a
south/southeasterly direction. Winds will turn southwest
overnight and then westerly Friday morning. Gusts into the lower
to mid 20kt range will be possible by mid/late morning and will
continue through sunset. cms

&&

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

&&

$$

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