317
FXUS63 KLOT 021141
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
641 AM CDT Tue Jul 2 2024

.KEY MESSAGES...

- Showers and isolated thunderstorms this morning, mainly north
  of I-88

- Additional showers/storms expected to reach the I-39 corridor
  this evening, then decreasing in coverage and intensity as
  they attempt to move east toward/east of the I-55 corridor

- Typical July warmth and humidity through the week with
  periodic chances for showers and storms late Wednesday night
  through Friday

&&

.DISCUSSION...
Issued at 302 AM CDT Tue Jul 2 2024

Through Wednesday:

Shortwave trough extending from southern MN to southwest Iowa
will continue to move east-northeastward today across the
western Great Lakes. There appears to be at least a couple of
small MCVs embedded within this larger trough with fairly
widespread convection from WI southwestward across IA. This
activity is being fed by an axis of higher moisture and
moderate elevated instability which is gradually spreading east
toward northern IL.

Observational trends and short range model guidance suggest
that this convection will outpace the better moisture and
instability as it spreads eastward. That, plus weakening
moisture transport and convergence due to the diurnal weakening
and veering of the low level jet should result in convection
weakening as IA convection spreads east into IL early this
morning. Given current radar trends and latest CAM guidance,
have bumped up pops this morning north of I-88 where some
showers and perhaps a couple thunderstorms are looking
increasingly likely, particularly western CWA.

Shortwave trough and embedded MCVs should push east of the area
this afternoon likely bring an end to the rain by late morning
or early afternoon. Clearing skies and strengthening warm air
advection with brisk southerly winds should help boost high
temps well into the 80s this afternoon most of the area.

Pair of shortwave troughs, one over southern Colorado and a
second more robust one over Wyoming, will both translate
east and northeastward into the Midwest later today. Combination
of these waves and heating/destabilizing air mass ahead of an
associated cold front should allow for strong-severe convection
to break out over northern MO into IA later this afternoon.

While this activity is expected to push eastward toward northern
IL this evening, guidance is in strong agreement that it will
encounter an increasingly hostile environment likely leading to
substantial weakening possibly its demise overnight. Have
sharpened up the pop gradient from east to west across the CWA
tonight, maintaining categorical pops by evening toward the
I-39 corridor, tapering off to just chance/slight chance pops
late tonight into northwest IN and adjacent portions of eastern
IL where rain is looking pretty unlikely tonight.

There has been a sizable trend in guidance toward pushing the
effective surface boundary safely south of our CWA Wednesday
afternoon, taking the threat of re-newed storm development along
the front during the afternoon out of our CWA. Did maintain
some chance pops southeastern CWA in case front slows and
guidance reverse course. Also, kept some slight chance pops for
showers in the morning, in case any lingering showers can
survive into the morning hours. Increasingly, it is looking like
all or most of our CWA will be rain free Wednesday.

While front will move across the area Wednesday, in true summer
cold front fashion, the cooler and drier air will lag well
behind the front. So with skies clearing out Wednesday, it
should be another warm to very warm day with highs well into
mid-upper 80s and moderate humidity with afternoon dewpoints in
the 60s.

- Izzi


Wednesday Night through Monday:

The front is expected to stall out south of our area Wednesday
night which should yield mostly dry conditions. Despite the area
being north of the front, dew points are expected to remain in
the mid to upper 60s Wednesday night which won`t allow for much
cooling with temperatures projected to bottom out in the upper
60s to around 70s by Thursday morning.

While our area remains dry, guidance continues to show a storm
complex developing across the central Plains and tracking
eastward along the stalled front Wednesday night into the day on
Thursday. Given that guidance continues to favor the front
stalling more into southern IL, the bulk of any associated
shower and/or storm activity from the aforementioned complex
should remain south of our area. However, the front will be
trying to lift back northward during the day on Thursday which
may allow a few storms to materialize in our southern CWA
Thursday afternoon. Therefore, have maintained some 30 to 40
percent chances for thunderstorms for areas south of I-80 on
Thursday, but suspect we may be able to reduce these values
further if new forecast trends continue to favor the southern
solutions. Regardless, many dry hours our expected for outdoor
July 4th festivities, especially for areas north of I-80, with
high temperatures in the low to mid-80s.

Heading into Thursday night, an upper-level trough is progged
to pivot into the Upper Midwest and eventually the Great Lakes
by Friday morning. As the trough pivots overhead, a cold front
is expected to develop near the Mississippi River and get pushed
southward through northern IL and northwest IN Thursday night
into the day on Friday. Given that the environment ahead of the
cold front is forecast to remain warm and humid, showers and
thunderstorms are expected to develop along the boundary and
track across the area. While there is still some timing
uncertainties as to exactly when the front and associated
precipitation will arrive Thursday night, the general consensus
is that the front should hold off until after 10 PM Thursday
evening. Therefore, suspect that conditions for viewing
fireworks should be decent aside from increasing cloud cover
ahead of the front but still recommend keeping an eye on
forecast trends in case something changes.

Showers and thunderstorms are expected to persist through much
of the day on Friday as the aforementioned front continues to
progress its way across the area. Though, precipitation is
forecast to gradually taper from northwest to southeast Friday
evening as the front exits. A surface high is then expected to
move overhead in the wake of the front Friday night and persist
through a good portion of the upcoming weekend. However,
guidance continues to show the upper trough stalling over the
Great Lakes through the weekend which may be able to kick off
the occasional shower and/or thunderstorm if sufficient
instability can materialize. Given that this period is still 5+
days out, have maintained the 20 to 30 percent POPs offered by
the NBM but suspect that these POPs may be overdone since the
region will be under northwest flow and beneath rather dry mid-
levels. Regardless of how the precipitation chances play out,
temperatures and humidity are expected to remain near typical
early July readings with daily highs in the low to mid-80s and
dew points in the 60s to around 70.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 641 AM CDT Tue Jul 2 2024

Forecast concerns for the terminals...

- Scattered showers this morning, especially near RFD

- Breezy south-southeast winds this afternoon

- Another round of showers and thunderstorms this evening and
  overnight with a cold front

- MVFR ceilings behind showers/storms overnight into Wednesday
  morning


An upper-level disturbance continues to pivot northeastward
across Iowa this morning which continues to generate a broad
area of showers and thunderstorms across the Midwest. However,
the coverage of thunderstorms is expected to diminish as the
disturbance interacts with the drier airmass overhead later
this morning. Therefore, the shower coverage is expected to
become more scattered with eastward extent with the greatest
coverage likely to remain near the IL-WI line and RFD.
Regardless, a few showers may get as far south as ORD and MDW
where a VCSH mention has been maintained for this potential.
Additionally, there is also an outflow boundary tracking
southward out of Wisconsin which may result in a north-northeast
winds shift if it reaches the terminals. Given how defuse the
boundary is on radar and the stout synoptic winds, confidence is
low that it will actually reach the terminals (especially ORD
and MDW) so no formal mention has been added to the TAFs but
will continue to monitor.

Showers will taper off by 18z this afternoon leaving rain-free
and VFR conditions through the rest of the afternoon. Though,
the south-southeast winds will increase as showers end with
gusts in the 20-25 kt range expected through sunset. As speeds
ease this evening, directions will gradually become south-
southwesterly ahead of a cold front where they will remain
through the night.

As the aforementioned cold front moves into northern Illinois
this evening, another round of showers and thunderstorms is
expected to develop along the front. While this second round of
showers and storms will be in a gradually decaying state as it
moves through the area, guidance continues to show a decent
coverage of showers with at least isolated thunder through the
night. Therefore, have maintained the existing timing for -SHRA
at the terminals which remains reasonable based on latest
guidance. Additionally, the PROB30s for thunder have also been
maintained but confidence on how widespread thunder will be,
especially with southward extent, has begun to diminish. So
there may need to be further adjustments made to the PROBs in
future TAFs.

Showers and storms tonight will taper toward daybreak Wednesday
morning, but MVFR ceilings are expected to develop on the tail
end of the showers and persist into Wednesday morning. Generally
guidance has been in decent agreement that most ceilings will be
in the 1500 to 2000 ft range, but there are some indications
that lower values, near the IFR range, may be possible
especially near RFD. Regardless of magnitude, the lower ceilings
are expected to scatter out by Wednesday afternoon and remain
VFR through the rest of this forecast period.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement until noon CDT today for ILZ006.

IN...None.
LM...Small Craft Advisory until noon CDT today for Winthrop Harbor
     to Wilmette Harbor IL.

&&

$$

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