004
FXUS63 KLOT 021754
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1254 PM CDT Tue Jul 2 2024

.KEY MESSAGES...

- 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 /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1254 PM CDT Tue Jul 2 2024

Key Aviation Messages:
- Gradual wind shift from SSE to SSW early this evening.
- SHRA with possibly some embedded TS late evening and early
  overnight.

A mid-level wave over central Kansas is expected to spread a
period of SHRA and perhaps some embedded TS across the area late
this evening into the early overnight hours. Confidence in
occurrence and associated coverage of TS at the Chicago
terminals remains low enough at this time to maintain a PROB30
mention roughly in the 06- 09Z window. However, have included a
short period (02-04Z) of VCTS at RFD where elevated instability
will be notably higher this evening.

Isolated to widely scattered -SHRA may persist to around
sunrise Wednesday ahead of an approaching cold front. A period
of post- frontal MVFR ceilings will follow through late morning
before ceilings SCT and lift into VFR levels through the
afternoon.

SE/SSE winds over 10 knots will begin to gust over 20 knots
this afternoon while gradually veering SSW through this evening.
Winds will begin to favor west of 180 degrees sometime in the
23-01Z window, especially with the development of a veering LLJ.
Winds will further veer SW overnight and WNW behind the cold
front after daybreak Wednesday.

Kluber

&&

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

&&

$$

Visit us at weather.gov/chicago