398
FXUS63 KLOT 120833 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
333 AM CDT Wed Jun 12 2024

.KEY MESSAGES...

- Decaying showers/storms may push into parts of far northern
  Illinois late tonight/early Thursday morning.

- 40-50% chance for thunderstorms Thursday afternoon and
  evening. Any storms that develop would be capable of damaging
  wind and hail and localized flooding.

- Hot and humid conditions are expected early next week, though
  thunderstorms may occur at times and modulate the heat.

&&

.DISCUSSION...
Issued at 315 AM CDT Wed Jun 12 2024

Through Thursday Night:

Generally zonal flow is in place across the northern Plains with
a west northwest flow pattern across the Upper Midwest. This will
set the stage for potential MCS development across the region the
next few days. As is often the case in these scenarios, the
forecast for each subsequent day is highly dependent on
thunderstorm evolution the day prior and results in lower than
normal confidence.

Today has the potential to be the warmest day of the year so far
as southerly warm and moist air advection remains established over
the region. Highs are currently forecast to reach the upper 80s
to around 90. An attempt at a lake breeze this afternoon may hold
high temperatures in the low to mid 80s near the immediate
lakeshore, however. Heat indices today look to remain mostly in
check with dew points mixing into the 50s and potentially as low
as mid 40s over the Chicago metro.

Several thunderstorm complexes will likely be underway across
portions of Minnesota/Wisconsin/Iowa by this evening and
overnight. Model guidance continues to favor any upstream storms
being in a decaying phase as they approach the area thanks to
strong capping and dry low levels which should keep most of the
area dry through tonight, with the best chance (20-30%) for a
shower/storm mainly across northwest Illinois and toward the WI/IL
stateline.

As for the severe thunderstorm potential late Thursday afternoon
into the evening, it remains highly conditional on whether
thunderstorms, in fact, develop. Questions remain regarding how
early Thursday AM decaying storms and resulting surface boundary
locations influence warming during the day on Thursday, and
accordingly impact the thunderstorm potential. As a result, a few
general scenarios of how Thursday might play out are becoming
apparent:

Scenario 1: Greater shower/cloud coverage persisting through the
morning hours and/or earlier surface frontal passage which would
limit our warming, keeping highs in the 80s and below the
convective temp needed to break the stout capping in place (per
00Z NAM/NAMNest/ECMWF). This would keep a large portion of the
local area free from convection altogether, including the Chicago
metro. This scenario does include the potential for storms that
develop across southeast Iowa to progress east southeast across
the southern third of the forecast area (generally south of a
Peru, IL to Rensselaer, IN line) with a damaging wind threat.

Scenario 2: Less cloud cover and/or a quicker recovery of the
warm airmass would result in temperatures warming into the lower
90s across the area. This would likely be warm enough to break the
capping inversion prior to sunset resulting in numerous
thunderstorms across the area (per 0Z HRRR, ARW, RAP, GFS, and 6Z
NAMNest). If this is the case, there is ample instability in place
areawide with 2000+ J/kg of MUCAPE. This combined with 35-40kts
of deep layer shear would be supportive of damaging wind and hail.

Scenario 3: The 0Z NSSL WRF is a bit of a combination of the two
above scenarios with storms developing generally along/south of
I-88 mid afternoon along the effective front as the mid-level wave
approaches, with less coverage (isolated to widely scattered),
any of which could be severe. At this range, this may be the more
likely of the three scenarios.

With all of the above said, the SPC severe threat level 2 of 5
for areas generally south of I-80 seems appropriate with a Level 1
of 5 risk for the rest of the area. Any storms that do develop
will also be capable of high rainfall rates and localized flooding
given PWATS potentially upwards of 2 inches. Continue to monitor
the latest forecast trends as it may not be until Thursday morning
that we get a decent handle on the thunderstorm potential.

Petr


Friday through Tuesday:

The two primary forecast concerns and challenges in the extended
are high temps and thunderstorm chances.

Friday and Saturday are expected to be dry with temps in the
lower/mid 80s inland, cooler near the lake.

An upper ridge is expected to build across the region Sunday
through the middle of next week and if this materializes, high
temps Sunday through Wednesday next week could be in the lower
to middle 90s each day.

However, precip chances have crept back into most periods of the
extended and from this distance, there is little skill in trying
to make changes. If thunderstorm clusters are able to form close
enough to or in the local area, they will impact temperatures
and their outflows/lingering boundaries will affect later
possible rounds of thunderstorms. There are two time periods of
note, the first is Sunday as the ridge is building north. There
does appear to be some chance for thunderstorms across northern
IL or perhaps just north of the area. In addition to the
uncertainty if precip will occur, there is also uncertainty of
when it might occur. The second time period is at the end of the
period, next Wednesday. There has appeared to be some consensus
for a frontal boundary moving across the area for the middle of
next week, which would bring a break from the heat and perhaps
a better chance of showers/thunderstorms when/if it moves across
the area. cms

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1223 AM CDT Wed Jun 12 2024

Only forecast concern is a chance of showers, possibly with
isolated thunder, early Thursday morning, at the end of the 30
hour ORD/MDW tafs.

A weak disturbance will move across the area early this morning
with a mid deck expected to develop across the area. There is a
small chance of sprinkles or a few showers toward daybreak this
morning but confidence is too low to include any mention with
this forecast and trends will need to be monitored over the
next few hours.

There is another chance for showers and possibly isolated
thunderstorms early Thursday morning. This activity will be
dependent on how widespread convection becomes northwest of the
area Wednesday night and how far it can persist before it
dissipates Thursday morning, with a better chance for RFD and
northwest IL. Confidence is too low to include anything for the
new ORD/MDW 30 hour tafs.

Light southerly winds to start will turn more southwest by
daybreak with speeds increasing to 10-12kt later this morning
with a few gusts possible. Winds are expected to turn more
westerly this afternoon then back southwesterly this evening.
Low level winds will increase tonight, though they appear to
remain below low level wind shear criteria. However, Its
possible some gusts into the 15-20kt range may develop tonight.
cms

&&

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

&&

$$

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