157
FXUS63 KLOT 150559
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1259 AM CDT Thu Aug 15 2024

.KEY MESSAGES...

- Period of showers with some embedded storms likely late
  tonight into Thursday morning.

- After a likely break in the rain most of Thursday afternoon,
  another round of showers and storms is possible Thursday night,
  with some threat for severe weather.

- Isolated to widely scattered showers and thunderstorms
  possible on Friday. A few strong storms are possible over
  parts of east central Illinois into northwest Indiana.

- After a few showers on Saturday, primarily dry and seasonable
  through mid next week.

&&

.UPDATE...
Issued at 853 PM CDT Wed Aug 14 2024

Fairly quiet conditions tonight over the city with easterly
winds helping to slowly bring temperatures down into the 70s.
Meanwhile, mid level moisture is moving in from the west-
northwest. Showers will continue to move in from the west and
have arrived as far as the Fox Valley at the time of this update
was published. However, observations of light rain making it to
the surface have been isolated at best. Instability is fairly
meager and mesoanalysis shows fairly widespread capping over
northern Illinois. There have been a few flashes in north
central Iowa on GOES GLM data, but confidence is lower for
lightning activity here for the next few hours. So thunder
chances were capped at just a slight chance (up to 24 percent)
through 7 AM, with the chance (up to 40 percent) for thunder
west of I-39 just before day break. Otherwise, no major changes
to the rest of Thursday`s forecast listed below, as thunder
chances increase as the front arrives.

DK

&&

.DISCUSSION...
Issued at 346 PM CDT Wed Aug 14 2024

Through Thursday Night:

This afternoon, we find ourselves on the southwestern periphery
of a surface high pressure bubble positioned over the Great
Lakes. To our west, a double-barreled surface low brews over the
central Plains, flanked to its east by an MCV over northern
Iowa and to its southeast by a warm frontal zone. The leading
wing of warm air advection aloft is only now encroaching upon
the Mississippi River, and it will eventually spread over our
forecast area this evening as it slowly presses eastward.
Gradual saturation of the tropospheric column will ensue as this
occurs, eventually yielding a northwest-southeast oriented arc
of showers that will provide us our initial raindrops tonight.

Later on in the night, a cluster of storms originating from the
central Plains should approach from the west. It will likely
reach our western counties around daybreak in a decaying phase,
having outrun the instability plume still centered west of the
Mississippi River. As a result, the expectation is that a band
of widespread rain showers will pass through our forecast area
over the course of the morning, likely clearing most or all of
the area by the afternoon. This band will still likely contain
some lingering embedded convective elements that will pose a
threat for lightning, but it is unlikely that the threat for
lightning tomorrow morning will be widespread with the lingering
convection expected to weaken with time and eastward extent as
it continues to separate itself from its parent instability
plume.

Going into the afternoon, there should be a general lull in
shower activity after the morning rain band departs due to a
lack of instability and forcing mechanisms and the presence of
large-scale subsidence on the back side of the departing
convectively-enhanced shortwave. Cloud cover lingering around
into the afternoon will hinder destabilization efforts a good
bit, but temperatures and humidity will still build throughout
the afternoon as a warm front lifts northward through the area.
High temperatures are currently forecasted to reach the 80s
across only the southwestern half of our CWA (with mid-upper 70s
across the northeastern half), but could potentially end up
being a little warmer if the rain departs earlier or if the
cloud cover scatters out faster and/or to a greater extent than
presently expected. Regardless of how much clouds end up
clearing out, between ongoing low-level warm/moist air advection
and mid-level lapse rates steepening markedly as an EML plume
drifts eastward into the area, it appears that the air mass in
northern Illinois should eventually become unstable to surface-
based air parcels, possibly by as early as the mid-late
afternoon.

While it is possible that storms try to get going in or near
our forecast area by the late afternoon, large-scale forcing
will still largely be lacking on the whole at that time, and
unless some sort of low-level convergence axis materializes for
ascent to focus along, most or all of our forecast area should
remain free of thunderstorms until after sunset. By mid-evening,
large-scale forcing for ascent will become much more abundant
as DPVA increases along the southern periphery of an upper-level
low swirling over the Upper Midwest, and convective coverage
will likely begin to increase across the area as a result, with
most of the latest CAMs focusing it along a surface cold front
in spite of relatively weak convergence along the front. Shear
profiles appear to be favorable enough to potentially support an
initial supercellular storm mode with a threat for large hail
and damaging winds. Shear vectors oblique to the front should
then tend to promote some degree of upscale growth with time,
potentially yielding at least a loosely-organized MCS that would
track across or just south of our area during the late evening
or overnight hours. While a slowly-stabilizing boundary layer
should begin to mute the severe weather threat to some extent
later on into the night, it likely won`t curtail the threat for
strong to damaging winds entirely with this storm cluster given
the overall degree of low-level moisture, instability, forcing,
and shear that will be in play.

One potential failure mode to keep an eye out for would be if
extensive convection were to develop well to our south (possibly
along an outflow boundary laid out by the morning convection)
during the afternoon or early evening. If that were to occur,
this convection could inhibit some of the northward moisture
return into the area, which, in turn, would likely limit to some
extent both the coverage and potential intensity of any
convection that we`ll see Thursday night. This scenario likely
still wouldn`t prevent thunderstorms from occurring here
altogether, though, with the stout large-scale forcing support
and mid-level buoyancy at our disposal likely to still be
sufficient enough to support at least isolated thunderstorms in
the area in spite of the better moisture remaining fenced off to
our south in this scenario.

Ogorek


Friday through Wednesday:

The mid-level low responsible for the active period of weather
on Thursday-Thursday night will slow over far northern
Wisconsin by Friday morning. An associated surface cold front
is expected to cross the forecast area Friday morning into the
afternoon. Meanwhile, guidance is in decent agreement that a
embedded trough axis will swing across southern Wisconsin and
into northern Illinois/Indiana Friday afternoon and evening.
This trough axis will intercept an air mass with temps in the
80s and dew points in the mid to upper 60s.

While the kinematic profile on Friday looks potentially
favorable for strong storms, dry-air advection and modest mid-
level lapse rates near and behind the front should greatly
diminish thunderstorm coverage and intensity. However, any
slowing of the front will result in increasing strong to severe
storm chances across the southeast CWA Friday afternoon. With
veered surface winds limiting convergence and dry air at the
mid-levels until evening, low afternoon PoPs in the 20-40%
range (highest far north and in parts of NW IN) appear
reasonable, for isolated to widely scattered showers and storms.

The upper-level low will drift eastward toward Lake Huron
through Saturday night. While some wrap-around precipitation is
possible on Saturday, substantial capping is progged to
develop above low-level cold air advection (CAA). Diurnally-
driven isolated to widely scattered shallow showers appear
plausible, supporting 20-30% PoPs over mainly the eastern 1/2 or
so of the CWA. Thunderstorm chances continue to look very low
due to the aforementioned capping.

Beyond Saturday, latest trends continue to suggest that
Tropical Cyclone Ernesto will remain far enough east that any
blocking or Fujiwhara effects with the upper-level low should
also remain well east of the forecast area. This favors a
generally quiet and seasonable period Sunday through Monday as
a weak ridge shifts across the western Great Lakes.

Overall ensemble guidance does suggest that the impacts of the
tropics will result in a highly amplified pattern and loose
omega block across North America toward the middle of next week.
While this could place us in a typically favored area for
active weather on the eastern edge of the ridge, moisture return
looks to be questionable while the better kinematic environment
remains displaced well northwest into south central Canada due
to expansive surface high pressure across the Great Lakes and
upper Midwest.

Kluber/Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1259 AM CDT Thu Aug 15 2024

Aviation key messages:
- Weakening SHRA this morning with a low (10-20%) of TS mid-
  morning
- MVFR ceilings this afternoon and evening with IFR ceilings
  possible west of ORD/MDW
- 30% chance of TS late this evening into early Friday morning

SE winds up to 10 knots will slowly veer SSE through sunrise
while gusting over 20 knots at times. Gusts around 20 knots
should then persist through much of the day as directions settle
S/SSE. A SW wind shift is then expected late this evening and
overnight.

Mid-level -SHRA and TS near the Mississippi River will continue
to drift eastward through the early morning hours as they
encounter an increasingly unfavorable environment. Expect
sprinkles or a few light showers over the Chicago terminals
prior to sunrise, with the sparse TS surviving far enough east
to near RFD through sunrise.

A secondary broken line of SHRA and TS over central Iowa will
also continue weakening while moving eastward early this
morning, ultimately crossing northern Illinois through the
daylight hours this morning. While some isolated TS cannot be
entirely ruled out over the Chicago metro mid-morning, coverage
and potential this far north and east appears too low to
maintain TS mentioned in the TAF at this time.

Isolated -SHRA should persist into the afternoon as ceilings
lower solidly into MVFR levels. Some brief periods of IFR
ceilings are possible at RFD. Another round of SHRA/TS is then
possible late this evening into early Friday morning as a mid-
level wave to the north brushes northern Illinois.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 9 AM this morning to 7 PM CDT this
     evening for the IL nearshore waters.

&&

$$

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