350
FXUS63 KLOT 141800
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
100 PM CDT Wed Aug 14 2024

.KEY MESSAGES...

- Period of showers and some t-storms likely late tonight into
  Thursday morning.

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

- Highly conditional chance for strong thunderstorms toward
  northwest Indiana and east-central Illinois Friday afternoon.

&&

.DISCUSSION...
Issued at 257 AM CDT Wed Aug 14 2024

Through Thursday Night:

Despite increasing cloudiness this afternoon, today should be a
warm day, though with comfortable humidity. A lake breeze will
provide for some modest cooling near the lake. Guidance has
trended slower with onset of rain tonight and it now looks like
most of the area should squeak out a warm and dry evening.

Still a strong signal in guidance that an MCS or two will
develop this evening across IA into southern MN, though likely
developing farther west than guidance suggested last night. This
activity should gradually spread eastward during tonight, with
some showers and perhaps a couple of thunderstorms spreading
into our western CWA during the early part of the overnight
hours, then possibly getting as far east as the Chicago area by
sunrise. By late tonight, the low level jet is progged to veer
and begin weakening, resulting in weaker moisture convergence.
In addition, as the convection moves east of the Mississippi
River it will encounter an increasingly stable air mass. This
should result in a weakening MCS as it moves across the CWA late
tonight into Thursday morning with chances of thunder dropping
off substantially over eastern IL and northwest IN.

Still appears as though Thursday afternoon will largely be
devoid of convection as convectively enhanced shortwave moves
east of the area and warm air advection weakens. Surface low
will track northeastward into western Wisconsin Thursday
afternoon into the early evening which should drag the synoptic
warm front northeast across our CWA with a much more humid air
mass overspreading the area. Meanwhile, westerly mid level
flow should advect a modified elevated mixed layer and steeper
lapse rates east into the CWA during the afternoon and evening.

How warm temperatures get Thursday will hinge on how much skies
clear out. Best chance of clearing appears to be western CWA
where temps should be able to climb into the 80s. Confidence in
how warm temps get farther east into the CWA decreases, though
with guidance trending less impressive with decaying morning
MCS, perhaps temps could overperform. Combination of steepening
mid level lapse rates, substantial increase in boundary layer
dewpoints, and at least some heating should result in moderate
to strong instability developing by late afternoon. Forcing
looks nebulous at best during the afternoon, so unless some
unpredictable at this distance outflow boundary kicks something
off, the chance of convection in our CWA during the afternoon
looks low.

Thursday evening guidance is in good agreement in digging a
developing/deepening closed mid-upper level low east-southeast
into the upper Great Lakes. DCVA in advance of this system will
result in strengthening synoptic ascent Thursday evening across
the area. Convergence along the weak approaching cold front is
very unimpressive, so it isn`t terribly clear where, when, and
even if convection will redevelop Thursday evening. However, if
it does, strengthening kinematic fields associated with the
tightening geopotential height gradient with the upper low
should result in increasingly favorable deep layer shear
profiles.

So if/where we get sufficient boundary layer destabilization
and if/where convective initiation occurs, the forecast shear
profiles would support a threat of severe thunderstorms and even
supercells. The synoptic forcing looks to arrive mostly after
dark in our area, but with such high dewpoints, boundary layer
based convective inhibition should be rather slow to increase
tomorrow evening. All that being said, confidence is low, but
there certainly is a conditional threat of severe weather in our
CWA Thursday evening and potentially even into the early
overnight hours. Any showers and storms that do occur, should
end late tomorrow night as weak cold front and trough axis shift
east of our CWA.

- Izzi

Friday through Tuesday:

The mid-level low responsible for the active period of weather
on Thursday will slow over northern Wisconsin by Friday
morning. An associated surface 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. While the
kinematic profile and modest mid-level lapse rates look
favorable for strong storms, dry-air advection 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.

The upper-level low will drift eastward toward Lake Huron
through Saturday night. Forecast guidance has gradually trended
drier with wrap-around precip on Saturday, especially with
substantial capping developing above low-level CAA. While some
diurnally-driven isolated to scattered shallow showers cannot be
ruled out Saturday afternoon and evening, thunderstorm chances
look to be extremely low.

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 into 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 north into southern Canada.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 100 PM CDT Wed Aug 14 2024

The main aviation weather concerns for the 18Z TAF period include:

* A period of showers and embedded thunderstorms that will occupy
  the majority of Thursday morning

* Gusty near-southerly winds on Thursday

* MVFR to IFR cigs expected for the better part of Thursday

A system of rain showers will approach northern IL from the west
late tonight. It appears that a few spotty lighter showers could
attempt to go up ahead of the main body of precip during the
overnight tonight. More widespread coverage is expected to make it
to RFD by around 10Z and the Chicago sites closer to 12/13Z. Rain
will remain in the area through the morning before fizzling away for
the afternoon. At least a few embedded thunderstorms are expected
although thunder coverage should be fairly isolated. Impacts to
vsbys will be possible underneath these showers during the morning.

There is high confidence for at least MVFR cigs to develop mid-late
morning with the rain. Lower cigs will then carry on through the
rest of the period. The signal is currently strongest for
predominantly MVFR during the day but IFR is certainly an option,
especially up at RFD.

Meanwhile, light E or SE winds will pick up just a tick this
afternoon as near-10 kt winds are expected to push off of the lake
and over the Chicago sites. SE winds will be mostly under 10 kt
tonight until the more widespread rain moves in. During the morning,
SSE winds should be regularly gusting to around 20 kt with
occasional gusts as high as 25 to 30 kt possible. Winds will remain
gusty and may want to teeter to west of south (190-200) for a time
during the afternoon.

Doom

&&

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

&&

$$

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