471
FXUS63 KLOT 141949
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
249 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.

- 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.

&&

.DISCUSSION...
Issued at 249 PM CDT Wed Aug 14 2024

Through Thursday Night:

To be issued shortly.

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 /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...Small Craft Advisory from 9 AM to 7 PM CDT Thursday for the IL
     nearshore waters.

&&

$$

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