024
FXUS63 KLOT 161138
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
638 AM CDT Fri Aug 16 2024

.KEY MESSAGES...

- Another round of scattered thunderstorms this afternoon and
  evening, some of which could be strong to severe with the
  threat of damaging winds and hail.

- Periods of scattered showers Saturday and Sunday afternoons.

- Below average temperatures expected next week with highs in
  the mid-70s to around 80.

&&

.DISCUSSION...
Issued at 410 AM CDT Fri Aug 16 2024

Through Saturday:

A broad upper-level low continues to spiral over northern MN
this morning with several shortwave impulses pivoting across the
upper Midwest and Great Lakes regions. One of these shortwaves
has been pivoting overhead overnight which has allowed a complex
of showers and thunderstorms to develop and progress across the
southern 2/3rds of our CWA. While these storms did have some
localized severe winds earlier, the gradually waning instability
has turned these showers and storms into primarily a heavy rain
threat as they continue into northwest and eventually northern
IN through daybreak. Therefore, mostly dry conditions are
expected to start our Friday with skies gradually becoming
partly to mostly sunny by late morning.

However, another one of the aforementioned shortwaves
(currently in southern MN) is expected to pivot into southern WI
this afternoon and develop more scattered thunderstorms in the
process. While most guidance remains in decent agreement on
storms occurring, there continues to be uncertainty on the
coverage. The reason is due to the models struggling to resolve
how low dew points will mix down this afternoon which will
dictate how much instability will be available for storms. The
general consensus seems to favor dew points dipping into the mid
to upper 60s this afternoon, but there still are a couple hi-
res models (namely the HRRR) which mixes dew points into the
lower 60s. Regardless, most guidance does show at least 1000 to
1500 J/kg of MLCAPE developing across northern IL and northwest
IN this afternoon which should be sufficient to support some
scattered thunderstorms especially for areas along and north of
I-80 where forcing should be maximized.

Additionally, forecast soundings continue to show around 25 to
30 kts of effective shear overhead this afternoon which may
allow a few storms to become strong to severe. Given that wind
profiles (and resultant hodographs) are largely unidirectional
the main threats with any severe thunderstorms looks to be
damaging wind gusts and maybe some hail depending on degree of
destabilization. However, low- level shear is forecast to
increase this evening as the low-level jet redevelops which
could support some supercell structures and possibly a localized
tornado threat. Though the tornado threat is much more
conditional on storms remaining surface based as the low- level
jet ramps up after sunset. That said, the Storm Prediction
Center has maintained the level 1 of 5 severe risk across the
area for today which covers these threats well.

Furthermore, PWATs this afternoon should also rebound into the
1.5 to 1.6 inch range as storms develop which will make the
storms efficient rain makers. While some instances of localized
flooding may occur if storms develop on top of already saturated
areas, the progressive and isolated nature of the storms should
limit this threat for most.

Thunderstorm coverage is expected to diminish after 10 PM this
evening as the upper wave moves east and a cold front pushes
through the area. Therefore, mostly dry conditions are expected
overnight with temperatures cooling into the mid to upper 60s.
However, the upper low is expected to be moving into Lower
Michigan during the day on Saturday. Since mid-level moisture is
expected to still be in place, scattered showers are expected
to develop during the afternoon as modest diurnal instability
develops. While the threat for thunder generally looks low, a
stray rumble or two of thunder cannot be completely ruled out
just yet.



Saturday Night through Thursday:

The upper low will continue to gradually move east across Lower
Michigan on Sunday as upper-level riding builds across the
western CONUS. As this occurs, a surface high is expected to
pivot into the Great Lakes Sunday night resulting in a period of
drier and below average conditions through the middle of next
week. However, until this high and associated dry air fully
arrive scattered showers will once again be a possibility on
Sunday. Though coverage should be somewhat less than on
Saturday. Additionally, the entering high pressure will also
tighten the pressure gradient across the area which will result
in breezy northerly winds for Sunday and Monday. These breezy
winds will also help to build waves at southern Lake Michigan
beaches during this period which will make for hazardous
swimming conditions, especially in northwest IN. So if planning
to head to the beach this weekend be sure to check the forecast
before hand.

Winds and waves are expected to gradually subside late Monday
into Tuesday as the surface high moves overhead. Therefore,
tranquil weather is expected through at least the middle of next
week with plentiful sunshine and high temperatures in the
mid-70s to near 80 each afternoon.

Yack

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 638 AM CDT Fri Aug 16 2024

Aviation Forecast Concerns:

- Brief period of MVFR/IFR CIGS early this morning.

- Breezy WSW winds later this afternoon with gusts near 20-25
  kts.

- Scattered TSRA potential again early this evening.

Some patchy MVFR/IFR ceilings have developed across northern IL
in the wake of the overnight storms. These CIGs are likely to
impact some of the northern IL terminals for a short period
through mid-morning. Thereafter, cloud bases will diurnally
improve back to VFR levels. West-southwest winds are also
expected to gradually increase through the morning, and may
gust into the 20-25 kt range during the afternoon hours behind a
weak cold front. Winds will then shift more westerly tonight
into Saturday.

Another mid-level disturbance will rotate across the area from
the northwest into early this evening. This feature may aid in
producing another round of scattered TSRA early this evening.
However, confidence in coverage remains somewhat low at this
time, so we have opted to maintain the going PROB30 mention at
this time.

KJB

&&

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

&&

$$

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