520
FXUS63 KLOT 160521
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1221 AM CDT Fri Aug 16 2024

.KEY MESSAGES...

- Several rounds of thunderstorms are possible from tonight into
  Friday. A few storms may be severe, and not all areas will see
  thunderstorms.

- Isolated showers and thunderstorms possible over the weekend,
  especially on Saturday.

- Temperatures early next week will be below average with highs
  in the 70s to around 80.

&&

.UPDATE...
Issued at 839 PM CDT Thu Aug 15 2024

There are spotty weak returns that remain on radar passing over
the area. However, as soon as they develop, the quickly fall
apart. Isolated to scattered sprinkles to light rain is possible
(15 to 20 percent chance) for the next couple hours. There is a
sharp instability gradient from northwest to southeast draped
just along our western forecast area. Given the lack of
instability presently, thunder is not expected until after 9PM,
first arriving around I-39 before moving east.

Most attention on radar is focused on the thunderstorm activity
over eastern Iowa and northeastern Missouri. These storms are
expected to move eastward through the overnight. As better
instability moves into the area, there is the potential for some
development out ahead of the current activity, but the overall
coverage of storms is expected to be scattered at best. Given
how cloudy it was during the day which inhibited heating and
provided weaker low level lapse rates as well as slightly weaker
forcing, confidence is lower on the risk for severe weather.
However, given stronger effective shear and enough moisture
and instability present, stronger storms could develop where
some small hail or localized wind gusts could necessitate
the issuance of a severe thunderstorm warning. That being said,
thunderstorm activity is expected through the overnight before
it slowly moves east of the forecast area after daybreak
allowing for improving conditions for the morning commute.

DK

&&

.DISCUSSION...
Issued at 230 PM CDT Thu Aug 15 2024

Through Friday Night:

Regional water vapor and radar imagery depicts an elongated area of
low pressure centered over the Midwest with several embedded
shortwaves pinwheeling around the perimeter of the circulation. The
first shortwave is currently moving through the Lower Great Lakes,
and continues to support scattered showers across northern Illinois
and northwestern Indiana. Mostly cloudy skies have prevented
highs from climbing much above the low to mid 70s, altogether
making for a cool, and somewhat wet, day. With the increasingly
tight pressure gradient being forced by the approaching surface
component of the broad upper-level low in the northern Plains,
south to southeast winds remain breezy and continue to build up
waves on Lake Michigan. A Beach Hazards Statement remains in
effect from Chicago to Waukegan (and points northward) though
this evening.

The next upper-level shortwave and associated upper-level jet speed
max is easily identifiable in radar and satellite imagery near
eastern Nebraska, and will approach our area after dark. Although
forecast confidence in exactly how and were thunderstorms develop
remains on the lower side of the spectrum, an ensemble of convective
allowing models favors at least scattered thunderstorms moving
through the general area from about midnight to daybreak along the
nose of the aforementioned jet speed max. While thunderstorms will
likely be rooted above the near-surface layer, cloud-layer shear >50
kt should support semi-organized clusters capable of producing
frequent lightning, heavy downpours, locally gusty winds, and small
non-damaging hail. One or two storms may briefly acquire severe
caliber hail or winds, though such a threat should be localized. The
SPC Level 2/5 threat level is probably a category too high, but
should cover the threat well.

All remaining showers and storm should scoot east and away from our
area after daybreak allowing for clouds to scatter by mid-afternoon.
Boundary layer warming with surface temperatures climbing into the
mid 80s and lingering low-level moisture should allow for a pool of
1000+ J/kg of MLCAPE to develop by mid-afternoon ahead of the third,
and final, shortwave (currently meandering across North Dakota).
With a stout pop of southwesterly (and veering) low-level winds
ahead of the shortwave, forecast kinematic profiles within the
instability axis tomorrow afternoon do raise eyebrows and appear
supportive of low-topped supercell structures, given some 25-30kt of
shear and 150-200 J/kg of SRH in the lowest 3 km (assuming a
southeasterly storm motion). As a result, thunderstorms tomorrow
afternoon and evening may carry a threat for localized severe
weather in the form of brief tornadoes, damaging hail, and damaging
winds.

Now, a clear failure mode for thunderstorms (both coverage and
intensity) tomorrow afternoon and evening will be the quality of low-
level moisture, as the normally trustworthy HRRR and RAP depict
aggressive downward mixing of mixed-layer dew points in response to
breezy west winds. Forecast soundings taken from more moist CAMs
also depict a notable dry pocket within the mixed layer, which would
lead to similar, correspondingly low, values of MLCAPE. With that
said, even just 500 J/kg of MLCAPE may be enough for sustained
convective attempts given the deep-layer shear won`t be overly
strong (to enhance the entrainment of dry air and "shear apart"
initial updrafts). However, the signal for less-than-stellar
quality moisture in the mixed layer does suggest coverage of
thunderstorms may be isolated to scattered at best, and
primarily confined to areas along and north of I-88 in closer
proximity to the core of forcing. For now, the SPC Level 1/5
threat level for tomorrow afternoon and evening appears
appropriate, as adjustments (both upward or downward) may be
needed as the integrity of mixed-layer moisture becomes more
clear.

Borchardt


Saturday through Thursday:

The upper trough responsible for today`s rain can be found
spinning over the North Dakota/Manitoba border this afternoon.
It will gradually work its way eastward over the next several
days traversing the Great Lakes over the weekend. After dragging
a cold front across the area Friday night providing a chance
for some stronger storms (see short term discussion above), the
low level circulation will remain over Lake Michigan during the
day on Saturday. The broad forcing for ascent will help pull
precip chances into this weekend. Forecast soundings also remain
relatively moist on the backside of the system which may make
it easier for any subtle forcing mechanisms to stir up some
showers. A couple of thunderstorms will be possible during the
afternoon as well after heating can destabilize the low levels
some. Thermo profiles though remain poor aloft and overall
unfavorable for robust convection, so none is anticipated at
this time. Coverage looks isolated to widely scattered but may
be maximized around the lakeshore nearer to the circulation.

Conditions will be seasonably cool over the weekend following
Friday`s cold front. Similar highs in the middle and upper 70s
are expected both days. Onshore flow will keep temperatures a
bit cooler near the lake on Sunday. A rather tight MSLP gradient
between the departing low and an impending high to the
northwest will bring about breezy conditions over the weekend as
well. As a result, building waves may make for dangerous
swimming conditions (mainly Sunday into Monday). A few
additional showers may go up during the day on Sunday, but with
high pressure and drier air encroaching from the west, chances
are primarily focused across the eastern half of the CWA.

After the trough finally makes it out of the region early next
week, a sizable upper ridge will dominate the western CONUS
spreading high surface pressure across the Great Lakes region.
This is expected to keep precip chances minimal and provide lots
of sunshine during the earlier half of the week. Highs will be
pretty steady in the middle to upper 70s each day before
conditions look to heat up after mid- week as the ridge meanders
eastward.

Doom

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1221 AM CDT Fri Aug 16 2024

Aviation Forecast Concerns:

- Scattered thunderstorms will affect the Chicago terminals
  early this morning, and may come close to KRFD. TSRA threat
  should wane after 09-10Z.

- MVFR/patchy IFR ceilings linger past sunrise, with some patchy
  MVFR fog (IFR perhaps at KRFD) early.

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

- Lower-confidence scattered TSRA potential again early Friday
  evening.

The forecast area remains within a moist low-level warm sector
to the south of an area of surface low pressure over northern
MN. A mid-level disturbance approaching from the west is
providing persistent warm/moist advection, which was resulting
in redevelopment of low (IFR/MVFR) clouds into the terminals as
well as the development of scattered thunderstorms. As of
midnight (05Z) the greatest coverage of thunderstorms was south
of KRFD and southwest of the Chicago area, near KPIA-KVYS. This
complex of storms was tracking mainly to the east, though
additional cells were developing along its northern periphery
into the KSQI-KRPJ-KDKB areas. These scattered TSRA are expected
to move east across the Chicago terminals roughly 06Z-10Z. KRFD
looks to be along the far northern periphery of this activity,
but have included a VCTS mention for the first few hours of the
TAF as they may get close.

After TSRA depart pre-dawn, MVFR/patchy IFR ceilings are
expected to persist through sunrise before lifting a few hours
later. Some MVFR fog is expected, especially away from the core
Chicago terminals with KRFD and possibly KDPA having the
potential for a period of IFR vis. This should all burn off
after sunrise. With clouds lifting/scattering to VFR, west-
southwest winds will gradually increase and may gust into the
20-25 kt range during the afternoon hours behind a weak cold
front. Winds will shift more westerly Friday night.

Another mid-level disturbance will rotate across the area from
the northwest toward Friday evening, and may aid in producing
some additional scattered TSRA early. Confidence in coverage is
somewhat low in this, as models differ in how much mixing out
of low level moisture occurs during the afternoon. Have
maintained a prob30 mention during the early-mid evening for
this potential.

Ratzer

&&

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

&&

$$

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