962
FXUS63 KLOT 151145
AFDLOT

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

.KEY MESSAGES...

- Showers and isolated t-storms expected this morning, likely
  decreasing in coverage/ending this afternoon.

- Second round of scattered showers/t-storms probable tonight,
  but details in timing, coverage, and location not particularly
  high.

- Scattered showers and t-storms expected later Friday
  afternoon into early evening, especially northern CWA.

&&

.DISCUSSION...
Issued at 320 AM CDT Thu Aug 15 2024

Through Friday Night:

Early morning radar mosaic across the Midwest shows just a
largely unorganized mess of showers and thunderstorms across the
region. This activity is being driven by low level theta-e
advection within broad 30-40kt southerly low level jet as well
as modest height falls and synoptic ascent associated with
approaching shortwave trough. Low level jet should weaken as it
begins to veer and flop over into northern IL this morning.
While low level jet is expected to weaken, continued (albeit
weakening with time) low level theta-e advection and eventually
ascent from the approaching trough should keep showers and
isolated to scattered t-storms going through the morning as the
activity spreads gradually eastward. Highest thunder chances
will likely be early this morning and focused more over the
western CWA closer to the better instability, though isentropic
ascent could still be enough for some "slantwise" convection
persisting east of the axis of upright instability.

Lead shortwave trough axis should be pushing into our eastern
CWA by early afternoon, with threat of showers/storms largely
ending from the west coincident with the passage of this trough.
Seems probable that eastern portions of the CWA could be socked
in with cloud cover much of the day, limiting heating, though
some breaks are more plausible western CWA this afternoon,
allowing a better chance for temps to rebound. NW-SE oriented
warm front will move across the area this afternoon, with a much
more humid air mass spreading across the area.

Tricky convective forecast for tonight. It is unclear if and
where boundary layer rooted convection will develop late this
afternoon or early this evening, but if it does, it seem
northeast MO, eastern IA, west central IL would be area most
likely to experience sufficient heating today to erode
convective inhibition and have the forcing necessary to initiate
convection. Farther east into our CWA, forecast confidence
wanes considerably.

It does seem probable  that some destabilization is likely as
a result of low level moisture advection beneath steepening of
mid-level lapse rates this evening. Cold front west of the
Mississippi River that could initiate BL based convection late
this afternoon or early this evening is forecast to weaken/wash
out as it tries to move east tonight. However, shortwave trough
axis is still progged to rotate around the slowly deepening mid
and upper level low as it wobbles southeast into the upper
Mississippi Valley.

A couple of the more probable of the possible scenarios for us
tonight include: 1) BL based convection west of us maintains
itself as it moves east into all or parts of our destabilizing
CWA. 2) BL based convection either doesn`t develop to our west
or stays south of the area and new, likely elevated convection
develops over our CWA tonight as synoptic ascent increases in
association with the approaching shortwave trough. There remains
a large spread in guidance with respect to magnitude of the
deep layer shear (and instability too), so severe threat tonight
is highly conditional and uncertain. There are scenarios that
could evolve that would result in a severe weather threat into
our area, but many more that would result in minimal severe
threat. Going to need to watch observational trends today to get
a better handle on severe threat, if any, tonight.

With the initial cold front expected to largely dissipate
tonight, it seems unlikely that there`ll be much of a scouring
out of low level moisture. Skies are expected to clear out
Friday, allowing the boundary layer to heat up and destabilize,
the sunshine and deeper mixing could also allow the dewpoints to
mix out some in the afternoon as well which could mute
instability somewhat. A secondary cold front and associated
trough axis pinwheeling around the upper low appear likely to
ignite at least scattered t-storms over southern WI or far
northern IL late in the afternoon. This convection should move
southeastward through the early evening before dissipating after
sunset. Greatest coverage/chances of convection look to be
north of I-80, with weaker/later arriving forcing farther south
likely limiting chances.

Certainly can`t rule out a severe weather threat late Friday
afternoon into Friday evening as 0-6km bulk shear is progged to
increase to 30 to perhaps as high as 40kt. Big uncertainties
regarding coverage of storms and magnitude of the instability,
so the marginal risk in SWODY2 is quite reasonable. If
confidence increases in convective coverage and threat of
dewpoints mixing out/limiting instability decreasing, then being
a window of severe wx potential will increase.

- Izzi


Saturday through Wednesday:

The upper-level low currently over North Dakota will drift ESE
to the eastern Great Lakes by Sunday night. Guidance continues
to waffle on whether sufficient low-level moisture will linger
far enough west to support diurnally-enhanced showers Saturday
afternoon or if gradual drying limits precip coverage. However,
guidance has trended more in favor with an embedded mid-level
trough rotating across the area on the southwest flank of the
upper low. Backward trajectories indicate that this is not a new
trough but actually today`s wave rotating completely around the
upper low over the next couple days. The latest forecast shows
scattered showers with perhaps a few thunderstorms Saturday
afternoon, though as is the case with upper lows, the details
and evolution of embedded impulses around the low likely will
not be fully realized beyond 24-36 hours, so some forecast
changes are quite possible.

Following suit with Saturday`s forecast guidance, there has
been a slower/westward shift with the upper low on Sunday.
Previously, Hurricane Ernesto was progged to shift far enough
west to pull the upper low farther east late weekend into early
next week. With the forecast track of the hurricane trending
farther east, little to no interaction between the two systems
is expected. Ultimately, this may result in continued isolated
shallow showers across far eastern Illinois and northwest
Indiana on Sunday.

Mid-level ridging and an approaching surface high will provide
quiet conditions across the area Monday into Tuesday. A loose
omega block is then expected to develop across much of North
America by the middle of next week. By this time, guidance shows
developing spread with the strength of this blocking pattern as
a strong wave over the northern Canadian Prairie provinces
potentially dampens the amplitude of the ridge. This may result
in a slightly more favorable set-up for a round or two of
convection to the north to come precariously close to our area
late next week as low-level moisture finally begins to return
northward. Otherwise, generally seasonable to seasonably cool
temperatures are expected through the period.

Kluber

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 644 AM CDT Thu Aug 15 2024

Aviation key messages:
- Band of -RA into mid-morning with continued periods of -SHRA
  late morning into mid-afternoon
- 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

SSE/S winds will gust 20-25 knots through much of this
afternoon. Directions should favor just east of south, but brief
periods with directions just west of south are likely at times.
A SW wind shift is then expected late this evening and
overnight.

A band of -RA will continue to shift across the terminals
through mid-morning. A 2-3 hour period with isolated -SHRA will
follow late this morning before coverage increases again for a
few hours early to mid-afternoon. During this period, there is a
10% chance for a couple showers to produce lightning.

Ceilings will lower solidly into MVFR levels across the Chicago
terminals this afternoon, while IFR ceilings are expected at
RFD mid-morning through the afternoon. MVFR ceilings may
persist well into tonight. Another round of SHRA/TS is then
possible late this evening into early Friday morning as a mid-
level wave brushes northern Illinois.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through this evening for ILZ006.

IN...None.
LM...Small Craft Advisory until 10 PM CDT this evening for Winthrop
     Harbor to Wilmette Harbor IL.

     Small Craft Advisory until 4 PM CDT this afternoon for
     Wilmette Harbor to Northerly Is. IL.

&&

$$

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