497
FXUS63 KLOT 052340
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 PM CDT Fri Jul 5 2024

.KEY MESSAGES...

- Showers and isolated storms this afternoon, mainly north of
  I-80. Chance of sprinkles/showers Saturday.

- Isolated to widely scattered showers and thunderstorms
  possible on Sunday, highest chance (~30-40%) near and
  northwest of a Sublette to Antioch line

- Unsettled Sunday night-Tuesday afternoon with periodic showers
  and thunderstorms, highest chance/coverage on Monday.

- Seasonable temperatures will continue through next week.

&&

.DISCUSSION...
Issued at 238 PM CDT Fri Jul 5 2024

Through Saturday night:

Isolated to widely scattered showers will continue this
afternoon--primarily north of I-80--as a robust vort max
pinwheels across southern Wisconsin. Have pared back lightning
chances to the immediate stateline locales where 600-500 mb
temperatures are a bit colder. Farther south, these temperatures
just look too warm to allow parcels to cool much below -10C.
There may be a brief window early this evening about 7-9 pm
prior to the diurnal loss of surface-based instability where
mid-level temperatures cool sufficiently to allow a slightly
better electrification potential. By this point, however,
suspect most activity will be on a downward trend with the loss
of heating.

Otherwise, we increased forecast cloud cover into Saturday
morning and afternoon as guidance is mixing into a lingering
moist layer above 900 mb. Additionally, a check of forecast
soundings shows that, while surface dewpoints may mix out during
the day, sufficient CAPE under the lingering 600 mb temperature
inversion exists to support the development of late-morning and
afternoon sprinkles/showers out of sufficiently deep Cu. Think
the lightning threat will remain quite low once again given the
warmer temperatures aloft, however. Any lingering activity
should diminish through the evening hours.

Carlaw


Sunday through Friday:

On Sunday morning, the remnants of a semi-organized cluster of
overnight showers and thunderstorms to the west of the MS River
may push into parts of northwest Illinois and Wisconsin. Some
potential exists for renewed widely scattered afternoon shower
and thunderstorm development as a weakly capped airmass
destabilizes. However, this would be most favored to occur well
to the northwest of I-55 in the vicinity of any remnant outflow
boundaries. As noted in the key messages, the highest
chances/potential convective coverage are forecast near and
northwest of a Sublette to Antioch line through mid afternoon.

Rising heights and an overall less favorable thermodynamic
environment farther to the southeast should largely curtail the
threat of storms for the southeast half to 2/3 of the CWA. In
addition, increasing subsidence should result in a lull in
convection overall from the late afternoon through the early to
mid evening hours. Highs on Sunday will be well into the 80s for
most (locally upper 80s), except perhaps far NW Illinois and
also along the Illinois shore due to a shore-hugging lake breeze
keeping highs near 80F.

The threat and coverage of showers and thunderstorms should
increase from northwest to southeast across the area Sunday night
through Monday night. This will occur as a slow eastward moving
cold front shifts into the area beneath a series of small scale
mid-level perturbations streaming over the area in the
southwest flow aloft. The highest chances for showers and storms
will remain near and northwest of I-55 Sunday night into Monday
morning, before chances ramp up farther east and southeast
Monday PM into Monday night. The main threat we will have to
keep an eye on with these storms is the possibility for some
locally heavy rainfall given the potential for some northeastward
training along the slow moving cold front and PWATs up to 150%
of normal.

Into Tuesday morning, the surface cold front should begin to
shift east of the area as the main mid-level trough axis makes
steady eastward progress into the Western Great Lakes. However,
in spite of the early frontal passage, chances for scattered
showers and storms will continue into Tuesday as relatively cool
mid-level temps under the upper trough shift over a still warm
and fairly humid post frontal airmass. Accordingly, we continue
to carry 30-50% PoPs. The threat of these showers and storms
should come to a quick end later in the afternoon, however, as
the trough exits east.

For the Wednesday-Friday period, the threat of organized
showers and thunderstorms looks to become rather low owing to
prevailing mid-upper level height rises, overall parched air
aloft, and surface high pressure centered over the northern
Great Lakes. We`ll be keeping a close eye on the remnants of
Beryl as it moves inland from the TX Gulf Coast mid next week.
The last few model cycles (operational models and a majority of
ensemble members) have kept the remnants safely south of the
local area, though there`s certainly plenty of time for large
track changes. Otherwise, expect seasonably mild temperatures in
the 80s for much of the upcoming week, with onshore flow
Tuesday PM through Friday keeping highs at locations near Lake
Michigan roughly in the 75-80F range.

KJB/Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 639 PM CDT Fri Jul 5 2024

The primary aviation weather concerns for the 00Z TAF period include:

* MVFR cigs expected overnight possibly stretching into Saturday
  morning

* At RFD, a potential for MVFR vsbys overnight in addition to the
  cigs

Westerly winds will continue to gust to around 20 kt through mid
evening before subsiding to closer to 10 kt for the rest of tonight.
Meanwhile, isolated pockets of non-impactful light rain will
continue floating across northern IL through mid evening. Periods of
MVFR cigs are looking likely overnight and possibly into Saturday
morning. Confidence in MVFR at any given time is currently too shaky
to warrant prevailing any such conditions in the TAF, although the
06-10Z timeframe does appear to be the overall fan favorite among
model guidance at the Chicago sites and a tad later at the suburban
sites. MVFR cigs may very well hang around beyond daybreak, perhaps
intermittently, but best guess at this point is predominantly VFR
through the morning. At RFD, light ground fog may develop overnight
and provide a period of MVFR vsbys in addition to the cig potential.
This fog does not look to last long after dawn.

Expect NW winds under 10 kt through at least the first half of the
day on Saturday. A lake breeze is progged to move onshore during the
day, although a great deal of uncertainty exists regarding timing
and how far the boundary will protrude inland. Best guess at this
point is that the boundary will cross MDW sometime in the mid
afternoon, although models appear to be trending closer to early
afternoon. The lake breeze looks to cross ORD sometime later in the
day, although guidance appears to be trending toward the idea that
the boundary may stay just out of ORD`s reach and winds will remain
NW through the afternoon. Behind the boundary, expect easterly winds
under 10 kt. A synoptic wind shift will eventually turn all sites to
easterly later in the evening.

Doom

&&

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

&&

$$

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