746
FXUS63 KLOT 060527
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1227 AM CDT Sat Jul 6 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.

&&

.UPDATE...
Issued at 834 PM CDT Fri Jul 5 2024

No significant changes to the going forecast this evening.

A seasonably-deep upper trough was over the western Great Lakes
region this evening, with a few smaller-scale mid-level
vorticity maxima rotating through the flow. Most notable of
these was exiting southeast WI over Lake Michigan with a
corresponding weak surface low pressure reflection. West-
northwest flow around the south and southwest periphery of the
trough/surface low was producing cool advection into the area,
with mid-level cooling noted in comparing ILX/DVN 00Z RAOBs, as
well as deepening weak instability depicted in RAP forecast
soundings. This has led to an increase in isolated showers from
northeast IA/southern WI int far north/northwest IL over the
last couple of hours, though sunset and the loss of diurnal
warmth in the low levels should allow these to fade in the next
1-2 hours. Until then, a few isolated showers look to be most
likely across our western counties, along/west of the I-39
corridor. Most areas however, will remain dry. Breezy west winds
still sporadically gusting 20-25 mph in some spots will also
diminish with the loss of boundary layer mixing with sunset,
allowing temps to settle into the lower 60s in most areas
overnight (mid-60s in the core of the Chicago area).

Going forecast has all of the above handled nicely, and other
than a few obs-based tweaks (most notably to decrease cloud
cover a bit this evening), no changes are needed.

Ratzer

&&

.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 /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1227 AM CDT Sat Jul 6 2024

Key messages:

- A brief (1 to 3 hour) period of MVFR CIGs and north-
  northeasterly winds early this morning.

- Easterly wind shift with lake breeze expected at main Chicago
  terminals this afternoon.

Currently a weak back door front is shifting southward across
northeastern IL. While this is not expected to cause a major
change in the going forecast, it does appear that a wind shift
to a north-northeasterly direction will occur at the main
terminals for a brief hour or two period just after 06z early
this morning as the boundary passes. This wind shift is also
likely to be accompanied by a short period of MVFR CIGs
(2,000-3,000 ft AGL). Opted to message this wind shift within
the going tempo for MVFR CIGs from 06-09z.

Otherwise, prevailing winds will settle back into a light (8kt
or less) northwesterly direction by daybreak, and will remain
that way through at least early this afternoon. Thereafter, a
lake breeze is expected to shift inland across northeastern IL.
This will result in an easterly wind shift at ORD and MDW
sometime in the 20-21z timeframe. Winds are then expected to
become very light (under 5 kt) from the south-southeast
tonight.

KJB

&&

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

&&

$$

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