066
FXUS63 KLOT 031743
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1243 PM CDT Mon Jun 3 2024

.KEY MESSAGES...

- Scattered thunderstorms expected this afternoon and evening,
  some of which could produce locally damaging winds and flash
  flooding.

- Potential for additional showers and storms Tuesday.

- Wednesday through midday/early afternoon will likely be the
  last day of the week with any meaningful thunderstorm chances.

&&

.UPDATE...
Issued at 1143 AM CDT Mon Jun 3 2024

Have made some refinements to the afternoon forecast as we
continue to assess thunderstorm potential/coverage. Remnant
circulation from earlier convection continues to track
northeast over SW WI triggering thunderstorms just to the
west/northwest of our forecast area. The few cells that did pop
over the NW forecast area did not sustain themselves very long.
Satellite imagery shows a large corridor of cu across northwest
and west-central IL and much of eastern IA and eastern MO with
an outflow/frontal boundary moving east toward the Quad Cities.
This boundary looks to be more diffuse than it had been with cu
not currently looking too impressive. It`s not readily apparent
on satellite, but there looks to be some sort of wave tracking
northeast across north-central or northeast MO which had earlier
triggered a few cells but is not currently producing much.
Between the cu, the boundary, the potential mid/upper wave, and
a destabilizing atmosphere overhead, it still appears that
isolated to scattered thunderstorms will still be possible this
afternoon into early evening. High res guidance has shifted
gears and shows little if any convection which does not
necessarily help confidence. However, do agree that chances are
not all that great so have reduced the eastward extent of
chances through the afternoon with the main focus area being the
NW quarter to third of the area or roughly west of a Woodstock
to Mendota line with quickly decreasing chances to the east
through the afternoon with some eastward spread of chances
across the Chicago metro into the evening, though this may be a
bit overdone. Isolated severe storms are still possible along
with a flash flood threat given high precipitable water values.

MDB

&&

.DISCUSSION...
Issued at 356 AM CDT Mon Jun 3 2024

Through Tuesday:

Surface low pressure was analyzed over central MN early this
morning, with a warm front extending south-southeast through IA
and MO, and a cold front trailing southwest across the central
Plains. The low will continue to lift northeast into western
Ontario today, in association with a low-amplitude mid-level
short wave trough propagating from the northern Plains into
Ontario and the northern Lakes. As this occurs, the warm front
will lift northeast across the forecast area and will bring a
warmer and more humid summer-like air mass to the region. Within
this higher Theta-E air mass, instability will increase
diurnally during the midday and afternoon hours, with
temperatures in the low-mid 80s and surface dew points into the
mid-60s expected.

Forecast soundings indicate MLCAPEs in the 1000-1500 J/kg range
by afternoon, with little or no capping present by early to mid
afternoon. While the primary short wave and associated surface
low are tracking well north of the area, the Midwest radar
mosaic depicts a well-developed MCV over northeast NE which
evolved from an MCS earlier last evening. This feature is
progged to continue to track east-northeast across IA this
morning and southwest/central WI this afternoon. Though run to
run and model to model differences persist, CAM guidance
generally continues to indicate convective development in
association with the MCV across southwest WI and
northwest/northern IL this afternoon. Areas north of the I-88
corridor appear to be most favored, though details of timing and
coverage remain of somewhat low confidence.

Background deep-layer shear is relatively weak (~20 kts),
though the MCV will likely enhance mid-level winds and increase
deep shear to perhaps 25-30 kts for a time across southern WI
and far northern IL this afternoon. The combination of diurnal
peak instability and this period of somewhat enhanced shear may
support a few severe thunderstorms with damaging wind gusts the
main threat. Though low level flow is not very strong, subtle
backing in the vicinity of the warm front or any outflow
boundaries could support a low-end tornado threat. Storms would
likely become somewhat messy with propagation tied to outflow
boundaries by late afternoon and evening, but will likely spread
east-southeast into parts of the Chicago metro area before
weakening with the loss of diurnal instability this evening.
Lingering storm threat may transition to more of a locally heavy
rainfall threat during the evening as a southwesterly low level
jet ramps up, and results in slow southward or even
backbuilding propagation.

Scattered thunderstorms remain possible on Tuesday as well,
though with somewhat more subtle forcing as a slow-moving weak
short wave lifts out of MO/downstate IL during the day. Better
chances look to be Tuesday night however, as a cold front and
more amplified short wave approach as outlined in the following
section of the discussion. Warm and humid summer-like conditions
will persist, with highs in the low-mid 80s.

Ratzer

Tuesday Night through Sunday:

There is good ensemble member support for showers and
thunderstorms affecting the region Tuesday evening and night,
resulting in high PoPs, especially overnight (70-90%). The key
large scale player will be a seasonably deep mid-upper level
trough from the Canadian Prairies to the Upper Midwest. Strong
surface low pressure deepening into the mid 980s mb over Manitoba
will drag its cold front toward the mid-upper MS Valley overnight
into Wednesday morning. While there is reasonable agreement in the
details above, convective trends are quite unclear for various
reasons.

The strongest forcing will be well off to our northwest to start
the evening. Thus, the presence of a convectively augmented
impulse or MCV lifting northeastward will likely be the key to
higher convective coverage during the evening hours to offset the
decrease of instability with sunset. Otherwise, any lingering
diurnally driven scattered showers and thunderstorms will likely
weaken/dissipate with the loss of heating and lack of deep layer
shear. The western half or so of the CWA may be in best position
to be affected by the potential MCV early enough for convective
maintenance, which is where the highest evening PoPs (~60-80%)
are located. Farther east in the CWA, confidence is lower (only
40-50% PoPs), and there may very well be a path to a relatively
quiet evening

Mid-level height falls will maximize to the north of our area as
the earlier mentioned deep trough axis ejects eastward, though
we`ll still see appreciable height falls (6+ DaM/12 hours at 500
mb) overspread the area after midnight Tuesday night. Unfavorable
diurnal timing suggests thunderstorm coverage may end up being
scattered, though with the approaching front and height falls,
shower coverage should be widespread enough to justify the 70-90%
PoPs. Outside of the (uncertain) convective trends, Tuesday night
will be fairly warm and muggy.

Showers and embedded scattered thunderstorms should continue to
translate eastward with the approaching cold front Wednesday
morning. Activity will likely be focused over the eastern half or
third or so of the CWA a few hours after sunrise. Unless
convective coverage is greatly diminished overnight and precip and
debris cloud cover ahead of the front is minimized Wednesday
morning through midday, the potential for meaningful
destabilization appears limited. This entails a low chance for any
organized/strong convection prior to the frontal passage.

By the late morning through midday on Wednesday, the cold front
will likely be east of I-57 and quickly sweeping eastward,
bringing any lingering showers and thunderstorms to an end for the
far east/southeast CWA by or shortly after 1PM CDT. The rest of
Wednesday will likely be drier, breezy, and increasingly sunny,
with seasonable temperatures in the upper 70s to lower 80s.

Deep upper level low pressure from the aforementioned strong
mid-week short-wave will then become quasi-stationary from the
northern Great Lakes across northeastern North America Thursday
through the weekend. The extended stretch of northwest flow in
this pattern will result in comfortable near to slightly below
normal temperatures (highs in the 70s to near 80F away from any
lake cooling, little chance of any meaningful rainfall, low
humidity (daytime dew points in the 40s and 50s), and breezy
daytime conditions. On this last note, Thursday will likely be a
windy day for June, with west-northwesterly gusts up to 30-35 mph.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1242 PM CDT Mon Jun 3 2024

Key Aviation Impacts for the current TAF period:

- Non-zero thunder chances tonight, but confidence was low
  enough to remove from the TAF

- Winds are expected to be prevailing 190, but occasional
  flopping to 170 is possible through Tuesday late morning

- Leading showers may arrive at Chicago terminals late Tuesday
  afternoon

Winds are slowly becoming more prevailing southwesterly;
however, some flip flopping to southeasterly is still possible
through the early afternoon. While there are storms just north
of KRFD moving to the northeast at the time this discussion was
published, the biggest change to the forecast was removing
thunder from the TAFs. There will still be enough instability to
generate a pop-up storm or two later today, latest guidance
keeps the best areas for storms well north of terminals. With
the probability less than 30 percent, it was decided to pull the
thunder from the TAF and monitor chances through the night.

There is low confidence in the exact wind direction overnight
(SE vs. SW), though gusts are expected to come down before
returning Tuesday afternoon. The next system that will bring
showers and storms to the region will likely arrive after 00Z
(and therefore outside the current TAF window). However, latest
high-res guidance suggests a weak boundary drifting northward in
the afternoon. Instability does not look impressive to add thunder,
and while timing may be pushed back in later TAF packages, felt
comfortable introducing -SHRA into the ORD/MDW TAFs.

DK

&&

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

&&

$$

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