617
FXUS63 KLOT 031144
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
644 AM 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.

&&

.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 /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 644 AM CDT Mon Jun 3 2024

Main Concerns:

- Low confidence TS potential today, with RFD area more
  favorable overall for one or two rounds

Convective trends remain highly uncertain today through this
evening. A remnant upper disturbance over north central IA from
overnight thunderstorms will move to the central Wisconsin lake
shore this evening. As this feature approaches the MS River,
widely scattered SHRA and TS may develop across northwest
Illinois and track east-northeast across far northern Illinois.
Maintained VCTS mention in the RFD TAF from 16-20z. If this
activity does occur, it may pass relatively close to the
northern vicinity of DPA and ORD.

From the mid to late afternoon into the evening, it`s unclear to
what extent additional SHRA and TS will develop, especially
into the Chicago metro terminals, where confidence is lowest.
Again maintained PROB30 mention at the Chicago area TAF sites,
with small timing tweaks from the previous issuance. The RFD
area has a higher chance of additional SHRA/TS from the late
afternoon into the evening, but still didn`t have enough
confidence to upgrade from PROB30 mention there.

South-southeast winds of 10 kt or less will increase and shift
to 180-200 deg by mid day with gusts to around 20 kt through the
afternoon. Lighter southerly winds are expected later this
evening into Tuesday morning.

Castro

&&

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

&&

$$

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