710
FXUS63 KLOT 030855
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
355 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 of 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 /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1258 AM CDT Mon Jun 3 2024

Main Concerns:

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

- Direction of breezy southerly winds Monday PM

Convective trends are highly uncertain today through this
evening. An upper disturbance associated with a SHRA/TS complex
over Nebraska early this morning will track across southern and
central Wisconsin during the early to mid afternoon. While
current TS in the complex should weaken prior to reaching the MS
River, additional scattered SHRA and TS developing near and
south of the disturbance will probably extend into northwest
Illinois. Favoring a 2-4 hour window of TS in the vicinity of
RFD from the late morning through the early afternoon.

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, where confidence is lowest. Maintained PROB30
mention at the Chicago area TAF sites. The RFD area probably
has an overall 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.

Light east-southeast to variable/calm winds prior to sunrise
will increase from the south-southeast by the mid morning. The
strongest winds, including gusts to 20 to perhaps 25 kt, will
likely be from the mid afternoon through sunset. Direction will
be near due south to south-southwest, ranging from 180-210 deg.

Castro

&&

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

&&

$$

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