309
FXUS63 KLOT 310830
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
330 AM CDT Fri May 31 2024

.KEY MESSAGES...

- Rain and embedded thunderstorms Saturday.

- Periodic shower and thunderstorm chances Monday-Wednesday.

&&

.DISCUSSION...
Issued at 329 AM CDT Fri May 31 2024

Through Saturday Night:

Early morning surface analysis depicts weak high pressure
across the Great Lakes region. The high will drift slowly off to
the east today, allowing low-level winds to turn southerly and
associated warm advection to help boost temperatures into the
upper 70s/near 80 in many spots today. Locations along the IL
Lake Michigan shore will be a little cooler (low-mid 70s) thanks
to an east-southeasterly wind component there and an afternoon
lake-breeze push. A persistent but thin cirrus layer aloft
should still allow for partly to mostly sunny skies through the
day.

Attention then turns to a slow-moving mid-level short wave
trough taking shape across the central/southern Plains and lower
Missouri Valley region. This disturbance is progged to lift
northeast across the mid-Mississippi Valley and western Great
Lakes region through Saturday, with an accompanying weak surface
low tracking across downstate IL Saturday and central IN
Saturday night. Guidance spreads deeper moisture northward into
the forecast area Saturday morning, setting the stage for a
rainy/showery day as the wave moves across the area. Various
models continue to show some spread with respect to the greatest
areal coverage of rain and QPF amounts, though the EPS and CMC
ensembles have been fairly consistent in favoring the southeast
half of the cwa, generally along and southeast of the I-55
corridor for highest pop and QPF focus.

Precipitable water values increase to 1.75" (165% of normal)
across these areas Saturday, suggesting the potential for some
1.00"+ amounts (especially noted in some CAM and deterministic
model QPF) though the EPS/CMC ensemble and the 06Z HRRR
generally place the axis of heaviest rain amounts off to the
south and southeast of the forecast area where convection looks
to be more favorable. Mid level lapse rates are not very
impressive per model forecast soundings, generally in the
5.5-6.5 C/km range, suggesting mainly an embedded thunder threat
within the broader area of rain/showers. Best chances for
thunder looks to be Saturday afternoon/early evening where some
weak (<500 J/kg) MUCAPE is depicted. WPC has the southeast 2/3
of the cwa in a marginal threat (level 1 out of 4) for excessive
rainfall for Saturday and can`t argue with that given the
higher pwats. Global ensemble QPF is generally in the 0.50-
0.80" range across our southeast cwa, which may be more
representative for rainfall amounts where convection is more
limited. Rain/showers should end Saturday evening across our
western counties, and gradually end from the west across the
remainder of the area Saturday night as the mid-level short wave
axis shifts east of the area.

After low temperatures in the 40s in spots this morning,
increasing clouds and south-southeast flow tonight will keep
overnight lows in the mid-50s in most areas. Considerable cloud
cover and rain on Saturday looks to limit daytime highs to
around 70 degrees, with lows in the 50s expected again Saturday
night. Can`t rule out some patchy fog or low stratus development
Saturday night due to moist ground conditions, light winds and
late clearing.

Ratzer


Sunday through Thursday:

Weak high pressure centered over Lake Michigan will bring a quiet
and seasonable day on Sunday. Some pockets of stratus and/or
fog may hold on past sunrise near Lake Michigan and in our
southeast CWA. Expectation for the afternoon is partly cloudy
skies for most of the area from fair weather Cu. Highs will
reach the upper 70s to around 80F inland and upper 60s to lower
70s near the lake (mid 70s a bit farther inland) due to onshore
flow and then lake breeze reinforcement.

Fairly widespread convection will get going Sunday evening well
to our west and northwest (MO Valley to the northern Plains and
upper Midwest) ahead of a cold front trailing from low pressure
tracking toward central Canada. This will occur as a short-wave
intercepts the frontal zone. A relatively sharp west to east
MUCAPE gradient from the mid-upper MS Valley to the Great Lakes,
plus the strongest large scale forcing peeling north of the region
will likely mean whatever convection pushes into areas near and
west of I-39 Monday morning should be in a weakening phase as
it tracks southeast. It`s plausible that the farther east you go
in the CWA, any decaying earlier convection could miss to the
west/southwest.

The presence of the decaying convection, the effects of any
outflow associated with it, and lingering debris cloud cover,
render the Monday forecast increasingly uncertain into the
afternoon. A warm front should be draped across the area,
conceivably merging with retreating outflow from the morning.
Given neutral height tendencies in the PM hours (modest at best
larger scale forcing from the weakening short-wave lifting
northeast), and items above that may limit destabilization across
at least portions of the area, the exact focus or trigger
mechanism for afternoon convective development is a bit unclear.
Pockets of heating near the warm frontal zone may be enough for
1000-2000 J/kg of MLCAPE to develop with minimal capping as temps
warm to the lower 80s with dew points well into the 60s.

Our forecast has likely (~60%) PoPs for the Illinois portion of
the CWA and chance PoPs for our Indiana counties Monday afternoon,
though this may be a bit broad-brushed in reality. Capped
thunderstorm mention at chance/scattered type coverage. Marginal
deep layer shear will result in mostly non-severe storms, but
can`t rule out a few feistier storms with downbursts and small
hail. Anticipating a relatively quick diurnal decay of showers
and thunderstorms lingering into the evening given lacking deep
layer shear and large scale forcing.

The next period of interest will be Tuesday PM and night, possibly
into Wednesday. As things stand now, our area may be yet again
caught in the middle timing wise for a higher chance of organized
convection. Isolated to widely scattered diurnal pulse-type
convection may be able to develop Tuesday afternoon as capping
erodes amidst mid 80s temps and dew points pushing 70F. A
seasonably strong short-wave trough and associated surface low
ejecting from the northern Plains into the Canadian Prairies will
drag its cold front eastward Tuesday night.

This looks to be yet another favorable synoptic pattern for
extensive severe thunderstorms to initiate late day Tuesday into
Tuesday evening out ahead of the cold front and near the
lifting warm front, from the Missouri Valley to the northern
Plains. For our area, if this sort of timing and evolution comes
to pass within the realm of the recent guidance consensus,
decaying convection may push into our area from the west in the
evening and overnight. While it`s too early for a firm
prediction, it doesn`t currently appear that the ingredients
will be in place to maintain a strong nocturnal MCS well
eastward across the local area Tuesday night.

The system cold front may also sweep across the area at a less
favorable time diurnally (Wednesday AM-mid day) on the heels of
the weakening overnight convection. Thus prospects for afternoon
redevelopment will likely hinge upon a slowing of the cold frontal
approach and passage. For now, maintained 40-50% chances of
showers and t-storms Wednesday PM, but a scenario of a primarily
dry afternoon and evening is firmly in the realm of possibility.
The upper level low from Wednesday`s frontal passage may become
trapped near or north of Lake Superior to close out the work week,
resulting in a cooler (how much cooler is uncertain) and drier
air mass into the following weekend.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1220 AM CDT Fri May 31 2024

Main Concern:

- Timing of likely lake breeze wind shift to easterly this
  afternoon at ORD, MDW, and GYY

VFR conditions will persist through this TAF cycle. Spotty VFR
showers may approach ORD and MDW toward 12z Saturday, with the
meaningful rain expected after the current TAF period.

Light easterly to VRB/calm winds early this morning will become
southerly after sunrise. Direction will probably vary between
160 and 200 degrees at ORD and MDW. As the lake breeze takes
shape this afternoon and slowly pushes inland, direction may
favor due south to just west of south. We`re still forecasting
the wind shift to easterly near 10 kt during the 22-23z hour,
though confidence is only medium. Some of the guidance is a bit
stronger with the opposing southerly winds and delays the shift
to a light southeast/east-southeast direction until after 00z at
ORD and MDW. While this isn`t the current most likely scenario,
it`s something to monitor for subsequent updates.

Castro

&&

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

&&

$$

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