546
FXUS63 KLOT 020843
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
343 AM CDT Sun Jun 2 2024

.KEY MESSAGES...

- Patchy fog possible early this morning, mainly near the IL/WI
  border and in northwest IL.

- Scattered thunderstorms possible Monday and Monday evening
  and Tuesday, with a possible period of higher thunderstorm
  coverage Tuesday evening in parts of the area.

- Last chance of thunderstorms for the week looks to be on
  Wednesday, mainly in the morning southeast of I-55.

&&

.DISCUSSION...
Issued at 343 AM CDT Sun Jun 2 2024

Through Monday:

Weak surface low pressure was over western lower Michigan early
this morning, with a weak surface cold front trailing across
the southeastern cwa counties and into southeast Missouri. A few
spotty showers were decreasing in coverage but were still
present along the front mainly across northwest Indiana as of 3
AM, and a slight (<20%) chance will persist southeast of a
Bloomington to Kankakee to Burns Harbor IN line until the front
clears the forecast area a few hours after sunrise. Behind the
front some patchy fog (briefly somewhat dense earlier) had
developed, though a combination of extensive low stratus and
9-12 kts or so of northerly flow above the boundary layer has
limited the extent of dense fog across the area.

We can`t rule out some patchy denser fog developing in spots
west of the Fox River Valley and north of I-88 across far
northern IL through sunrise, where a couple of holes were noted
in the stratus deck and boundary winds are lighter, though the
threat of any widespread dense fog appears low. After sunrise,
stratus and any fog should gradually lift and break up through
mid-late morning, with partly cloudy skies by this afternoon.
Temperatures should rebound into the mid-upper 70s in most
areas, though east-northeast lake breeze winds will limit highs
to the mid-upper 60s along the Lake Michigan shore.

Quiet weather will continue this evening, with attention then
turning to our west/northwest where several clusters of
thunderstorms are expected to develop ahead of a cold trailing
from surface low pressure which is progged to lift northeast
across northern MN by Monday morning. While the best large-scale
forcing for ascent looks to be focused well to our northwest,
several models suggest that evening storms over the mid-Missouri
Valley may generate an MCV/convectively enhanced vort which
will then track into the IA/WI/IL border region toward sunrise,
accompanied by weakening convection (poor diurnal timing,
outrunning better MUCAPE to the west). Some of these may spread
across the I-39 corridor as in a decaying mode around sunrise,
with coverage and thunder potential decreasing farther east into
the Chicago metro area.

Scattered thunderstorm potential will likely increase by Monday
afternoon however, as low-level southerly flow strengthens
ahead of the cold front off to our west, and another mid-level
short wave (also potentially convectively-enhanced by storms
across the TX/OK panhandle region the previous evening)
approaches during move favorable diurnal conditions and
increasing low-level instability (warmer and more humid Monday
with temps in 80s/dew points in the mid- 60s), especially across
western portions of the forecast area. Various CAM runs have
depicted regeneration of storms across parts of western IL by
mid-late afternoon, though some spread remains and confidence in
details/evolution remains low at this time. Background deep
shear is not too impressive in the global guidance, but would
likely be enhanced if a well-developed MCV were present. SPC has
included our western cwa in a marginal (level 1/5 risk) for
severe weather in their day 2 outlook for Monday into Monday
evening.

Ratzer


Monday Night through Saturday:

- Scattered thunderstorms possible Monday evening and Tuesday,
  with a possible period of higher thunderstorm coverage Tuesday
  evening in parts of the area

- Last chance of thunderstorms for the week looks to be on
  Wednesday, mainly in the morning

On Monday evening, any lingering scattered thunderstorms past
sunset should dissipate with the diurnal loss of heating and
instability. See the short term discussion above for details on
the lower end (level 1/marginal) severe threat Monday PM.
Primarily quiet conditions are expected overnight, though can`t
completely rule out a few isolated showers or non-severe
thunderstorms (~20% PoPs).

Tuesday`s forecast has been oscillating a bit regarding the
potential for daytime convection. On the most recent (00z 6/2)
model suite, there was a shift toward a coherent mid-level
impulse lifting northeast across the area during peak heating.
Much more favorable mid-level moisture with this feature
entails a quicker erosion of capping as temps rise into the mid
to upper 80s with dew points in the upper 60s to around 70F.
Even with the mid-level wave traversing the area, height falls
and mid- level cooling will be modest at best, as well as only
20-30 kt of southwest flow from 700-500 mb (sub-marginal deep
layer shear). Uncapped 1500-2000 J/kg of MLCAPE intercepted by
the mid-level impulse should result in scattered pulse-type
convection. The lack of wind shear but moderately strong
instability and steep low- level lapse rates may support a
localized downburst threat. Relatively slow storm motions and
seasonably high PWATs between 1.5-1.75" also suggest the
potential for ponding on some roads.

Turning ahead to Tuesday evening and overnight, the lead impulse
will be exiting to the northeast, with a much stronger short-wave
impinging upon the northern Plains and upper Midwest. Extensive
convection out ahead of the system`s cold front west of the MS
River will translate southeastward into a less supportive
environment (diurnal decrease in instability and marginal wind
shear). Thus following the probable dissipation of afternoon pulse
convection, it`s a bit unclear how robust upstream storms will be
as they approach from our west in the late evening and overnight.
While we do have PoPs peaking in the 60-80% range given the
overall global model and ensemble member signal for convection
during this time, there conceptually appears to be a path toward
lower convective coverage surviving eastward overnight.

The cold front will sweep across the area Wednesday morning.
Unless overnight convection dissipates much quicker and sets up
for destabilization ahead of the front by the mid to late morning
across much of the area, it appears that areas southeast of I-55
have the best chance for any renewed development ahead of the
front, or intensification of existing showers and embedded
thunderstorms pushing eastward. 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. This will bring a period of near to
slightly below normal temperatures (highs in the 70s away from
any lake cooling, primarily dry weather, comfortably low
humidity (daytime dew points in the 40s and 50s), and breezy
daytime conditions.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1252 AM CDT Sun Jun 2 2024

Main Concerns:

- IFR to LIFR CIGs and IFR to MVFR VSBY early this morning.

It appears the chance of dense fog has diminished enough to go
a bit more optimistic in the TAFs. Barring the development of a
larger break in the low clouds over the area, enough northerly
flow aloft and not quite saturated low levels should result in
low CIGs being more of the issue, with IFR to LIFR expected
until steady improvement after sunrise. Still expect IFR VSBY at
times at MDW, GYY, and possibly ORD.

Light north to north-northwest winds will shift to north-northeast
after sunrise (except VRB at RFD) and then northeast by mid day
with speeds possibly near 10 kt at ORD, MDW, and GYY. Light
easterly winds are in store tonight.

Castro

&&

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

&&

$$

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