137
FXUS63 KLOT 011926
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
226 PM CDT Sat Jun 1 2024

.KEY MESSAGES...

- Showers continue into the early evening but become more
  scattered in nature. Main threat for an isolated storm west
  and south of Chicago.

- Patchy dense fog possible tonight.

- Periodic chances for showers and thunderstorms Monday into
  Wednesday morning.

&&

.DISCUSSION...
Issued at 220 PM CDT Sat Jun 1 2024

Through Sunday Night:

Looking at decreasing coverage of a more cohesive region of
showers/rain across NE IL and NW IN through mid afternoon as the
primary region of low-level warm advection on the nose of a stout
40 knot southerly 850 mb jet moves east of the region. Through
the rest of the afternoon: scattered showers and perhaps an
isolated storm or two will develop, preferentially in the
vicinity of a surface convergence axis which currently arcs from
near Springfield, IL, northeastward to Pontiac. In this region,
visible satellite shows a bit more vertical growth, with echoes
evident off regional radar mosaics. This convergence axis will
meander northeastward towards a La Salle to O`Hare line through
the late afternoon as a diffuse low/surface circulation scoots
into NW Illinois.

Did add a low mention (15-20 percent chance) for a storm or two
in the vicinity of this convergence axis, and in particularly
closer to central IL where mid-level temperatures will be the
coolest. Given the moist-adiabatic nature to the thermal profile
today with lapse rates generally near or under 6 C/km, not
expecting much in the way of lightning, but there`s just enough
vertical growth to our southwest to support an isoT mention this
afternoon. Given very weak flow in the column, cell motions will
be slow, resulting in a threat for highly localized/isolated areas
of more appreciable rainfall (on the order of 1-2 inches).
Finally, can`t totally rule out a funnel cloud this afternoon in
the vicinity of La Salle/Livingston counties where the low-level
CAPE/lapse rate axis will impinge on the aforementioned
convergence axis (have already seen a picture of one sent from far
west-central Illinois).

Activity will diminish through the evening, although suspect we
may not fully lose spotty shower activity until a cold front
sweeps through the region overnight. With moist low-levels, fog
development is possible although a recent trend towards somewhat
stronger post-frontal northwest winds casts uncertainty on this
part of the forecast. Did not make many changes to the inherited
patchy fog grids, with pockets of dense fog possible overnight.

Sunday looks dry and pleasant, with highs in the mid 60s lakeside
to the upper 70s/near 80 farther inland, and morning stratus
should gradually erode, lift, and scatter. A complex of
thunderstorms is forecast to press across Iowa Sunday evening and
overnight but with little/no deep layer shear nearby and the
primary MUCAPE axis relegated well to our west, any MCS should
weaken appreciably as it approaches our W/NW towards daybreak
Monday morning.

Carlaw


Monday through Saturday:

A slow-moving trough within relatively weak WSW flow aloft will
track from the central Great Plains into the western Great
Lakes through Monday night. While the evolution of upstream
convection on Sunday will have bearing on the forecast across
the area Monday into Monday night, some decaying convection
should survive into the western CWA late Monday morning. The
combination of a slightly capped environment and some forcing
from the weakening wave suggests isolated to scattered
thunderstorms should develop across most of the forecast area by
late afternoon and into the evening hours. A few strong storms
with small hail and gusty winds cannot be ruled out,
particularly across the northern CWA and into Wisconsin
conditional on any notable enhancement of mid-level flow ahead
of the wave.

By Tuesday, subtle height rises, little organized forcing, and
increasing low-level capping should keep most of the area dry
through at least mid-afternoon before a modest mid-level wave
fosters a band of showers and storms across the CWA Tuesday
night. While dry conditions are likely for most of the day, have
maintained chance PoPs in the forecast given the potential for
upstream convective debris with weak MCVs to interact with
steepening mid-level lapse rates. If a few storms were able to
develop, locally strong winds would be possible.

The cold front will begin to usher in a cooler airmass and
decreasing shower and storm chances on Wednesday. Deeper
troughing across the far northern Great Plains will then expand
into a broad upper-level low across Ontario and the Great Lakes
region late in the week. Seasonably cool temps are likely
Thursday through Sunday, with a period or two of diurnally
enhanced showers possible during this time.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1251 PM CDT Sat Jun 1 2024

Key Aviation Messages:
- IFR ceilings this afternoon through tonight, with periods of
  LIFR ceilings possible tonight.

Periods of RA/SHRA will continue this afternoon as ceilings
steadily lower from low-end VFR levels into IFR levels. While
precip coverage and intensity will greatly diminish this
evening, IFR ceilings will lower into LIFR levels at times late
this evening into the overnight hours. Expected FG over Lake
Michigan should remain east of ORD/MDW overnight given NNW
winds, but may begin to advect inland around daybreak as winds
veer NNE. While diurnal trends favor rising ceiling heights
through the morning, it is possible that IFR ceilings persist
through at least mid-morning if upstream FG over the lake
becomes more widespread. Ceilings will ultimately lift to VFR
levels and/or SCT by Sunday afternoon.

Winds should remain E/SE around or under 10 knots through the
afternoon, then become VRB at 5 knots or less this evening as a
surface trough drifts over the area. Winds behind the trough
will settle NNW for several hours overnight before veering
NNE/NE Sunday morning.

Kluber

&&

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

&&

$$

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