-- Discussion containing changed information from previous version are highlighted. --
889
FXUS63 KLOT 012352
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
652 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 /00Z TAFS THROUGH 06Z MONDAY/...
-- Changed Discussion --
Issued at 652 PM CDT Sat Jun 1 2024

Forecast concerns include...

Ifr/lifr cigs/vis through daybreak Sunday.
Wind directions this evening.
Rain ending this evening.

A frontal boundary extends from near PIA to ARR to ORD and is
expected to slowly shift southeast tonight as an area of low
pressure moves along this boundary. Rain will slowly dissipate
this evening but there will likely still be showers near the
boundary through mid/late evening. Locally heavy rain will be
possible with any of these showers.

South of this boundary, winds are generally southerly and
gusting at times. North of the boundary, winds are generally
light north/northeast. As this boundary transitions south, a
period of variable winds can be expected at MDW and GYY. Winds
may stay light northerly tonight and then drift to the north/
northeast Sunday morning and then more easterly Sunday afternoon
into Sunday evening.

Ifr/lifr cigs are widespread north of the boundary and will
spread south tonight. While visibilities have improved where
the rain has stopped, areas of fog are expected to develop
tonight but confidence remains low. If and where cigs clear,
most likely across northwest IL and RFD, dense fog is possible.
Along the boundary, Chicago terminals and areas to the
southeast, dense fog is also possible but with prevailing lifr
cigs likely, vis may remain in the 1sm range. Confidence is too
low to make changes to the current forecast, other than to add
lower vis at RFD. Cigs/vis are expected to steadily improve
after sunrise Sunday morning. cms
-- End Changed Discussion --

&&

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

&&

$$

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