890
FXUS63 KLOT 010827
AFDLOT

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

.KEY MESSAGES...

- Rain/showers overspread the area this morning and persist
  through the day. Some localized rainfall amounts of an inch
  or more are possible, which could cause some ponding of water
  in typical low spots this afternoon.

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

&&

.DISCUSSION...
Issued at 327 AM CDT Sat Jun 1 2024

Through Sunday:

Surface low pressure was analyzed over central Missouri early
this morning, associated with a slow-moving mid-level short wave
trough which was evident in GOES vapor imagery across
Missouri/Arkansas. The surface low will gradually fill while
lifting east-northeast across downstate Illinois and central
Indiana through tonight, as the mid-level trough tracks across
the region.

Despite dry air in the 850-500 mb layer noted in DVN and ILX
00Z RAOBs from last evening, southerly 35-40 kt winds above the
surface (925-700 mb) ahead of the approaching mid-level trough
will help to transport deeper moisture (precipitable water
values approaching 1.70" or 160 percent of normal) into the area
later this morning. This will set the stage for a rainy/showery
Saturday. HREF and EPS ensembles depict average rain amounts in
the 0.35-0.75" range through this evening, with highest amounts
generally forecast across the eastern 2/3 of the cwa.
Deterministic (HRRR/RAP) and ensemble (HREF max QPF) CAM
solutions continue to indicate the potential for some higher
(1-2 inch) amounts however, roughly in the Peru/Pontiac to
Chicago corridor. While confidence in these higher amounts and
their placement is somewhat low, it has been a fairly consistent
signal and can`t be ruled out given the available moisture and
the relatively slow movement of the mid-level wave. Guidance
trend toward less thunderstorm potential (poor lapse rates and
MUCAPES only <250 J/kg mainly across the south this afternoon)
may help to keep 1"+ amounts isolated. Ponding of water in low
spots would be possible where these heavier rains occur this
afternoon.

Rain/showers should gradually taper off from west to east
across the area this evening, though moist low levels may
maintain some spotty shower/drizzle potential through midnight
before the mid-level trough axis passes. Have maintained a
patchy fog mention overnight (perhaps as early as later this
afternoon over Lake Michigan) with moist ground and moist low-
level conditions persisting. Some indication that low stratus
may persist much of the night however, which could limit how
thick fog becomes.

Dry weather returns to the area Sunday, as weak surface high
pressure drifts in from the west. Clouds may linger especially
during the morning, though at least partly sunny conditions
should develop. Temperatures today will likely be limited to the
upper 60s/near 70 in most spots due to thickening overcast and
the arrival of rain. Highs Sunday look to rebound to the upper
70s/around 80, though with onshore winds off of Lake Michigan
limiting shore temps to the mid-60s.

Ratzer

Sunday Night through Friday:

Quiet weather will persist locally on Sunday night due to the
continued influence of weak high pressure centered over Lake
Michigan. Meanwhile, 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 east-southeast. It`s likely that
the farther east you go in the CWA, particularly the eastern
half, 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 remains unclear.
Pockets of heating near the warm frontal zone may be enough for
1000-1500 J/kg of MLCAPE to develop with minimal capping as temps
away from the immediate Illinois shore warm to the lower to mid
80s with dew points well into the 60s.

Given the uncertainty regarding convective coverage Monday
afternoon, we`ve lowered our forecast PoPs into the ~30-50% range,
highest western half of the CWA. The best chance for scattered
thunderstorms will probably be in the mid-late afternoon into the
early evening when capping may be eroded. Marginal deep layer
shear will result in mostly non-severe storms, but can`t rule out
a few gusty storms with 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 only element that might keep pockets of elevated
convection going (and possibly focused farther south than implied
by the official PoPs) beyond typical evening diurnal decay would
be another weak mid-level short-wave translating from central
Illinois into Indiana.

The next period of interest will be Tuesday PM and night, possibly
into Wednesday morning. As things stand now, our area may be
yet again caught in the middle timing wise for a higher chance
of organized convection. Despite the presence of dry air at the
mid-levels (h7-h5 RH 25% or less) and likely at least some
capping and a lacking large scale trigger mechanism, isolated
to widely scattered diurnal pulse-type convection may be able to
develop Tuesday afternoon as heating erodes the capping amidst
mid 80s temps and dew points pushing 70F. Suspect our Tuesday
afternoon PoPs in the 40-60% range may be a bit aggressive.

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 another potentially favorable synoptic pattern for 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 still 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`s cold front will likely 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 continue to hinge upon a slowing of the
cold frontal approach and passage. Maintained 30-50% chances of
showers and slight chance of t-storms Wednesday PM, but a
scenario of a primarily dry afternoon and early 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
and drier air mass into next weekend. There may be a few showers
with the digging short-wave and secondary cold front Wednesday
evening and night, likely followed by primarily dry conditions
thereafter through day 7 (Friday) and the start of the weekend.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 107 AM CDT Sat Jun 1 2024

Main Concerns:

- Rain developing by the mid to late morning with VSBY as low as
  2-3SM during the afternoon-early evening

- Deteriorating CIGs expected, with IFR probable during the
  afternoon and at least periodic LIFR in the evening and overnight

- Fog/BR and lingering -SHRADZ after the rain ends this evening,
  with 1-2SM or lower VSBY possible for the Chicago metro sites

The good flying conditions will come to an abrupt end today as a
weather disturbance affects the area. Initial light rain/showers
should start at the terminals by around 15z, with embedded
higher rates likely knocking VSBY down to MVFR at times toward
midday. Prevailing light to moderate rain with 2-3SM VSBY (3SM
indicated in the TAFs) will then persist until a bit after
sunset. This will be followed by occasional drizzle or light
showers until more pronounced drying occurs in the pre-dawn
hours of Sunday. With the surface low moving overhead tonight,
the period of -SHRADZ looks to be accompanied by IFR BR and
potential is there for LIFR dense fog in spots.

CIGs will likely trend downward to MVFR and then IFR in the
afternoon, though temporary scattering of the IFR CIGs will be
possible until sunset. After sunset, there is a strong signal in
the guidance for LIFR CIGs at the Chicago metro terminals, with
TEMPO mention in the TAFs for now. South-southeast winds through
this morning will become easterly to east-northeast this
afternoon and evening and then light northerly to variable late
this evening and overnight.

Castro

&&

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

&&

$$

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