191
FXUS63 KLOT 010607
AFDLOT

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

.KEY MESSAGES...

- Rainy Saturday.

- Periodic shower and thunderstorm chances Monday-Wednesday.

&&

.UPDATE...
Issued at 859 PM CDT Fri May 31 2024


Cirrus clouds continue to expand across northern IL northwest
IN this evening as an upper-level shortwave and its associated
surface low inches into central IL. Recent radar out of eastern
MO shows a broad area of light to moderate showers along the
eastern fringe of the surface low and it will be this axis of
rain that will spiral into our area around daybreak tomorrow
morning. The expectation is still for widespread off and on
showers to persist through tomorrow evening as the
aforementioned wave and low pivot through the area, but there
is a growing concern for the threat of isolated embedded
thunderstorms.

Latest 18z guidance has started to show some pockets of better
instability developing across our southern CWA (roughly areas
south of I-80) tomorrow afternoon which may be sufficient to
produce at least a few stray lightning strikes. However, this
instability continues to look rather weak, only around 200-300
J/kg of MLCAPE, which in combination with the limited coverage
does not offer enough confidence to add a formal thunder
mention to the forecast. Though, this will be a forecast point
that I will pass along to the midnight shift to take another
look at. Regardless, of the coverage of storms Saturday
afternoon no severe weather is expected.

Additionally, there is also the continued threat for an axis,
possibly two, of heavier rainfall. While the latest guidance
continues to very on the exact location of the heavy rainfall,
it seems the growing consensus for areas near a Pontiac, IL to
Gary, IN line to be within the crosshairs of at least one of the
bands. Though there is lesser consensus on where, or even if,
the second band develops. Regardless, rainfall totals in excess
of an inch seem like a plausible outcome within the heavier
bands which could lead to some instances of ponding if this axis
is to occur over any urban areas.

Given that these aforementioned points seem to be well handled
in the prevailing forecast I saw no need to make any adjustments
aside from mentioning for the midnight shift to take a closer
look at thunder potential.

Yack

&&

.DISCUSSION...
Issued at 312 PM CDT Fri May 31 2024

Through Saturday Night:

Upper trough over the southern Plains this afternoon will move
northeast and across the mid Mississippi Valley Saturday. Dry
conditions are expected this evening and most of the overnight
hours with some rain possibly moving into southwestern portions of
the CWA toward dawn Saturday. Showers are expected to overspread
the remainder of the CWA Saturday morning and continue much of the
day.

Cloud cover and widespread rain look to prevent any meaningful
destabilization, so thunder chances appear quite low with this
system. Despite that lack of thunder, rather strong forcing, a
moist air mass (PWATs >150% of normal), and the slow moving
nature of the trough could result in some formidable rainfall
totals. Widespread rainfall of a half inch to around an inch and a
quarter are likely, with some locally heavier totals possible.
The steadier nature of the rain should limit any significant
flooding concerns, but a many areas will see a good soaking with
standing water in typical poor drainage areas. Rain should end
from west to east late Sat afternoon into Saturday evening.

Given the expected cloud cover and rain, have lowered high temps
for Saturday, with highs now generally expected to be in the mid-
upper 60s (possibly still a bit generous). Given the expected
rainfall and likelihood of winds going calm, will need to watch
for some patchy fog overnight Saturday night.

- Izzi

Sunday through Friday:

Weak high pressure over Lake Michigan will bring seasonable
weather to the area on Sunday. Cloud cover will decrease
throughout the day with temperatures rising into the high 70s
and low 80s. Winds will be light, however, onshore flow will
allow for the development of a lake breeze, keeping temperatures
a little cooler along the lakeshore (high 60s to low 70s).

Late Sunday evening, a shortwave will move across the Plains
with an associated surface low lifting through the northern
Plains and into Canada. Convection will fire along the cold
front of this system well to our west and begin advancing east
overnight. The instability gradient appears to be fairly sharp
and with the surface low and cold front lifting into Canada, the
storms will be in the weakening phase by the time they reach
the western portions of the CWA early Monday morning. The
precipitation will continue to dissipate as it advances eastward
and it appears possible that areas in the eastern portion of
the CWA may not see any precipitation Monday morning.

Monday`s forecast has a lot of uncertainty still remaining with
it due to the remnant convection that will be present Monday
morning and the associated leftover cloud cover from the
dissipating showers and storms. A warm front will lift across
the area on Monday with southerly flow bringing increased dew
points. If the sun is able to peak out for a bit then pockets of
1000-1500 J/kg of CAPE may be able to develop along the frontal
boundary. Forcing for convection looks uncertain as large scale
forcing appears to be limited and poorly placed with the
instability as the shortwave continues to lift northeast. Any
storms that are able to develop will likely not be strong as
deep layer shear will be weak, however, it cannot be ruled out
that storm is able to produce some strong winds or small hail.

Diurnal heating and moisture return on Tuesday will once again
allow for CAPE values to grow Tuesday afternoon. However, pretty
stout capping appears to be in place. If the cap is able to
erode throughout the afternoon, it is possible that some storms
may be able to form, however, deep layer shear will not be
present, so there will not be any severe storms.

On Tuesday another shortwave will push across the Northern
Plains and Canada with a surface low in southern Canada. A cold
front will push east across the Mississippi River Valley
bringing convection into the area late Tuesday night. The exact
timing is still uncertain, but it appears that the cold front
and associated convection will move through later Tuesday night
or early Wednesday morning. There appears to be potential for
severe storms across the Mississippi Valley Tuesday afternoon
and evening, however, instability will be quickly decreasing
after dark Tuesday night, so the convection will likely be
weakening as it reaches the CWA.

Following the passage of the cold front early Wednesday
morning, storm chances look marginal Wednesday afternoon. PoPs
chances were left in the forecast, but it looks possible that
Wednesday could be a completely dry day following the overnight
convection and cold frontal passage. The overall synoptic
pattern will shift to a primarily northwest flow pattern as the
low gets stuck in southern Canada, near Lake Superior. This will
allow cooler temperatures and dry conditions through the
remainder of the work week.

Carothers

&&

.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.

&&

$$

Visit us at weather.gov/chicago