-- Discussion containing changed information from previous version are highlighted. --
155
FXUS63 KLOT 312342
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
642 PM CDT Fri May 31 2024

.KEY MESSAGES...

- Rainy Saturday.

- Periodic shower and thunderstorm chances Monday-Wednesday.

&&

.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 /00Z TAFS THROUGH 06Z SATURDAY/...
-- Changed Discussion --
Issued at 642 PM CDT Fri May 31 2024

Forecast concerns include...

Rain developing Saturday morning.
Mvfr/Ifr cigs Saturday afternoon/evening.
Possible fog Saturday evening/early Sunday morning.

Areas of rain are expected to spread across the area after
sunrise Saturday morning and start times will likely need some
refinement as trends emerge. The rain is expected to continue
through mid/late afternoon and then taper off to showers by
sunset. The rain may be moderate/heavy at times. There is a
small chance for thunder, especially Saturday afternoon, but
confidence is too low to include mention in the tafs.

Cigs will lower through mvfr and likely into ifr Saturday
afternoon. Only medium confidence for timing for ifr cigs, which
could remain sct/bkn in the afternoon, with prevailing ifr cigs
expected through most Saturday evening. Mvfr vis is expected
with the rain Saturday afternoon but after the rain ends
Saturday evening, some fog development is possible. Confidence
is low, but after sunset Saturday evening into early Sunday
morning will need to be monitored for the potential for areas of
fog and possibly dense fog.

Southeast winds around 10kt with a few higher gusts will
continue through sunset this evening then speeds will diminish
under 10kts. Winds will turn more easterly Saturday and then
there is uncertainty for wind directions Saturday evening, which
will be dependent on where an area of low pressure tracks. cms
-- End Changed Discussion --

&&

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

&&

$$

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