769
FXUS63 KLOT 071147
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
647 AM CDT Sun Jul 7 2024

.KEY MESSAGES...

- Chance (30 to 40 percent) for scattered showers and
  thunderstorms today, mainly north of I-80.

- A cold front moves over the area on Monday providing another
  chance for scattered showers and thunderstorms as it moves
  west to east over the forecast area.

- Increasing heavy rain potential for at least portions of the
  area Tuesday night and Wednesday.

- Increasing heat and humidity next weekend.

&&

.DISCUSSION...
Issued at 330 AM CDT Sun Jul 7 2024

Through Monday:

This morning will start out dry with higher clouds moving in
from the west. A small complex of showers and thunderstorms is
gradually moving northeastward from eastern Iowa into
southwestern Wisconsin ahead of a an upper level short wave
swinging over Minnesota. As this moves east, it will increase
the probability for the development of showers and
thunderstorms later this morning for areas along the state
line, particularly around the Rockford Metro Area.
Unfortunately, what happens after noon is fraught with
uncertainty as models are still struggling with convection for
that time period.

There is another complex of thunderstorms that has a slight
signature of an MCV in northern Missouri that is slowly making
its way to southeastern Iowa. The HRRR is still suggesting that
as it moves eastward, it will slowly decay and fall into just
lightly scattered showers, keeping most of the area dry. Other
high res models like the NAM Nest, the RRFS and RDPS are
suggesting that a weak surface front extending from the Quad
Cities to Milwaukee will develop and provide enough surface
convergence to support the development of additional showers
and thunderstorms, especially north of I-80. On top of that,
models keep winds out of the south to southwest through much of
the day, though there is the potential for a weak lake breeze to
develop. Most model soundings, including the drier HRRR
mentioned above, suggest that destabilization will occur, the
question will be what exactly will be the forcing mechanism to
set it off. The good news is that wind shear looks to be 20
knots or less and mid-level lapse rates should be less than 6
degrees C/km, so severe weather is not expected.

While confidence remains low with the areal coverage of storms
across northern IL this afternoon, we have opted to spread
increased PoPs of 30 to 40 percent farther east into the Chicago
metro area, especially north of I-80. Lastly, it does appear
that these showers and storms (if they do develop) look to be
more transient as they pass over. So perhaps the strongest cells
could provide some momentary ponding on roads, the flooding
risk remains low. The only area that is under the Level 1 out of
4 risk for excessive rainfall is a small portion of westernmost
Lee, Ogle, and Winnebago counties due to the potential more
frequent showers through the day where there are higher p-wats.

The long wave upper level trough narrows, deepens, and becomes
more positively tilted Sunday night and it slowly sags into
northern Illinois during the overnight period into Monday
morning. There could be a brief window of drier conditions after
6 PM tonight as it loses day time heating. However, as Monday
approaches the probability of scattered showers and
thunderstorms increases. Persistent southwesterly steering flow
will continue to advect in p-wats around 1.5 inches with a
surface cold front moving over the area to provide the forcing
mechanism. Severe weather is not expected, but depending on what
transpires with the rainfall today, potentially Monday could
have some flooding issues to the west. This system will move in
from the west and gradually pass to the east through the day
Wednesday providing a nice soaking rain with seasonal
temperatures in the 80s.

DK


Monday Night through Saturday:

Over the past 24 to 48 hours model and ensemble guidance have
continued to favor a faster and farther north track of the post
tropical remnants of Beryl into the Ohio Valley late Wednesday
into early Thursday. Accordingly, there is an increasing threat
for moderate to heavy rainfall and breezy winds affecting at least
parts of our area late Tuesday night and Wednesday. In
collaboration with neighboring offices, we have thus opted to
increase POPs from the NBM into the likely category (~60%). These
likely POPs cover areas roughly along and southeast of the I-55
corridor, where the highest threat of heavy rain and potential
flooding concerns currently reside. Farther refinements to the
forecast for Tuesday night and Wednesday will undoubtedly be
needed as we continue to hone in on the exact path Beryl takes
into the mid-week period. Stay tuned!

Following the passage of Beryl`s remnants later Wednesday, the
remainder of the week is looking to be a bit more tranquil.
However, with aggregate upper troughing lingering across the
Great Lakes region Thursday into Friday, we cannot completely rule
out the low (~20% chance) possibility some isolated to widely
scattered afternoon showers and storms. However, a good amount of
dry time is looking likely both days.

Temperatures through Thursday are expected to remain seasonable,
with daily highs generally in the upper 70s to low 80s, coolest
near the lake. Thereafter, heat and humidity looks to build into
next weekend as a large subtropical ridge builds across the
central Rockies into the Mid-Mississippi Valley. If this upper
ridge builds far enough northeastward into our area next weekend
we could be looking at at least a short period of some significant
summer heat later next weekend. Assuming we end up remaining far
enough removed from any ridge riding thunderstorm complexes in
this pattern through the weekend, we could end up with
temperatures reaching well into the 90s (heat indices possibly in
excess of 100) for next Sunday.

KJB

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 647 AM CDT Sun Jul 7 2024

The main forecast concern today continues to center around the
threat and timing of any thunderstorms over or near the
terminals this afternoon.

All is quiet early this morning, with the exception of a small
cluster of showers and storms across far northwestern IL. This
activity will shift eastward and will be around KRFD through
about 14z. Beyond this current activity, the trends with
thunderstorms today remains of low confidence.

The airmass is expected to destabilize across the area through
the morning in advance of multiple weak impulses that will be
streaming into the area from IA. These impulses may thus be able
to foster some isolated to widely scattered thunderstorms
across northern IL later this morning into the afternoon.
However, because confidence remains low (~30%) with the
coverage of storms, and the fact we cannot forecast a PROB30
within the first 9 hours of the TAF, we have opted to only
mention a VCSH after 17z this afternoon at this time. If
confidence increases, we will need to either add a VCTS or a
tempo group for TS with later updates. The potential timing for
any storms around the Chicago terminals could be as early as
midday, and as late as 22z this afternoon. Brief heavy downpours
and some locally gusty winds would be the primary threats from
any storms.

The threat of storms looks to wane for a period this evening,
though more showers and storms may move into northwestern parts
of the area sometime late tonight into Monday. With the better
threat looking to be closer to RFD tonight, we did opt for a
PROB30 mention for TSRA after 05z tonight.

Prevailing wind directions today area expected to be from the
south-southwest, though the winds in and around any
thunderstorms will become more chaotic. It appears the lake
breeze may struggle to move inland through the day, so we have
opted to leave a wind shift out of the TAF until the larger
scale flow shifts southeasterly for a period this evening.

KJB

&&

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

&&

$$

Visit us at weather.gov/chicago