293
FXUS63 KLOT 070609
AFDLOT

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

.KEY MESSAGES...

- Chance (25-45 percent) of showers and thunderstorms Sunday,
  mainly northwest of a La Salle to Lake County line. Slight
  chance (15-25 percent) southeast of that line.

- Continued periods of unsettled weather Sunday night-Tuesday
  afternoon with periodic showers and thunderstorms, highest
  chance/coverage on Monday.

- Seasonable temperatures will continue for the coming week.

&&

.UPDATE...
Issued at 816 PM CDT Sat Jul 6 2024

Evening water vapor imagery shows a minor-amplitude mid-level
short wave trough tracking east-northeast in the vicinity of
southern Lake Michigan. While a few isolated showers developed
with peak afternoon heating across parts of northern IL,
thankfully coverage and intensity remained minimal due to weak
lower and mid-level winds (and resulting lack of deep-layer
shear) and unfavorable mid-level lapse rates. Weak
subsidence/mid-level height rises in the wake of this feature
and loss of diurnal low-level instability will therefore result
in dry and mostly clear to partly cloudy conditions for the
rest of the night.

Farther to the west, an upper trough was evident across the
Northern Plains, with another mid-level short wave propagating
through its southern periphery across NE/KS. Several clusters of
strong/severe thunderstorms were occurring from KS northeast
into NE/IA at this hour. Current RAP forecasts indicate this
activity may convectively enhance the aforementioned short-wave
(potentially with an embedded MCV), which will track northeast
across IA through early Sunday morning, and move into
WI/northwest IL by mid-morning. The main low-level moisture
(mid-60s surface dew points) and associated instability axis
looks to remain mainly across MO/IA into WI during much of the
day, though some elevated moist advection and weak instability
associated with enhanced southwest low level jet flow does look
to provide some showers/thunderstorms into our northwest cwa.
CAMs continue to focus this mainly northwest of about a Mendota-
Highland Park IL line. May then be a lull for our area during
the early afternoon, before forcing increases again later in the
day and evening as the main short wave eventually approaches.

Going forecast captures these trends pretty well, and other
than some previous tweaks to near term elements based on obs
trends, no changes to the forecast appears needed this evening.

Ratzer

&&

.DISCUSSION...
Issued at 316 PM CDT Sat Jul 6 2024

Through Sunday...

Overcast skies have kept most locations in the upper 70s to around
80 early this afternoon, and a lake breeze pushing west through the
metro area has supported some even slightly cooler temperatures.
Sunday, with more sunshine expected, looks to be several degrees
warmer with a bit more humidity as well.  The exception would be
toward the Rockford area where shortwave energy exiting the base of
a shallow upper trough would support increased cloudiness and
scattered showers or thunderstorms.  It is possible that some of
these showers could also work east toward the Chicago metro during
the afternoon or evening. Elected to maintain just a slight chance
mention in that region for now.  However, more favorable mid-level
lapse rates and low level moisture profiles do spread across the
area later in the day as southerly surface winds turn southwesterly
for a while and tap into the better moisture to our west.

Sunday Night through Saturday...

The weakly forced and seasonably mild pattern...at least by July
standards...continues into the early part of the coming week.  On
Monday a frontal zone to our west makes very slow progress toward
our area but increases the overall low level moisture convergence.
Lapse rates don`t look overly impressive but should be enough to
support scattered thunderstorms, especially by the afternoon.  The
lack of strong forcing along with generally unidirectional SW wind
profiles would not appear to pose a severe threat, but the
possibility of flooding may need to be monitored if storms manage to
track repeatedly across the same area.  As with Sunday, the greatest
concern would be for the northwest half of the forecast area.

The weak front...maybe more of a surface trough...moves through the
area Tuesday.  Surface winds turn west then northwest but copious
low level moisture remains in place, which in conjunction with the
upper trough trailing the front will support ongoing chances (25-45
percent) of thunderstorms into Tuesday evening.

Wednesday presently looks quieter with subtle height rises aloft in
the wake of the passing trough, but Thursday into next weekend are
more of a question.  The path of Beryl remnants may play some part
in how the rest of the week evolves.  Models suggest the low center
passes to our south, which could provide the primary focus for
convection late in the week. A different track might allow more
of the moisture to work farther north. For now will maintain
slight chance mentions of precip, with highs very gradually
warming toward the upper 80s.

Lenning

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 109 AM CDT Sun Jul 7 2024

The main forecast concern today is the threat and timing of any
thunderstorms in or near the terminals this afternoon.

All is quiet early this morning, with all the thunderstorm
activity expected to remain well west of the area across parts
of IA. However, as we head into the late morning and into the
afternoon, the airmass is expected to destabilize across the
area in advance of multiple weak impulses expected to stream
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. Confidence on
the coverage of any storms continues to be on the low side, but
with an increasing signal we felt it prudent to add a PROB30
group for TSRA. It appears the best timing for these would be
from roughly 17-20z at RFD and 19-23z at the Chicago area
terminals. Brief heavy downpours and some locally gusty winds
would be the primary threats from any storms.

Prevailing wind directions today area expected to be from the
south-southwest, though the winds in and around any
thunderstorms will become more chaotic. A lake breeze may also
result in an east-southeasterly wind shift sometime later this
afternoon at the main Chicago terminals. However, the exact
timing of this remains of low confidence, especially
considering that outflow from any storms could significantly
alter the behavior of the lake breeze boundary.

The threat of storms looks to wane for a period this evening,
though more showers and storms may move into the area sometime
late Sunday night into Monday.

KJB

&&

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

&&

$$

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