751
FXUS63 KLOT 070116
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
816 PM CDT Sat Jul 6 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 /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 631 PM CDT Sat Jul 6 2024

There are no major aviation weather concerns through the 00Z TAF
period.

While they still have some subsiding to do, SE winds around 5 kt or
less can be expected during the bulk of this evening and into
tonight. Winds may go light and variable at times overnight, but
direction should be predominantly SSE. Wind direction during the day
on Sunday is somewhat uncertain, although SW looks like a safe bet
at least into the afternoon across Chicagoland. RFD is a bit more
uncertain and while SW appears favored, there is noteworthy support
for predominantly E or SE during the day. Wind speed during the day
should remain primarily under 10 kt with gusts into the teens.
Another lake breeze is progged to move onshore tomorrow but more
uncertainty exists on when it will pass over the Chicago sites and
turn winds easterly. Most model guidance suggests sometime in the
mid-late afternoon.

Additionally, a complex of showers and thunderstorms will impact
northern IL Sunday afternoon and evening. RFD is likely to see
periods of showers during the day. With no apparent timeframe for
more appreciable rain chances, went with a broad brushed VCSH during
the day to cover the scattered coverage and intermittent shower
activity. While most of the activity should remain west of the
Chicago metro, it`s very possible, if not likely, that at least some
rain makes it INVOF the Chicago sites at times during the day. Could
be as early as the early afternoon, but better chances arrive during
the late evening and especially just beyond the current period
overnight Sunday night. Confidence isn`t quite there yet for a
formal mention in the TAF, but look for precip to be included in
future TAF issuances.

Doom

&&

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

&&

$$

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