906
FXUS63 KLOT 161123
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
623 AM CDT Mon Sep 16 2024

.KEY MESSAGES...

- Hot and dry conditions will prevail through the weekend.

- Long-range models advertise a pattern shift during the last
  week of the month with a return of chances for rain. Time
  will tell if such a pattern change occurs.

&&

.DISCUSSION...
Issued at 213 AM CDT Mon Sep 16 2024

Through Sunday:

We remain locked in a doldrums between aggregate upper-level
troughing across the western United States and expansive Rex
blocking over the Ohio River Valley. Persistence is the way to
go in this regime, favoring highs today through Wednesday in the
upper 80s to around 90 and overnight lows in the upper 50s to
lower 60s. Daily lake breezes will keep shoreline locations some
5 to 10 degrees cooler during the afternoon and evening hours.
Lobes of dry air originating from the Ohio River Valley will
propagate into our region today through Wednesday, leading to
very low humidity levels (relative humidity of only 20 to 30%
each afternoon). The exception will be in the wake of the
aforementioned lake breezes, where locally higher humidity
levels will prevail (dew points over Lake Michigan are in the
low to mid 60s).

Toward the end of the week, an upper-level trough currently
propagating over Aleutian Islands of Alaska will dive into
southern California while "kicking" the current western US trough
northeastward into the northern Plains. At the same time, the
Rex block (and associated broad surface cyclonic flow) across
the Ohio River Valley will get nudged toward the Atlantic Coast.
The net result will be an eastward drift of upper-level ridging
toward the Great Lakes and the reestablishment of a feed of
low-level moisture from the Gulf of Mexico toward the Upper
Mississippi River Vally. For this reason, will advertise rising
humidity levels by this weekend with a continuation of above-
average temperatures in the mid to upper 80s.

Prospects for widespread, soaking rainfall currently look slim to
none through the weekend as upper-level forcing remains focused to
our west. So, when will it rain again? Conceptually, would have
to think that the elongated upper-level jet streak currently
stretching over central Russia will intensify and shift eastward
over the Pacific Ocean toward the end of the week, causing the
upper-level pattern across the US to become more progressive
thereafter. It is for this reason that multi-run ensemble
meteograms of QPF have been showing a return of precipitation
chances, as well as a gradual stair-step downward in
temperatures, to our area during the last week of September.
Time will tell if such a pattern change will take place.

Borchardt

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 623 AM CDT Mon Sep 16 2024

No forecast concerns through the period. Light east/southeast
winds to start will increase into the 6-9kt range by mid/late
morning. A lake breeze is expected to move inland this afternoon
turning winds more east/northeast for the Chicago terminals
with speeds perhaps around 10kts for a few hours. Speeds will
diminish back to 5kts or less this evening. cms

&&

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

&&

$$

Visit us at weather.gov/chicago