916
FXUS63 KLOT 092306
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
606 PM CDT Mon Sep 9 2024

.KEY MESSAGES...

- Warming trend with summer-like daytime temperatures through
  the work week.

- Persistent stretch of dry conditions will yield an increased
  threat for grass and brush fire starts, especially south and
  western locales.

- Rain chances increase by the weekend, as the remnants of
  Tropical Storm Francine move into the region.

&&

.DISCUSSION...
Issued at 200 PM CDT Mon Sep 9 2024

Through Tuesday night:

Delightful September weather continues across the Midwest with cool
mornings and above normal daily highs expected to continue for
several days. There will be increased fire weather risk through
mid week mainly due to low humidity.

Water vapor satellite imagery depicts upper level flow out of the
northwest across the lower Great Lakes, with very dry conditions
through the atmospheric column over a large portion of the
continental United States. This is consistent with upper air
radiosondes which have precipitable water values across the region at
or below 25th percentile readings for early-mid September. Upper
level flow will shift westerly as a seasonally strong ridge axis
across the plains meanders eastward and begins to flatten. The other
interesting features on the satellite analysis include: 1) a weak
upper low in the northern/central Plains and then 2) the tropical
disturbance across the western Gulf which will influence our local
weather this weekend. The local area is currently under the
influence of high pressure at the surface with seasonal temperatures
in the upper 70s and low humidity.

The main weather phenomenon of interest will be both increasingly
dry conditions, which may contribute to an increased brush fire
risk. Forecast soundings depict continued subsidence and an even
further drying of the lower atmospheric column on Tuesday, and this
will continue into midweek as temperatures continue to inch upward,
in spite of a slow increase in deep layer moisture. With
surface high pressure in place, any influence from the weak
upper low across the plains may be some increased high clouds,
with any light precipitation heading north of the area. Low
level flow will be out of the southwest, and thus temperatures
will increase through the 80s, whereas normal highs are
generally in the upper 70s. With these high temperatures and
dewpoints mixing down into the 40s for most areas tomorrow,
relative humidity readings will dip to as low as 20 percent in
some locations, and this is quite dry. Fortunately wind gusts
should remain under 20 mph as the pressure gradient over the
area will remain fairly weak except toward the Rockford area.

A secondary feature of note will be the advection of smoke aloft
back into the lower Great Lakes mainly from a more concentrated of
smoke noted extending from MN southward to Arkansas. Winds both in
the mid and upper levels will shift more westerly as the upper ridge
axis flattens. Therefore some of the smoke will advect back toward
the lower Great Lakes later Tuesday into Tuesday night. Cross
section analysis from the HRRR seems to be biased a bit low in terms
of how close to the surface smoke will extend, but the HRRR
depiction of an increased smoke concentration aloft late Tuesday is
reasonable.

KMD


Wednesday through Monday:

Main long-term forecast concerns continue to focus on warm, dry
weather mid-week with potential increased brush fire risk, then
rain trends with the remnant circulation of what is now
Tropical Storm Francine in the western Gulf of Mexico.

Medium-range guidance/ensembles develop upper-level ridging
across the Midwest/western Great Lakes Wednesday into Thursday,
to the northeast of the remnant circulation of TS Francine which
combines with a weak upper low currently over Mexico. This is
expected to maintain dry weather and well above average warmth
across the forecast area mid-week, with daytime highs in the
mid-upper 80s (near 90 in our warmest metro locations).
Subsidence and dry easterly low-level flow is expected to
maintain low dew points and relative humidities during this
period, especially during the midday and afternoon hours of best
mixing and warmest surface temps. Have maintained the trend
from earlier shifts, blending lower NBM 10th percentile and
ensemble MOS guidance both Wednesday and Thursday, yielding
afternoon RH values around 25 percent. While winds will not be
particularly strong, combined with our antecedent dry conditions
this will likely present an elevated risk of grass and brush
fires through Thursday.

Attention then focuses on the eventual approach of the remnants
of TS Francine from Friday through the weekend. GFS/ECMWF
ensembles are in decent agreement in bringing the mid-level
circulation across the lower Mississippi Valley Thursday, and
eventually into the lower Missouri/Ohio/Tennessee Valley regions
where it then is progged to wobble slowly east Friday through
Monday. NBM pops continue to appear a bit too high/too far north
especially in the Friday through early Saturday period based on
current guidance (the GEM/CMC ensemble are more phased with
short wave energy emerging from the central Rockies and are
significantly farther N/NW with the circulation). Have generally
lowered NBM pops for the Friday through Saturday AM period to
match our going forecast, which could still eventually end up
being too high. With decreasing confidence in location/track of
the slow-moving circulation later Saturday into Monday,
maintained NBM chance pops through that period. Increasing cloud
cover and easterly low-level winds Fri-Mon along with loss of
upper ridging overhead should support more typical temperatures
by the late week/weekend period.

Ratzer

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 605 PM CDT Mon Sep 9 2024

There are no aviation concerns through the TAF period. West
winds at press time will back modestly toward the southwest
overnight and remain so through tomorrow. Pockets of upper-level
wildfire smoke will occasionally meander over the terminals, as
well.

Borchardt

&&

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

&&

$$

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