519
FXUS63 KLOT 111120
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
620 AM CDT Wed Sep 11 2024

.KEY MESSAGES...

- Mid summer-like daytime temperatures through at least Thursday.

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

- Most of the appreciable rain with Tropical System Francine`s
  remnants should remain south of the area, with chances of
  widespread beneficial rain appearing low through the weekend.

&&

.DISCUSSION...
Issued at 223 AM CDT Wed Sep 11 2024

Through Thursday Night:

Mid-summer like daytime warmth but otherwise quiet and dry
conditions are in store. Western North American wildfire smoke
will continue to be present aloft at varying thicknesses.
Expecting some effect on temperatures today, reducing the chance
of any locations hitting 90F, though mid to locally upper 80s
are forecast away from Lake Michigan, where some modest
afternoon lake breeze cooling will occur. As has been the case
in this dry pattern, undercut raw model and blended guidance
low temps for tonight and trended toward MOS, with low-mid 50s
prevalent outside of Chicago.

High level cloud cover will stream northward in advance of
Francine`s remnants Thursday afternoon and night, which combined
with the smoke aloft, will slightly limit the ceiling on inland
temps Thursday afternoon. A synoptic easterly component and
lake breeze influence will cap shoreline highs in the upper 70s
to around 80F. Forecast lows Thursday night are in the upper 50s
to lower 60s outside Chicago and mid-upper 60s in and near the
city, milder due to the increased high clouds plus smoke aloft.

We`ll continue to message a threat for grass and brush fire
starts due to the very dry conditions and warm temperatures
today and Thursday.

Castro


Friday through Tuesday:


The forecast later this week for our area will largely be
dependent upon the evolution of the remnants of Francine
following its landfall later this evening on the LA Gulf Coast.
Ensemble guidance continues to favor a northward trajectory up
the Lower Mississippi Valley into Thursday. Thereafter, it
appears rapidly building upper-level heights and a building
ridge across the Great Lakes (Rex block pattern) will largely
limit its northward progress, essentially causing it to become
quasistationary while gradually weakening across the TN Valley
into this weekend.

While a notable increase in tropospheric moisture will exist in
the vicinity of Francine`s lingering circulation, a wall of
exceptionally dry air will exist on its periphery, with modeled
700 mb dewpoints on Friday morning in the -15 to -25 C range
across northern Illinois. An initial push of locally-enhanced
DCVA is forecast to arrive south of I-80 through Friday morning
along with generally modest upper divergence owing to relatively
weak flow above 500 mb. Given the tremendously dry airmass
across northern Illinois, it seems likely that we`ll struggle to
moisten the lower-levels appreciably, resulting in a general
erosion of the northern terminus of any precipitation
shield/bands of showers. Have continued to generally confine
slight to chance PoPs well south of I-80 during the day Friday
as a result. Certainly can`t rule out some light activity
develop farther to the north, but chances appear low at this
time.

Any activity should tend to refocus inland, with increasing
moisture the farther west you go with dry east to northeasterly
trajectories off the lake continuing near the lake into Friday
night and Saturday. With limited large scale forcing overhead
on Saturday, precipitation coverage may remain pretty spotty,
and most locations/hours look to remain precip-free. From Sunday
and beyond, run-to-run differences in the handling of a
secondary moist surge are evident, with some signal this ends up
largely missing the region with another plume of very dry mid-
level air sneaking in from the east. For now, have continued to
trim PoPs to generally advertise on slight chances Sunday into
Monday given uncertainties.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 620 AM CDT Wed Sep 11 2024

VFR with no significant concerns.

SSW to SW (200-230 deg) winds will increase to 7-10kt away from
the lake later this morning. A lake breeze will gradually push
inland from the Illinois shore this afternoon, while hugging the
IN shore, which may briefly result in an east-northeast wind
shift at GYY in the late afternoon. MDW`s TAF favors an east-
southeast wind shift (7-8 kt speeds) at around 23z with medium
confidence. The boundary will probably wash out near ORD around
sunset/00z this evening, and winds will likely turn light
southeasterly even if the lake breeze doesn`t make it across the
airport.

Light/VRB to calm winds will return to outlying terminals with
sunset. Initially light easterly winds will gradually increase
Thursday morning, to near 10 kt by midday. The varying thickness
smoke layer aloft will remain at about 15kft AGL.

Castro

&&

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

&&

$$

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