Rosemont, Illinois Weather Forecast Discussion

579
FXUS63 KLOT 040826
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
326 AM CDT Sat Oct 4 2025
.KEY MESSAGES...
- July-like warmth will continue through Monday.
- Stronger winds and continued dry conditions will result in
  heightened fire danger and areas of blowing dust Sunday
  afternoon.
- Cold front will bring rain chance Monday and especially Monday
  night, followed by more seasonable temperature for the rest of
  the upcoming work week.
&&
.DISCUSSION...
Issued at 326 AM CDT Sat Oct 4 2025
Another unseasonably warm day is on tap for today. Over the past
24 hours, guidance has trended about 1C warmer with 925mb temps,
now progged to be pretty close to where they were Friday
afternoon. High temps today will probably be close to or maybe a
degree or so less than Friday, but still in the mid to upper 80s
area wide with a few isolated 90 degree readings possible.
Remnant shear/deformation axis that was responsible for Friday
afternoon/evening`s convection lingers across central IL early
this morning. Background mid-level flow will become more
southerly today and should advect this weakening shear axis
north into northeastern IL and northwest IN today. Almost all
available guidance is dry today, however the NSSL WRF and ARW
both are convecting this afternoon over NE IL/NW IN. Both of
these models are likely too high with sfc Tds and resultant
instability, so opted to keep the forecast dry today. However,
both of these models performed well with Friday`s isolated
convection downstate, so as a nod to them, I did add some 10-14%
pops with no formal mention of rain over eastern half of the
CWA this afternoon.
Sunday will be another unseasonably warm day, with a tightening
pressure gradient also leading to stronger winds, particularly
over western portions of the CWA. Given the expected sunny skies
and very dry conditions, boundary layer should become deeply
mixed and quite efficient at transporting down some stronger
gusts from late morning through late in the afternoon. Could see
a couple hours with peak gusts to around 30 mph, especially
western/northwestern half of the CWA. Given the very dry
conditions and increased farming activity, the gusty winds
tomorrow will probably result in some areas of blowing dust in
open areas. Not anticipating a big enough problem to require a
blowing dust advisory, but will probably mention the blowing
dust in the SPS that will likely be issued later today or
tonight for fire wx concerns. More on that in the fire wx
section below.
Upper trough will gradually move eastward into the upper
Mississippi Valley and western Great Lakes Monday with
associated sfc cold front gradually sagging southward into far
northern IL Monday afternoon. Ahead of the front, it will be
another warm day with a good deal of sunshine, so highs should
once again climb into the 80s.
Anafrontal nature of the cold front should result in most of the
precipitation occurring behind the front Monday night and
probably lingering into Tuesday morning. Guidance continues to
depict a pretty favorable set-up for rain and possibly even a
couple of embedded thunderstorms with a post-frontal axis of
rain Monday night. There is strong agreement that our area will
end up in the right entrance region of 100kt+ upper level jet
with some models suggesting some coupling with the left exit
region of a weaker southern stream jet streak, which could
further enhance ascent.
In addition to the strong synoptic ascent, the thermally direct
circulation in the right entrance region of the upper level jet
will aid in strengthening low-mid level frontogenesis. This
could result in some mesoscale enhancement to the precipitation
rates, which given PWATs progged to be near or above 200% of
normal (around 1.50"), does certainly open the door to the
potential for some locally heavy rainfall Monday night. Drought
begets more drought, so far too early to lock in any kind of
solution that points toward heavy rainfall putting a dent in the
developing drought, but Monday night`s set-up does bear some
watching.
Cooler, more seasonable temperatures are expected Tuesday
thought much of the upcoming work week. After rain ends
Tuesday, dry weather will resume for the remainder of the
week. The general idea of a trough-ridge-trough longwave
pattern across the North American continent late next week into
next weekend continues to garner support among the various
medium range ensemble suites, but there are notable differences
in the longitude of where these longwaves will set up, resulting
in considerable uncertainty locally. A more eastward
positioning of the ridge (EPS) could set the stage for a return
to above average temps, while a farther west ridge (GEFS) would
keep temps cooler (closer to normal). Given the uncertainties,
no changes were made to NBM out for later in the week and next
weekend.
- Izzi
&&
.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1210 AM CDT Sat Oct 4 2025
VFR with no significant concerns through the period.
Light southerly winds early this morning will increase to around
10 kt from the south-southwest (190-220 deg) by the mid morning
and then gust to near/around 20 kt this afternoon. The wind
direction will return to southerly around or shortly after
sunset as speeds diminish. With strengthening flow not far above
the surface overnight Saturday night, can`t rule out sporadic
15-20 kt gusts at the Chicago terminals.
Castro
&&
.FIRE WEATHER...
Issued at 326 AM CDT Sat Oct 4 2025
Very little change in thinking regarding potential fire weather
concerns this weekend. Winds will pick up a bit today with some
gusts to around 20 mph, especially west of the Fox River Valley.
Min RH values are one again expected to bottom out mostly in the
30-35% range.
Of greater concern continues to be Sunday. Stronger winds are
expected Sunday, again favoring areas west of the Fox River Valley
the most. In addition to the stronger winds, it continues to look
favorable for dewpoints to mix out Sunday afternoon, resulting in
min afternoon RH values between 20 and 30 percent. There remains
significant spread in model guidance with regards to min RH values
Sunday, with some guidance substantially higher than our
forecast, and a couple of models a bit lower. Have continued to
lean toward the lower end of the guidance spectrum given the
worsening short term drought and expectation for deep mixing.
There has been a subtle trend toward slightly higher winds and
slightly lower RH, but still appears winds will remain solidly
below the red flag warning criteria of 20 mph sustained 20 ft
winds (roughly 23 mph sustained 10m winds).
We will likely end up issuing a SPS for heightened fire weather
concerns later today or tonight.
- Izzi
&&
.CLIMATE...
Issued at 821 PM CDT Fri Oct 3 2025
High temperatures may approach record highs this weekend. Here are
the current records for October 4th and 5th:
Saturday       Sunday
Chicago    90 (1951)     88 (1997)
Rockford   90 (1922)     90 (1922)
- Izzi/Ratzer
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion