Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
610 AM CDT Thu Sep 5 2024
KEY MESSAGES...
- A cold front moves through the area this evening bringing
scattered showers and a slight chance of thunderstorms (20%)
- Showers may linger into Friday morning across northwest
Indiana, with isolated showers possible areawide in the
afternoon (20%)
- Lake effect rain showers develop late Friday into Saturday,
particularly into northwest Indiana.
- A big cooldown arrives heading into the weekend with highs in
the 60s before warming back up early next week.
&&
DISCUSSION...
Issued at 308 AM CDT Thu Sep 5 2024
Through Friday:
Low level moisture continues to feed into the area early this
morning along the western periphery of a departing expansive
surface high. This has allowed for an increase in cloud cover
for areas generally near and east of I-55 early this morning. In
fact, a few spotty showers/sprinkles have managed to develop in
far southeastern Benton County and accordingly can`t rule out a
stray sprinkle through the morning hours south of the Kankakee
River Valley. Continued warm and moist air advection today
should allow for temperatures to easily warm into the mid 80s
today. Locally upper 80s can`t be ruled out in the urban core of
the Chicago metro.
A cold front is still progged to sweep across the area late
this afternoon through the evening hours in advance of an
approaching upper trough. Model guidance has been consistent in
developing isolated to scattered showers along and behind the
front. However, for those looking for meaningful rainfall, total
accumulations do not look overly impressive, with less than
0.25" forecast though Friday morning. Localized swaths of higher
amounts can`t be ruled out beneath any more robust showers. The
thunder threat continues to look rather unimpressive with poor
lapse rates limiting instability to under 500-750 J/kg. While
organized thunderstorms don`t appear likely, do suspect there
will be at least occasional lightning strikes this
evening/overnight.
Strong northerly to northwest winds will set up over the lake
in the wake of the front allowing for waves to quickly build
over Lake Michigan and result in another round of dangerous
swimming conditions at area beaches Friday and Saturday.
Temperatures on Friday will be notably cooler with highs only
in the upper 60s to lower 70s. The aforementioned upper trough
will begin to swing through the area on Friday and could stir up
isolated to widely scattered showers in the afternoon.
Petr
Friday Night through Wednesday:
The long term forecast picks up Friday evening when we`ll find
the base of a deep upper trough dropping south through northern
IL. The encroaching cold core of the trough will destabilize
conditions aloft which could generate a few elevated showers
during the evening, especially across parts of the Chicago metro
north of I-80 as well as over the lake. As ambient near-surface
conditions cool during the evening, instability will quickly
build over the relatively warm lake surface setting us up for
some lake-effect shower development beginning in the evening.
The highest potential should focus toward the northwest IN
lakeshore given the northerly flow, but some showers may very
well push across the IL shoreline during the evening. Overnight,
the potential becomes basically confined to northwest IN as the
low level flow backs to NNW. Instability will continue to build
into the night and is such that we may even see some
waterspouts or lake-effect thunderstorms develop, especially
near the IN lakeshore. The shower chances will continue to
gradually shift eastward around the shoreline until the
potential exits our CWA to the east by the end of the morning.
Saturday morning will be one of the coldest in some time with
lows forecast to drop into the lower 40s outside of the internal
Chicago metro. Highs should only make it into the 60s despite
mostly sunny skies, except over far northwest IN where the lake-
effect will keep some added cloud cover around.
The upper low will eject to the northeast during the day on
Sunday. Upstream, an upper high will extend eastward from the
Desert Southwest into the Plains and eventually evolve into a
quiet split flow regime across the central CONUS. This
translates to expansive surface high pressure that should keep
rain chances rather minimal through the middle of next week.
Some good news for those who aren`t ready for fall just yet,
this quiet pattern will also feature a big warmup that will
bring 80s back to the area for several days next week. We`ll be
back in the lower to middle 70s to round out the weekend. 80s
may return as early as Monday with middle or even upper 80s
expected mid and late week.
Doom
&&
AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 610 AM CDT Thu Sep 5 2024
The main aviation weather concerns for the 12Z TAF period
include:
* A broken line of showers and possibly some thunderstorms
Thursday evening.
* MVFR cigs expected late Thursday night into early Friday.
Winds will be light SSW to start the day before veering to a SW
gusting into the teens kts by late morning. We may see some
sporadic gusts to around 20 kt, especially during the latter
part of this afternoon. A cold front will push across the area
later today first getting INVOF RFD early evening. Along the
front, a broken line of showers is expected. A couple of heavier
showers with associated vis reductions or even a few
thunderstorms are possible, but coverage of such conditions
looks widely scattered. This activity will approach the Chicago
sites closer to late evening. The better rain coverage should
only remain over a given area for a couple of hours as a
majority of the precip is expected to be confined to this narrow
line of activity along the front.
Winds will veer from SW to NW as the front passes over, and
eventually onto NNW overnight. Sustained winds may stay close to
10 kt during the overnight and we might even see some sporadic
gusts as high as 20 kt. Additionally, MVFR cigs are expected to
move over all TAF sites late Thursday night. VFR looks to return
by mid-late morning. A couple of additional showers are
possible during the latter part of the morning, although
confidence at this time is too low to include in the TAF.
Doom
&&
LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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