Evanston, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

219
FXUS63 KLOT 080826
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
326 AM CDT Tue Apr 8 2025

.KEY MESSAGES...

- Periods of cold rain are expected on Wednesday, following
  lingering showers Wednesday night into Thursday.

- Temperatures will warm to above normal early next week.

&&

.DISCUSSION...
Issued at 326 AM CDT Tue Apr 8 2025

While today will be chilly (some 10 degrees below normal),
nearly full sunshine (aside from some tufts of fair weather
cumulus) and lighter winds will make things feel a bit more
comfortable than yesterday. A lake breeze will turn winds east
to southeasterly this afternoon near the lake holding
temperatures in the upper 30s/near 40 degrees lakeside, with mid
to upper 40s prevalent inland. Tonight also looks largely
uneventful, with gradually increasing cloud cover ahead of the
next disturbance. With the thickest cloud cover likely to hold
off until very late tonight/early Wednesday morning, light winds
and dry air (dewpoints in the teens/20s) should allow
temperatures to fall readily into the 30s and upper 20s.

A modest surface low embedded within broad surface troughing
across western Nebraska will scoot eastward into Iowa late
tonight and Wednesday morning, while steadily weakening/filling.
A low-level mass response in advance of this feature will
encourage the development of an intensifying southwesterly
925-850 mb LLJ which in turn will yield an expansion of
isentropic ascent across parts of the Upper Midwest. In general,
the trend has been towards a slightly slower evolution, with the
main batch of WAA-induced precipitation holding off until after
about 6 AM Wednesday, and have commensurately pulled back on the
late overnight precipitation chances.

Interestingly, some fairly striking differences in the handling
of moisture depth, orientation of f-gen and warm advection,
etc. continue across the guidance suite, even at this relatively
short lead time. This appears to be resulting in some notable
timing discrepancies with the development of the initial batch
of WAA-driven precipitation, with the GFS producing widespread
precipitation across the forecast area a few hours before
everything else (RAP, ECMWF, GEM, HRRR, NAMNest). While this
timing difference is only on the order of ~3 hours, this has
meaningful implications to the initial p-type forecast, as
wetbulb temperatures through 9 AM would support nearly all snow
across roughly the eastern half of the forecast area. If
precipitation develops as early as the GFS advertises (and can`t
really argue against it given the intense nature of incoming
ascent), a brief period of wet snow may materialize as precip
initially breaks out through mid-morning Wednesday. A rapid
transition to all rain would then occur through mid-late morning
as stout southwesterly winds aloft steadily warm the column.
Even if the more aggressive GFS solution verifies, any
accumulations would likely be minimal and relegated to grassy
surfaces with air temperatures expected to remain at or above
freezing (and steadily warming with time).

Precipitation chances will continue through Wednesday, although
things should trend more "drizzly" and lighter with time as
drier air filters in aloft during the afternoon. Can`t rule out
a few lightning strikes during this period, although chances
still seem a bit too low to justify an expansion of thunder
probs. While another flip to snow or a rain/snow mix remains
possible across far northern Illinois Wednesday evening/night,
this signal has diminished considerably across the deterministic
and ensemble guidance given a weaker and generally more
progressive surface low.

A secondary surface trough will approach the region Wednesday
night, with additional sprockets of mid-level vorticity set to
meander overhead. With lingering 0-2 km moisture, there`s some
potential for low stratus to gradually build down into a fog
bank overnight into Thursday morning, particularly north of I-80
where the low-level flow is forecast to be the weakest. No
mention in the gridded forecast at this time given lingering
placement uncertainties, but something we may have to introduce
in future updates.

On Thursday, there`s a fairly sizable bust potential on the
temperature forecast as a cold front will eventually push across
the southern end of the lake and into northern Illinois and
Indiana. Timing remains unclear, however, and the
coverage/placement of any lingering fog and low stratus will
additionally play a critical role in the temperature
distribution. For this forecast package, have cut temperatures a
bit under the NBM offering, but it`s plausible (likely?) that
temperatures will need to be cut further, particularly across
northeast Illinois. A sliver of surface-based instability is
forecast to develop during the afternoon, mainly across central
Illinois, but if the front is slower, this instability axis
could develop into our southern locales. Additional vorticity
maxima traversing overhead could provide the impetus for some
afternoon thunderstorm develop as a result. At this time, the
main thunderstorm chances look to remain to our south, but is
another item we`ll keep an eye on.

Broad troughing will linger overhead through the end of the
week, although the introduction of drier air largely looks to
suppress precipitation chances late in the week and into the
weekend. Temperatures will remain below normal on Friday,
coolest lakeside, then return to seasonable inland (upper
50s-low 60s) on Saturday. There`s a signal for above normal
temperatures Sunday-Monday with highs reaching into the 70s, and
this warmth possibly making it all the way to the lakeshore for
a change.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1207 AM CDT Tue Apr 8 2025

A surface high pressure system will slowly move southward
through the terminal airspace through the duration of the TAF
period. Generally light and variable winds are hence expected,
with a preference for northwesterly to westerly direction by
late morning. A lake breeze will cause winds to shift east
northeasterly at ORD/MDW/GYY this afternoon and DPA after
sunset, before winds gradually turn southerly toward the end of
the TAF period. A few VFR stratocumulus clouds will continue
near the lakeshore before being replaced by mid-level clouds
this evening.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 AM CDT this morning for the IL
     nearshore waters.

     Small Craft Advisory until 4 PM CDT this afternoon for the IN
     nearshore waters.

&&

$$

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