Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261946
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
246 PM CDT Tue Mar 26 2024

.KEY MESSAGES...

- Scattered showers persist into this evening, with a chance of
  flurries overnight.

- West-southwest winds remain blustery tonight, though gradually
  diminish into Wednesday.

- Another period of active weather likely late week into a least
  early next week.

&&

.DISCUSSION...
Issued at 246 PM CDT Tue Mar 26 2024

Through Wednesday Night:

Early afternoon surface analysis displays deep 988 mb low
pressure lifting north-northeast across Michigan`s upper
peninsula toward Lake Superior. An occluding cold front extended
south across Lake Michigan/lower Michigan into eastern Indiana,
while a secondary cold front trailed into eastern Iowa. The low
will continue to track off to the northeast into Ontario
tonight in response to a deep mid- level short wave which will
lift northward from IL/WI early this evening, while the trailing
secondary cold front moves east across the forecast area.

The thunderstorm and severe weather threat has moved east of
the forecast area early this afternoon, as the occluded front
has cleared the cwa. Ambient synoptic winds will remain blustery
from the southwest with gusts reaching 35-40 mph at times
within a strong low-level cold advection regime and the tight
surface pressure gradient associated with the low to our north.
These winds will become westerly tonight with the passage of the
secondary cold front, and will remain blustery though will
gradually diminish overnight into Wednesday. Temperatures have
already fallen sharply into the low-mid 40s this afternoon
behind the initial occluded frontal passage, and will continue
to fall into the 30s behind the secondary cold front this
evening. Overnight lows are expected to dip below freezing area-
wide, from the mid-20s to near 30 degrees. Scattered rain
showers will persist across the area through early evening, as
the mid-level vort transits the area. GOES visible imagery
depicts extensive low cloud cover upstream however, which is
expected to linger into Wednesday morning before scattering out.
Despite subsidence developing behind the short wave, a few
flurries may fall from this cloud deck later this evening and
overnight as the column continues to cool and low-level cold
advection maintains steep low-level lapse rates. No accumulation
is expected however, with the saturated cloud layer otherwise
fairly shallow.

Persistent dry, cold low-level advection should allow clearing
skies by Wednesday afternoon. Despite the return of sunshine,
temperatures look to only reach the low-mid 40s for afternoon
highs (a good 5-10 degrees below our average lower 50s for late
March). Winds will continue to add a chill to the air however,
with westerly gusts still in the 15-20 mph range during the day.
Another chilly night is then in store for the area Wednesday
night, with sub-freezing overnight lows in the mid-upper 20s
outside the core of the Chicago metro where readings will bottom
out around 30 degrees.

Ratzer


Thursday through Tuesday:

Broad mid-level ridging will cross the region Thursday through
Friday night, bringing a period of warming conditions amid dry
weather. A LLJ directed just west of the forecast area Thursday
night will support increasing WAA with a stronger baroclinic
zone across the forecast area. Moisture appears to be rather
deficient for any notable precip chances, so have maintained dry
conditions at this time.

By Friday, the low-level baroclinic zone noted above will only
slowly lift NE ahead of an open mid-level wave quickly moving
through the departing and weakening ridge. While there remains
some question on whether ample moisture will arrive in time
ahead of broader forcing, the rapid transport of moisture within
the LLJ combined with modest mid-level lapse rates suggests at
least some elevated shower and storm chances across mainly the
north half of the forecast area Friday night. Conditional upon
an earlier arrival of higher moisture (e.g. Pwats over 1"), a
localized heavy rain risk may exist with training storms.

Beyond Saturday, guidance begins to diverge with solutions
regarding the evolution of expansive ridging centered over the
southeast U.S. and an upper-low attempting to cutoff across the
Great Basin region. It does appear amplifying mid-level
westerlies in response to a formidable trough digging SE from
south-central Canada will yield a more active period of weather
somewhere around or south of the region early next week. At this
time, rain and thunderstorm chances appear highest Sunday night
into Tuesday morning. However, the potential of any phasing of
the northern trough with the upper-low to the southwest will
likely continue to result in substantial model discrepancies for
at least a few more days.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1240 PM CDT Tue Mar 26 2024

- Period of -RADZ late afternoon/early evening with associated
  MVFR-near IFR VSBY/CIG reductions

The earlier line of gusty showers has since moved over the lake
and into northern Indiana. A few showers and patchy drizzle
remain across northeast Illinois and northwest Indiana and may
linger a bit beyond 18Z before clearing out.

There is a separate region of rain and drizzle over the Quad
Cities that will gradually push east across the area this
afternoon. There remain some uncertainty as to how long the IFR
VSBYS/CIGS persist with eastward extent. Opted to introduce
TEMPO groups to account for this potential, beginning earliest
at RFD-19Z, DPA-21Z, and ORD/MDW/GYY-22Z. Have hinted at some
MVFR VSBY and low-end MVFR CIGS for the Chicago area terminals,
though IFR can`t be ruled out if the drizzle is more persistent.
After this period of lower CIGS, expect MVFR stratus to then
persist the remainder of the overnight and linger into Wednesday
morning before eventually scattering out. A few flurries can`t
be ruled out during the pre-dawn hours Wednesday morning, but
the potential remains too low to include in the TAFs.

Winds will remain gusty through this afternoon out of the
southwest with gusts up to 30kt. Winds turn more westerly as
the area of rain and drizzle moves through late afternoon/early
evening with gusts then easing some overnight.

Petr

&&

.MARINE...
Issued at 246 PM CDT Tue Mar 26 2024

Southwesterly gales of 35-40 kts will persist this afternoon,
before becoming westerly and gradually easing tonight. A Gale
Warning remains in effect for the Illinois and Indiana nearshore
waters through 7 PM this evening. A follow-up Small Craft
Advisory will be needed into early Wednesday morning while
winds and waves subside.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning until 7 PM CDT this evening for the IL and IN
     nearshore waters.

&&

$$

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