557
FXUS63 KLOT 052320
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
620 PM CDT Sat Oct 5 2024

.KEY MESSAGES...

- Low chance (under 20%) for a few showers/storms mainly along
  and north of I-90 tonight

- Very elevated fire danger Sunday due to gusty winds and dry
  conditions.

- Dry weather continues next week, with above average
  temperatures especially mid-late week.

&&

.DISCUSSION...
Issued at 208 PM CDT Sat Oct 5 2024

Through Sunday:

Water vapor loops reveal an impressive mature cyclone pinwheeling
across Manitoba which is driving a robust cold front eastward
into Minnesota. In the near term, the main effects from this
system in our area will be gusty south to southeasterly winds this
afternoon, with gusts peaking around 30 mph. This evening, the
mass response ahead of this approaching cyclone will facilitate
the development of an intense low-level jet with winds expected
to increase to nearly 55 knots in the 1-2 kft layer. While
surface winds may briefly diminish as the boundary layer attempts
to decouple immediately after sunset, winds should gradually ramp
back up with time as the LLJ intensifies overhead, with at least
intermittent gusts near 25 to 30 mph through the overnight hours.

Today`s guidance is universally precipitation-free in our region
with the cold FROPA tonight. However, we continue to hold onto
some very low (15-20 percent) chances for showers/storms across
parts of northwest and northeast Illinois. The reason for this is
the presence of a very steep mid-level lapse rate plume and not
insignificant forcing with 50 to 70 m/12 hour 500 mb height
falls. Forecast soundings do indeed appear capped to surface and
elevated effective parcels, but not by much, particularly closer
to the Wisconsin state line. Chances are better than not that
things will remain precipitation-free tonight, but didn`t see
enough of a change in the guidance to support removing PoPs
entirely.

In the wake of the front on Sunday, west to west-northwesterly
winds will increase quickly in the morning as cold advection
fosters steepened near-surface lapse rates. Gusts look to peak in
the 25 to 30 mph range during the late-morning and afternoon
hours. The main focus Sunday is on the likelihood of a period of
elevated to near-critical fire weather conditions during the
afternoon. For more details on those specifics, please see the
Fire Weather Discussion below.

Lingering cold advection will help send temperatures into the
upper 30s near/west of the Fox Valley and our typical cool spots
Sunday night. Some of the raw model output (NAM/NSSL WRF for
example) is outputting near freezing temperatures, but suspect
this is a bit overdone at this point. Winds also look to stay a
touch elevated through the night, only falling off immediately
prior to daybreak on Monday. As a result, not hitting the frost
potential at this point, but something we`ll need to keep an eye
on.

Carlaw


Sunday Night through Saturday:

The forecast in the long term picks up on what will be a chilly
Monday morning with early morning lows progged in the upper 30s
and lower 40s. Some of the cooler solutions out there, namely
the NAM, WRF, and ARW, even suggest lows in the lower to middle
30s west and northwest of the Chicago metro, although lower 30s
are probably a bit aggressive. Monday looks to be the coolest
day of the period with highs forecast in the middle 60s despite
mostly sunny skies overhead. This cold is the result of a
departing storm system that will drag a cold front across the
CWA late tonight. Efficient cold advection on the backside, both
near the surface and especially aloft, will continue through
Monday. Looks as though surface high pressure scoots to our
south late Monday which should pump just enough mild return flow
into the area to get us back into the lower 70s on Tuesday,
with the help of more sunny skies.

Conditions will remain overall quiet through the week. The
leeward side of a large upper ridge will hang out in the region
through mid week before the ridge centers itself over Ontario
later in the week. At the surface, this translates to several
days under the influence of expansive high pressure. This will
keep precip chances minimal while a gradual warming trend is
expected through the week. Temperatures look to return to the
upper 70s to even lower 80s near the end of the week.

Doom

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 620 PM CDT Sat Oct 5 2024

Key messages:

* Two periods of gusty winds to 25-30 kt: SW late evening/early
  overnight hours, then NW mid Sunday AM through the afternoon.

* Strong LLJ develops overnight resulting in a period of LLWS
  during sporadic lulls in surface gusts.

A brief lull in the higher wind gusts is occurring as we
approach sunset but are expected to pick back up later this
evening out of the SSW as a we begin to tap into a strong low-
level jet (LLJ) developing overhead in advance of an approaching
cold front. This is accounted for with a low-level wind shear
(LLWS) mention for SW winds to 50 kt at 1500 ft. While
prevailing surface gusts may end up mainly in the 25-30 kt range
overnight, given the strength of the LLJ overhead, sporadic
gusts in the 30-40 kt range can`t be ruled out.

Winds turn W to NW behind the cold front late tonight through
the pre-dawn hours Sunday. This will coincide with another brief
reprieve from the stronger gusts within the vicinity of the
front along with a period of increased high cloud coverage.
However, expect wind gusts to ramp back up after daybreak out of
the WNW to around 25-30 kt before easing again after sunset.

Petr

&&

.FIRE WEATHER...
Issued at 210 PM CDT Sat Oct 5 2024

While temperatures on Sunday will be around 10 degrees cooler
than today, dewpoints are expected to settle into the mid and
upper 30s which will result in afternoon relative humidities in
the 20 to 30 percent range. The driest conditions/lowest RH
values will occur south of about I-80. Gusty winds from the west
to west-northwest will develop quickly through the morning,
peaking towards midday and early afternoon in the 25 to 30 mph
range. At this time, sustained 20-foot winds are forecast to
come in just under formal Red Flag criteria, and 10-hr fuel
moisture values may only briefly tag 8 percent. As a result,
have refrained from issuing a Fire Weather Watch or Red Flag
Warning on this shift, but will continue to highlight a
heightened fire danger in our products and graphical messaging
suite. Winds will diminish rapidly near sunset.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 7 AM CDT Monday for the IL
     nearshore waters.

     Small Craft Advisory until 10 AM CDT Monday for Calumet Harbor
     to Gary IN.

     Small Craft Advisory until 4 PM CDT Monday for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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