513
FXUS63 KLOT 230240
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
840 PM CST Fri Nov 22 2024

.KEY MESSAGES...

- Mostly cloudy skies will prevail tonight and tomorrow with
  relatively muted temperature swings (lows in the mid 30s,
  highs in the mid 40s).

- There is a chance of rain on Monday, with some threat for rain
  and snow Wednesday into Thanksgiving Day particularly for
  areas south of Interstate 80.

- Temperatures will turn much colder later next week and
  beyond.

&&

.UPDATE...
Issued at 840 PM CST Fri Nov 22 2024

Nighttime microphysics satellite imagery shows an extensive
swath of stratus clouds blanketing the region, who`s back edge
is over in central Iowa. Some lowering of cloud decks are
possible through the overnight. However, with persistent and
relatively breezy northwest winds within the cloud layer,
mechanical mixing looks like it will reduce the chances for
stratus to build down into a fog layer into tomorrow morning.

The pockets of patchy drizzle that has lingered through the
evening, particularly on the southeastern tip of Lake Michigan,
is slowly diminishing. Parts of Lake and Porter county in
Indiana may still get random spits into early Saturday morning,
but nothing of any real note. As the upper level ridge keeps
northwesterly flow over the area into Saturday morning, models
are suggesting that increased moisture in the mid levels could
move down with an associated wave during Saturday morning. The
RAP was notably aggressive suggesting a short window of light
rain or even drizzle, but most recent runs have backed off.
Maybe there were be some isolated pockets of drizzle tomorrow
morning, but the probability was too low to add it in the grids
for now. Otherwise, the rest of the forecast mentioned below
remains on track.

DK

&&

.DISCUSSION...
Issued at 209 PM CST Fri Nov 22 2024

Through Saturday night:

Regional satellite imagery depicts a scene quintessential of
the cool season with pervasive cloud cover from the Great lakes
to Northwest within broad upper-level cyclonic flow.

As the back edge of the upper-level cyclonic shear axis of the upper-
level trough shifts eastward and away from our area tonight, neutral
to modestly positive upper-level height tendencies and associated
mid-level subsidence should encourage the base of the low-level
inversion to gradually lower. Accordingly, the blanket of stratus
draped across our area won`t be going anywhere anytime soon.
Interestingly, the HRRR/RAP have been persistent in depicting
"splotchy" QPF typical of drizzle during the overnight hours.
Considering BUFKIT soundings depict only filaments of
discernible lift and/or wind shear within the marginally-deep
stratus layer, will keep any mention of drizzle out of the
forecast. Cold air advection facilitated by (albeit weakening)
northwesterly winds should make up for an otherwise muted
diurnal drop in temperatures, with lows expected to drop into
the mid 30s.

Tomorrow, a surface high pressure system will traverse across the
Ozarks allowing for low-level steering flow across the Lower Great
Lakes to turn west southwesterly. Accordingly, the back edge of the
stratus should gradually lift northeastward across our area
throughout the day in spite of the continued lowering (via mid-level
subsidence) and eventual strengthening (via low-level WAA) of the
inversion. Even as (if?) low-level stratus clears, mid-level clouds
will be in the neighborhood as a low-amplitude upper-level shortwave
propagates southeastward across the Upper Mississippi River
Valley. For these reasons, will hold onto mostly cloudy skies
all day tomorrow and advertise limited diurnal warming with
highs in the mid 40s. Of course, if clouds manage to clear
(particularly during the afternoon), temperatures may verify
above our forecast.

Tomorrow night, upper-level clouds will spill over the Great Lakes
as a series of upper-level shortwaves propagate across the
northern Plains. Overnight lows should fall into the mid 30s.

Borchardt


Sunday through Friday:

Warm advection and a seasonably mild starting point will yield
above normal temperatures on Sunday. The main uncertainty in
Sunday`s forecast is the effect of cloud cover on how mild
temperatures get. Thick overcast this time of year is a common
recipe for under-performing temps, so confidence is a bit lower
than usual in forecast highs (officially 48-55F range, warmest
southwest). Regardless, it will be a fairly nice day for this
time of year for outdoor work and holiday shopping.

In a stark change from recent cool seasons, the shortened
Thanksgiving week and into the beginning of December will mark a
transition to a decidedly wintry pattern. Pronounced high
latitude riding over the northeast Pacific near Alaska will
allow for plenty of cold air to surge southward from our source
region (Canada and the northern Plains) later week and beyond.

The first cold front in the pattern change will be accompanied
by a wave of weak low pressure developing in the frontal trough
Sunday night into Monday. Spotty light rain or sprinkles/showers
are conceivable for parts of the area late Sunday night into
Monday morning. Then on Monday afternoon, a period of wind-
whipped (primarily light) rain is looking more favorable as a
northern stream short-wave trough digs in behind the departing
weak surface low. PoPs up into the 30-40%+ range for most of the
area appear reasonable for now. At this time, the cold air
advection looks to lag enough to preclude a changeover from rain
to snow before the precip ends Monday evening. High temperatures
will tumble from above normal levels on Monday to only mid to
upper 30s on Tuesday, following lows in the 20s to around 30F
early Tuesday morning.

For the all important busy Thanksgiving travel period, there are
a few features of interest. Most notably, a fairly moisture
laden southern stream wave is likely to emerge from the
southern Rockies. Strongly confluent flow over northeastern
North America caused by a deep PV lobe near James Bay may
initially have some suppressive influence on our southern stream
wave of interest. Meanwhile, a strong northern stream short-wave
is forecast to approach into Thanksgiving, packing mid-winter
type cold building in for the start of December.

The above described pieces could plausibly yield a higher impact
weather event for our area, though the general tendency over
the past few model cycles is higher precip. chances to be
favored generally farther for our southern CWA and points south.
If the southern stream and northern stream waves don`t end up
constructively interfering with each other to bring a stronger
surface low farther north and west, the northern stream trough
could bring a chance of light snow Thanksgiving night into early
Friday, associated with an Arctic frontal boundary. Stay tuned
for updates regarding the Wednesday-Thursday forecast!

Friday looks to be windy and cold (highs only in the upper
20s-lower 30s!) for the busy holiday shopping day. The lake
effect machine will undoubtedly get going in this pattern,
though likely primarily east of our CWA on Friday due to west-
northwest flow.

Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 531 PM CST Fri Nov 22 2024

Key messages:

- MVFR cigs tonight and into Saturday. Some potential for
  stratus to build down to IFR, particularly at RFD late tonight
  and Saturday morning.

- Small chance for drizzle tonight--too low for a mention in the
  TAFs.

- Low confidence in the return of VFR conditions late Saturday
  afternoon and evening.

Expansive low stratus blankets the region and extends back into
central Iowa and northeast Missouri at this time. MVFR cigs will
remain locked in place tonight and through much of the day on
Saturday. Have added a mention of IFR cigs at RFD where the
potential for some stratus build down is greatest.

Pockets of drizzle will be possible tonight, mainly across parts
of northwest Indiana in the vicinity of onshore flow off the
lake, as well as west and south of c90 where low-level
moisture is expected to remain the deepest. As a result,
chances for -DZ at the TAF sites remains a bit too low for a
mention at this time, but will continue to monitor trends.

Winds will turn westerly and eventually southwesterly on
Saturday. As this occurs, the back edge of the stratus deck
should inch towards the region. Most guidance suggests VFR
conditions will return by Saturday afternoon. However, given the
expansiveness of the low cloud shield, have maintained MVFR
conditions through 00z Sunday, although will note this is a low
confidence from group.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 4 AM CST Saturday for Winthrop
     Harbor IL to Gary IN.

     Small Craft Advisory until 10 AM CST Saturday for Gary to
     Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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