151
FXUS63 KLOT 222009
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
209 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.

&&

.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 /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1118 AM CST Fri Nov 22 2024

As the backside of a surface low continues moving east away from
the area and mid-level heights rise, inversion heights will
gradually lower through the TAF period. As a result, ongoing
stratus is expected to prevail with ceiling heights gradually
lowering below 2000 feet overnight. (Any holes in the stratus
that develop this afternoon, such as near ORD/MDW/RFD, should
fill back in by sunset).

High res guidance such as the HRRR/RAP depict a weak signal for
pockets of drizzle overnight, though felt the increasingly
marginal depth of the stratus and an absence of
robust/persistent low-level forcing should keep drizzle
isolated. For this reason, will withhold any formal mention of
DZ (and associated drops in visibility to 1-2SM) out of the TAF
package. Breezy northwest winds will ease overnight and
gradually shift toward the west by daybreak Saturday.

This time of year, it`s always tricky to pinpoint when stratus
will finally erode. With inversion heights lowering through the
TAF period, the depth of the stratus will become increasingly
thin and susceptible to mechanical mixing of dry air beneath
cloud bases after sunrise tomorrow. As a result, it`s possible
stratus begins to scatter out in the last 4 to 6 hours of the
TAF. For now, will hold onto BKN to OVC status through the
duration of the TAF as a course of least regret.

Borchardt

&&

.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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