572
FXUS63 KLOT 221719
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1119 AM CST Fri Nov 22 2024

.KEY MESSAGES...

- Lake effect rain showers to prevail in northwest IN today,
  especially in Porter County and points east.

- Moderating temperatures for Sunday, then turning colder again
  Tuesday through the Thanksgiving Holiday.

&&

.DISCUSSION...
Issued at 315 AM CST Fri Nov 22 2024

Through Saturday:

The upper-level low that brought us the snow yesterday
continues to spiral over the Mid-Atlantic and eastern Great
Lakes this morning. While most of the precipitation has come to
an end across northern IL, some pockets of drizzle continue to
be observed near between I-39 and I-355. The drizzle is
expected to linger through daybreak before tapering by mid-
morning. However, a mid-level vorticity maximum is expected to
pivot overhead this afternoon and interact with the persistent
low-level moisture which may lead to another bought of patchy
drizzle this afternoon. Since the coverage is somewhat lower
confidence for this afternoon have decided to forego a formal
mention in the forecast at this time, but did boost POPs around
10% to account for this potential. Regardless, temperatures are
expected to remain above freezing (in the upper 30s to mid-40s)
today which will keep the any impacts low.

Aside from the drizzle, there is also the ongoing lake effect
showers in northwest IN to discuss. The lake effect is currently
riding along a north-south oriented convergence axis that will
continue to drift eastward into northern IN as winds become more
west-northwesterly later today. So while lake induced
instability will remain sufficient (ELs in the 5000 to 6000 ft
range), the coverage of showers should focus into far eastern
Porter County and points east by midday. In terms of
precipitation type, forecast soundings continue to show that
virtually all of the effective moisture to be underneath the
DGZ (-12 to -20 C layer) which will keep the showers in the form
of rain. However, a couple of snowflakes may mix with the rain
prior to daybreak where temperatures are still hovering near
freezing.

The lake effect showers and any drizzle today will conclude
this evening as the aforementioned upper-low progresses east and
allows modest upper ridging to build overhead. Therefore, dry
conditions can be expected for tonight and through the day on
Saturday with otherwise seasonable conditions with highs in the
40s and overnight lows in the low to mid-30s. However, lingering
low-level moisture trapped beneath the ridge does look to
maintain mostly to completely cloudy skies through Saturday as
well.

Yack


Saturday Night through Thanksgiving:

Focus in the extended forecast period is on a brief warm-up to
start the upcoming week before a cold front early Monday ushers
in a period of slightly below normal temperatures through the
Thanksgiving holiday with additional wintry-precipitation
chances. Much colder temperatures are then potentially in store
over the extended holiday weekend.

Heading into Sunday morning the surface ridge axis will begin
to translate east of the area bringing a return to southerly
low-level flow (aka warm advection) with continued dry
conditions. While there will likely be a fair amount of cloud
cover overhead, temperatures should still manage to warm back
above normal with forecast highs in the lower to mid 50s. Be
sure to get out and enjoy it if you can!

A cold front subsequently moves through the area early Monday
ahead of an approaching weak mid-level shortwave. This will
bring the next chance for rain to portions of the area; however,
questions remain as to whether sufficient moisture will be
available locally given the displacement of these two features.
In fact, a faster frontal passage could keep most areas dry.
Opted to not make large changes to the previous forecast for now
with the best potential (20-40% chance) mainly near the lake
and east of I-55 during the day on Monday. This cold frontal
passage then ushers in a period of slightly below normal
temperatures for late November with highs generally in the mid
30s to lower 40s and lows in the 20s to lower 30s forecast
through Thanksgiving.

The upper level pattern also turns more quasi-zonal toward
midweek next week as the jet stream sags a bit further south
latitudinally. This would place us just to the north of the
favored system track, which could bring additional wintry
precipitation chances toward midweek, including the Thanksgiving
holiday. Stay tuned.

Looking ahead there remains a fairly strong signal for a period
of much colder temperatures over the long holiday weekend into
the first week of December with the Climate Prediction Center
forecasting a 70-80% chance of below normal temperatures across
much of the region.

Petr

&&

.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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