Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
000
FXUS63 KLOT 230807
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
307 AM CDT Sat Mar 23 2024

.KEY MESSAGES...

- Scattered lake effect snow showers for portions of northeast
  IL and northwest IN this morning

- Seasonably cool conditions for the weekend

- Next rain chances arrive Sunday night into Tuesday. Potential
  for locally heavy rainfall late Monday afternoon and evening.

- Breezy to windy conditions at times, particularly on Monday and
  Tuesday.

&&

.DISCUSSION...
Issued at 307 AM CDT Sat Mar 23 2024

Through Sunday:

High pressure is beginning to drift into the upper Midwest
beneath a broad upper trough that continues to pivot over the
Great Lakes this morning. While precipitation associated with
this upper trough has largely come to an end across the region,
scattered lake effect snow showers have developed across the
southern third of Lake Michigan and are starting to wiggle their
way onshore near the IL-IN line and points east. Guidance
unfortunately continues to struggle with the handling of the
showers likely due to fact that most guidance is too dry in the
low-levels. Though the RAP is starting to get a better handle
and does show some modest lake induced equilibrium levels around
6500 ft persisting through mid-morning which should continue to
support scattered lake effect snow showers across portions of
northwest IN until instability wanes. However, forecast
soundings show very limited moisture in the dendritic growth
layer which I suspect will keep snowflake size small and limit
any accumulations to under a tenth of an inch if any occurs at
all.

Otherwise, expect skies to gradually clear through the morning
as the aforementioned high takes hold to our north. However,
winds will remain out of the north-northeast which will continue
to advect in cooler air and cap highs this afternoon in the
mid-30s to lower 40s. Though warmer temperatures are expected
for Sunday as winds turn southeasterly and warm advection takes
hold.

While this will allow temperatures to reach into the 40s to
lower 50s Sunday afternoon, the warm advection also looks to
generate some showers across portions of northeast IA and
northwest IL tonight into Sunday morning. Guidance continues to
favor that the bulk of this shower activity should remain west
of the forecast area but given the modest forcing and
instability noted in forecast soundings, I have decided to
maintain the 20-25% chances for precipitation along and west of
I-39. If this precipitation does pan out, it should fall in the
form of snow and could even lead to some minor accumulations
(a dusting) primarily on grassy and elevated surfaces.
Additional showers may also occur Sunday afternoon across
northwest IL as warm advection continues to strengthen ahead of
our next weather system but elsewhere should remain dry.

Yack

Sunday Night through Friday:

By Sunday night, attention will turn to a maturing low across
Kansas and Nebraska. Persistent modest pressure falls and a robust
40-50 kt LLJ streaming overhead will yield intermittent 25 to 35
mph gusts during the overnight hours. With it looking like the
main elevated instability/moisture axis will likely hang west of
the Mississippi River through the overnight, precipitation chances
may remain fairly low, at least through the late
overnight/morning hours on Monday. Thereafter, broad ascent from
a combination of increasing DCVA and upper jet divergence will
arrive as the surface low barrels towards Iowa. Shower coverage
will commensurately increase through Monday morning and afternoon.
There may also be just enough elevated instability (MUCAPE) at
the eastern fringes of the the main steepened mid-level lapse
rate plume to supper a few embedded thunderstorms, although
coverage at this time appears a bit too low to justify a formal
mention in the gridded database.

A trend in guidance over the last 24 hours has been to shift the
system`s modest warm sector a bit farther to the west on Monday
afternoon with widespread/persistent stratus and intermittent
shower activity across our forecast area. Winds will likely also
become intermittently pretty gusty through Monday morning and
early afternoon, although just how strong these get is a bit
unclear as low stratus will likely limit the depth of boundary
layer mixing. For now, have capped gusts around 35 to 40 mph.

The most intense and cohesive large scale forcing for ascent will
arrive across our area Monday afternoon and into the evening as
strongly-diffluent flow broadens in advance of a bifurcated upper
jet. Precipitable water values look to surge past an inch which
is around 200 percent of normal for this time of year. This will
yield increasing warm cloud depths and precipitation efficiency.
Thankfully, this system will remain somewhat progressive, with
the mid-levels drying out readily late Monday night. That said,
this looks like a pretty favorable setup for localized corridors
of heavier rainfall, and it`s not surprising to see 12-hour
precipitation amounts nearing 2 inches in spots from the global
guidance.

A cold front will sweep across the region on Tuesday, although
moisture doesn`t look like it`ll be entirely scoured out until
later in the day which will keep at least some shower activity
going. Depending on the front`s exact timing, temperatures will
probably hold steady or even fall through the day. After two
chillier nights Tuesday night and Wednesday night, high
temperatures look set to moderate back into the 50s and 60s
through the end of next week.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1200 AM CDT Sat Mar 23 2024

The only aviation weather concern is the likelihood of
intermittent lake effect snow showers at ORD, MDW, and GYY
tonight and potentially through Saturday morning.

Regional radar mosaics reveal lake effect snow showers
developing upstream across Lake Michigan. Steering flow looks
like it`ll primarily take this activity towards MDW and GYY
although brief intermittent vsby reductions at ORD are also
possible tonight. Farther upstream, additional, somewhat more
intense snow showers are pushing off the Door Peninsula in NE
Wisconsin. Some indication that this activity may intensify a
bit and move across the Chicago-area terminals during the 12-16z
timeframe. Most guidance is not handling this activity well, and
this lowers overall confidence at this time. For the time being,
have left additional TEMPO groups for MVFR -SHSN at ORD, MDW,
and GYY, but may have to make this addition in amendments if
upstream trends continue.

Otherwise, northerly winds will trend northeasterly on Saturday
and eventually southeasterly Saturday night. MVFR cigs will also
gradually lift to VFR through Saturday morning.

A brief period of flurries/light snow appears possible,
particularly INVOF RFD but potentially including the Chicago-
area sites. Chances remain too low at this time for formal
mentions, however.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.