Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 231528
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
1028 AM CDT Sat Mar 23 2024

.KEY MESSAGES...

- Gusty southerly winds will return Monday. There is currently an
  80% chance that gusts range 35-50 mph along our East-to-West
  highways (e.g. I-74, I-72, and I-70). This may pose a travel
  risk for high-profiled vehicles.

- Periods of moderate-to-heavy rain will accompany a cold frontal
  passage between Monday and Tuesday. Regional rainfall totals may
  be highly variable, but a reasonable low-end amount may range
  between 0.25"-0.50", while a reasonable high-end amount may
  range between 1.25"-2.0". Widespread flooding is not a concern.

- Isolated thunderstorms may pose a severe weather risk on Monday
  in areas north of the Illinois River Valley, though the
  probability remains low (about a 5% chance).


&&

.UPDATE...
Issued at 1027 AM CDT Sat Mar 23 2024

Subsidence behind the departing upper trough has resulted in
low level moisture getting trapped beneath a pronounced dry layer
evident on the 12z ILX raob around 800-850mb. Stratus along/east of
I-55 which formed with a southwest-moving moisture plume off of Lake
Michigan should gradually burn off as the solar angle increases late
morning-early afternoon, allowing for efficient radiational warming
that should bring highs into the 40s - possibly 50 in some spots
south of I-70. Hourly forecast temps are looking a lot like sfc obs,
so no adjustments were needed there; only slight edits were made to
keep the sky forecast in-line with satellite obs.

Bumgardner

&&

.DISCUSSION...
Issued at 1254 AM CDT Sat Mar 23 2024

A quick glance at the regional satellite depiction shows a large
bank of stratus extending from eastern Kansas to the lower Great
Lakes. Short-term guidance is in excellent agreement that low
clouds will scattered out by midday as a drier and more subsident
airmass filters in. The notable exception will be portions of
eastern Illinois where stratus may linger longer due to a healthy
northeast fetch off Lake Michigan. Afternoon temperatures will be
held down by these northeast winds, with highs topping out in the
mid 40s at most of our climate sites. Chilly conditions are then
anticipated overnight with deterministic NBM guidance suggesting
sub-freezing temperatures along and north of a Galesburg-to-Terre
Haute line.

Another noteworthy satellite feature evident this morning is a
deepening upper- level low positioned off the coast of the Pacific
Northwest, with multiple shortwave disturbances pinwheeling along
its southern periphery. The rest of this discussion will focus on
the evolution of these features as they begin to impact central
Illinois.

By early Sunday morning, the upper-level low will have begun to
open and dig southward as a strong jet core rounds the base of
the upper trough. As this process occurs, a pair of embedded mid-
level shortwaves will phase together and lift across Baja California,
with preceding shortwave energy ejecting into the Central Plains.
The net effect will be a deepening area of low pressure that
develops in the lee of the Rockies and begins to move
northeastward with well-defined frontal zones.

A few of the hi-res models depict light precip lifting across
portions of western and central Illinois early Sunday morning.
Low-level dry air will be the limiting factor, but as winds veer
southerly and begin overrunning a surface warm front draped across
the Plains, some degree of isentropic/moist ascent could support a
brief period of precip. Thermal profiles do appear cold enough to
support a transition to sleet or snow. This scenario has been
added to the gridded forecast, but given the brevity of the
situation, we do not anticipate any accumulations/impacts.
Temperatures will also be steadily climbing by Sunday afternoon,
with forecast highs in the 50s within a broadening warm advection
regime.

The focus then shifts toward the Monday-Tuesday timeframe as a
parade of upper-level shortwave troughs and jet streaks help
reinforce what will become a powerhouse early spring storm system.

For central Illinois, it will start Monday morning as the surface
pressure gradient tightens in between deepening low pressure
(990mb) positioned over the Northern Plains and strong surface
high pressure (1036mb) over eastern Canada. While the gradient
wind looks to support sustained winds of 15-20 kts, occasional
gusts to 40 kts look to be supported once the boundary layer
mixes into the low-level jet. This will put most of central
Illinois in consideration for a Wind Advisory, and this thinking
is further supported by both blended and ensemble guidance, with
each offering a high probability (80% chance) of gusts ranging
between 35-50 mph. Some of those higher-end gusts may become less
frequent by Monday afternoon with the onset of rain.

While flooding is not a big concern Monday into Tuesday, there
will be periods of moderate to heavy rainfall as a moisture-laden
airmass pools along the decelerating cold front. The probability
for 2 inches or more of rain over a 48-hr period remains low
(generally less than 20%), but both ensemble and multi-model
guidance reveal mean QPF values between 0.75" - 1.50" across our
forecast area.

An attendant severe weather risk bears watching Monday afternoon
in areas north of the Illinois River Valley where an intersection
of key synoptic features come together. These features include:
(a) an upper-level trough over central Iowa, which will steepen
mid- level lapse rates and increase potential instability; (b) a
strong mid-level jet nosing in from the southwest, which will help
increase deep layer shear; and (c) backed surface winds in the
vicinity of a surface warm front, which leads to humped hodographs
and increased SRH. With where these features currently lie, the
severe weather threat favors mostly central and eastern Iowa;
however, should a slower evolution occur within model guidance,
these features would be shifted further east into far western
Illinois.

Thunderstorm potential may then bleed into Tuesday across far
eastern Illinois before the cold front departs into Indiana. Drier
and much colder weather will filter in Tuesday night - Wednesday
night with overnight lows in the 20s. But, temperatures bounce
much warmer late in the week (60s Thursday-Friday) amid strong
warm air advection and increasing heights.

MJA

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 507 AM CDT Sat Mar 23 2024

Short-term model guidance continues to indicate stratus
scattering-out at KSPI/KBMI by 15z. The exception will be at
KCMI/KDEC where a narrow corridor of stratus (due to the healthy
northeast fetch coming off Lake Michigan) will help support MVFR
cigs until about 18z. All central Illinois terminals will return
to VFR this afternoon, though some mid-level cirrus will begin
moving in late tonight ahead of a frontal system.

A few models signal light wintry precip moving into western
Illinois beyond 06z, but confidence and coverage are both too low
at this time to include a mention in the TAFs.

Surface winds will maintain a northeast component into the
afternoon with sustained speeds around 10 kts, becoming east
beyond 06z.

MJA

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$


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