Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 241456
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
956 AM CDT Sun Mar 24 2024

.KEY MESSAGES...

- Strong south winds will gust between 40-45 mph on Monday. A few
  gusts could exceed 50 mph (greater than a 50% chance) in areas
  north of I-70. These gusts may pose a travel risk to high-
  profiled vehicles traveling along east-to-west highways.

- An axis of heavy rainfall will develop Monday afternoon,
  bringing 1-2 inches to parts of the region through Monday night.
  Some forecast uncertainty remains in the placement of this
  axis, but recent model trends have favored locations along and
  west of I-55. Widespread flooding is not a concern.

- The risk for a hard freeze returns to areas north of I-70 by the
  middle of the week as nightly temperatures dip into the 20s. Any
  early-season vegetation will be susceptible to damage.

&&

.UPDATE...
Issued at 952 AM CDT Sun Mar 24 2024

The current forecast was looking a lot like observations this
morning, so few changes were needed. PoPs and sky conditions were
freshened with the the 12z HRRR and newest NBM, as were winds and
gusts through tomorrow evening which may necessitate the issuance
of an advisory.

Bumgardner

&&

.DISCUSSION...
Issued at 1236 AM CDT Sun Mar 24 2024

Wind Risk:
Cyclogenesis is already well underway along the lee of the
Rockies, and the surface low will continue to deepen today as
strong temperature advections help augment its areal footprint.
With the surface pressure gradient tightening today across the
Midwest, sustained wind speeds will increase to 15-20 mph. Model
soundings generally indicate mixing heights up to about 900mb,
limiting gusts to around 25-30 mph today. However, should a drier
boundary layer evolve this afternoon, mixing heights could extend
up to 850mb, supporting occasional gusts to 35 mph--mainly in
areas west of I-55.

Stronger winds will arrive Monday morning as the surface low lifts
into the Central Plains, and a 45-55 kt LLJ veers into Illinois.
There does appear to be brief window before the steady rain
arrives Monday afternoon in which the boundary layer will mix
into the LLJ and help support widespread 40-45 mph gusts. And,
after consulting both blended (NBM) and ensemble (HREF) guidance,
there is some support (> 50% probability) for sporadic gusts
exceeding 50 mph. Gusts should theoretically diminish Monday
afternoon from west to east as rain spreads in and the near-
surface layer becomes too stable to appreciably mix into the LLJ.
We will continue to monitor model trends over the next 12-24
hours, but a Wind Advisory may be needed for most if not all of
our CWA on Monday.


Flood Risk:
An extended period of moderate rainfall will commence Monday
afternoon as the surface low becomes occluded and the cold front
decelerates. A moisture-laden airmass (90th percentile PWATS) will
reside along the front for 12+ hrs on Monday as the mean wind
(850mb-500mb) becomes parallel to the front amid strong synoptic
ascent. The net effect will be an axis of heavy rainfall that
develops along the cold front.

Latest ensemble guidance from both the LREF Mean QPF and HREF
LPMM QPF supports a narrow axis of 1-2 inches along and west of
I-55, with amounts diminishing further east. This is juxtaposed to
the NBM Mean QPF guidance, which is much more broad in its area
of > 1 inch--extending it all the way to the IL/IN state line.

With us now being within 48 hours of this rainfall event, we
generally lean heavily towards short-term, hi-res ensemble QPF
(HREF PMM or HREF LPMM). We will continue to monitor trends in
this guidance and adjust the axis of heaviest rainfall
accordingly.

Regardless of where the axis of heaviest rainfall develops, we are
unlikely to experience widespread flood impacts, with 6-hr flash
flood guidance at around 2.5 inches. Per the latest HREF, there is
less than a 10% probability of us exceeding that guidance.


Freeze Risk:
While the growing season is not fully underway across central
Illinois at this juncture, there have been plenty of reports of
pre-mature budding or flowering. This early-season vegetation will
be susceptible to damage by mid-week as nightly temperatures north
of I-70 fall below freezing both Tuesday and Wednesday nights. Per
the latest NBM deterministic guidance, temperatures will fall into
the mid-to-upper 20s, and we see no reason to stray from this
guidance given the cold 1024mb high progged to settle over the
region.

Drier and progressively warmer temperatures return by the end of
the week amid strong warm advection and increasing mid-level
heights. Afternoon temperatures will reach into the 60s both Friday
and Saturday.

MJA

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 347 AM CDT Sun Mar 24 2024

Southeast winds will increase this morning to 10-15 kts, with
occasional 25 kt gusts this afternoon. VFR conditions will remain
in place throughout this TAF period, though cigs begin to lower
toward MVFR across western Illinois by the end.

Low-level wind shear may become a problem overnight, though a
tightening pressure gradient and increasing surface gusts may
limit the regional extent. Nevertheless, have added mention of
low-level wind shear by 05z across all terminals as a 45-55 kt
low-level jet veers into Illinois overnight at about 3 kft AGL.

MJA

&&

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

$$


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