168
FXUS63 KILX 111704
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1204 PM CDT Tue Mar 11 2025

.KEY MESSAGES...

- Above normal temperatures continue through the week, with highs
  generally in the mid 60s to mid 70s, although Friday could see
  temperatures climb into the upper 70s.

- A late week storm system will result in windy conditions Friday
  through Sunday. The strongest wind gusts are expected on
  Saturday, out of the southwest, when gusts up to 50 mph are
  possible (30-60% chance).

- A line of storms is likely Friday night (90% chance), and these
  storms could be severe (15-30% chance of severe weather within
  25 miles of a point).

&&

.DISCUSSION...
Issued at 300 AM CDT Tue Mar 11 2025

*** TODAY - THURSDAY ***

A warm and mostly dry week continues across central IL. Analysis
of 00z RAOBs reveals a split flow pattern in the upper
troposphere, with mostly zonal flow along both the northern and
southern US borders, while upper lows sit over the Carolinas as
well as just off the SW California coast. The low off the
California coast is not the late week system that we`ve been
closely monitoring, that wave is still over the Pacific.

The main forecast challenge today relates to the evolution of an
approaching cold front, which was draped across parts of central
NE and IA as 230am/0730z. No precip is expected with this front
owing to the dry airmass in place ahead of it (PWAT values less
than 0.5"), but it will result in a shift from SW winds ahead of
the front to NNE winds behind it. This wind shift is most likely
to occur north of I-72 today, and becomes less likely near/south
of I- 70 (at least during the daytime hours).

CAMs also depict a reinforcing backdoor cold front surging
southwest from Lake Michigan this afternoon. Unlike the initial
cold front passage, this one will feature quickly falling temps
behind it, along with breezy NE winds gusting 20-25 mph. Guidance
has been consistent in showing this front reaching the I-74
corridor during the late afternoon/early evening, and continuing
SW in time, reaching areas like Springfield/Mattoon by late
evening (9-11pm,02- 04z). Temps could drop as much as 30 degrees
in a few hours following this frontal passage (aided somewhat by
the diurnal timing), for example, going from the upper 60s at KCMI
mid-afternoon to low 40s by 7-8pm, while areas still south of
this front are progged to still be in the upper 50s at that time.
After several warm days, the brisk NE winds and cooler temps
behind this front will feel quite crisp.

On Wed, the frontal zone is expected to lift back north, although
latest guidance is less aggressive in the northward progression.
At any rate, we should stay mild on Wed, with highs in the mid 60s
to low 70s. That aforementioned upper low off the CA coast will
shift towards the TN Valley late Wed night into Thurs, and
scattered showers/isolated storms could extend as far north as the
I-70 corridor (15-25% chance). Models show modest MUCAPE values
present which could support some rumbles of thunder with this
precip activity. Thurs remains well above normal as well, with SE
sfc winds in response to lee cyclogenesis from our developing late
week system. Highs should be in the low to mid 70s.


*** LATE WEEK - WEEKEND ***

The previous messaging regarding the late week system largely
remains on track, and forecast confidence is steadily increasing.
Ens guidance continues to depict an intense mid-latitude cyclone
tracking from the central Plains to the Upper Midwest between
Thurs night and Sat night.

As the low deepens over the central Plains on Fri, stout SE flow
develops across the ILX CWA on Fri, with sustained winds of 20-25
mph and gusts of 30-40 mph. This flow will help push high temps
into the upper 70s, and perhaps even the low 80s in west-central
IL (20- 40% chance).

* FRI NIGHT SEVERE STORM POTENTIAL *

The parameter space continues to appear conditionally favorable
for severe storms in IL Fri evening, with 50th percentile MUCAPE
values from the Grand Ensemble approaching 750 J/kg and deep layer
shear around 60 knots. Through 7pm Fri/00z Sat, fcst soundings
continue to suggest a cap in place, preventing DCI (locally) and
allowing dry conditions to prevail during the daytime hours on
Fri. Into the evening, linear forcing along an a front combined
with increasing synoptic scale ascent (height falls, DPVA from the
upper low) should support widespread convection along the front.
As the front/line of storms push into west-central IL (after 7pm
Fri/00z Sat, likely closer to 10pm/03z Sat), deep layer shear
vectors appear largely parallel to the front (crossing angle less
than 45 deg), which also supports a linear storm mode. To what
extent, if any, discrete storms are able to develop ahead of this
line remains unclear, but the presence of the capping inversion
should limit the discrete cell potential somewhat.

Damaging winds will be the top concern with this line of storms,
although the robust kinematic fields (0-3 km shear around 50
knots) could support spin-ups within the line. The main question
is how long sufficient surface-based instability will persist into
the late evening/overnight hours. The latest deterministic GFS
soundings do show around 40 J/kg of 0-3 km CAPE, which is
marginally supportive of mesovortex generation. The latest
guidance did not exhibit any significant shifts in timing, but
adjustments are certainly still possible in the coming days. For
now, it looks like roughly 9pm Fri to 4am Sat (02z-09z) as the
most likely window for strong/severe storms. There are questions
about how far east the line will be able to maintain strong/severe
intensity during the overnight hours, so in a relative sense I
think the severe potential is higher across western IL compared to
eastern IL, but certainly not ruling out a severe threat anywhere
in the ILX CWA.

* GRADIENT WINDS *

Precip could linger across eastern IL Sat (20-30% chance east of
I-57), but guidance continues to focus the bulk of the precip
south and east of our area, across IN/OH/KY/TN. The biggest
concern on Sat is the gradient winds/gusts, as deeper mixing
following FROPA Fri night will allow stronger winds to mix to the
sfc. Will note that the ens mean peak gust has trended a tad
weaker, which is consistent with the deterministic GFS trending
the sfc low weaker as well. (To be clear, `weaker` in this sense
refers only to the guidance trend, not strength of the low or the
expected gusts on Sat). Winds will be out of the SW on Saturday.
EC ens members have an average peak gust between 45-50 mph, while
the NBM has a 20-40% chance of 50 mph gusts west of I-55, and a
40-60% chance east of I-55. These values are supported by fcst
soundings, which show mean boundary layer winds around 40 mph, and
winds at the top of the channel approaching 55 mph. These
soundings indicate the strongest gusts can be expected late
morning through early afternoon, then the wind fields start to
gradually weaken by late afternoon as the sfc low continues to
lift away.

Breezy conditions continue on Sun, although not nearly as intense
as Sat, with winds out of the WNW and peak gusts around 30 mph.
Temps will be much cooler by Sun, with highs only in the 40s as
cold air continues to advect around the departing sfc low.

Erwin

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1204 PM CDT Tue Mar 11 2025

A frontal boundary has stalled over central Illinois near the I-72
corridor. Winds will be light and variable within the frontal
trough and north to northeasterly north of the front. An enhanced
lake breeze from Lake Michigan will overspread the terminals late
this afternoon into the evening reinforcing the NE winds across
the area and causing a brief period of gusts to around 20kt. VFR
conditions are expected through the period.

Deubelbeiss

&&

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

$$