865
FXUS63 KILX 121036
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
536 AM CDT Wed Mar 12 2025

.KEY MESSAGES...

- Warm and mostly dry conditions through the daytime hours on
  Friday, with high temperatures generally in the 70s. There is a
  low chance (20%) for showers south of I-70 on Thursday.

- A line of storms is expected (95% chance) Friday evening into
  Friday night, and there is an enhanced risk (level 3 of 5) that
  these storms will be severe. Damaging winds are the top concern,
  although hail and a few tornadoes are also possible. The most
  intense storms could contain straight-line wind gusts over 75
  mph. The most likely time for these storms is between 7 pm
  Friday and 2 am Saturday.

- Strong winds are expected on Friday (out of the southeast) and
  Saturday (out of the southwest). The probability for gusts over
  45 mph is 40-50% on Friday, and 70-90% on Saturday.

&&

.DISCUSSION...
Issued at 318 AM CDT Wed Mar 12 2025

* TODAY - THURSDAY *

The main forecast challenge over the next 48 hours is tracking the
meridional fluctuations of a weak front that sagged into the area
on Tues. The front was located near the I-70 corridor as of
230am/0730z Wed AM. The latest CAMs vary in how far north this
front lifts today, which will impact the MaxT forecast as well as
the (light) wind direction. Some bring the front as far north as
I-74, while others keep it closer to I-72. High temps are likely
to climb into the low 70s south of the front, with mid 60s north
of the front. The NBM probability of exceeding 70F today reflects
this uncertainty, with a 70-90% chance south of I-72, dropping
quickly to the north of I-72, with just a 20-40% chance north of
I-74. Winds will be southerly to south of the front, and
northeasterly north of the front, but in both cases wind speeds
should generally be less than 10 mph.

A shortwave will pass by to the south of the region Wed night into
Thurs, providing a chance (15-30%) for scattered showers/storms,
mainly south of I-70. The weak frontal boundary should lift
northward once again, and while there`s some uncertainty, the
thinking is Thurs will be warmer than today, with a greater than
70% chance for highs in the 70s area-wide.

* FRIDAY WINDS *

A potent upper wave will eject from the SW US late this week, with
lee cyclogenesis ramping up Thurs night into Fri, and a 975mb sfc
low expected over KS/NE by midday Fri. An expansive wind field
will develop, including breezy SE flow across IL through the day
Fri. PBL mixing is usually a question mark in WAA regimes, and
while fcst soundings do suggest some potential for diurnal Cu,
deep mixing is present. This will allow for vertical mixing of
stronger winds to the sfc, and the fcst soundings from both the
GFS/NAM make gusts of at least 35-40 mph appear likely Fri PM
(which is right in line with the ens mean peak gust). Winds atop
the PBL approach 50 mph during the afternoon, offering a glimpse
at what the more sporadic, higher end gust potential is. The SE
flow and resultant WAA should push temps into the upper 70s, and
there is a low chance (10-30%) for temps in the low 80s (mainly
south of I-70 or west of I-55).

Into the evening, ahead of an expected convective line (more on
that below), soundings continue to show steep low-level lapse
rates while LLJ forcing leads to increases in the low-level wind
speeds. Confidence is lower that mixing will be sufficient after
7pm Fri/00z Sat to transport these winds to the sfc, but there is
some potential for a window of up to 50-60 mph synoptically-driven
wind gusts BEFORE any storms move in. This analysis was based on
the fcst soundings/modeled wind fields, but the 12.00z NAM
corroborates this analysis, showing a broad swath of 50-60 mph
gusts prior to convection. Unfortunately, it`s difficult to parse
out the ensemble signal for this threat due to convection
overlapping in the same 6 hour period. The NBM potential for 40+
mph gusts at any point on Fri/Fri eve is 70-90%. A wind advisory
may be needed for Fri.

* SEVERE STORM THREAT *

The biggest change regarding the severe storm threat is a trend
towards slightly faster storm arrival, perhaps as early as 7pm
Fri/00z Sat across west-central IL. Most models still bring storms
in closer to 9 pm, but did want to note the potential for earlier
arrival.

Fcst soundings continue to exhibit substantial capping through
the daytime hours on Fri (anywhere from -50 to -150 J/kg of
SBCIN), which will keep the area dry. Storms are expected to
initiate to the west, across central MO/IA, along a dryline.
Strong synoptic forcing, strong linear forcing, and a low crossing
angle between the deep-layer shear vectors and the front all
favor a linear or quasi-linear convective mode, which has been
well- supported by model`s depiction of the convection.

As storms push into the ILX CWA, the parameter space will feature
seasonably moderate instability (MUCAPE values around 750 J/kg)
and robust shear, as the jet core at each level is focused over
the ILX CWA and the upper low sits just to the west. This will
result in deep layer shear values over 60 knots, and 0-3 km shear
around 50 knots. Despite the loss of diurnal heating, the strong
SE flow will help limit temp reductions after sunset, resulting in
only subtle decreases in ambient theta-e ahead of the advancing
line. The NAM/GFS vary in their LCL (cloud base) heights, but both
show a fairly well-mixed PBL ahead of the line. This PBL
structure increases downdraft CAPE/damaging wind potential, and
both the NAM and GFS have DCAPE values approaching 1000 J/kg.
Between this signal, the robust wind fields (won`t take much to
vertically mix stronger winds to the sfc via downdrafts), and the
expected storm mode, damaging winds remain the top concern with
the storms Fri night. The 12.00z NAM also shows hints of a meso-
high behind the bowing portion of the convective line, an
impressive signature given the resolution of that model, and one
that raises concern that a mature squall line capable of
significant severe gusts (>75 mph) could impact parts of our CWA.
The latest SPC convective outlook expanded the enhanced risk
(level 3 of 5) to include our entire CWA, and notes potential for
significant severe gusts everywhere within the enhanced risk area.

The tornado threat will also need to be monitored given the robust
wind fields. While low-level hodograph curvature is somewhat
limited in the lowest km, there is still forecast to be over 250
J/kg of 0-1 km storm-relative helicity. The 50 knot 0-3 km shear
vectors will be oriented out of the SSW, which should be largely
parallel to a good portion of the convective line. However, models
suggest a concave/bowing line, so in areas where the 0-3 km shear
vector becomes more perpendicular to the line, either due to the
larger- scale bowing or localized surges within the line, these
will be areas to monitor for localized tornado potential. Some
factors that make the tornado potential less certain include
questions about how long into the night sfc instability will
persist, as well as the fact that the PBL structure discussed
above is not the most favorable for tornadoes. While I expect the
wind probabilities to drive the convective outlook category, I do
not expect the tornado probabilities to be zero. CSU`s machine
learning guidance supports this notion, with a 5-8% tornado threat
west of I-55 and a 2-5% tornado threat east of I-55.

One final note on the storms...the line will be moving very fast,
with storm motions over 60 mph. At these speeds, the apex of the
line could traverse the ILX CWA in as little as 3-4 hours.

* SATURDAY INTO NEXT WEEK *

While the primary upper low/occluded sfc low gradually lift
northward on Sat, another deep trough follows quickly behind
resulting in a renewed, broad area of shower/storm development
across IN/OH/KY/TN. This precip is mostly expected to be focused
SE of the ILX CWA, but some low end precip chances (15-30%) linger
across eastern IL into Sat (east of I-57).

The main concern for Sat is continued breezy winds, potentially
meeting advisory criteria. Ens probs have gradually trended
downward with the peak gusts, but fcst soundings still suggest
sufficient mixing during the late morning/early afternoon for a
period of 35-50 mph gusts out of the southwest. The best chance
for 50 mph gusts is north of I-74/east of I-39, where there is a
60-70% chance. Probabilities decrease with southwestward extent,
such that there is only a 30% chance along a Galesburg-Lincoln-
Mattoon line. Winds remain breezy during the afternoon, but should
gradually trend downward after early afternoon. Will need to
monitor the potential for elevated fire danger Sat, but the Fri
night storms may sufficiently wet fuels to limit the danger.

CAA results in a much colder airmass pushing in by Sun, with highs
only in the 40s across the entire CWA. Breezy conditions continue,
although nothing like the previous days, with gusts around 25 mph
out of the west-northwest. There is a low chance (20%) for light
rain or snow Sun AM, but minimal impacts are expected from this.

The CONUS pattern remains progressive/active into next week.
Current guidance suggests a dry start to the week, with southerly
flow in advance of another potential mid-week cyclone resulting
in rebounding temperatures. For Day 7 of the forecast, there is
strong model consensus that temps return to the upper 60s or 70s
by Tues. In terms of another potential system during the middle of
next week, guidance spread is considerable. For now, WPC`s
probabilistic hazard outlooks have a slight chance (20%) for heavy
precipitation and strong winds for IL sometime during the Wed Mar
19 - Thurs Mar 20 period.

Erwin

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 536 AM CDT Wed Mar 12 2025

VFR conditions and scattered high clouds are expected through the
period, along with wind speeds less than 10 knots. A weak front
will lift north today, and will likely be draped near the I-72
corridor, resulting in variable wind direction during the daytime
hours. Further north, at KPIA/KBMI, northeasterly winds will
prevail through the period. The northeasterly winds will spread to
the other terminals after 00z, and winds should remain
northeasterly through the overnight hours.

Erwin

&&

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

$$