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000
FXUS63 KILX 101723
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1223 PM CDT Wed Apr 10 2024

.KEY MESSAGES...

- Rain moves in from south to north late this afternoon and will
  continue into the day Thursday. The highest rainfall amounts are
  expected east of I-55. There is a 60% chance of over 1" of rain
  along the I- 55 corridor, increasing to a 90% chance of over 1"
  east of I-57. Localized rainfall amounts of over 3" can`t be
  ruled out (10% chance east of I-55).

- Breezy northwest winds develop on Thursday and Friday, with
  gusts of 35-45 mph on Thursday and 30-40 mph on Friday.

&&

.UPDATE...
Issued at 1012 AM CDT Wed Apr 10 2024

Periods of scattered showers will gradually spread northward today
as disturbances eject off of a low over east TX, interacting with
a frontal zone south of I-70. While instability looks to be
minimal, there could be a few thunderstorms developing as
afternoon solar heating takes place. Dry air evident in the
morning sounding profile should limit precipitation intensity
through the afternoon, especially north of I-70, with heavier
amounts expected this evening into Thursday morning. Temperatures
should reach the mid 60s over most of central and southeast IL
this afternoon, with the cooler side of mid 60s expected in the
cloudier/rainier areas near I-70, and warmer end north of I-72.
With the frontal zone expected to remain nearly stationary and the
approaching main low increasing pressure gradients, a gradually
increasing northeasterly wind is expected through this evening,
around 10 mph late morning to early afternoon, and 15 mph by this
evening. The morning forecast package looks to be on track with
these features, and main update has been to trim back
precipitation amounts north of I-70 this afternoon to amounts
under a quarter inch (and under a tenth north of I-72).

37

&&

.DISCUSSION...
Issued at 252 AM CDT Wed Apr 10 2024

Early Wed AM, a slow-moving cold front was positioned near the
I-70 corridor with scattered light rain ongoing near/just south
of the front. Showers will continue along this corridor into the
morning, but an appreciable northward shift in precip is
anticipated until late this morning into the afternoon. Elsewhere,
a sfc low near the southern tip of TX will deepen and lift
northward today, bringing widespread rain to the area tonight into
Thursday.

Noted a westward shift in guidance yesterday, but the latest runs
have nudged back a bit to the east this evening. Despite the
wobbles in the exact track, models seem to be converging on a
track somewhere between the two solutions discussed in recent days
(the western one being a track from the MO bootheel to NW IN, and
the eastern one being a track up the OH River Valley). This
envelope of solutions favors the highest rainfall totals across
eastern IL. There will likely be a sharp gradient in precip totals
on the western side of the system, so confidence in rainfall
amounts is lowest between I-55 and the IL River Valley.

Rain will gradually spread north this afternoon, but most high-res
models keep precip south of I-72 through 6-7pm this evening. Much
of the environmental analysis related to rainfall potential
remains on track, with anomalously high PWAT values approaching
1.3", which will support a locally heavy rain threat. Regarding
precip efficiency, there is not much in the way of instability,
but forecast soundings do show deeply saturated profiles. The warm
cloud layer is around 8 or 9kft deep, which is moderate, but
concern about flash flooding is greater when the warm cloud layer
is deeper (over 10kft). One factor that could enhance precip rates
is that the track of the low is such that the FGEN/deformation
zone on the NW side of the low is oriented parallel to the track.
That could result in a narrow corridor of prolonged enhanced rates
persisting over the same areas. Current guidance suggests this
would be most likely near/east of I-57.

In terms of rainfall probabilities, there is a 60% chance of over
1" of rain along the I-55 corridor, increasing to a 90% chance
east of I- 57. The probability of 2" of rain is still highest over
southeast IL, with a 30% chance along the I-72 corridor
increasing to a 70% chance along the I-70 corridor. High-end
rainfall amounts (90th percentile, or a 10% chance of occuring)
are in the 3-4" range. WPC has issued a slight risk (15% chance
within 25 miles of a point) of flash flooding for most area east
of I-55 through Thursday morning. Current 6-hour FFG ranges from
2.25-3" across the CWA, which is a lofty threshold to reach,
although that may decrease as this rainfall event unfolds.

This will be a dynamic system, with the sfc low potentially
deepening to below 990-mb as it moves across IN Thurs PM. As we
transition to the CAA-regime on the back-side of the low, deeper
mixing will make it quite breezy. GFS forecast soundings for Thurs
PM show 35+ mph winds just 700 feet above the sfc, which should
easily and frequently mix down. The NBM probability for wind gusts
exceeding 40 mph on Thurs are 70-90% east of I-55 (slightly lower
probs to the west where the pressure gradient and wind fields are
a tad weaker). The probability of exceeding 45 mph gusts (wind
advisory criteria) is lower, but non-zero, with a 20-50% chance
east of I-55. Winds remain gusty on Friday, although it`s worth
noting the forecast soundings show weakening low-level wind
fields. Gusts near 35 mph are likely Fri, but the likelihood of
exceeding 40+ mph seems a bit lower based on those forecast
soundings.

A warm-up remains on track for the weekend after a sfc pressure
ridge shifts east of the area Fri night and results in southerly
flow developing. Forecast highs climb into the 70s Sat and remain
there into next week. Guidance continues to show a weak
disturbance moving by north of the area Sat night into Sun. The
associated weak front could kick off isolated showers/storms
(MUCAPEs 500-750 J/kg Sat night into Sun AM). Will have to keep an
eye on that front Sun, which could linger just east of our area.
Instability progs increase during the day Sun (40-60% of over 1000
J/kg SBCAPE per the latest GEFS, but the upper levels do not
appear overly supportive for storm development, with height
rises/subsidence occuring as a developing upper ridge amplifies
and approaches from the west. The NBM prob of precip during the
day Sun is less than 15%.

Looking ahead to next week, there are signs that another potent
mid-latitude cyclone emerges during the mid-week. Ensembles have
a 20- 40% chance of a sub-990mb sfc low over the central Plains by
Tues AM. The forecasted track of this system is solidly north of
the ILX CWA, so warm sector hazards (severe, flooding, synoptic
winds) will need to be monitored but are all low confidence at
this range.

Erwin

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1223 PM CDT Wed Apr 10 2024

Rain will gradually spread over the area this afternoon and
evening, although northwesternmost extent look to be close to
KPIA. Heavier precipitation rates and MVFR-IFR cigs look to hold
off until around 02Z, and most likely for only KDEC and KCMI.
Winds NE around 10 kts, gradually increasing and shifting toward
NW through the period, becoming 15-20 kts with gusts 25-35 kts by
15Z.

37

&&

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

$$