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035
FXUS63 KILX 231034
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
534 AM CDT Thu May 23 2024

.KEY MESSAGES...

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- There is a 70-80% chance for thunderstorms Friday, with a level
  2 of 5 (slight) risk some of these storms become severe. The
  strongest storms could produce damaging wind gusts and large
  hail.

- Scattered thunderstorms are likely (80% chance) on Sunday.
  Confidence has increased in severe weather potential, but the
  timing and location of greatest risk remains uncertain.
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&&

.DISCUSSION...
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Issued at 233 AM CDT Thu May 23 2024

Current mosaic radar shows a couple showers drifting across
southeast Missouri and southern IL, ahead of a weak mid-level wave
which deterministic models suggest will ride into southeast Illinois
later this morning. The HRRR is the most bullish of the CAMs, but
unfortunately also the most accurate to current radar, and has a
couple thunderstorms lifting into our area near and south of I-70
mid morning; meanwhile, the other CAMs are either dry or have very
light reflectivities suggestive of only sprinkles reaching the
surface through the very dry column which, per the 00z ILX raob,
features a PWAT of only 0.49 (roughly the 10th percentile from
climatology). The CAMs are thereafter looking dry across our area,
with the arrival of the next wave of thunderstorms slated to be
sometime between 9 and 11 am Friday near the IL River and between
noon and 2 pm at I-57; this was looking at the 25% onset time from
HREF given the recent faster trend with precip arrival from the
newer iterations of the highres models. High temps will thus make a
return to slightly above normal across central Illinois this
afternoon, hitting 80+ degF in most locations with only gentle south-
southeast winds around 5-10 kt.

The pleasant conditions will come to an abrupt end tomorrow morning,
however, as the aforementioned thunderstorms arrive from the west.
CAMs suggest a decaying line of thunderstorms will enter our area
shortly after sunrise, but vary on their depiction of the line`s
evolution as it encounters (1) the drier air we`ll have in place and
(2) the nocturnal temp inversion which will be gradually eroding
east to west across our area with increasing insolation after
sunrise. The HRRR and WRF-ARW suggest the line will hold together,
producing gusty but mostly sub-severe wind gusts and pockets of near
severe hail as it travels across our area, while the NAMNest and
Highres FV3 suggest it`ll break up and scattered storms will linger
along the boundary left in its wake. Given the actual cold front at
the surface isn`t slated to cross central/southeast IL til quite
late (9pm at the IL River to 3am at I-70, if you believe the GFS),
there`s potential for additional storms to fire late in the day,
though guidance suggests these storms would be mostly sub severe
given lack of deep layer shear. Tomorrow`s highs are tricky, and
this forecaster is getting a sinking feeling they`re going to be
noticeably lower than NBM`s forecast (mid 80s) northwest of I-55
where the 00z HRRR and NAMNest each have us struggling to get above
the low-mid 70s behind the storms...although, given the warm
advection ahead of them we could briefly sneak up close to 80 during
the morning.

All this being said, I did want to add with the morning aviation
update the new wrinkle added to the forecast from the 06z
iterations of both the HRRR and NAMNest - the two models which
were previously showing the line of storms entering the area
during the mid morning hours: Now they`re each suggesting the line
falls apart, and then at some point during the afternoon storms
redevelop near/west of the IL River along the morning storms`
outflow boundary. Each are also a bit more aggressive with
convection during the afternoon and evening...And no wonder, given
3000 to 3500+ J/kg SBCAPE. Deep layer shear will be running near
30 kt or a little less, which isn`t particularly favorable for
organized storms, but given the high instability, steep lapse
rates between 850mb and 500mb, and mid-level dry air favoring
precip loading (PWATs forecast to climb to over 1.5"), it should
be easy for any of these storms to produce both severe hail and
wind in wet downbursts, with wind becoming a bigger risk if any
storms grow upscale into a linear multicell cluster. This also
means temps could wind up being warmer than forecast, though we`ll
hold off on making any changes given uncertainty. After all, this
is just one iteration of each of these models, and it`s not one
of the synoptic hours which includes fresh raob data; it`ll be
interesting to see what the 12z iteration of each of these models
show, along with the other HREF members.

Saturday, dry conditions will return as surface high pressure builds
into the region, and given weak cool advection during at least the
first half of the day temps should stay in the 70s for highs north
of I-70. Then, thunderstorm chances return by Sunday, when a warm
front is slated to lift across the area ahead of a surface low which
will approach central Illinois during the evening. The deterministic
ECMWF forecasts prolific lightning activity with scattered storms
along that warm front during the morning-early afternoon, and then
suggests scattered thunderstorms continue across areas near and
south of I-72 right through the evening. Things are very subject to
change between now and Sunday, and last minute shifts (leaving our
area virtually severe weatherless) have been a pattern this season
thus far, so we don`t want to prematurely sound alarms. However,
this system is going to have our attention the next several days,
as steep mid level lapse rates associated with the EML promote
fast updraft parcel accelerations through the hail growth zone,
and 0-6km bulk shear values reach 40-55 kt to favor sustained
storms. Forecast hodographs in the vicinity of the warm front and
on the nose of the LLJ take on the classic "hockey stick" look,
implying sufficient directional shear in the low to mid levels for
rotating storms that during the morning will pose a risk for
large to very large hail, and during the afternoon will pose a
conditional tornado threat. This will all depend on the exact
track of the low (the ECMWF is a bit further north than the GFS)
and how far north the warm front lifts during the morning/early
afternoon, with middle of the road (or "average of the extremes")
guidance suggesting it`ll be near our northeast CWA boundary by
peak diurnal heating/destabilization when surface based storms
carrying that conditional tornado risk would be most likely to
happen. Storms and clouds will suppress highs a bit Sunday for
parts of the area, making the high temp forecast tricky; in fact,
NBM`s 10th-90th range is 68 to 87 degF in Lincoln.

Behind that system, though, temps will moderate and we`ll encounter
a near to cooler than normal (and drier) airmass. Tight gradients
behind the low will also make for breezy conditions on Memorial Day,
with a 60-70% chance for west/northwest gusts surpassing 30 mph
during the afternoon.

Bumgardner
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&&

.AVIATION...  
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(For the 12z TAFs through 12z Friday Morning)
Issued at 534 AM CDT Thu May 23 2024

Light and variable winds early this morning will become more
consistently south-southeasterly around 5-10 kt this afternoon
while FEW to SCT cumulus develop along and south of I-72.
Additional, higher cloud cover will overspread the area from the
west near and after the end of the forecast period (12z/7am
Friday) as a decaying line of storms approaches from the
northwest.

Bumgardner
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&&

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

$$