257
FXUS63 KILX 261510
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
1010 AM CDT Sun May 26 2024

.KEY MESSAGES...

- Multiple rounds of strong to severe thunderstorms are expected
  today, with a level 3 of 5 risk for much of the area. The first
  round moves through between 4 AM and noon, posing a risk of hail
  and strong winds. The second round should develop by late
  afternoon, with all hazards possible. The best chance for severe
  storm development this evening is south of I-72.

- Localized rainfall amounts exceeding 2-3" could result in
  isolated flash flooding today.

- Weather impacts should be relatively low during the upcoming
  week, with a low chance (20%) for showers/storms north of I-74
  Monday and Tuesday afternoons, followed by mostly dry weather
  Wednesday through Friday.

&&

.UPDATE...
Issued at 1010 AM CDT Sun May 26 2024

There has started to be some gradually sky clearing in west-
central Illinois this morning as the rain exits to the east.
Environmental recovery following this round will be hard to
achieve but some recovery is possible. CAMs continue to suggest the
best chance for severe storms this afternoon/evening would be
near/south of the I-70 corridor. The new Day 1 outlook will be
issued around 1130/1630z. Also the flooding risk has shrunk to
areas near and south of I-70 for the round of storms this
afternoon

Copple

&&

.SHORT TERM...  (Through Late Tonight)
Issued at 226 AM CDT Sun May 26 2024

A messy and challenging forecast entails over the next 24 hours.
Two rounds of strong to severe thunderstorms are expected across
the region today - one during the morning (5 AM to Noon) and
another during the late afternoon/evening (5 PM to 10 PM), with
the best chance of PM storms being south of I-72.

*** MORNING STORMS ***

Regional radar shows areas of thunderstorms ongoing from IA south
to AR, the leading edge of which extends roughly from Des Moines,
IA, towards Quincy, IL, at 2am/07z. To this point, IR satellite
doesn`t show particularly cool cloud tops with these updrafts, but
storms within this zone have occasionally pulsed to near svr
limits. More robust storms are ongoing near the KS/OK/MO/AR
borders, pushing east. The latest runs of the HRRR seem to be
catching onto the idea that these become dominant and progress
east (staying south of the ILX CWA) through the morning. This
could have substantial implications on the chances of svr wx
across the local area both this morning and this evening.

26.00z soundings from SGF and ILX sampled steep mid-level (700-
500mb) lapse rates, 8.5 degC/km at SGF and 7.6 degC/km at ILX.
Southwesterly mid-level flow in place across the Midwest will help
advect these steeper lapse rates northeastward from SGF, while
height falls from the approaching short wave also enhance the
elevated instability. The RAP supports this assessment, showing a
pocket of 8.5 degC/km mid-level lapse rates reaching the ILX CWA
ahead of this morning`s convection. 00z HREF members show mean
CAPE values of 1500-2000 J/kg present, and forecast soundings show
around 35-40 knots of eff. bulk shear. The HREF members also show
a signal for scattered updraft speeds greater than 20 m/s (45
mph), which are speeds that are loosely associated with nickel to
quarter sized hail.

Forecast soundings suggest these storms would be elevated which
should help limit the tornado threat, but a wind threat is also
possible as HREF member max gusts show isolated to scattered gusts
over 60 mph with these storms and a few 00z CAMs depict stronger
bowing segments developing. As noted above though, recent radar
trends and the newest CAMs suggest any such bowing segments this
morning would likely stay south of the ILX CWA. There is still
some concern that as the storms progress into eastern IL later in
the morning that sufficient surface destabilization could occur to
support all hazards, however, the expectation for stronger storms
across southern IL/MO this morning should limit northward
moisture advection which will impact instability. Overall,
confidence is higher in the presence of a hail threat with these
AM storms, whereas confidence is much lower regarding the wind
threat.

In terms of timing, most CAMs had storms reaching the western
portions of the ILX CWA around 5-7am/10-12z, which is too slow
seeing as they`re already approaching the MS River at 2am. I
expect the storms to reach the western portions of the CWA between
3-4am/8- 9z, then progress east across the area, eventually
reaching the I-57 corridor by reaching the I-57 corridor by
10am-12pm/15-17z.

These storms will be fairly progressive, which should largely
mitigate flooding issues, at least with this initial wave of
storms. There is a 30-50% chance of exceeding 1" of rain from the
AM storms, but localized amounts approaching 2" can`t be ruled
out. 3-hr FFG ranges from 1.6" to 2.25", lowest south of I-72
where the heavy rain occurred on Fri. Once this initial wave of
storms pushes into IN, a several hour break from precip is likely
as subsidence occurs in the wake of the storms. Most of the 00z
CAMs show very little in the way of precip across the ILX CWA
between noon and 3 or 4 PM.

*** EVENING STORMS ***

The specific placement of the evening storms will remain uncertain
until we`re able to assess the impacts of this morning`s storms
(i.e. how far north atmospheric recover occurs this afternoon).
It`s worth noting that high-res guidance has generally focused the
evening storm threat south of I-72, while the 00z CAMs have
trended even further south (near and south of I-70. Wherever the
warm sector does develop this afternoon, conditions will be quite
favorable for svr storms capable of all hazards - with high
instability (over 2500 J/kg of CAPE), 45-50 knots of deep layer
shear, and 0-1km helicity approaching 200 J/kg. Models show rather
quick upscale growth, which would favor damaging winds. Again,
the best indications right now are that this threat will exist
near/south of I-70 between 5pm- 10pm/22z-03z.

The threat of training storms is a bit higher with the evening
storms given the expected storm orientation being roughly parallel
to the deep layer shear vector. There is a 40-60% chance of
6-hour rainfall amounts exceeding 1" south of I-70, with a 20-30%
chance of exceeding 1.5". The southward trend of the PM storms
does shift the expected corridor of heaviest rains away from I-72,
which would help limit the flash flood potential quite a bit.
Still, with two rounds of convective rains expected south of I-70
today, some isolated flooding issues could arise.

Erwin

.LONG TERM...  (Monday through Saturday)
Issued at 226 AM CDT Sun May 26 2024

Overall, the potential for hazardous weather throughout the
upcoming week is low. Upper troughing will remain in place over
the Great Lakes, with a shortwave progressing towards the ILX CWA
Mon PM. This could kick off a few shower/storms, mainly north of
the I-74 corridor. Instability is rather weak (less than 1000
J/kg), but still sufficient to support lightning which could pose
a threat to outdoor activities. Forecast soundings also show an
inverted-V profile in the boundary layer, so isolated stronger
gusts will be something to keep an eye on with these storms. DCAPE
values are pretty marginal though, less than 700 J/kg (above 1000
J/kg is more common with severe wind events). Another wave will
pass just north of the area Tues afternoon, again supporting a low
chance of showers/storms during the afternoon/evening mainly
north of I-74.

Into the latter half of the week, sfc high pressure settles into
the area, resulting in a multi-day stretch of dry weather Wed
into Fri. Can`t completely rule out some showers/storms starting
to return to western IL by Fri, and the ensembles do show PoPs in
the 20-30% range across the western half of the CWA, consistent
with the NBM. However, forecast soundings look quite dry and my
inclination is that the PoPs are too aggressive and that Fri will
be mostly dry. The upper ridging/sfc high pressure pattern
eventually breaks down over the weekend, with an approaching
trough resulting in more robust rain chances returning to the area
next weekend.

Temps will be slightly below normal for much of the week, with
highs in the mid 70s, but a gradual warming trend is expected and
highs will return to around 80 Fri into the weekend.

Erwin

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 614 AM CDT Sun May 26 2024

Scattered to widespread thunderstorms are possible across the
region through 16-17z, and will end from west to east. MVFR
ceilings and visbys could accompany the storms, otherwise VFR
conditions are expected through the day. A few strong storms are
possible late in the afternoon into the evening, but recent models
suggest these stay south of the I-72 corridor, away from the
terminals. Breezy southerly winds are expected today, turning to
northwesterly (and becoming less gusty) by late in the period as
a front moves through overnight.

Erwin

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late tonight for ILZ044>046-049>057-061>063-
066>068-071>073.

&&

$$