188
FXUS63 KILX 190212
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
912 PM CDT Sat May 18 2024

.KEY MESSAGES...

- Above normal temperatures will continue through at least
  Tuesday, with highs in the upper 80s. Tuesday looks like the
  most humid day of the stretch, resulting in heat indices in the
  low 90s.

- Isolated storms are possible (20% chance) Sunday afternoon.
  While much of the area will stay dry, locally heavy rainfall
  over 1.5" could occur beneath the storms that do develop.

- A stormy pattern will exist early next week, with potential for
  severe thunderstorms Monday through Wednesday. During that time
  frame, the greatest risk is for damaging winds west of I-55
  Tuesday evening.

&&

.UPDATE...
Issued at 912 PM CDT Sat May 18 2024

Skies are clear and winds are light southwest. Current forecast
has a good handle on this with temps expected to drop into the
middle 60s across the CWA. Therefore, no official update is
planned at this time.

Auten

&&

.DISCUSSION...
Issued at 220 PM CDT Sat May 18 2024

For the rest of today, upper level height rises will continue to
produce large-scale subsidence. While some areas of agitated Cu
are evident on satellite, vertical development seems suppressed
and I kept the forecast dry. A cold front is progressing across
WI/IA this afternoon, although to this point, storm development
has been confined to WI where there is better upper level forcing.
CAMs still show storms forming along the front in IA by late
evening, but these quickly dissipate after sundown and are
unlikely to reach the NW part of the ILX CWA.

A summer-like pattern will exist through the middle of the
upcoming work week, with above normal temperatures and storm
chances, some of which could be severe. The Tues night-Wed
timeframe understandably continues to be the main highlight, but
there are other storm chances before then that also warrant
discussion. High temperatures will be in the mid 80s to near 90F
through Tues, while dewpoints will be in the mid 60s through Mon
before climbing to near 70F on Tues. Due to those higher
dewpoints, the highest heat indices are anticipated to occur on
Tues, with values in the low to mid 90s.

** SUNDAY **

The chance for storms on Sunday is fairly low, as the area remains
beneath an upper level ridge axis, but there are still a few
potential mechanisms via which storms could form. The weak cold
front NW of the area today is progged to move into the CWA late
tonight and stall across the area on Sun. While upper support will
not be conducive, forecast soundings do show an uncapped
environment with moderate instability (2000-2500 J/kg of CAPE) and
that front could provide sufficient lift to kick off storms. The
latest CAMs suggest this scattered activity, if it were to
develop, would occur between the IL River and I-70. Hodographs
look crumpled (very weak shear), so similar to Friday I expect any
storms to be relatively short-lived, but also slow-moving which
could result in very isolated instances of heavy rainfall. HREF
24-hour LPMM supports that assessment, with a few pockets where
rainfall exceeds 1.5".

The other, lower confidence way that storms could impact the area
on Sunday is via a convective complex that develops over the
west- central Plains late tonight and rolls east across the
Plains, potentially reaching west-central IL by Sun evening.
Latest CAMs are not as bullish on this idea compared to previous
cycles, now showing the complex dissipating across IA as the
nocturnal LLJ weakens and cloud-top warming occurs after sunrise.
Nevertheless, the instability gradient will be oriented such that
if these storms hold together they would progress towards areas
north of a Macomb- Bloomington line by Sun evening.

** MONDAY **

By 12z Mon, a shortwave and an associated weak sfc low are progged
to be lifting towards the upper Midwest, located NW of the ILX
CWA. As this disturbance lifts north, there could be sufficient
ascent to aid t-storm development late Monday morning through
Monday evening in areas NW of the IL River. CAPE-Shear parameter
space for that area appears marginally supportive of strong/severe
storms, with moderate instability (again between 2000-2500 J/kg)
and marginal shear (25-30 knots of bulk shear), so a low-end hail
or wind threat could develop.

** TUES & BEYOND **

On Tues, the next upper trough begins to emerge from the lee of
the Rockies, with a sfc low forecast to reach southern MN by
midday. Guidance continues to show storm development along the
associated cold front to the NW of the ILX CWA Tues afternoon,
with storms eventually progressing into the area late Tues
evening/Tues night. The latest deterministic GFS is faster than
the other global models, and is the only one to bring storms into
the western parts of the ILX CWA by 00z Wed (7pm Tues). Forecast
soundings show steep mid- level lapse rates, as well as
moderately-high LCLs (just above 1000 feet) with an inverted-V
profile beneath the cloud base. The strong linear forcing of the
cold front combined with that low-level thermo profile favoring
cold pool development suggests upscale growth into a linear
segment by the time the storms reach the ILX CWA. Accordingly,
damaging winds are the top concern, but a threat of hail and
tornadoes will also exist, especially if any discrete cells manage
to form ahead of the primary line (which is difficult to assess
at this range). Exactly how far east a severe threat will persist
into the overnight hours remains a point of uncertainty.

Wed`s severe storm potential will depend on how far south the cold
front or outflow boundaries from Tues night`s storms push. If the
front is not completely through the ILX CWA, another round of
severe storms could develop along/ahead of the front on Wed as the
airmass ahead of the front remains quite unstable. The best
chance for this to occur in our CWA would be near/south of I-70,
but it is possible the front pushes completely south of the ILX
CWA. Forecast soundings for Wed do show a more unidirectional
shear profile than Tues (lower tornado potential), and deep layer
shear vectors are roughly parallel to the front, so a linear mode
would be favored once more. Given mesoscale uncertainties such as
boundary placement, SPC`s confidence was too low to introduce a
15% risk area for Day 5 (Wed). CIPS analogs and CSU machine
learning guidance both have the highest severe probs for Wed
focused south of I-70.

As sfc high pressure settles in on Thurs behind the cold front,
temps will be a bit cooler (highs in the 70s) and mostly dry
weather should prevail. Some precip chances linger in the forecast
across SE IL due to proximity of the stalled front, but it`s
possible these PoPs will be removed as confidence in the frontal
placement increases over the coming days. Further into the
extended, model consistency decreases, resulting in low confidence
in any precip chances beyond Thurs.

Erwin

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 638 PM CDT Sat May 18 2024

VFR conditions will prevail at all TAF sites next 24hrs. Skies
will be clear through the night with scattered CU developing again
tomorrow morning. Winds will be light and variable through the
24hr period.

Auten

&&

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

$$