419
FXUS63 KILX 201045
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
545 AM CDT Mon May 20 2024

.KEY MESSAGES...

- Heat indices will return to the low 90s tomorrow, posing a risk
  for heat-related illness in vulnerable populations.

- Periods of thunderstorms will continue through Tuesday night
  north of I-70 and Wednesday further south. Any of these storms
  could turn severe, but the greatest risk (level 3 of 5,
  "enhanced") will be near and west of the IL River late Tuesday
  evening.


&&

.DISCUSSION...
Issued at 132 AM CDT Mon May 20 2024

Latest radar shows an absence of appreciable reflectivities across
central Illinois, but also a stretched-out and weakening MCS whose
MCV is centered near Kansas City. The past several iterations of the
HRRR suggest renewed showers and storms associated with the southern
portion of that MCS could erupt 4-8am this morning mainly north of I-
72, but disagree on eastward extent of thunderstorm activity and
fortunately all suggest storms stay sub-severe (though isolated wind
gusts over 40mph could occur). The CAMs suggest the remnant MCV will
lift northeastward across eastern Iowa this morning, leading to
regeneration of convection to our north mid to late morning and
across our area along its trailing boundary after around 1-2pm. It`s
uncertain how much sunshine we`ll get late morning into early-mid
afternoon for destabilization as HREF shows abundant mid to high
"convective debris" clouds, and soundings generally appear less
unstable than yesterday with a weak capping inversion in place.
However, forcing for ascent and increased shear along the
aforementioned boundary - wherever it tracks - could prove
sufficient to initiate and sustain storms, which, given dry mid
levels and modest (7-7.5C/km) 700-500mb lapse rates, could
generate hail and strong wind gusts (especially if a linear storm
mode evolves along the boundary). Virtually all of the CAMs show
something across our area during the 3 to 7 pm timeframe, so we`ll
be keeping a close eye on things this afternoon and evening.

Tuesday`s forecast is quite the challenge, as this afternoon/evening
activity will (1) modify the environment into which the low is
moving, affecting its development for better or worse...and (2) have
an especially large influence over the location of the warm front
which could be as far north as Milwaukee or south of I-80 during the
day. If it ends up being on southern end of the ensemble envelope,
we could be dealing with severe weather as early as Tuesday morning
across our northwest; in fact, the 00z HRRR suggests several rounds
of convection could ripple along the warm front from northeast
Missouri to northeast Illinois (clipping our far northwest CWA)
throughout the entire day Tuesday, with each storm carrying a risk
for large hail and afternoon/evening storms posing a threat for
tornadoes and intense straight-line winds as well. However, if one
of the model solutions featuring a farther-north warm front set up
materializes, we could go the entire day without seeing any precip.
Forecast soundings suggest a loaded gun-type thermodynamic profile
suggestive of a strong capping inversion which should prevent
convective development outside of some rather strong forcing
mechanism, until the upper low starts to draw near during the mid to
late evening leading to steepening mid level lapse rates (over
8C/km, if you believe the NAM) and increasing shear (45-55 kt west
of the IL River by 7pm).

The 00z deterministic ECMWF suggests a broken line of storms reaches
Schuyler County around 6-7pm, I-55 around 9-10pm, and I-57 around
11pm, and given the LLJ stretching hodographs for 150-250 m^2/s^2 0-
1km helicity, concern exists that these storms could pack potential
for more than just straight-line winds even if the storms wind up
being linear. However, if a large enough component of 0-3km shear
(30-40 kt) is perpendicular to the cold front, storm mode might be
more supercellular (or hybrid) than QLCS...and if that happens we`ll
definitely be concerned about all severe hazards, especially west of
the IL River.

Wednesday, there`s some uncertainty in how far south the cold front
will make it before stalling out, and given another mid level wave
lifting northeast along and south of I-70 we could wind up with
another round of severe storms there during the afternoon and
evening. We`ll be watching model trends with the placement of this
wave and the frontal boundary to get a handle on Wednesday`s storm
potential as well as the northward extent of PoPs. Northwest of the
cold front it`ll be near to slightly below normal with highs in the
low to mid 70s, while south of it the airmass will remain quite warm
and sticky. Thursday into the weekend, predictability decreases as
models differ in their depiction of a parade of MCSs riding along a
quasistationary boundary draped roughly from the Central Plains to
Lower Ohio or Tennessee Valley, bringing potential for heavy rain,
strong wind gusts, and isolated hail to a large portion of the
country. Since we`ll be near or just north of that boundary, and
that boundary will move under the influence of MCS outflows, both
precipitation chances and temperatures are impossible to pin down at
this juncture.

Bumgardner

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 545 AM CDT Mon May 20 2024

VFR conditions will be predominant throughout the forecast period,
however an isolated thunderstorm could (20% chance) impact one of
the terminals resulting in brief gusty winds and visibility
reductions from heavy rain at any point after around 15z/10am.
The terminals at greatest risk will be those along I-74 (namely,
PIA, BMI, and CMI) between 21z Monday and 01z Tuesday (i.e., 4-8pm
CDT). Around mid morning, south-southwest winds will abruptly
increase due to mixing, with frequent gusts of 20 to 25 kt
through around 00z/7pm when the lower atmosphere decouples and
winds quickly diminish.


Bumgardner

&&

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

$$