469
FXUS63 KILX 191938
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
238 PM CDT Sun May 19 2024

.KEY MESSAGES...

- Temperatures remain above normal through Tuesday, with highs in
  the upper 80s or low 90s. Peak heat indices will be in the low
  90s each day.

- Scattered thunderstorms, mainly north of I-72 (20-30% chance),
  through this evening. Severe weather is unlikely but funnel
  clouds or a brief landspout can`t be ruled out. Slow storm
  motions could lead to locally heavy rainfall exceeding 1.5".

- Severe thunderstorms are possible Monday through Wednesday,
  although the areas at risk vary each day. The greatest risk is
  the potential for damaging winds northwest of the IL River late
  Tuesday evening, as well as south of I-70 on Wednesday
  afternoon.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 234 PM CDT Sun May 19 2024

As of 2pm/19z, the diffuse frontal boundary that`s been draped
across the area today does seem to have lifted back to the north,
with NE winds at Pontiac and Peoria, but a southerly component to
the wind at Lincoln/Bloomington/Champaign. To this point, Cu
development had been fairly limited, but day-cloud phase satellite
imagery shows sporadic signs of glaciation occurring in Vermilion
and Menard County. We`ll likely see increasing vertical
development eventually leading to deep convective initiation over
the next few hours, which is consistent with what the CAMs have
shown. The latest RAP mesoanalysis is still showing a corridor
near that frontal boundary where non-supercell tornado parameter
values range from 0.5 to 1, so a funnel or isolated landspout
could occur as cells deepen this afternoon. With weak shear,
storms should be relatively short- lived, but they also won`t be
moving fast and if an updraft manages to persist longer than
expected locally heavy rainfall could occur (isolated totals over
1.5" possible). These storms should wane after sunset.

A shortwave is progged to lift NE from the Plains towards the
upper Midwest early Mon AM, which could produce a few
showers/storms NW of the IL River before midday. The forecast gets
a little more complicated into the afternoon. Zooming out and
stepping back for a moment, explosive thunderstorm development is
expected over KS this evening. This matters for us because CAMs
depict a remnant MCV lifting NE thru IA toward N IL, which could
provide additional forcing for storms Mon PM, even though the
primary upper wave will be positioned well north of the area by
that time. The environment does look marginally supportive of
severe weather, with moderate instability and weak shear (less
than 30 knots). Forecast soundings show an inverted-V profile
below the cloud deck, with over 1000 J/kg of DCAPE. The weak shear
means storms will likely lack organization, but that thermo
profile could support sporadic hail or strong wind gusts. As is
often the case with MCVs, there is still a good deal of
uncertainty as to where it ultimately tracks, how expansive the
associated cirrus shield will be, and what impacts that
subsequently has on the destabilization. For now, most of the CWA
north of I-70 is in a marginal risk (level 1 of 5) for severe
weather, but far northern portions of the CWA (Marshall and
northern Woodford/McLean Counties) are in a slight risk (level 2
of 5).

Erwin

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 234 PM CDT Sun May 19 2024

Severe t-storm potential exists across portions of the ILX CWA
both Tues evening (level 3 of 5 risk west of IL River) and Wed
(best chance south of I-70), driven by a volatile environment
ahead of an advancing cold front. Unfortunately, we`re at a time
of year where the mesoscale details become quite murky more than
24 hours in advance, owing to multiple rounds of convection
progressing across the Plains and Midwest between now and
Wednesday.

The current expectation is that an emerging upper level trough
will result in a deepening sfc low across the west-central Plains
by late Mon, which eventually lifts northeast to the upper
Midwest on Tues. WAA could result in a few elevated (not surface-
based) storms Mon night into early Tues AM. Forecast soundings
don`t look particularly impressive, but there could be a low-end
hail threat from these storms. For Tues afternoon, forecast
soundings show strong capping across the entire ILX CWA, owing to
a +18 to +20 degC warm nose. Storms are expected to form along a
cold front across IA Tues afternoon and eventually progress SE
into the CWA, but they likely won`t move in until after 7pm
Tues/00z Wed. Given the expected storm mode (linear or quasi-
linear), damaging winds remains the top concern, but all hazards
are possible. Forecast hodographs do become elongated in the low-
level as the LLJ kicks in, with nearly 50 knot winds just 1km AGL
Tues eve, resulting in roughly 30 knots of 0-1km shear and around
200 J/kg of 0-1 SRH. 0-3 km shear values are around 40 knots,
which gives some wiggle room for embedded surges within the line
to hit 30 knots of line-normal 0-3 km shear. All of those
aforementioned low-level shear/helicity values suggest a tornado
threat will exist in addition to the damaging wind potential.
Exactly how long, and how far east, these threats persist into the
night remains uncertain, and this is well reflected in the Day 3
convective outlook which gradually lowers the risk category with
eastward extent.

Wednesday`s severe threat will still ultimately depend on where
the front/outflow from Tuesday night`s storms set up. Some
guidance has been trending slower/further north with the frontal
placement, so while the most likely area for severe weather is
still south of I- 70, the I-72 corridor could wind up being at
risk as well. Between the mesoscale uncertainty and the northward
trends, was not as comfortable reducing the northwestward extent
of PoPs compared to previous forecast cycles. Ahead of the front,
the environment is expected to remain quite volatile with ample
instability (approaching 2500 J/kg). Despite the primary upper
wave associated with Tuesday`s storms lifting well off to the
north, a secondary short wave will place a new strong mid-level
jet streak over the region (60+ knot flow at 500mb), resulting in
40-50 knots of deep layer shear across the warm sector. Shear
profiles continue to look rather unidirectional, and deep layer
shear vectors parallel to the front suggest a linear storm mode
with damaging winds as the primary threats, while hail is also a
concern.

Temperatures cool off a bit behind the front, with highs in the
upper 70s/low 80s for the end of the work week. The upper wave
pattern remains quite progressive into next weekend, so the
extended forecast continues to be littered with precip chances -
but model spread in the timing and placement of any specific
disturbance is quite large. The CPC 6-10 day precip outlook (May
25-29) shows a 40- 50% chance of above normal precip.

Erwin

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1202 PM CDT Sun May 19 2024

A weak, stalled front is draped east-west across the region, and
continues to make the wind direction forecast tricky through the
afternoon. There is also a 20-30% chance for showers/storms this
afternoon. The addition of a PROB30 for -TSRA was considered,
mainly for the 20-23z time frame, but with storms expected to be
widely scattered, opted to stick with just VCSH. Winds will shift
to SE overnight, before shifting back to SW after sunrise Mon. VFR
conditions should prevail through the period.

Erwin

&&

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

$$