418
FXUS63 KILX 181427
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
927 AM CDT Sat May 18 2024

.KEY MESSAGES...

- Heat indices in the upper 80s to low 90s are forecast each day
  Sunday through Tuesday. This will pose at least a small risk for
  heat-related illness in vulnerable populations.

- There is a 20-30% chance for thunderstorms Sunday afternoon and
  evening. While the threat for severe weather is low, there is a
  nonzero chance for localized strong wind gusts and hail with
  any storm that develops.

- Thunderstorm chances increase early next work week, peaking at
  70-80% Tuesday night and diminishing Wednesday. Some of these
  storms could be severe, especially northwest of I-57 Tuesday
  evening and night.

&&

.UPDATE...
Issued at 926 AM CDT Sat May 18 2024

A warm and mostly sunny morning across central Illinois today,
with temperatures already in the mid 70s. Visible satellite
imagery shows that any lingering fog across far eastern Illinois
has dissipated. NAM Cu- rule and HREF cloud cover both suggest
diurnal Cu will blossom across the area by late morning, and there
are already hints of that occurring south of I-70 as of
920am/1420z. A weak shortwave was positioned near the bootheel of
Missouri and could perhaps support an isolated shower in far SE
IL, but elsewhere mid-level height rises associated with an
amplifying ridge should result in enough subsidence to prevent
shower development. The 12z models remain in good agreement that
any precip activity associated with a cold front over IA/WI today
will remain northwest of the ILX CWA.

Erwin

&&

.DISCUSSION...
Issued at 130 AM CDT Sat May 18 2024

At 1:30am, long-wave IR satellite imagery shows the last wisp of
convective debris clouds from thunderstorms that quickly dissolved
near the I-72 corridor within the past couple hours. Nighttime
microphysics suggests a lack of any other clouds, which will
enhance radiational losses through daybreak, and, together with
light to calm winds, may result in patchy fog. The CAMs are not as
aggressive nor extensive in their depiction of diminished
visibilities this morning, but both the HRRR and HiResARW have
been highlighting an area along and south of a roughly
Jacksonville to Danville line with transient sub- quarter mile
visibility from 5-8am. We`ll monitor surface observations to
determine whether a DFA could become necessary before insolation
"burns off" whatever fog develops, though at this juncture that`s
not looking particularly likely given the brevity and rather low
(20-35%) probabilities for sub half mile visibilities.

A ridge of high pressure will continue to build into the region
today to maintain dry conditions across central and southeast IL,
although a decaying cold front accompanied by thunderstorms
upstream (these will develop late afternoon) will attempt to
intrude tonight, offering a 10-15% chance for a shower northwest
of Peoria. Afternoon highs are forecast to reach the mid 80s as
deep mixing transports drier mid-level air to the surface
bringing dewpoints down into the low 50s. The second half of the
weekend is tricker to forecast, however, as a piece of upper level
energy lifts out of the Plains and attempts to flatten the ridge
over IL. Given the remnant boundary from tonight will be
somewhere in or near central Illinois, we might just have a
trigger for afternoon storms which could significantly lower high
temps Sunday. However, if storms either (1) don`t develop or (2)
develop after peak heating, deep mixing during the afternoon could
once again reduce low level moisture and aid temps in warming to
around 90 degF, so we`d offer a reminder to those planning
strenuous outdoor activities to stay hydrated and take breaks as
needed in the shade or AC. Soundings would suggest a classic
"inverted V" thermodynamic profile with 1500-3000 J/kg of SBCAPE
(highest west of I-55) per the HREF and NBM mean, suggesting a
favorable atmosphere for at least isolated severe wind and hail
with any pulse storms that fire tomorrow. However, it`s distinctly
possible the boundary from Saturday night is completely gone by
tomorrow afternoon and/or the incoming wave is too far northwest
of our area to initiate any storms and we consequently remain dry.
We`ll have to wait and see.

Unfortunately, convective potential (and thus the dependent high
temp forecast) becomes even less clear on Monday, as it will depend
on any residual boundary (or lack thereof) from tomorrow`s storms.
However, NBM suggests chances for measurable precip are highest
west, at 60-75% beyond the IL River tapering to less than 25% east
of the I-57 corridor. A low level temp inversion along and north of
a warm front which will be lifting into our west Monday
evening/night should limit convective potential during the day when
shear is weak (0-6km generally less than 20 kt), but as the LLJ
strengthens toward Tuesday morning we could have a couple storms
drift into our area from the west, posing some risk for locally
severe hail. Models` depiction of this system continue to slow and
shift everything west, so most of the day Tuesday could very well
wind up dry, but there`s still quite a bit of model spread in the
evolution of the surface cyclone. Nonetheless, deterministic
models continue to advertise an unseasonably strong and quickly
deepening low lifting out of the Plains and into the Upper Great
Lakes late Tuesday into Wednesday, leading to increasing shear and
moisture fostering a primed environment for severe thunderstorms,
especially west of I-55 where the NBM suggests 20-40% chances for
more than 3000 J/kg of SBCAPE and the deterministic GFS depicts
25-30+ kt of 0-1km bulk shear Tuesday evening. Storm mode is a bit
difficult to pin down at this time range, but given the warm
front will be well to our north and surface low to our
west/northwest, we could wind up with some sort of broken line
comprised of supercell-like structures capable of all hazards
(again, highest risk west of I-55) Tuesday
evening/night...although something more isolated could always fire
along an outflow boundary during the afternoon further east.

Depending on cloud cover, Tuesday could be the hottest day in the
period, but it will more likely be the most humid day as
dewpoints climb into the 70s and heat indices consequently reach
the low (to possibly mid) 90s. Fortunately, the same southerly
winds advecting that rich low level moisture will also offset the
oppressiveness of the early season warmth as gusts during the
afternoon frequent the 20-25 mph range. Nevertheless, because
Sunday through Tuesday will be the warmest conditions of the
season (in other words, since early last autumn) so far and hence
folks may not be quite acclimated to it just yet, it will pose
some risk for heat-related illness in vulnerable populations.

Wednesday through Friday should be a bit cooler in the wake of the
front`s passage Tuesday night or early Wednesday, with Thursday and
Friday highs forecast to hit the mid to upper 70s in a drier airmass
(dewpoints in the 50s). NBM`s 15-30% precip chances lingering
through the end of the forecast period may be a bit overdone both
temporally and spatially, but given a stationary boundary won`t be
particularly far to our southwest during this time period it`s tough
to justify lowering/removing PoPs; just be aware it`s highly
unlikely to be a total washout during the late Wednesday through
Friday period.

Bumgardner

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 541 AM CDT Sat May 18 2024

Guidance suggests there is a 20-30% chance that visibilities drop
into the MVFR category at DEC and CMI, and lower chance at PIA,
through 13z (8am CDT) this morning given small dewpoint
depressions, clear skies, and light winds fostering patchy fog.
Otherwise, VFR conditions should be predominant throughout the
forecast period, with 5-10 kt south-southwesterly winds becoming
light after sunset.

Bumgardner

&&

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

$$