959
FXUS63 KILX 112028
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
328 PM CDT Tue Jun 11 2024

.KEY MESSAGES...

- Hot and humid on Thursday, with highs in the low 90s and peak
  heat indices approaching 100F west of the Illinois River.

- There is a 40-50% chance of t-storms Thursday evening, some of
  which could be strong to severe (level 2 of 5 risk). Large hail
  and damaging winds are the primary threats.

- Above normal temperatures persist well into the extended
  forecast, with hot and humid conditions (peak heat indices in
  the upper 90s or low 100s) returning Sunday into early next week
  (and potentially beyond).

&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 326 PM CDT Tue Jun 11 2024

An upper low remains positioned over the Canadian Prairie Tuesday
afternoon, while a band of cloud cover associated with the weak
front is pushing into the NW parts of the CWA as of 20z/3pm.
Maintained a slight chance of sprinkles as this front progresses
east across the CWA tonight, but recently there has been very
little in the observational data to suggest anything reaching the
sfc across SE IA or W central IL. Lows will be about 10 degrees
warmer tonight, in the upper 50s or low 60s.

On Wednesday, upper level height rises and southwesterly sfc flow
will continue the warming trend. The 850mb airmass is oriented
such that warmer temps are present further west, resulting in
highs ranging from near 90 NW of the IL River to the mid 80s south
of I- 70. Afternoon dewpoints near 60F mean that peak heat
indices will be similar to the high temperatures. Forecast
soundings and NAM-based Cu rule both suggest scattered diurnal Cu
development is possible during the afternoon, mainly east of the
IL River, and this aligns well with the HREF cloud cover
depiction. The EPA is forecasting air quality becoming unhealthy
for sensitive groups across southeastern IL (south of I-70) on
Wednesday due to ozone. These air quality forecasts only go one
day into the future, but given the transition to a more summer-
like airmass, it would not be a shock to see additional air
quality issues arise on future days.

Late Wed PM, an MCS is expected to develop across MN and progress
south/east. Not surprisingly, CAMs have a broad spread in where
this MCS subsequently tracks, but it`s not out of the question
that it moves into the northwestern portions of the ILX CWA early
Thurs AM as it is decaying. The evolution of this MCS, and its
outflow, could play a role in determining what latitude at which
Thursday`s severe threat develops, so even if it doesn`t bring
precip into our area, the evolution of this disturbance will
require close watching.

Erwin

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 326 PM CDT Tue Jun 11 2024

By Thurs AM, the upper pattern will feature ridging over the
southern Plains with a shortwave moving through the upper Midwest,
and the associated cold front will result in a chance of severe
storms during the evening. As noted near the end of the short term
section, the exact latitude at which this front will be situated
by late Thurs afternoon remains uncertain, but at least compared
to yesterday`s forecast the trend has been a southward shift.
Ahead of the front, breezy SW winds are expected through the day,
which will both continue the warming trend (with highs in the low
90s area- wide) and advect moisture northward.

As is often the case, the NAM is exhibiting a moist bias,
depicting dewpoints in the mid 70s NW of the IL River. The NBM
had dewpoints approaching but not quite into the 70s, which seemed
a tad low and some edits were made to nudge these up slightly.
The expectation is that the moist low-levels will result in peak
heat indices approaching 100F west of the IL River and strong
instability developing (MLCAPE approaching 3000 J/kg). In the
upper levels, the 500mb jet will be displaced north of the area,
but with sufficient flow to produce 40-50 knots of shear across
the northern portions of the CWA. However, deep layer shear values
decrease with southward extent. Shear profiles are quite weak and
unidirectional in the low- levels (with some forecast soundings
showing less than 5 knots of 0- 1 km shear), so the tornado
potential appears low, unless storms stretch sfc vorticity
associated with the frontal boundary and produce landspouts.
Recent hail forecasting studies (such as Nixon et al. 2023) have
emphasized that CAPE values don`t provide utility for discerning
max hail size, but one factor that can make large hail more likely
is weak low-level shear, which is present in this case, and thus
despite high freezing levels around 15 kft I am still concerned
about the large hail potential with these storms. Forecast
soundings also show DCAPE values around 1000 J/kg, and given the
linear forcing a wind threat will also exist.

In terms of timing, storms are progged to develop along the front
late afternoon/early evening, likely north of I-72, and eventually
sag south across the area. The severe threat will wane into the
nighttime hours as instability weakens and storms progress south
towards weaker shear. The forecast PWAT values are still around
2", and thus locally heavy rainfall is also a concern. Today`s 12z
run of the NAM actually had a few localized instances of rainfall
exceeding 3". We`ll get a better sense of the maximum rainfall
potential (and the probabilities of exceeding certain thresholds)
as this event comes into range of the HREF.

Regarding temperatures, the main shift in the forecast has been
for slightly cooler temperatures on Friday as northerly flow
persists behind the front (highs in the mid 80s). Temperature and
moisture content is expected to rebound as upper level ridging
shifts through and strengthens, with highs returning to the low to
mid 90s Sun- Tues, and dewpoints in the mid 60s to near 70
resulting in peak heat indices near 100F. The HeatRisk for these
days pushes near or into the "high" category (level 3 of 4) during
the afternoons. While we`re still a few days away from being in
the "heat advisory" issuance time frame, one may eventually need
to be considered due to the multi-day nature of the heat.

Looking further into the extended, the heat is likely to continue,
with WPC highlighting the potential for excessive heat across
Illinois during the Jun 19-Jun 23 timeframe. We have begun
monitoring the potential for flash drought development: rainfall
over the last 30 days has been below normal, we`re entering a
period of both above normal temps and near or below normal precip
over the next two weeks, and we`re at the climatological peak of
solar radiation. The most uncertain part of the flash drought
potential is the rainfall forecast - the signal for below normal
rainfall is not particularly strong, and subtle shifts in the
synoptic features could result in the area seeing more rainfall
than currently forecast.

Erwin

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1209 PM CDT Tue Jun 11 2024

VFR conditions prevail through the period. Southwesterly winds
today will shift to southerly overnight, then back to
southwesterly after sunrise on Wednesday. Wind speeds will be
light, generally below 10 knots. A weak front will result in
broken mid-level cloud cover moving in from west to east this
evening. This front could produce a stray sprinkle and perhaps
some virga, but overall, precipitation is unlikely.

Erwin

&&

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

$$