001
FXUS63 KILX 191424
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
924 AM CDT Sun May 19 2024

.KEY MESSAGES...

- Heat indices in the low 90s are forecast today and again
  Tuesday. This will pose at least a small risk for heat-related
  illness in vulnerable populations.

- There is a 20% chance for thunderstorms near and north of I-72
  between 4 and 9 pm this afternoon and evening. While most of the
  area will remain dry, any of these thunderstorms could generate
  localized strong wind gusts and heavy rainfall surpassing 1.5
  inches.

- Thunderstorm chances increase Monday into Tuesday, peaking
  around 80% Tuesday night and diminishing late Wednesday. Any of
  these storms could produce severe weather, though the greatest
  risk will be from damaging winds Tuesday night west of the IL
  River.

&&

.UPDATE...
Issued at 923 AM CDT Sun May 19 2024

At 920am/1420z, a weak, stalled front was positioned roughly near
the I-72 corridor (NE winds at Quincy, SW winds at
Springfield/Decatur/Champaign). This front may nudge northward
during the day. The upper level support remains unfavorable for
precip development, with a ridge axis draped across IL and modest
height rises anticipated today. However, forecast soundings
continue to show moderate instability (~2000 J/kg) developing by
the afternoon, with little in the way of capping, and CAMs suggest
that forcing near this weak front will be sufficient to kick off
scattered storms this afternoon. Added a corridor of 30% PoPs
roughly from Macomb to Champaign, rather than just a broad 20%
chance north of I-72, because the CAMs have been consistent in
showing development in each of the last several model cycles.

Erwin

&&

.DISCUSSION...
Issued at 150 AM CDT Sun May 19 2024

At 145am, nighttime microphysics satellite imagery shows a narrow
band of broken stratocumulus around 5 to 8 kft - per sfc obs - is
drifting slowly southeast near the IL River along a dissolving
cold front, the remains of which will stall across central
Illinois through this afternoon. The HRRR continues to suggest sfc
temps, currently in the mid to upper 60s, will rapidly warm
toward 90 degF this afternoon in the vicinity of that boundary as
deep mixing (NBM mean for mixing heights is nearly 7 kft) dries
the low levels a bit - resulting in a classic inverted V-type
forecast sounding with 2000-2500 J/kg SBCAPE, per HREF mean. If
storms fire along that boundary, as virtually every CAM in the
HREF suggests, they`ll be short-lived given the weakly sheared
environment (HREF`s ensemble 90th percentile is generally less
than 20 kt 0-6km shear south of I-74, while the mean is closer to
10 kt). However, since lapse rates in the mixed layer are dry
adiabatic in forecast soundings, storms will pose a risk for gusty
wind and torrential rain as they collapse, and new storms forming
along their outflow boundaries could move over the same areas
increasing the risk for localized hydrological issues. At this
time, the greatest chance for thunderstorms this afternoon is
between 4 and 9 pm along and just north of I-72, with high
(80-90%) confidence in dry conditions area-wide prior to 2pm. Heat
indices today should remain near maximum sfc temps since
dewpoints will fall slightly with afternoon mixing, however
because this afternoon will be the warmest we`ve seen since autumn
it may catch some folks off guard and pose a risk for heat-
related illness to vulnerable populations, including those
participating in strenuous outdoor activities.

Tonight, the LLJ will set up to our west where scattered storms -
some possibly generating severe hail - will percolate mainly across
Iowa and northern Missouri, although a storm or two could approach
the western fringe of our area; we`ll keep one eye on this.
Tomorrow, whether and where storms form will be a function of where
the outflow boundary from tonight`s activity lands and whether it
gets turned over by a combination of mixing and increasing low level
southerly winds which, at the surface, will gust 20+ mph at times.
Anywhere from 15 to 50% - the greatest percentage along the I-74
corridor, nearer the warm front - of LREF membership produces
measurable rain across our area during the afternoon and/or evening
tomorrow. Since we`ll have at least a little shear (20-30 kt, per
HREF mean) to work with given the proximity of the shortwave trough
lifting into northern IL/WI during the afternoon, any of these
storms could turn severe posing mainly a hail risk though possibly
also an isolated damaging wind gust (given steep low level lapse
rates once again). Highs tomorrow are tricky given potential for
convection and convective debris clouds offsetting insolation, and I
would not be surprised if we underachieve a bit in spots; we`re
still forecasting mid to upper 80s, though some locations could very
well struggle to surpass 80 degF.

The warmth is expected to return, however, for Tuesday as moisture
advection strengthens to bring dewpoints into the upper 60s to low
70s, though winds gusting to 25 to 30 mph during the afternoon
should make it feel a bit less oppressive compared to today. While
there`s a small (10%) chance something fires in the warm sector, a
strong capping inversion should keep us dry during the daylit hours.
Model guidance continues to trend slower and farther west with this
system, and it makes one wonder how significant the severe threat
will be for us after all. At this point, the greatest risk
appears to be along and west of the IL River where the latest
iteration of the ECMWF brings the leading edge of the line at
around 10pm. Nonetheless, it maintains rigorous lightning
activity with those storms all the way to I-57 (ETA 1am
Wednesday), which suggests the low level jet could sustain storms
well into the night with all hazards (though straight-line winds
the most common) still in play given strong forcing along the cold
front, 40-50 kt deep layer shear, and steep mid level lapse rates
from the EML. And it`s worth mentioning the GFS is several hours
faster, bringing the line to I-55 around 7pm Tuesday, suggesting
the entire area would see storms and many areas west of I-57 would
see severe weather. The ECMWF has been more consistent in its
depiction of this entire system for the past 5-6 days, so I`m
inclined to lean in that direction but in any case severe weather
is a concern area-wide with the greatest confidence west of the IL
River.

Fortunately, the surface low will be rapidly occluding as it lifts
well to our north on Wednesday, causing shear along and ahead of the
cold front to dramatically weaken. Nonetheless, we don`t want to
quite rule out a couple feisty storms in our far (south)eastern
counties (mainly southeast of I-70) late Wednesday morning-mid
afternoon if the cold front should tarry long enough there. As
strong cool advection overspreads the region, it`ll feel chilly on
Wednesday with northwest winds gusting 25-35 mph - highest along
and northeast of the I-74 corridor - and temps only warming into
the low to mid 70s for highs during the afternoon. We hang on to
20-35% PoPs Thursday into Friday, though the boundary Thursday
will be far enough to our south that most, if not all, of the area
should stay dry, and with a little sunshine we could make a run
for 80 degF. Friday`s a bit more uncertain as models and their
respective ensembles struggle to handle the evolution of the next
system which 50-70% of NBM membership suggests will bring
measurable rain to the region by Saturday.

Bumgardner

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 533 AM CDT Sun May 19 2024

VFR conditions will be predominant throughout the forecast period,
though briefly there could be visibility reduction if one of the
few sparse thunderstorms that develop this afternoon happen to
move directly over a terminal. Storm coverage will only be around
20% at best, peaking between 22z Sunday and 02z Monday (5-9pm
CDT). Scattered cumulus around FL050 will develop after around
17z (noon) Sunday and dissipate after 01z Monday (8pm Sunday),
while light and variable winds become more consistently
southeasterly during that time frame.

Bumgardner

&&

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

$$