456
FXUS63 KILX 170802
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
302 AM CDT Fri May 17 2024

.KEY MESSAGES...

- Patchy fog will cause transient visibility reductions through 8
  to 9am this morning, with the greatest potential for visibility
  less than half a mile (50-70%) west of the IL River.
  Visibilities this low can increase the risk of pedestrian and
  automobile accidents, so drivers should use extra caution early
  this morning.

- Each day Sunday through Tuesday, increasing warmth and humidity
  will lead to heat indices in the upper 80s to low 90s. This
  will pose at least a small risk for heat-related illness in
  vulnerable populations.

- Thunderstorm chances increase early next week, peaking at 80% late
  Tuesday. Storms Monday night through Tuesday night could be
  severe with damaging winds and hail, but uncertainty is high in
  timing and precise location of severe weather.

&&

.UPDATE...
Issued at 254 AM CDT Fri May 17 2024

Surface observations showed quickly decreasing visibilities along
and a bit south of the I-74 corridor, so a Dense Fog Advisory was
issued through 9am to reach a broader audience with the message.
Confidence is low in whether these reduced visibilities will
persist (and whether widespread sub half mile visibilities will
materialize at all in some places), especially east of I-55 where
mid to high clouds continue spreading in from the southwest.
However, we will continue to monitor conditions and can expire the
Advisory early if necessary. Drivers should leave early this
morning, take it slow, maintain extra following distance, and use
fog headlights if possible.

Bumgardner

&&

.DISCUSSION...
Issued at 205 AM CDT Fri May 17 2024

Current nighttime microphysics satellite imagery shows cirrostratus
lifting northeast across areas mainly east of the IL River as an
elongated upper disturbance meanders through the Ozarks and into the
Tennessee River Valley. A piece of enhanced vorticity associated
with this wave is forecast to lift northward towards southern IL
this afternoon, sparking a few garden-variety storms. These storms
will be both unorganized and slow-moving given the low shear
environment, and given tall and skinny CAPE profiles (favoring warm
cloud processes) with 1000-2000 J/kg SBCAPE they could certainly
produce some briefly heavy rain, but they`ll be transient enough
that I`d be shocked if there were any hydrological issues. In the
more near-term, though, the main forecast concern is fog; it`s
likely to be most persistent and dense west of the IL River where
those mid-high clouds are less dense, but given yesterday`s rain the
low levels are quite moist so more transient visibility reductions
are in play elsewhere. HREF guidance has probabilities for
visibilities less than a half mile peaking from 15-30% along I-70 to
50-70+% west of the IL River around 6-8am, and then quickly dropping
off thereafter as surface heating erodes the stuff.

Tomorrow, a piece of upper energy will drift southeast through our
area, but given a ridge will be building in quickly behind it
synoptic forcing for ascent should considerably limit shower
coverage. The CAMs are not completely dry, but they`re developing
very little with coverage being generally less than 10-15%. Sunday
is a bit more questionable, given the ridge axis shifts east and an
upper trough passes too close for comfort to our northwest. The GFS
does clip areas west of the IL River with some light QPF as
convection that develops over the Central Plains comes here to die
with waning instability/shear on the eastern fringe of a warm front,
and the NAM (though the most aggressive at this juncture) advertises
2000+ J/kg of SBCAPE with an inverted-V type sounding in Galesburg
Sunday afternoon/evening. However, other guidance is dry; needless
to say, convective potential is conditional Sunday (and would most
likely be sub-severe again), but it`s not something we`re going to
take our eyes off of just yet.

The better opportunity for storms will come with the stronger trough
ejecting out of the Desert Southwest and across the Plains on
Monday, and approaching central Illinois late Tuesday. Given the
origin of this system and its strength, an EML should make its way
into the region to foster steep mid level lapse rates and
consequently at least elevated instability. So after perhaps a
couple more garden variety storms early on Monday, the warm front`s
arrival -- sometime between Monday evening and lunchtime Tuesday,
depending on which model you look at -- will mark an intrusion of a
more volatile environment conducive to stronger storms posing a
diurnal risk for all hazards and nocturnal risk for large hail.
Current thinking is that Monday night into early Tuesday we could
have a few large-hail producing storms, with more widespread strong-
severe storms capable of all hazards on Tuesday with increasing
shear as the surface low and its attendant cold front (forcing
mechanism) approach. However, it`s still too early to say how things
will materialize; we`ve certainly dodged several bullets this year
as storms further south gobbled up our moisture/instability, which
is always a possibility with this event. We`ll continue to monitor.

Depending on convection and associated cloud cover, temps could
become quite warm (upper 80s degF, perhaps touching 90 in a few
spots) Sunday through Tuesday, and with increasing low level
moisture (dewpoints likely to hit 70 degF most areas at some point)
heat indices could certainly climb into the 90s. The LREF suggests
probabilities for this are generally 15% or less (highest on
Tuesday), but I`m not convinced the chance is truly this low given
our forecast dewpoints are running a 2-4 degrees lower than the
deterministic models would say. Oftentimes, especially on days like
Sunday and Monday when we don`t have strong low level moisture
advection, mixing will decrease afternoon dewpoints by a couple
degrees, but since this can also lead to overperforming temps I
would not be surprised to see heat indices overachieve on any of
these days. It`s been since autumn that we`ve been quite this warm,
so the lack of acclimation to these temps and humidity will pose
at least some risk for heat-related illness to vulnerable
populations such as outdoor workers, the elderly, very young,
chronically ill, and unhoused.

Behind the system on Wednesday, the area is forecast to cool to near-
normal and a more stable airmass should, at least briefly, build
into the region offering a reprieve from convection. Late next
week, forecast confidence decreases as model differences become
increasingly vast in the timing and strength of shortwaves
rippling across the Midwest in a quasi-zonal flow regime.

Bumgardner

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1142 PM CDT Thu May 16 2024

VFR conditions will prevail at all TAF sites; however, still
expecting fog to develop at sites that have seen more clear skies
this evening...which would be PIA. CMI has already gone down to
below 1 mile earlier, but now it is over 2sm. Given the amount of
rain they have had...this seems reasonable at the moment. CMI will
go back down toward morning. All sites will see IFR conditions,
but PIA will see LIFR as vis goes down to 1/2sm at times. Will
continue the TEMPO groups in the morning for all sites, but then
improve slowly in the morning for PIA and BMI, but quickly for
SPI, DEC, CMI. Clear skies expected tomorrow evening. Winds will
be light and variable through the 24hr period.

Auten

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CDT this morning for ILZ027>031-
036>038-040>046-053.

&&

$$