429
FXUS63 KILX 091512
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
1012 AM CDT Tue Jul 9 2024

.KEY MESSAGES...

- Through tomorrow morning, the remnants of Hurricane Beryl are
  forecast to bring a swath of heavy rainfall to some part of
  central or southeast Illinois. The highest chance (30-40%) for
  more than 3 inches of rain is near and southeast of I-55. This
  would result in localized flooding of poor drainage, urban
  areas, and possibly streams and creeks overflowing their banks.

- East of an Shelbyville to Paris line, there is a conditional
  risk for tornadoes between roughly 4 and 11 pm this evening. Chances
  increase further southeast, with a level 2 of 5, slight, risk
  for severe weather southeast of I-70.

- North to northwest winds will increase as this system departs tonight
  into tomorrow mainly near and southeast of I-57. There is a 30%
  chance wind gusts surpass 45 mph in these areas, which would
  cause difficult driving conditions for high profile vehicles on
  west- east roadways and may result in some downed tree branches.

- Heat and humidity will build back into the region this weekend,
  with a greater than 70% chance for heat indices over 100
  degrees by Monday. This would increase the risk for heat-related
  illness in vulnerable populations and those attempting
  strenuous activities outdoors.

&&

.UPDATE...
Issued at 1011 AM CDT Tue Jul 9 2024

Latest surface map shows the center of the low associated with
Beryl`s remnants is centered just northeast of Little Rock AR.
Large rain shield extends northeast to about a Kankakee-Quincy
line, with some heavier amounts over an inch occurring along the
leading edge. Core of the overall heavier rain still looks to be
moving in between I-55 and I-57 this afternoon and overspreading
east central Illinois this evening. Minor adjustments were made to
the storm total precip forecast, but nothing that goes against the
existing messaging in the heaviest rain axis, and no changes are
needed to the Flood Watches at this time.

Regarding the severe potential, main focus in our area appears to
be south of I-70 in the 4-9 pm time frame, though a few stronger
storms can`t be ruled out as far north as Champaign and Danville.
Radar mosaics show some thunderstorms starting to get going over
northeast Arkansas, and these are likely to be the ones of concern
as the low center moves into the lower Ohio Valley late this
afternoon. Morning HREF guidance is still coming in, but goes out
far enough to capture the event, and does show about 200 m2/s2 of
0-1km storm relative helicity spreading northeast through early
evening. Brief tornadoes will be possible in the fast-moving
storms, with the greatest threat over the lower Ohio Valley.

Concern also exists for winds approaching advisory levels
beginning late this evening, on the back side of the low. Morning
HREF probabilities of over 45 mph wind gusts are around 40% by
midnight centered along US-51, shifting to the I-57 corridor by 4
am and increasing to around 50-60%. Will continue to evaluate this
potential for any headlines in the afternoon forecast issuance.

Geelhart

&&

.DISCUSSION...
Issued at 258 AM CDT Tue Jul 9 2024

At 3am, post tropical depression Beryl was centered in
southwest/central Arkansas, with bands of heavy rain to its
northeast across northern Arkansas and western Tennessee. Well
ahead of this system, mid- level vorticity continues streaming
northward across the region providing support for some widely
scattered (10-15% coverage) and generally light showers, but as it
draws nearer more persistent rain is expected to overspread
central and southeast Illinois near and east of the Illinois River
mid morning into early afternoon. HREF`s Localized Probability
Matched Mean (LPMM) is highest near and just southeast of I-55,
but this remains a blend of (underdispersive) high res guidance
characterized by considerable spread with some CAMs depicting the
heaviest totals nearer the IL River and others suggesting it`ll be
nearer I-57. At this time, however, our concern is greatest along
and just southeast of I-55 where HREF suggests 30% or greater
probabilities for more than 4 inches of rain by the time the
system departs tomorrow morning. The period of greatest concern
for hydrological issues would be mid afternoon today through
around midnight tonight when the heaviest rain is falling, though
some wrap around rain could linger into early tomorrow afternoon
northeast of a Peoria to Mattoon line. We`ve expanded the flood
watch north to our I-55 counties to highlight the potential for
localized hydrological issues, such as ponding of water in
urbanized areas and rivers and streams overflowing their banks.

Unfortunately, it`s not just rain we`ll have to be watching this
evening, but also the potential for tornadic thunderstorms northeast
of the surface low which is slated to track from roughly Effingham
to Paris from around 8 pm to midnight. North and northeast of the
surface low, between roughly 4pm (bottom three counties) and 11
pm (vermilion County), CAMs suggest one or more broken bands of
convection lifting through an environment characterized by
marginal instability, but also extreme (GFS suggests 40-50 kt in
lowest 3 km) low level wind shear, low LCLs, and meso-a scale
uplift with strong vorticity advection. The column looks quite
moist during this time frame, and lapse rates are poor (5 to 6.5
C/km), so hail is not going to be the issue, but if we have severe
weather the number one concern is tornadoes, with a marginal
damaging wind threat in any bowing line segments. The risk for
tornadoes will be highest just to our southeast across the Ohio
Valley, but areas near and south of an Effingham to Danville line
are not quite out of the woods this evening; we`ll be watching
closely.

As if heavy rain and tornadoes were not enough, there`s one more
thing we`re going to have to watch this evening (I-55 corridor)
through early tomorrow afternoon (I-57 and east): the potential for
strong gradient winds on the back side of the departing surface low.
Both global and high res guidance continues to suggest pressure
rises will be sharp as the low shifts east, and while I can`t fully
buy into what the 09.00z HRRR has been showing - widespread 45 to 60
mph wind gusts east of I-55 late this evening through tomorrow
afternoon - it bears watching. The reasons for dismissing this as
overdone are compelling: (1) there`s no strong signal for it in
global ensemble guidance, with the 00z ENS mean only bringing gusts
to 35 mph in our east counties; and (2) mixing of the strong winds
aloft (around 40-50+ kt at 925 mb) is generally not favored
overnight, especially with so much cloud cover around. However, with
dry air entrainment on the west side of this system, evaporational
cooling may foster some downward momentum transfer. In addition, the
gradient remains tight across areas near and east of I-57 into early
tomorrow afternoon when, diurnally, mixing may be a little more
favored. It`s tough to ignore the signal for high winds in the CAMs,
chiefly the 00z HRRR, given its potential impacts (trees are in full
leaf, soils are wet, it could be an extended period of strong gusts,
etc.), but we`re not quite ready to bite off on gusts to 55+ mph as
gospel just yet. We`ll keep an eye on upstream observations and
local trends to assess whether we need to issue a headline
highlighting the wind potential, but at this point we`re holding off
- especially in light of the 06z HRRR run which is much lower with
winds (and the fact that upstream sfc obs are lower than the 00z was
forecasting).

Behind the system tomorrow afternoon, the 09.00z HRRR and NAMNest
suggest we might have an isolated shower/storm, which, given the
steep low level lapse rates and increasing dry air in the mid
levels, could pose a risk for gusty winds. The greatest risk for
something would be after 1 pm across our west as a mid-level
disturbance approaches from Iowa, but even there coverage will only
be 10-20%.

Additional isolated-scattered storms may redevelop Thursday morning
and continue through the afternoon, when forecast soundings suggest
1000-2000 J/kg SBCAPE but LREF mean gives us only around 20 kt of 0m-
500mb bulk shear. Cannot rule out a feistier storm or two, but
organized severe weather is not expected at this time. Friday is a
bit of a toss up at this point, as the deterministic models are
showing a shortwave passing near/through our area and dotting some
portion of the Prairie State with precip, although run to run and
model to model consistency is lacking in whether, where, and when
we`ll get precip. At this point, NBM probs of 12-20% seem
reasonable, so those PoPs haven`t been touched.

This weekend, a ridge will slowly build into the region favoring
gradually warming temperatures and increasing humidity, with
forecast highs around 90 degF Saturday and in the low 90s Sunday.
There will be around a 20-30% chance for thunderstorms Saturday as
another shortwave disturbance dives southeast along the northeast
periphery of the building ridge, although the better chances for
precip with this wave are going to be to our northeast as capping
gradually strengthens across our area. The biggest concern late this
weekend and into early next work week is going to be the heat and
humidity, with dew points consistently well into the 70s and likely
touching 80 degF at some point Sunday or Monday when NBM`s 10th-90th
percentile range for high temps in Peoria spans 91 to 97. Such temps
and humidity would easily result in heat indices over 100 (70+%
chance) both days, with some potential (20% chance) we briefly touch
110.

Bumgardner

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 614 AM CDT Tue Jul 9 2024

Conditions will gradually deteriorate mid morning through early
afternoon to MVFR as rain lifts north from the remnants of
Hurricane Beryl. IFR visibilities are expected at DEC and CMI, and
possibly as far northwest as the I-55 terminals, in the heavier
rain between mid afternoon and early evening. Visibilities and
ceilings should both improve after around 03z/10pm, as
precipitation departs to the northeast, although northerly winds
will be increasing around that time. Northwest gusts tomorrow
morning are expected to surpass 20kt along and southeast of I-55,
and could reach 30kt at DEC and especially CMI by the end of the
forecast period.

Bumgardner

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Wednesday afternoon for ILZ038-042>046-
051>057-061>063-066>068-071>073.

&&

$$