486
FXUS63 KILX 080732
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
232 AM CDT Mon Jul 8 2024

.KEY MESSAGES...

- Tomorrow into Wednesday, 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 (around 20%) for more
  than 3 inches of rain is south of a Litchfield to Danville line.
  This would result in localized flooding of poor drainage, urban
  areas, and could cause some streams and creeks to overflow
  their banks.

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

&&

.DISCUSSION...
Issued at 231 AM CDT Mon Jul 8 2024

At 2am, Hurricane Beryl continues its northward track toward the
Texas Gulf Coast. Meanwhile, a couple weakening showers are
spreading northeastward into southwest Illinois along a remnant
convective boundary. The CAMs, while not in agreement as to the
precise area, each suggest renewed convection will develop somewhere
in the ILX CWA this afternoon, with the most overlap between models
evident in the 08.00z HREF`s corridor of maximum neighborhood probs
for >40dBZ 1km AGL reflectivity which is between the Illinois River
and I-57 mid afternoon-early evening. At any given location, chances
for precip run from roughly 35-50% during this time frame, with
tapering chances after dark when radiational heating ceases and
instability subsequently wanes; we can`t fully rule out a stray
shower overnight, however, as pieces of mid-upper level energy
breaking off of post-tropical Beryl and getting swept up in the
southwest flow ahead of an incoming trough lift through
central/southeast Illinois.

Better chances for precipitation arrive tomorrow as Beryl`s remains
approach the region from the southwest. Flow will turn northeasterly
over the region ahead of the system, which will bring slightly drier
low level air into its northern edge - potentially delaying the
onset of its stratiform rain. However, eventually the system is
slated to track near or through a portion of central or southeast
Illinois to bring a narrow band of heavy (3 inches or more)
rainfall. Current thinking is that resolution of the global guidance
is too coarse to capture the range of precipitation totals various
locations throughout the CWA will receive by the time the system
departs late Wednesday; some spots are likely to receive only a few
hundredths (or nothing at all, as NBM 25th percentile would suggest
north of I-72), while others could receive drought-busting totals
locally surpassing 4 inches (roughly the NBM 95th precentile).
Currently, the greatest overlap in LREF members` heaviest precip
totals is near the I-70 corridor where NBM suggests the chances are
highest, at 10-20%, for more than 3 inches of rainfall, although
that could certainly shift considerably (south of our area, or to
closer to I-55). Given highest confidence in at least measurable
rain there, and some pre-conditioning rains across our bottom six
counties a few days ago, we`ve elected to issue a Flood Watch to
highlight this potential.

One thing we`ll be watching: southeast of the surface low, which
LREF mean brings through the extreme southeast edge of our CWA,
directional shear will become quite high (08.00z NAM depicts a
narrow plume of 55-65 kt 0-6km bulk shear). With most of this
focused in the lowest km, low LCLs, and 1000-2000 J/kg SBCAPE, a
severe threat for tornadoes and damaging gusts could materialize.
The biggest question mark is just the track of Beryl`s remnants,
which is highly uncertain; most guidance would suggest the severe
threat will be just to our southeast across northwest KY and
southern IN, although a small subset of the ensemble envelope would
bring the risk as far northwest as I-57 so we can`t rule it out just
yet. Timing would be Tuesday afternoon into Tuesday night.

Behind this system, it`ll turn breezy on Wednesday, especially near
and east of the I-57 corridor where the highest chances (20-25%)
exist for northerly wind gusts surpassing 40 mph. The strong cool
advection may limit our temps a bit Wednesday afternoon and evening,
but with cloud cover departing to the east areas west of I-55 could
be warmer than NBM`s deterministic output suggests (upper 70s to
around 80 degF). Certainly tomorrow will be cooler, with so much
cloud cover around to significantly limit radiational warming for
highs in the low to mid 70s, except perhaps northwest of the
Illinois River.

One or more shortwave disturbances will pass near or through the
region amidst west-northwest flow with the ever-so-slowly departing
upper trough Thursday into Friday, keeping some cloud cover around
and maintaining perpetual - albeit slight - precip chances. The West
Coast ridge will gradually expand eastward heading into the weekend,
fostering rising 500mb heights (LREF mean reaches 593 dm by 1pm
Sunday) and warming 850mb temps (35% are warmer than 24 degC over
Galesburg by Monday). During the weekend and especially heading into
early next week, there are two things that catch our eyes at this
time period: (1) heat and (2) severe ridge-riding convection.
Considerable uncertainty surrounds each on any given day, as the two
depend on each other, not only in our CWA, but also upstream. By
Monday, NBM`s 10th-90th percentile for high temps in Peoria spans 90
to 97 degF, which isn`t that hot around here for mid-July; however,
with evapotranspiration contributing to low level moisture
(especially if we get a lot of rain from Beryl), one shouldn`t be
surprised if dew points climb to 80 degF or higher Sunday or Monday
which could in theory bring heat indices to 110 degF or hotter
(Excessive Heat Warning criteria). Right now, thinking is we`ll
probably need at least a Heat Advisory either Sunday, Monday, or
Tuesday, although LREF probs for heat indices reaching criteria (105
degF), which are highest Monday afternoon, are only around 20% at
this point. And again, an MCS diving a little further south than
most guidance would suggest could keep us 10 or more degrees cooler
than even currently forecast. Stay tuned.

Bumgardner

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1108 PM CDT Sun Jul 7 2024

A weak frontal boundary will be slow to move across the region
Monday with a few showers and storms around, but confidence in
coverage and timing is too low to include in TAFs. Besides an
isolated storm, conditions should be VFR through Monday evening.
Light to calm winds tonight will pick up from the southwest near
10 kt after 15z.

25

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Tuesday morning through Wednesday afternoon for
ILZ066>068-071>073.

&&

$$