549
FXUS63 KILX 161625
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
1125 AM CDT Tue Jul 16 2024

.KEY MESSAGES...

- Another round of potentially severe thunderstorms is possible
  this afternoon/evening south of I-72, with damaging winds once
  again the main concern. There is currently a Slight (level 2)
  risk of severe storms from I-70 southward, with a Marginal
  (level 1) risk just north.

- A stretch of below normal temperatures and mostly dry conditions
  arrives Wednesday and continues through the weekend.

&&

.UPDATE...
Issued at 1125 AM CDT Tue Jul 16 2024

An area of training thunderstorms stretches just south of the
forecast area near/north of I-70, migrating ESE. A shortwave
forecast to track eastward along this area is progged by most high
resolution guidance to clip the southeast counties of ILX forecast
area from around I-70 southward this afternoon into evening, where
precipitable water exceeds 2 inches, and 24-hour rainfall
estimates are generally 1 to 3 inches so far, pre-conditioning
soils for excessive rainfall. A flood watch continues for tonight,
but with a sped up timing from current trends and models, will be
changing the effective time to include this afternoon as well.
Rainfall rates in the training area and developing shortwave/MCS
are commonly 2-4 inches per hour according to recent estimates.

Severe weather is also a threat especially I-70 southward this
afternoon and evening, with a Slight Risk designated due to ample
shear associated with the shortwave/MCS and strong instability
likely to result from the rich moisture over the area.

Otherwise, temperatures seem to be lagging the overnight forecast
package a couple of degrees, especially in southeast IL due to
extensive cloud cover, so have trimmed a few degrees off and highs
are now forecast in mid to upper 80s across the area.

37

&&

.DISCUSSION...
Issued at 320 AM CDT Tue Jul 16 2024

Early Tuesday morning, a LLJ was resulting in warm, moist air
overrunning a cold pool produced by a damaging squall line that
moved across central/northern Illinois Monday evening. The low-
level flow has been oriented roughly parallel to the axis of
thunderstorms resulting in relatively slow movement and training,
which when combined with efficient rainfall rates up to 2"/hour at
times has led to a corridor of flash flooding. The greatest
impacts thus far appear to be across Fulton Co., where radar
estimates suggest 5-8" of rain have fallen. As these thunderstorms
continue to progress southeast, heavy rain/scattered flash
flooding remain a concern, so the flood watch was expanded
southward and now includes cities like Springfield, Shelbyville,
and east to Marshall. A gradual diminishing of convective activity
will occur after sunrise as the LLJ forcing wanes.

Weather impacts are possible again on Tues as a cold front over
IA/MO pushes SE across IL. Much of the 00z CAM suite did not look
particularly concerning for severe storm potential across the ILX
CWA on Tues, however, this guidance had a very poor handle on how
things were/did evolve with the Monday night/early Tues AM
systems. The 06z guidance has started to paint a much
different...and more concerning...picture about the severe
potential today. The 06z HRRR and NAMNest both depict much
stronger instability developing (MUCAPE approaching 4000 J/kg)
compared to their earlier iterations. Whether or not that actually
occurs is up for debate given multiple rounds of convection
across central IL Monday night (some of which is still ongoing),
but nevertheless it is a signal we can`t ignore. As the cold front
drops south this afternoon and interacts with this potentially
volatile atmosphere, models also depict an MCV developing from
convection currently ongoing across KS and tracking east,
providing additional forcing for ascent and perhaps locally
enhancing the otherwise marginal shear profiles. These CAMs
ultimately depict another organized thunderstorm complex that
would be capable of producing damaging winds tracking across areas
south of I-72 this afternoon/evening. The Storm Prediction Center
initially issued a marginal risk (level 1) in their Day 1
outlook, but did note the potential need for an upgrade to slight
risk (level 2), and the most recent guidance would seem to support
an upgrade. light risk (level 2) area introduced if confidence
increases.

In terms of flooding potential this evening, forecast PWAT values
remain above 2", and deep layer shear vectors oriented close to
parallel to the front could support training. The uncertainties
regarding convective evolution discussed above also apply to the
flooding potential, so the flood watch that is in effect for areas
south of I-70 Tuesday night was left unchanged with this forecast
package.

Depending on how quickly (or not) the front shifts SE of the area,
a few lingers showers could occur on Wed (mainly south of I-70).
Otherwise, minimal weather impacts are anticipated for the latter
half of the work week and into the weekend. The synoptic pattern
becomes highly amplified, with upper ridging extending from the SW
US up into western CAN, while deep troughing develops over the
upper Great Lakes. This pattern translates to broad sfc high
pressure becoming the dominant weather feature Thurs through at
least Saturday. Forecast high temps call for highs in the mid 70s
to low 80s, which is about 5-10 degrees below normal. Afternoon
dewpoints in the low 60s should also feel quite refreshing
compared to the mid/upper 70s dewpoints that accompanied the heat
this past Sunday/Monday. Most of the area will be free of precip
Wednesday through at least Saturday.

Erwin

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 544 AM CDT Tue Jul 16 2024

Variable cloud heights are present across the region this morning
in the wake of thunderstorms that have shifted south of the area.
Patchy MVFR ceilings and perhaps some very isolated instances of
IFR ceilings are possible during the first few hours of the
period. Otherwise, VFR conditions should prevail. A few scattered
storms are possible along a cold front this afternoon, but
coverage seemed too low to warrant a mention in the TAF at this
time. That front will result in winds gradually shifting from SW
to NNW during the period.

Erwin

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 7 PM CDT this evening through Wednesday morning
for ILZ066>068-071>073.

&&

$$