966
FXUS63 KILX 141516
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
1016 AM CDT Sun Jul 14 2024

.KEY MESSAGES...

- Hot and humid conditions through Monday, with highs in the low
  to mid 90s and heat indices in the 100s.

- An unsettled pattern continues with occasional disturbances
  producing periodic thunderstorm chances through Tuesday, some of
  which could be strong to severe.

- Much cooler air and a stretch of mostly dry weather is expected
  from Wednesday into the weekend.

&&

.UPDATE...
Issued at 1016 AM CDT Sun Jul 14 2024

A MCS is progressing through northern Illinois and is now mostly
east of the I-55 corridor. Some scattered showers have dropped
further south into central Illinois this morning along the outflow
boundary of the complex to the north. The main risk for any
damaging winds and flooding is to the north of the forecast area.
However, a stronger storm is not out of the question through 1pm
today. Heat indices of 100-110 degrees are still forecast for
today and a Heat Advisory is in effect through 8pm Monday.

Copple

&&

.DISCUSSION...
Issued at 235 AM CDT Sun Jul 14 2024

A multi-day stretch of oppressive heat and humidity is the top
weather headline across central IL through early next week. Upper
level ridging remains anchored over the Four Corners region, while
weak sfc high pressure sits over the SE US and lower sfc
pressures are present over the upper Great Lakes. This sfc pattern
will result in continued southwesterly flow and WAA through
Monday, continuing a gradual warming trend.

The pattern supports continued MCS activity primarily north of the
ILX CWA, and three are ongoing at the start of the forecast period
(N IL, cntrl MN, SD). The one over cntrl MN is expected to
progress SE towards N IL by mid-morning. CAMs have not been
handling the mature MCS over SD well (which is also tracking SE).
Most CAMs have it dissipating in a few hours, but it seems likely
to maintain beyond that. It still seems like a stretch that this
system would persist all the way to IL, but it bears monitoring,
and it could transition into a remnant MCV that leads to
scattered t-storm develop this afternoon/early evening. Confidence
in the evolution of these features is quiet low. The other avenue
via which scattered showers could development is WAA, focused
mainly across IA/MO where t-storms have formed recently (as of
2am/07z), but this activity could clip west-central IL this
morning.

While we`ve been monitoring the potential for these MCS to provide
some cooling cloud cover, the latest model guidance makes it seem
like widespread cirrus is unlikely, with cloud cover generally
forecast to be partly cloudy or less. So it`s full steam ahead on
those hot/humid conditions today, with highs in the low to mid 90s
and heat indices of 100-105 degrees. The heat peaks on Monday, as
temps warm a few degrees relative to Sunday and heat indices push
into the 105-110 degree range. The nighttime periods will also
offer little relief, as heat indices stay in the mid to upper 70s.
A heat advisory remains in effect area wide through Monday
evening. Additional convective development appears possible Sun
night as a LLJ ramps up and 850mb moisture axis focuses over
northern parts of IL. As has been the case with all of the
previous MCS activity, CAMs suggest the bulk of the precip stays
north of the ILX CWA, with areas near/north of I-74 having the
best chance of seeing any rain Sunday night.

Into early next week, a deeper upper low digs towards the Great
Lakes, sending a cold front pushing towards the ILX CWA which will
break the heat wave and provide a chance for thunderstorms Mon
night (mainly north of I-72) into Tuesday. Forecast soundings for
Tues continue to show a highly unstable environment ahead of the
cold front, but with varying levels of deep layer shear depending
on the guidance source. At any rate, the deep layer shear vectors
are nearly parallel to the front and forecast soundings show steep
low- level lapse rates resulting in DCAPE values over 1000 J/kg.
Collectively, these factors favor upscale growth of t-storms and
if a severe threat develops it would likely be a wind threat. The
exact area at risk will depend on the frontal timing, but for now
it appears the threat will be focused south of I-72 Tues PM. There
is also some potential for locally heavy rain/isolated flash
flooding, with PWAT values forecast to approach or exceed 2" and
deep layer flow oriented to the boundary resulting in the
potential for training convection.

The forecast for the latter half of the work week and into the
weekend remains largely unchanged. As the front gradually shifts
SE, precip could linger near/south of the I-70 corridor on Wed,
but otherwise mostly dry conditions will prevail into the weekend.
Large sfc high pressure develops behind the front and slowly
sinks southeast, reaching IL by Friday. Below normal temperatures
remain highly likely from Wed into the weekend, with highs near
80F which is about 5-6 degrees below normal.

Erwin

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 551 AM CDT Sun Jul 14 2024

Southwesterly winds prevail through the period, with gusts around
20 knots from mid-morning through the afternoon. VFR conditions
should largely prevail, although some isolated ground fog could
result in MVFR visbys at KCMI through 13z. The main challenge this
period was low confidence regarding multiple thunderstorm chances
and whether or not they will reach the terminals. A line of
storms is tracking into northern Illinois as of this writing, and
could impact the northern terminals (KPIA/KBMI/KCMI) between
13-16z if the line does not dissipate before then. If these
thunderstorms do reach the terminal, expect reduced visibility,
MVFR ceilings, and gusty northeasterly winds. Another rounds of
thunderstorms is possible overnight, late in the 12z TAF period.
Again, confidence in how these thunderstorm chances evolve is
lower than normal.

Erwin

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT Monday for ILZ027>031-036>038-
040>057-061>063-066>068-071>073.

&&

$$