534
FXUS63 KILX 121836
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
136 PM CDT Sun May 12 2024

.KEY MESSAGES...

- In the Monday to Tuesday timeframe, there is a 40-60% chance for
  more than an inch of rainfall. This rain will contribute to area
  rivers and streams already running high, increasing the risk
  for localized inundation near riverbanks. This could perhaps
  also further delay planting and inhibit germination of
  agricultural seedlings.

- Another system will bring rain to the area late Thursday into
  Friday, though at this time rainfall totals from that system are
  forecast to be lower than those with the first one. Chances for
  more than an inch are only around 10-20%.

&&

.DISCUSSION...
Issued at 133 PM CDT Sun May 12 2024

After yet another dry and pleasant spring day in central Illinois,
precipitation chances will increase tomorrow with the arrival of a
low pressure system from the west. Visible satellite at 130pm
shows the cirrus shield associated with that system drifting
eastward across Missouri, with a few gusty thunderstorms to our
northwest across Iowa where mesoanalysis suggests over 1500 J/kg
SBCAPE has materialized despite a rather dry and deep mixed layer.

Deep mixing this afternoon across our neck of the woods has kept us
even drier with dewpoints in the 40s, but low level moisture will
increase overnight into tomorrow with 80-100% of the HREF ensemble
bringing PWATs to over 1.3" (the 90th percentile from SPC`s sounding
climatology) by tomorrow afternoon. This, together with tall,
"skinny" CAPE profiles conducive to warm cloud processes, will
increase potential for efficient rain producing showers and
thunderstorms tomorrow afternoon/night, particularly north of I-70
where MUCAPE values (700-1200 J/kg, per the HRRR) are highest and
the HREF`s LPMM is well over an inch in spots. The same ensemble
shows spread in rain totals through Tuesday morning will be
considerable given the convective nature of this precip, though;
the ensemble max, a reasonable high-end amount which one or two
locations could see if there`s any training of the heavier cells,
is upwards of 3 inches in a few spots, while the 10th percentile
(which should be representative of the many locations which get
missed by the heavier showers) is only several hundredths.

While the risk for widespread hydrological issues is negligible
(less than 2%), many rivers and streams across the region are
already running on the high side, so localized inundation near
riverbanks could occur with the more flashy rivers (in addition to
the ongoing flooding near Havana and Beardstown along the
Illinois River). This rain could also delay planting even further
for farmers, and could inhibit germination of already-sown
seedlings. On the bright side, given weak shear (15-25 kt, per
HREF mean) and weak instability (with low NCAPE values indicating
slow updraft parcel accelerations), severe weather is not a
concern at this time.

Once we`re rid of this pesky system Wednesday morning, ridging will
attempt to build back in but will be rushed out by another trough
on its heels, southwest flow ushering in moisture and rain
chances once again by late Thursday as an ill-defined surface low
passes to our north. This system will be a bit more progressive
than the Monday- Tuesday night one, so initial thinking is
rainfall won`t be overly troublesome. However, a large component
of shear in the cloud bearing layer will be parallel to the
southeastward-moving cold front favoring some training of showers
and storms, and EPS mean brings PWATs back up to 1.2 to 1.3
inches, so some locations could certainly see an inch or more of
precip once again (NBM probabilities are around 10-20%) to once
more delay seedling germination and raise river levels.

Cluster analysis suggests there`s not a whole lot of model spread in
the upper pattern this upcoming weekend and beyond, with middle of
the road guidance suggesting 500mb flow turns rather zonal. This may
be washing out potential shortwaves due to model
timing/amplification differences, but there`s certainly not a strong
signal for a large and dynamic system at the end of the forecast
period.

Bumgardner

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1234 PM CDT Sun May 12 2024

VFR conditions should continue through the forecast period, with
scattered afternoon cu fading this evening while cirrus arrives
from the west. The incoming system will bring scattered showers
to the area near the end of the forecast period, with some thunder
possible with the heavier cells later Monday afternoon. Southwest
winds gusting 15-20 kt this afternoon will ease and back to
southerly this evening, but then pick back up from the southwest
mid to late Monday morning.

Bumgardner

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$