702
FXUS63 KILX 171827
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
127 PM CDT Thu Jul 17 2025

.KEY MESSAGES...

- A few strong thunderstorms are possible this afternoon...with a
  5-15% chance for damaging wind gusts along and south of I-70.

- An active weather pattern will develop this weekend, resulting
  in daily chances for thunderstorms Friday night through Monday.
  Severe weather and locally heavy rainfall will be possible.

- Heat and humidity will build next week...with heat index values
  soaring well above 100 degrees Tuesday through Thursday.

&&

.DISCUSSION...
Issued at 127 PM CDT Thu Jul 17 2025

...Severe Weather Risk This Afternoon...

The latest Day 1 Convective Outlook from SPC highlights locations
along/south of I-70 for a Marginal Risk (5-15% chance) for severe
weather today. 17z/1pm surface analysis shows a cold front along
I-70: however, regional satellite/radar mosaic indicates the
primary forcing for widespread thunderstorms appears to be a bit
further south along the Ohio River where a few clusters of strong
convection have already developed. Current mesoanalysis shows
MLCAPEs of greater than 1000J/kg along/south of I-70 which are
expected to increase to greater than 2000J/kg in the next few
hours. Mitigating factor for severe weather will once again be a
lack of deep-layer shear as 0-6km values will remain under 20kt
across this area.  CAMs generally show scattered showers/storms
forming along/south of I-72 this afternoon...with the greatest
areal coverage across the far SE KILX CWA. This makes sense given
current synoptic/mesoscale set-up...so have focused likely PoPs
along/south of I-70 from mid-afternoon into the early evening.
Isolated damaging wind gusts of 50-60mph will be possible with
the strongest cells.


...Active Weather This Weekend...

The cold front will settle southward and stall near the I-64
corridor tonight through Friday. As a result, think the northern
two-thirds of the CWA will remain mostly dry on Friday...with
better rain chances confined to locations along/south of a
Rushville to Robinson line. As a short-wave trough skirts through
the Northern Plains into the Great Lakes, it will nudge the
boundary back northward into the area Friday night into Saturday
and provide the necessary forcing for an uptick in convective
coverage. Questions still remain as to how organized the
convection will become and its exact timing. The 12z HRRR suggests
an MCS may develop ahead of the trough across Minnesota/Iowa late
Friday night, then spill into at least north-central Illinois by
Saturday morning. Meanwhile other models such as the GFS are not
as bullish with the possible MCS, but develop numerous
showers/storms near the frontal zone both Friday night and
Saturday. Will need to monitor later CAMs regarding the possible
MCS, but for now will increase PoPs to likely Friday night into
Saturday.

After that, additional nocturnal convection may develop
both Saturday night and Sunday night. Early projections suggest
the Saturday night into Sunday morning storms may focus
along/north of I-74...while the Sunday night into Monday morning
convection may be across the heart of central Illinois. This will
be a largely mesoscale-driven event, so the details regarding the
individual waves of convection will likely shift slightly with
subsequent forecast packages. For now, suffice it to say the
weekend will be unsettled at times...with the potential for severe
weather and locally heavy rainfall. The SPC Day 3 outlook places
all of central and southeast Illinois under a Marginal Risk (5-15%
chance) for severe weather on Saturday...while WPC highlights
locations along/north of I-74 with a Slight Risk (15% or greater
chance) for excessive rainfall. The Slight Risk for excessive
rainfall spreads southward across the entire CWA for Sunday.
Initial projections suggest rainfall of 1-2 inches along and
northeast of a Galesburg...to Lincoln...to Lawrenceville line.
Given the recent dry conditions, the rainfall will be quite
beneficial: however, excessive rainfall rates within the stronger
storms may lead to scattered flash flooding of urban areas and
low-lying spots prone to flooding.


...Building Heat Next Week...

The upper ridge is progged to steadily strengthen this weekend and
by early next week will shift the "ring of fire" northward into
the Upper Midwest and Great Lakes. As a result, rain chances will
decrease on Monday...then will disappear by Tuesday and Wednesday.
Thanks to rising upper heights, lesser rain chances, and more
sunshine...temperatures will climb into the lower 90s by Tuesday
and into the middle 90s by Wednesday. Given the expected rainfall
this weekend, the moist soils and actively growing crops will
contribute to dewpoints in the upper 70s and perhaps lower 80s in
a few spots. This will likely push heat index values well above
100 degrees and into Heat Advisory/Warning territory by next
Tuesday through Thursday.


Barnes

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1246 PM CDT Thu Jul 17 2025

1730z/1230pm radar imagery is showing widely scattered convection
beginning to develop just east of the Illinois River. Based on
satellite trends and latest HRRR, think this activity will
develop/spread E/NE across the area this afternoon. CAMs have
consistently focused the bulk of the convection along/south of the
I-72 corridor, so have opted to leave thunder out of the forecast
for both KPIA and KBMI. Further south, have added a PROB30 for
thunder at KSPI/KDEC/KCMI through early evening. Some models
linger or re-develop scattered convection after dark, but am
skeptical of that solution and will not include in the TAF at this
time. Low VFR ceilings will prevail this afternoon: however, both
the NAM and HRRR suggest ceilings will lower along/north of a
frontal boundary stalled near the I-64 corridor tonight into
Friday morning. All sites will drop to MVFR by 07z, and even lower
IFR ceilings are possible closer to the front at KSPI and KDEC.
Have given KSPI a period of MVFR from 09z to 15z accordingly, but
left KDEC at MVFR. The low clouds will scatter along I-74 by mid
to late morning, but will likely linger at KSPI/KDEC through
midday.

Barnes

&&

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

$$