Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 261800
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
100 PM CDT Sat Jul 26 2014

.DISCUSSION...
ISSUED 1045 AM CDT Sat Jul 26 2014

Update the forecast to include the mention of severe thunderstorms
possible late this afternoon sw counties and across area this evening.
SPC has moderate risk of severe storms late this afternoon and
evening across central IL except slight risk over Stark, Marshall
and northern Knox counties. Has 5% risk of tornadoes, 30% risk of
large hail (10% risk of 2 inch or larger hail) and 45% risk of
damaging wind gusts.

Also trended pops lower through early afternoon and higher late
this afternoon and evening. Most of central IL has been dry so far
this morning. Low clouds have spread into nw counties by late
morning and this may limit heating and instability. Kept highs in
the upper 80s and lower 90s with afternoon heat indices peaking
from 95 to 103F, lowest north of Peoria where more low clouds found.

A frontal boundary over NW IL and near the IA/MO border will press
se into central IL later this afternoon and tonight and collide
with an increasingly unstable airmass with CAPES projects to
surpass 3000 j/kg and bulk shear rising to 40-50 kts by 00Z/7 pm.

07

&&

.AVIATION...
ISSUED 1259 PM CDT Sat Jul 26 2014

Main concern with forecast is timing of thunderstorms that
potentially could be strong to severe late this afternoon into
mid evening. Leaned on the HRRR model for timing of convection
which appears to have a good handle on convection over MO. Have
22-24Z timing for western TAF sites of PIA and SPI, 23-01Z for DEC
and BMI and 00-02Z for CMI. Kept strong wind gusts of 35-40 kts
with thunderstorms and vsbys 1-3 miles with heavy rains and MVFR
ceilings. Convection currently moving east into central/nw MO.
Frontal boundary over nw IL nw of the IL river could develop
convection sooner this afternoon at PIA so will monitor this.
Otherwise frontal boundary to push se through central IL into this
evening with sw winds of 7-12 kts and few higher gusts turn WSW
this evening and lighten up to 4-7 kts and veern WNW and increase
to 10-15 kts Sunday morning with gusts 15-20 kts after 15Z/10 am.

07

&&

.PREV DISCUSSION...
ISSUED 312 AM CDT Sat Jul 26 2014
Unsettled weather anticipated today as low pressure and associated
fronts make their way through the Midwest. Plenty of fuel for
storms as hot and humid air pools over the region, but there is
much derision in the models as to initiation and timing and
location of waves to the west to move along the boundaries in the
region and trigger the storms/MCS that will bring the potential
for severe weather to Central Illinois. Warm front currently to
the southwest and expected to lift into the area today, leaving
ILX in the warm sector for much of the day. Depending on the speed
of the cold front expected this afternoon/evening will depend on
how much instability it taps into. Sfc dewpoints into the upper
60s and lower 70s just on the other side of the Miss River Valley
this morning are expected to shift eastward with high temps
approaching 90F. Beyond stormy today and tonight... remainder of
the forecast cooler behind the boundary slowly modifying through
next week to more seasonable temperatures ahead of the next system.

SHORT TERM...Today and tonight...
Plenty of instability for the area with a Moderate Risk in place
for just that reason. 18z forecast soundings showing 4000-4500 j/kg
CAPE as a reflection of the increasing moisture at the sfc and
climbing temps in the warm sector as the warm front lifts
northward. Warm front expected to be northeast and east by midday
as the cold front and actual low center makes its way into the
region. As similar situations have set up over the fa recently,
trouble will be fighting the cap in place for much of the day...
and the approach of the front being enough to break it. Not only
the front itself but models have persisted with a couple waves
moving into the region. Major issues with this are beginning to
show their hand this morning as the locations of the waves this
morning on sat imagery are not initializing in the same locations
as many of the models. Bottom line is... plenty of fuel for
storms, and the triggers are there, but timing and exact locations
of impact are troublesome at best. Llvl helicity better in the
early evening with the proximity of the cold front and the low
itself...but cannot ignore the impact of the warm front and its
location should it not progress further north, storms crossing the
boundary could also be impacted by the turning of the winds for
the front. Once the cap breaks, the forecast soundings/CAPE are
indicative of rapid development of storms. Once going, the very
high CAPE values remain in place well into the evening up to 06z.
Best directional shear is actually in place for the late
afternoon/early evening lending itself to supercells in the
beginning of the event...transitioning to more of a wind event
with time. Large hail will also be a threat mostly with supercells
and long sustained updrafts as the freezing levels are more than
15k ft. Some tornadic potential early with supercells and invof
both outflows and frontal boundaries.

LONG TERM...Tomorrow through Friday...
Event coming to a close from west to east on Sunday early...but
proximity to upper trof over the Great Lakes will keep an enhanced
threat for instability showers under cyclonic flow, esp Sunday
afternoon. Cooler temps in place for Monday in particular as 850mb
temps drop to closer to 10-12C. Moderating temperatures for the
week as northwesterly flow establishes itself in the region.
Majority of waves alluded to in the models remain north of here
and forecast remains mostly dry, though subtle shifts with those
waves could easily shift the forecast. Next major system impacting
the Midwest Thurs late into Saturday with an open wave aloft and
more cyclonic flow bringing threat for instability showers yet
again.

HJS

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$







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