Hail and Damaging Winds in Illinois, June 14, 2017

Area Forecast Discussion
National Weather Service Lincoln IL
1256 PM CDT Wed Jun 14 2017

.UPDATE...
Issued at 1040 AM CDT Wed Jun 14 2017

Warm sector destabilization is being aided by surface dewpoints in
the low 70s. Scattered showers and isolated storms have already
bubble up along and west of I-55, with slow movement to the NE.
There remains a large spread in the high res model solutions as to
timing and coverage of storms the rest of the day today. Will
continue with our trend of PoPs ramping up to a peak in the last
half of the afternoon and into early evening as a squall line
moves from west to east through central IL, which is closer to the
NAMnest solution. HRRR has broken lines of storms, and much less
organized, with storms generally east and southeast of our
counties by 00z/7pm. MUCAPEs are projected to climb into the
2500-3500 J/kg for areas west of I-57, with no CIN and 0-3km
helicity of 150-200 m2/s2. A few of the stronger updrafts may
begin to rotate this afternoon, but the primary hazard still looks
to be damaging downburst winds.

ILX will be sending up a special 18z/1pm upper air balloon to aid
SPC in severe potential analysis early this afternoon. That data
will give us a better handle on local shear and stability as we
approach peak heating.

Main updates this morning were minor adjustments to PoPs and
weather. High temps look on track, due to limited morning
showers/storms and plenty of sunshine across the area. Updated
forecast info is already available.

&&

.SHORT TERM...(Today and tonight)
ISSUED AT 340 AM CDT Wed Jun 14 2017

08z/3am radar mosaic shows several bands of convection ahead of a
slowly approaching cold front from Minnesota southward across
western Iowa into Kansas.  This activity is generally moving N/NE
and is expected to remain well west of central Illinois through
dawn.  As outflow from these storms slowly approaches from the west,
scattered thunderstorms will develop across the Illinois River
Valley by mid to late morning, then further east across the
remainder of the KILX CWA this afternoon.  With SBCAPES progged to
reach the 3000-4000J/kg range, any storms that occur later today
will develop vigorous updrafts and will thus be capable of
producing large hail and damaging wind gusts.  Heavy rainfall
will also be a concern with the storms, as slow movement and high
precipitable water values of 1.75-2.00 contribute to a localized
flash flooding threat.  Based on model consensus, it appears the
primary time frame for potential severe weather will be from noon
to 8pm.  Have carried high chance PoPs across the board with
afternoon high temperatures reaching the lower 90s.  30kt 850mb
jet will likely enhance areal coverage of showers and
thunderstorms late tonight, so have bumped PoPs to likely from
late evening through the overnight hours.

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 340 AM CDT Wed Jun 14 2017

Warm and unsettled weather will prevail through the weekend, before
a significant pattern change brings cooler/drier weather by early
next week.  The initial surge of convection from tonight will be
ongoing across the E/SE CWA Thursday morning, then will push into
the Ohio River Valley by afternoon.  Subsidence in the wake of this
system will then provide dry weather through Thursday night.

A series of short-waves will pass through the region Friday through
Sunday, bringing periodic rain chances.  It appears the strongest
waves and thus the greatest rain chances will arrive Friday night,
then again Saturday night into Sunday.  Have therefore focused
highest PoPs during those times with just slight chances elsewhere.
The final wave will carve out a significant trough over the Great
Lakes and push a strong cold front through central Illinois on
Sunday.  Based on the timing of the boundary, storms will become
most widespread along/south of I-70 by Sunday afternoon.  Once the
front sinks south of the area, cooler and drier weather will be on
tap early next week.  High temperatures will only reach the lower
80s by Monday and Tuesday, with overnight lows dipping into the
upper 50s and lower 60s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1256 PM CDT Wed Jun 14 2017

Boundary layer destabilization becoming more pronounced, with
storm coverage expanding. Still plenty of uncertainty with
convective mode and timing of any line of storms. Based on current
storm coverage, a broken line of storms appears more likely. Have
made an attempt to focus on the most likely 2 hour period each
TAF location will be affected by any strong/severe storms. Those
periods are concentrated this afternoon for all but CMI, that
extends to 01z. Will make adjustments to timing once the
convection approaches any terminal site.

There should be a significant reduction in storm coverage after
01z/03z, based on NAM-nest and RAP updates. Have left in a VCSH
overnight just in case a few showers bubble up behind the line.

Winds should generally remain at or below 10kt from the southwest,
outside of any thunderstorm wind gusts. Little change in air mass
expected throughout this TAF period, so no prevailing wind shifts
expected.

&&

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

UPDATE...Shimon
SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...Shimon

Hazardous Weather Outlook
National Weather Service Lincoln IL
1200 PM CDT Wed Jun 14 2017

ILZ027>031-036>038-040>043-047>053-151100-
Cass-Christian-De Witt-Fulton-Knox-Logan-Macon-Marshall-Mason-
McLean-Menard-Morgan-Peoria-Sangamon-Schuyler-Scott-Stark-
Tazewell-Woodford-
1200 PM CDT Wed Jun 14 2017

This Hazardous Weather Outlook is for portions of central
Illinois and west central Illinois.

.DAY ONE...THIS AFTERNOON AND TONIGHT...

Thunderstorms will increase in coverage and intensity this
afternoon. There is a slight risk that thunderstorms from early
this afternoon through evening may contain locally damaging wind
gusts and large hail, with a low probability of an isolated
tornado or two mainly northwest of the Illinois River. Locally
heavy rain and cloud to ground lightning will also be hazards.

Heat index values of 96 to 100 are expected this afternoon.

.DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY...

Periods of thunderstorms are forecast Friday through Sunday
morning. There is a marginal risk of severe thunderstorms for
Friday into Friday night, and potential for additional severe
thunderstorms Saturday into Saturday night.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon and evening.

ILZ044>046-054>057-061>063-066>068-071>073-151100-
Champaign-Clark-Clay-Coles-Crawford-Cumberland-Douglas-Edgar-
Effingham-Jasper-Lawrence-Moultrie-Piatt-Richland-Shelby-
Vermilion-
1200 PM CDT Wed Jun 14 2017

This Hazardous Weather Outlook is for portions of central
Illinois, east central Illinois and southeast Illinois.

.DAY ONE...THIS AFTERNOON AND TONIGHT...

A chance for thunderstorms will develop by middle afternoon, with
occasional thunderstorms possible through Thursday morning. There
is a slight risk that thunderstorms from middle afternoon
through evening may contain locally damaging wind gusts and large
hail. Locally heavy rain and cloud to ground lightning will also
be hazards.

Heat index values of 96 to 100 are expected this afternoon.

.DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY...

Periods of thunderstorms are forecast Friday through Sunday
afternoon. The highest thunderstorm chances are Friday night and
Saturday night into Sunday. Although severe weather is not
expected at this time, strong storms are possible.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon and evening.

Barker