Area Forecast Discussion
Issued by NWS Central Illinois

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787
FXUS63 KILX 280526
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1226 AM CDT Sun May 28 2017

.UPDATE...
Issued at 932 PM CDT Sat May 27 2017

With several convective systems off to the south of central IL
in southern MO through southern IL, KY, and TN, subsidence in
central IL has inhibited any convection so far. Although enough
instability overnight will continue that storms can`t be entirely
ruled out, looks like any significant chances will hold off until
morning when a shortwave arrives from the west. For tonight, any
significant chance for rain/thunderstorms is from around I-70
southward. Severe thunderstorm outlook has been diminished to just
a marginal risk for tonight. Have sent updates for the above
mentioned featured.

&&

.SHORT TERM...(This evening through Sunday)
ISSUED AT 328 PM CDT Sat May 27 2017

20z/3pm surface analysis shows stationary frontal boundary extending
from southeast Kansas eastward into the Ohio River Valley. Dewpoints
south of the front are currently in the lower to middle 70s,
creating a highly unstable environment characterized by CAPE values
of 3000-4000J/kg along/south of the I-64 corridor.  Meanwhile north
of the front, a much more stable airmass is in place across much of
central Illinois where lower to middle 60s dewpoints are yielding
CAPEs of only 1000-1500J/kg.  Latest radar mosaic is showing several
clusters of thunderstorms developing along/south of the front, with
the most organized linear structure over central/southern Missouri.
This feature will race eastward along the boundary, largely
remaining south of the KILX CWA this evening.  Based on radar timing
tools, the northern fringe of this system will brush the I-70
corridor between 00z and 04z...with the main widespread damaging
wind threat focusing further south across southern Illinois and
western Kentucky.  HRRR appears to have a good handle on the current
situation and shows this scenario unfolding over the next several
hours.  As a result, have updated PoPs to go with likely along/south
of a Jacksonville...to Taylorville...to Robinson line this evening.
Will need to closely monitor areas from Effingham and Robinson
southward, as the potential still exists for damaging wind gusts and
hail this evening.  Further north, will only carry slight to chance
PoPs for showers/thunder elsewhere around central Illinois.  The
main line of storms will quickly pass into southern Indiana/Kentucky
by around midnight, with only isolated showers expected overnight.

Low pressure will track across central Illinois on Sunday, keeping
rain chances alive throughout the day.  Instability/shear parameters
are meager across the area, with the greatest instability developing
further southeast across the Ohio River Valley.  As a result, the
latest Day 2 Convective Outlook from SPC has shifted the slight risk
out of the SE CWA.  Will carry chance PoPs throughout the day, with
rain chances quickly coming to an end from west to east by late
afternoon and early evening.

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 328 PM CDT Sat May 27 2017

Pronounced upper low will dig southward through Ontario into the
Great Lakes early next week, resulting in a cooling trend across
central Illinois.  A weak surface trough will sink southward around
the parent low on Monday, potentially bringing a few showers.
Convergence along the boundary will be weak and with the main upper
cold core still to the north, think areal coverage of precip will
remain low.  As a result, have only mentioned slight chance PoPs for
showers/thunder along and north of I-74 Monday afternoon.  Cool/dry
conditions will prevail Monday night into Tuesday before the next
potential system spreads showers across south-central Illinois
Tuesday night.  After that, mainly dry weather is expected until a
stronger system brings the next significant chance for rain by
Friday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1225 AM CDT Sun May 28 2017

Central Illinois is covered by expansive high cloud blowoff from
convection south of Illinois, which will complicate the first part
of the forecast. HRRR guidance suggesting a period of MVFR
ceilings and visibilities between about 08-13Z, but this is
uncertain given the higher cloud shield. Areas near KPIA would be
most likely to see this, as the high clouds are most likely to
thin out enough to allow the lower clouds to form. But with the
intense storms pushing south into the lower Mississippi Valley,
have included some brief MVFR conditions at all sites as some
erosion of the cirrus blowoff can`t be ruled out. A cold front
will be arriving near KPIA about 15Z and should be east of KCMI by
22Z, with a couple hours of scattered storms expected with its
passage. Late in the period, clouds will scatter out as skies
become mostly clear.

&&

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

$$

UPDATE...37
SHORT TERM...Barnes
LONG TERM...Barnes
AVIATION...Geelhart



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