Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 212347

Area Forecast Discussion
National Weather Service Lincoln IL
647 PM CDT Fri Jul 21 2017

.SHORT TERM...(This evening through Saturday)
ISSUED AT 300 PM CDT Fri Jul 21 2017

19z/2pm surface analysis shows a 1006mb low over northeast
Nebraska...with a warm front extending eastward along the I-80
corridor. A large cluster of showers/thunderstorms has been
ongoing throughout the day northeast of the low over southeast
Minnesota into Wisconsin. As a result, an impressive instability
gradient has developed from north to south across Iowa, with
SBCAPEs of 5000-6000J/kg south of the boundary in southern Iowa.
A new cluster of storms has recently developed along this boundary
near the Iowa/Nebraska border. The HRRR has been consistently
showing convection firing in this area for the past several runs,
with the storms then developing/tracking E/SE along the boundary
into north-central Illinois this evening. Given the extreme
instability feeding into the storms, think they will quickly form
robust updrafts and become severe across central Iowa over the
next couple of hours. Once a cold pool can develop, the storms
will then begin racing E/SE along the instability axis into
north-central Illinois by mid to late evening. While the main push
will likely be along the I-80 corridor, think areas as far south
as I-74 will feel the impacts as well. Based on latest HRRR, have
introduced likely PoPs across the NW KILX CWA around Galesburg by
02z/9pm. Have then spread likely PoPs E/SE along/north of I-74
into the overnight hours. As the initial convective system pushes
east, a trialing band of showers/thunder will gradually sink
southward to I-72 toward dawn Saturday. Further south, have kept
the southern half of the CWA dry through the entire night. As it
stands now, think the biggest threat for severe weather and
potentially damaging wind gusts will focus along/north of I-74
between 9pm and 3am.

Scattered showers and thunderstorms will linger across the
northern CWA through mid-morning Saturday before dissipating.
May see some re-development along the cold front later in the
afternoon: however, subsidence in the wake of the morning
convective system may act to suppress widespread development
initially. As a result, will only carry low chance PoPs
during the afternoon. Areal coverage of convection will likely
increase across the southern half of the CWA Saturday evening as
the front sinks further south. High temperatures on Saturday will
once again climb well into the 90s...with peak afternoon heat
index readings of 105-115 degrees.

.LONG TERM...(Saturday night through Friday)
ISSUED AT 300 PM CDT Fri Jul 21 2017

Front will sink into the Ohio River Valley on Sunday: however, a
trailing weak trough axis may trigger isolated thunderstorms. Once
this feature passes, high pressure will build into the region
early next week...allowing a cooler/less humid airmass to take
control. High temperatures will drop into the lower to middle 80s
for both Monday and Tuesday, then will slowly climb back into the
middle to upper 80s by Wednesday. There are still some
discrepancies among the synoptic models later next week, but
consensus suggests the next significant chance for rain will hold
off until late Wednesday night through Friday as a slow-moving
frontal boundary comes into the picture.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 646 PM CDT Fri Jul 21 2017

Main short-term aviation concern will be convective trends through
the evening. Latest radar imagery continues to show a cluster of
thunderstorms over eastern Iowa, moving E/SE along a stationary
frontal boundary draped across northern Illinois. Based on radar
timing tools, it appears these storms will remain north of the
I-74 terminals later this evening. Once this initial surge of
storms passes to the north, trailing cells will gradually settle
southward overnight into early Saturday morning. As a result, have
introduced VCTS at KPIA/KBMI/KCMI between 09z and 14-15z. A cold
front will drop into central Illinois on Saturday, triggering
scattered thunderstorms by late afternoon. Based on expected
location of boundary at peak heating, have included VCTS at
KSPI/KDEC after 21z. Winds will initially be from the S at less
than 10kt this evening, then will veer to the W/NW by Saturday
afternoon as the front arrives.


Excessive Heat Warning until 8 PM CDT Saturday for ILZ027>029-

Heat Advisory until 8 PM CDT Saturday for ILZ030-031-038-043>046-



LONG TERM...Barnes
AVIATION...Barnes is the U.S. government's official web portal to all federal, state and local government web resources and services.