Area Forecast Discussion
Issued by NWS Central Illinois

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FXUS63 KILX 220526

Area Forecast Discussion
National Weather Service Lincoln IL
1226 AM CDT Sat Jul 22 2017

Issued at 901 PM CDT Fri Jul 21 2017

Current forecast grids have already been updated once, but will be
making further adjustments to mainly pops/wx and severe watch. All
convection has remained north of the CWA this evening and based on
current radar trends looks like it will remain north of the CWA
remainder of the evening and possibly into the overnight hours.
However, additional precip is expected to develop along the
boundary out west and then move into northern IL again overnight.
Convection therefore is still possible in northern part of the CWA
overnight and will maintain some pops up there. Will be making
adjustment to evening pops across the north to account for current
trends. Update should be forthcoming.


.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 06Z TAFS through 06Z Saturday night)
Issued at 1225 AM CDT Sat Jul 22 2017

Large convective complex from northeast Iowa into northern Indiana
has largely been tracking north of the central Illinois TAF sites
this evening. Outflow boundary from them should be passing through
KBMI very soon with some temporary northwest gusts of 20-30 knots
or so, and the TAF was amended for this possibility. Highest gusts
will likely be before 06Z, so the TAF starting at 06Z will only
have some north gusts of 20-25 knots for a couple hours. Have also
maintained the VCTS mention at KPIA/KBMI from about 08-14Z as the
storms track southeast from Iowa. In addition, have added LLWS
mention during that time for these two sites with a lower level
jet ramping up just south of the convection.

After that, mainly quiet conditions expected through the
afternoon. A cold front will be moving through late afternoon and
evening between about 22-03Z. Some scattered storms possible along
this boundary, but too uncertain to add at this point.


Excessive Heat Warning until 8 PM CDT this evening for

Heat Advisory until 8 PM CDT this evening for ILZ030-031-038-



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