624 
FXUS63 KILX 210449
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1149 PM CDT Fri Aug 20 2021

.SYNOPSIS...
Issued at 300 PM CDT Fri Aug 20 2021

A cold front will move through Illinois on Saturday and Saturday
evening, bringing scattered showers and thunderstorms later 
tonight into Saturday evening. Dry and less humid weather is 
expected Sunday behind this front. But the heat and humidity will 
return during the work week, with heat indices over 100 possible 
during the afternoon hours Monday through Wednesday.

&&

.UPDATE...
Issued at 910 PM CDT Fri Aug 20 2021

The cold front at 9 pm extended from western Iowa to eastern
Kansas, with slow but steady progress to the east. The latest runs
of the HRRR and RAP are both pointing toward a slower arrival of
the line of storms into our western counties, and also a more
complete dissipation of the line of showers/storms by 15z/10am.
There is enough agreement in the HREF/SREF guidance to trend in
that direction with the evening update. Have slowed down the onset
and advancement after midnight, and reduced the coverage of PoPs
during the morning into early afternoon. There could be a sizable
break in any rain in our CWA from 10 am to 3 or 4 pm, before the
front energizes during peak afternoon heating for areas east of
I-55. Even that coverage could be reduced with future updates,
based on the latest trends. Updated forecast info is already 
available online. 

&&

.SHORT TERM...(This evening through Saturday)
ISSUED AT 315 PM CDT Fri Aug 20 2021

Isolated convection has been staying out of central and se IL so
far today, focused more in southern IL into se MO and sw KY. Also
a few isolated showers/thunderstorms over central and ne IN into 
western Ohio. Some subsidence over area with 587 dm 500 mb high
over IL and eastern WI keeping it dry and more stable despite the
warm and moist air mass with temps in the mid to upper 80s and 
dewpoints in the upper 60s to mid 70s. Heat indices at mid
afternoon ranged from 89 to 96 over CWA. 

1000 mb low pressure in east central ND has a cold front moving
into eastern SD/NE. Models have trended a bit slower with bringing
this cold front east into nw IL Sat morning. A broken band of
convection ahead of the cold front to push into the IL river
valley during overnight mainly after 2-3 am and get as far east as
I-55 by 12/7 am as it weakens. SPC day1 outlook keeps risk of
severe storms west of IL through tonight. Lows overnight mostly in
the lower 70s, with some upper 60s in east central IL and near the
IN border/Wabash river.  

Convection to redevelop over eastern IL during Saturday afternoon
and diminishes during Sat evening as cold front moves through. SPC
day2 outlook does not have any risk of severe storms over IL
tomorrow due to weak wind shear around 25 kts or less. Have some
pockets of more unstable air in eastern IL Sat afternoon so could
be a few thunderstorms with gusty winds and locally heavy rains.
Highs Sat in the mid to upper 80s, warmest in southeast IL. Muggy
dewpoints in the 70s Sat gives afternoon heat indices in the 90s.

.LONG TERM...(Saturday night through Friday)
ISSUED AT 330 PM CDT Fri Aug 20 2021

Convection chances diminish over eastern IL during Sat evening
after sunset to 20-30%, then dry overnight Sat night and Sunday
with upper level ridge nosing back into IL. Getting a short break
from the high humidity as dewpoints slip into the mid to upper 
60s nw CWA and lower 70s in southeast IL. Lows Sat night range
from low to mid 60s over the IL river valley, to around 70 in
southeast IL. Highs Sunday in the mid 80s central IL and upper 
80s southeast IL. Lows Sunday night in the mid to upper 60s,
coolest from I-74 ne. 

Another heat wave sets up over the area from Monday through 
middle of next week as subtropical ridge takes hold. Some
disturbances/short waves ridging over top of the ridge could bring
period chances of showers/thunderstorms Monday through Friday. The
higher chances will be overnight Monday night into Tue especially
ne CWA. Highs Mon-Wed are in the lower 90s with dewpoints back in
the low to mid 70s, giving afternoon heat indices in upper 90s to
around 105F. Heat advisory may be needed during those afternoons 
especially over southern CWA. The forecast builder brings a bit
cooler temps Thu/Fri with highs in the mid to upper 80s, with
se IL near 90 yet on Thu. Convection chances look more isolated on
Friday as upper level ridging nosing back toward IL. 

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1148 PM CDT Fri Aug 20 2021

A steadily approaching cold front will advance to near the
Iowa/Illinois border by 12z this morning. Some pre-frontal 
showers and storms will develop with support from a SW LLJ around 
30kts. Convection could reach SPI by 11z and the IL river 
valley/PIA toward 12Z. The line is still projected to gradually 
weaken as it moves toward I-55 Sat morning, leaving a break in the
rain chances from late morning into early afternoon. MVFR ceilings
are expected to develop early to mid-morning from west to east
with time, as the front slowly advances across central IL. An
improvement to VFR cloud ceilings is likely in the wake of the
front Saturday afternoon and evening. However, additional storm
chances are indicated for areas east of I-55, as the front re-
energizes in the heat of the day. Those storms could produce
strong winds due to moderate instability.

SE winds will remain in the 5-7kt range tonight, then become 
S to SSW at 7-10 kts Sat. 

&&

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

$$

UPDATE...Shimon
SYNOPSIS...07
SHORT TERM...07
LONG TERM...07
AVIATION...Shimon