Area Forecast Discussion
Issued by NWS Central Illinois

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000
FXUS63 KILX 210501
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1201 AM CDT Mon Aug 21 2017

.UPDATE...
Issued at 1014 PM CDT Sun Aug 20 2017

Spotty thunderstorms are possible across central and southeast
Illinois overnight, although most locations will remain dry.
Otherwise, warm and humid conditions are expected. Made some
tweaks to PoPs for the expected trends, but most other adjustments
were very minor.

&&

.SHORT TERM...(This evening through Monday)
ISSUED AT 305 PM CDT Sun Aug 20 2017

Old outflow from an earlier MCS was moving east near I-57 at mid
afternoon and had an isolated thunderstorm along it in west
central Clay county. A few showers were in eastern Marshall and
Woodford counties while more widespread in northern/nw IL closer
to MCV over east central IA. More widespread thunderstorms were
over sw IA into northern/nw MO and in southern MO and ne AR. More
sunshine in eastern IL while more clouds west of I-57 behind
outflow boundary. Temps at 20Z/3 pm ranged from around 80F north
of Peoria to the lower 90s from Effingham to Lawrenceville south
where heat indices were topping 100F with muggy dewpoints in the
low to mid 70s. Dewpoints around 70F were as far north as
Galesburg and Lacon.

Latest forecast models still have varying solutions with handling
convection through Monday over IL. The MCV over east central IA is
forecast to track east toward Chicago in ne IL by late evening
with its swirl of convection mainly impacting northern half of IL.
Also have the system in northern/nw MO that tracks into sw IL
after sunset and to give southern counties some isolated
convection tonight. So have carried 20-30% pops over most of
central IL tonight with only isolated convection in southeast IL
until sunset due to old outflow boundary and very unstable
tropical airmass. SPC day1 outlook has marginal risk of severe
storms over all but sw 4 counties due to high CAPES above 1800
j/kg from IL se (as high as 3000-4000 j/kg in southeast IL) while
slight risk of severe storms was over IA. Muggy lows tonight
around 70F.

Convection chances increase to between 20-40% during the day
Monday with highest pops nw CWA over IL river valley. Another
very warm and humid day with highs in the upper 80s and lower 90s
and afternoon heat indices peaking from the mid 90s to near 100F,
highest in southeast IL by Lawrenceville. Sky cover during solar
eclipse around 18Z/1 pm Monday is 50-70% with least amount of
clouds in southeast IL and also the clouds will be higher in
southeast IL with filtered sunshine. Solar eclipse peaks in our
area around 120 pm cdt with 93-97% of sun covered by the moon.

.LONG TERM...(Monday night through Sunday)
ISSUED AT 305 PM CDT Sun Aug 20 2017

SPC day2 outlook has marginal risk of severe storms from I-72
north on Monday night with slight risk of severe storms over parts
of Knox, Stark and Marshall counties and to the nw. The marginal
risk of severe storms shifts to se CWA on Tue from Paris to
Shelbyville se while slight risk from Lawrenceville se during Tue
afternoon. A cold front will track se through central IL on Tue
likely bringing a round of showers and thunderstorms, and
lingering over southeast IL southeast of I-70 early Tue evening.
Highs Tue of 80 to 85F with warmest temps in southeast IL. One
more humid day on Tue with dewpoints in the upper 60s and lower
70s.

A nice stretch of late summer weather expected from Wednesday
through Sunday as large surface high pressure ridging from eastern
Canada southwest into IL, and upper level trof over the northeast
U.S. Expect below normal temperatures during most of this period
with highs in the upper 70s and lower 80s while fairly comfortable
dewpoints with east to ne flow around high pressure and giving
some cooler nights. 12Z extended forecast models are showing some
qpf se over the Ohio river valley next Sunday afternoon and also
some qpf with low pressure system moving into the upper Midwest,
with most of our CWA remaining dry yet on next Sunday. Some models
show temperatures warming into the mid 80s next weekend, but
stayed close to model consensus for temperatures along with dry
conditions.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1201 AM CDT Mon Aug 21 2017

Convection currently over northeast Illinois associated with a
leftover MCV from last night MCS over Iowa should move east out
of the area over the next few hours. The boundary that has been
planted across the forecast area the past few days remains evident
in the wind fields near I-74 at 04z. Some pooling of low-level
moisture along that boundary will likely enable some reduced vsbys
and will include as least some MVFR/IFR BR in KPIA-KBMI-KCMI
toward sunrise.

Convection has developed quickly over western Iowa this evening
associated with a weak wave and the development of the low-level
jet (30 kts at 925mb on KEAX and TMCI VWPs). As this system
moves/develops east it will likely move just north of the
aformentioned boundary eventually impacting central Illinois
terminals after 12z. Although the 00z NAM and 02z HRRR both have
the overall evolution expected they seem to be running a bit
behind temporally. Will also need to watch for the potential for
isolated convective devlopment along the boundary ahead of the
wave.

As LLJ dies after sunrise, trend should be weakening but residual
VFR clouds should remain over the area much of the day. Scattered
convection will likely persist during the afternoon fed by a very
moist airmass interacting with where the boundary lays out in the
wake of the wave. Latest CAMs suggest that the boundary may drop
south of terminals by mid-afternoon. This may be a bit far south
given its current position but that will depend on the strength of
the convection during the morning.

Stronger wave and associated cold front will be approaching the
area monday night. However, appears the terminals will see impacts
mainly after TAF valid time.

&&

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

$$

UPDATE...Bak
SHORT TERM...07
LONG TERM...07
AVIATION...Barker


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