Area Forecast Discussion
Issued by NWS Central Illinois

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284
FXUS63 KILX 250200
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
900 PM CDT SAT SEP 24 2016

.UPDATE...
Issued at 900 PM CDT Sat Sep 24 2016

A high pressure ridge axis aloft remains over central IL this
evening promoting subsidence and keeping skies clear this evening
after dissipation of diurnally driven cumulus from this afternoon.
Afternoon dewpoints in the mid to upper 60s across the area
indicate enough moisture for at least patchy fog tonight given
expected lows in the mid to upper 60s. Have updated sky cover
through late tonight for clearer skies in the subsident region
well ahead of the cold front approaching from the central Plains,
followed by increasing clouds mainly west of I-55 before dawn.

&&

.SHORT TERM...(This evening through Sunday)
ISSUED AT 258 PM CDT Sat Sep 24 2016

A strong upper level ridge over the MS river valley including
IL is providing another very warm and somewhat humid summerlike day,
with temps in the mid 80s to near 90F and dewpoints in the 60s.
Scattered diurnally driven cumulus clouds with bases of 5-7k ft over
CWA this afternoon should dissipate around sunset today leaving fair
skies much of the night. Light winds combined with fairly moist
boundary layer will allow some fog to form again overnight spreading
from ne to sw across CWA and lift by mid Sunday morning.
The fog could get locally dense in spots over southeast IL where
winds will be lightest. Lows overnight should be in the mid to upper
60s with coolest readings in east central IL.

A 1004 mb surface low pressure over west central ND has a cold front
extending southward into western Nebraska/SD. The 12Z models bring
some qpf into far western/nw CWA late tonight where slight chance of
showers and thunderstorms, but much of area will stay dry thru the
night. A 564 dm 500 mb low along the eastern MT/WY border will lift
into southern Manitoba by Sunday afternoon and push the cold front
east into the IL river valley late Sunday afternoon. Will see
increasing chances of showers and thunderstorms from west to east
during the day Sunday, though far eastern/southeast IL should stay
dry until Sunday evening. One more warm summerlike day in the mid to
upper 80s (except lower 80s by Galesburg) on Sunday and dewpoints in
the mid 60s to near 70F. SPC day2 continues a marginal risk of
severe thunderstorms with 5% risk of hail and gusty winds north of I-
72 Sunday afternoon.

.LONG TERM...(Sunday night through Saturday)
ISSUED AT 258 PM CDT Sat Sep 24 2016

The 12z model runs are showing a slightly slower progression of the
cold front Sunday night. There is good agreement between the
GFS/ECMWF/GEM/NAM on showers continuing Sunday evening in a majority
of our forecast area, especially east of I-55 where we went with
likely PoPs. Thunder potential will wane quickly Sunday evening, as
the front departs into Indiana and instability params weaken
considerably by 03z. Showers appear likely east of I-57 after
midnight, while rain ends west of I-55.

Some post-frontal showers could linger into Monday morning southeast
of I-70, but all rain should end in our southeast counties by mid-
day. Much cooler air will limit highs on Monday to the upper 60s N
of I-74, with low 70s in the rest of our forecast area. The occluded
surface and upper level low pressure systems north of Lake Superior
will drift southeast Monday night through Wednesday, allowing the
cooler air mass to remain entrenched in Illinois due to deep layer
NW flow. Highs may climb a couple degrees each day on Tuesday and
Wednesday, but still remain in the low to mid 70s. Low temps will
dip into the upper 40s to low 50s each morning.

Instability showers due to steep mid-level lapse rates should
remain northeast of our counties through that period, even as the
upper trough deepens just east of Illinois Wednesday into Thursday.
The ECMWF cuts off a low in the base of the trough on Thursday,
while the Canadian and GFS are a little more progressive and shift
the trough toward the East Coast. The later consensus was followed
by the model blend, which should coincide with colder air shifting
east of Illinois. However, the increase in high temps on Thursday
and Friday was decreased slightly from previous model runs.
Southerly surface winds may not become established until Saturday,
delaying the arrival of warmer conditions until next weekend. High
temps even on Saturday are only projected to reach the mid to upper
70s. The slower onset of return flow winds will also set the stage
from dry conditions from Monday afternoon through next Saturday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 641 PM CDT Sat Sep 24 2016

Scattered cumulus clouds will dissipate around sunset 00Z/7 pm
leaving fair skies this evening. Patchy fog continues to look
likely overnight as increasingly moist conditions develop with
clear skies aloft. Have continued MVFR vsbys after 06Z and tempo
groups down to 1-1.5 miles with scattered clouds below 1k ft. A
cold front over the central Plains will approach central IL from
the west Sunday morning with showers and thunderstorms ahead of
the front. Added shra/vcts in TAFs starting 17Z at KPIA to as late
as 23Z at CMI for this approaching feature. Winds gradually
turning southerly through Sunday morning...with speeds increasing
to around 10 kts and gusts 15-18 kts Sunday afternoon.


&&

.ILX Watches/Warnings/Advisories...
NONE.
&&

$$

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



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