Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 220441

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1141 PM CDT Sat Oct 21 2017


Issued at 305 PM CDT Sat Oct 21 2017

Strong cold front extended from the extreme eastern Dakotas to
central KS. Southerly winds were bringing a very warm and moist
airmass to much of the area east of the front. Temperatures were
well into the 70s to lower 80s with dewpoints in the lower to mid 60s.
There has been plenty of sunshine across the dvn cwa for much of today.

At 2 pm a line of thunderstorms had formed ahead of the front, and
extended from western MN to along the NE/IA border. More storms
were firing in eastern KS. Individual storms were moving northeast
at 40 mph as the line gradually shifts eastward.


.SHORT TERM...(This Evening through Sunday)
ISSUED AT 305 PM CDT Sat Oct 21 2017

Forecast focus on severe thunderstorm potential this evening and
into the overnight hours.

Tonight: NAMNEST/HRRR brings in a line of thunderstorms into our
far western cwa early this evening (about 7-8 pm) with the line
gradually moving eastward across the cwa overnight. Eventually the
line stratifies out into a large area of rain/embedded storms as
low pressure rides up along the front. Rainfall amounts of at
least 1/2 to 1 inch seem likely across much of the cwa, locally
higher amounts possible in stronger storms. The cold front will
arrive in our western counties after midnight. Lows will range
from the lower 50s far west to the lower 60s east.

SPC maintains the slight risk west of the MS River with a marginal
risk into our IL counties. As the previous shift stated in their
afd, strong dynamics, a jet entrance region developing over the
area, and strong low level shear will lead to a chance of severe
weather. Model consensus of surface winds staying up after sunset
and backing to east of south as the cold front approaches leads to
1km low level shear of 30 to 40 knots. More than enough for
tornadoes. CAPE is forecast to be near 800-1000 J/kg, so another
HSLC situation. In this case, the best vertical motion due to CAPE
is closer to the layer where the highest shear exists. If any
updraft can tilt this shear into the vertical, then there would be
a tornado threat, especially with embedded circulations within
the squall line. At the current time it appears that the best
chances for this are across the western cwa between 7 pm and
midnight. With low level winds increasing to around 45 to 50
knots, and a mixed boundary layer due to higher surface winds,
damaging winds will also be possible with these storms.

Sunday: The cold front will be pushing eastward, exiting our far
east by noon. Widespread rain/isolated storms should continue
across much of the cwa in the morning, gradually ending in our
eastern cwa in the afternoon as weak low pressure pushes off to
our northeast. Highs will range from the upper 50s to lower 60s.

.LONG TERM...(Sunday Night through Saturday)
ISSUED AT 305 PM CDT Sat Oct 21 2017

Cool Fall weather will dominant the entire extended forecast,
though the upper patter is active enough to bring some fairly wide
swings in temperature. The initial cold push Sunday night will bring
"normal" temperatures to the area, with lows in the lower 40s and
highs in the lower to mid 60s Monday. The cold advection will
increase Monday afternoon, driving a colder air mass into the
region, as noted by 850mb temperatures dropping to zero to -4 over
the area. This sharply colder air has not been felt thus far this
Fall, and the resulting blustery day Tuesday will be a shock to
those used to the current run of pleasant early Fall weather. Though
the blended solutions of data we`re using do not directly reflect it
yet, the pattern supports a risk of a headline frost in the Tuesday
night period, and beyond day 7, a hard freeze. This night seems
likely to have clear skies, but other this upcoming week are far
less likely to see frost as passing disturbances in northwest flow
bring some clouds and possible light rain showers to the area. Any
rain that occurs will be very light, and should mainly provide a few
chilly daytime hours as we wetbulb the air for a short period.



.AVIATION...(For the 06Z TAFS through 06Z Sunday Night)
ISSUED AT 1137 PM CDT Sat Oct 21 2017

A large area of showers and isolated embedded thunderstorms is moving
across the terminals overnight as a cold front passes. Periods of
mostly MVFR conditions can be expected, mainly with any precipitation
into the morning hours. Winds will shift from south to the west and
northwest at 5 to 15 mph by 3 AM at all the terminals. Skies will become
VFR with fair skies of high clouds by or before the afternoon hours.





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