Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 162317

Area Forecast Discussion
National Weather Service Quad Cities IA IL
517 PM CST Sat Dec 16 2017


Issued at 315 PM CST Sat Dec 16 2017

High clouds drifted across areas along and north of Interstate 80
through the early afternoon hours. A southwest breeze boosted
afternoon temperatures into the 50 to 55 degree range over much of
the area. Locations along the Highway 20 corridor east of
Independence were in the 40s.


.SHORT TERM...(This Evening through Sunday)
ISSUED AT 315 PM CST Sat Dec 16 2017

The main forecast issues in the short term period are potential
for localized dense fog overnight, and possibly some slick spots
from fog freezing on roadways.

In the near term, winds and temps will drop off quickly this
evening. A front will extend across the forecast area, with a
light northeast wind along and north of I-80 and a light south
breeze south of I-80. Favorable northeast flow and low level
moisture aided by melting of lingering light snow cover will focus
the main fog potential along and north of Highway 30. Localized
dense fog would be favored in far northwest Illinois where the
most snow occurred recently. Any dense fog that occurs would
likely combine with the sub-freezing temperatures to produce a
thin layer of ice on roads, making for a slick morning commute.
Will add a mention of these potential threats in the Hazardous
Weather Outlook, with later shifts monitoring for any significant

Attention then turns to the compact H5 wave that will track
through the region during the day Sunday. Precipitation, mainly in
the form of light rain, will be favored south of Highway 30 with
the highest QPF likely south of I-80. There may be a light mix of
precip along the northern fringe, but any that does occur should
be brief with no significant impacts. With the increasing cloud
cover, have continued favoring the lower end of temp guidance.

.LONG TERM...(Sunday Night through Saturday)
ISSUED AT 315 PM CST Sat Dec 16 2017

Sunday night and Monday...The main sfc boundary to sink southward
and settle along and south of the I80 corridor as Sunday night
progresses, but placement and progression dependent on any
additional sfc wave rippling along it, if even a minor/weak one.
Light moisture laden convergent LLVL flow in the VCNTY of this
feature to drive areas of fog, and most of the latest fcst soundings
suggest just enough moisture depth to allow for at least some spotty
drizzle getting wrung out at times overnight. Expect low clouds and
patchy to areas of fog to plague the entire CWA into Mon morning. If
drizzle persists far enough northward along and north of I80 after
midnight when temps may dip to or below freezing, there may be some
freezing drizzle issues in those areas that may linger toward the
Monday morning commute. Will have to focus on this issue/possibility
in later shifts. There is the possibility that the american models
are too deep in moisture depth and thus too bullish on the drizzle
production. After the morning "grunge" of clouds and possible
drizzle breaks up, warming southwest sfc winds to make for a
seasonably mild afternoon with highs in the mid 40s to lower 50s
acrs much of the area...some uncertainty though of an afternoon
clear out.

Tuesday and Wednesday...Another mild day Tuesday before a temporary
chill down Tue night into early Wed. Then flattened out westerly
steering flow acrs the mid CONUS to lee of digging intermountain
west upper trof make for a split system flow into early Thu. One
system shuttles east well to the south of the region, while LLVL
cyclogenesis blossoms over the southern Rockies into southwestern
plains. The other system in the form of a strong elevated warm air
advection signal and nose of southerly moist conveyor, acrs the
northern plains into MN/northern WI Wed night. The slower and
adjusted north 12z ECMWF is dry acrs the DVN CWA through 12z Thu,
while the 12z GFS clips the far northwestern CWA with some WAA snow
or wintry mix late Wed night/early Thu morning. Milder night on tap
with non-diurnal temp trends possible as the area becomes further
entrained in the developing western plains system`s warm sector.

Thursday through next Saturday...The latest suite of medium range
models continue with phasing and handling descrepancies of the main
pieces of wave energy that will look to eject out in southwest flow
toward confluence, lee of whatever trof phasing occurs acrs the
eastern Rockies and plains early in the period. The 12z GFS is much
further south than the fresh 12z ECMWF run, with 06z Fri sfc low
placement near St Louis, as opposed to the ECMWF having it near
Burlington IA. The GFS then tracks to the main sfc low to the
eastern basin of LK Erie, while the 12z Euro is acrs the west
central coast of lower MI at the same time. The net results are that
the GFS forcing and track supports accumulating snows at least of
advisory levels acrs much of the northwestern 2/3`s of the DVN fcst
area, while the 12z ECMWF further northwest pull out of the low has
just marginal accumulations acrs the northwestern third of the CWA by
mid Friday morning. The 12z GEM low placement and timing more
resembles the ECMWF. Despite the discrepancies making for some
uncertainty/low confidence on snow amounts and placement, the signal
has been there that some type of system will affect portions of the
local area and will have to keep rather high POPs from later Thu
into Fri morning for now. Longer range upper jet placement and mid
to later December low track climatology slightly favors the latest
ECMWF track.

Initial cold dump out of Canada looks to be in store acrs the
western GRT LKS by this weekend, under developing deep cold core L/W
trof pattern acrs the mid CONUS. This after whatever system rolls
through the Midwest on Thu/Fri.  A clipper regime with re-enforcing
potentially very cold shots may be on tap through the early next
week Christmas period.    ..12..


.AVIATION...(For the 00Z TAFS through 00Z Sunday Evening)
ISSUED AT 508 PM CST Sat Dec 16 2017

Couple of forecast features to focus on for the 00z TAFs. First is
potential for fog and low clouds advecting into areas mainly
along/n of Hwy 30 late tonight through Sunday morning. Highest
confidence is at DBQ and maintained IFR conditions, although
LIFR/VLIFR is possible after looking at some of the latest hi-res
guidance. Added MVFR conditions to CID developing by mid to late
Sunday morning as northeast flow develops. The second focus is
with system ejecting from the Southern Plains on Sunday spreading
light rain across portions of Eastern IA, Northeast MO and West
Central IL during the day. The best coverage is expected along and
south of I-80 therefore have continued to hit BRL TAF site the
hardest with prevailing light rain and MVFR conditions.




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