Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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297
FXUS63 KDVN 130520
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1120 PM CST Tue Dec 12 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 330 PM CST Tue Dec 12 2017

The latest water vapor loop was indicating the next clipper wave to
impact the region rolling southeastward acrs Alberta, with deep cold
core L/W trof aligned acrs the eastern CONUS. Steep northwest flow
on the western flank of this trof to shuttle the clipper down acrs
the western GRT LKS on Wed.

&&

.SHORT TERM...(This Evening through Wednesday)
ISSUED AT 330 PM CST Tue Dec 12 2017

Tonight...Elevated warm air advection wing to the southeast of this
clipper system/vigorous vort complex, to drop in from the northwest
tonight with increasing cloud cover out of MN/Dakotas. Backing H85
MB flow of 40-50 KTs to surge warm air aloft of +3 to +8 C acrs the
CWA late tonight into early Wed morning. Some chance of flurries
getting generated out of this cloud deck, but will bank on dry LLVLS
to limit much of anything reaching the surface. But will go with at
least a slight CHC of snow in the NW IL-Freeport area after 3 AM for
the potentail of the southeast portion of the lead snow deck
streaking acrs MN and WI to clip these areas late tonight. Will have
to monitor for some of this snow making it even further to the
southwest. 12 hr low temps will likely occur this evening a few
hours after sunset, before backing sfc flow to the west-southwest
increases into the overnight for non-diurnal temp trends into the
late night-early morning hours.

Wednesday...Latest model runs and SREF suite suggests the main
clipper sfc low pushing 997 MB acrs far southeastern MN at 12z Wed,
dropping southeastward to just southwest of Rockford IL by 18z
possibly deepening below 995 MB on the way...a rather strong sfc
low. the system then progged to shuttle away quickly acrs
northeastern Indiana by 00z Thu. This path and mid level forcing keep
a strip of significant snow accums well to the northeast of the DVN
CWA acrs WI into lower MI late tonight and Wed. Some concern still
for a further southwest jog of the main clipper low pulling toward
the stong vort lobe aloft to the southwest, as well as southwestern
upper jet direct circulation effect. But even if it progresses/tracks
as currently progged, a stong satellite vort lobe and upper jet
energy will look to pass further to the southwest and acrs the local
area. Thus a strong passing lift/pos omega swath to drive a band of
wraps around precip acrs the CWA from northwest to east-southeast
from mid morning through mid afternoon. The clipper will initially
draw warm air up acrs the area Wed morning with areas generally
along and south of I80 possibly getting into the 40s. But then
dynamical and evaporative cooling will produce falling temps as the
afternoon progresses.

As for the precip type, early expectations are initially  snow in the
northwest as it arrives, with a mix or even periods of all rain
along and south of I80 during late morning and midday. But similar
to yesterday`s precip trends, a good chance that the same dynamical
and evaporative cooling processes turn much of this precip band into
mainly wet snow. The extent of lift and saturation in the dendritic
layer suggests that the band of snow may produce a quick half inch
to 1 inch of snow accumulation as it passes through, with the speed-
progression of the band and coverage orientation limiting much of
any more accumulation than that...less pivoting over the same area
to allow higher snow accums like what happened yesterday in NW IL.
But if the clipper comes further southwest and swings east, will
then have the potential for bands producing more accumulation than
those minor amounts discussed earlier. The passing band to possibly
temporarily reduce visibilties and produce quick slush accums on
roads for localized slick spots. Most of the snow of impact will
look to move out of the area by 2-3 PM, with just lingering flurries
into the late afternoon hours.

Besides possible slick spots and quick snow accumulations with
passing the snow band/s, the other main weather story will be the
strong winds wrapping in behind the departing clipper/tight pressure
gradient from midday and especially into the afternoon. Could see
west to northwest winds increasing to 25 to 32 MPH sustained, and
gusts up to 45+ MPH. Areal coverage still at question but some signs
of the stronger winds north of I80. Will hold off any wind headline
issuance for now and allow the night shift to assess one last model
run to fine tune where an advisory should be, or if one will be
needed at all. If the snow were to last longer, could see wrapping
the concern of the winds and snow impacts into a winter weather
advisory for portions of the CWA.    ..12..

.LONG TERM...(Wednesday Night through next Tuesday)
ISSUED AT 330 PM CST Tue Dec 12 2017

Overview: Storm track over the next 7 to 10 days expected to remain
north, east and south of the dvn cwa. These storm system`s should
remain far enough away from the cwa that only minor accumulations,
if any, are expected.

Temperatures will be normal to above normal during this extended
period with several days in the 40s.

The system to keep an eye on is the potential for a somewhat more
significant storm in the southern stream for this Sunday.
However, low confidence this far out as it`s all about the
phasing. The GFS/Canadian models are run-to-run consistent in
keeping this system well to our south, leaving the cwa totally
dry. The ECMWF model has been flip-flopping on the track with the
12z/11 run impacting the dvn cwa with accumulating snow, the
00z/12 run much farther south and keeping things dry, and this
morning`s run 12z/12 grazing our se cwa with some snow
accumulation. This will be monitored as the week progresses.

Haase

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday Night)
ISSUED AT 1115 PM CST Tue Dec 12 2017

South winds to southwest winds will increase the rest of tonight,
with clouds at all sites should be above 5000 ft AGL. Towards
morning, a strong low pressure system will quickly move southeast
through the region, bringing snow to locations mainly northeast of
Dubuque. Some isolated snow showers and rain showers are possible
over eastern Iowa and northwest Illinois, but at this time,
confidence is low, due to most model guidance keeping the system
too far northeast to produce snow this far south. That said, as
winds turn northwest and increase to 20 to 30 kts, possibly higher
Wednesday late morning, there could be a brief flurry or snow
shower. By sunset tomorrow, winds will decrease to under 20 kts.
As cold air arrives after 18Z, some MVFR cigs are likely in the
CID and DBQ regions, but farther south, this is less likely, and
VFR cigs are forecast for now.

ERVIN

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...12
SHORT TERM...12
LONG TERM...Haase
AVIATION...Ervin



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