Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 172349
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
549 PM CST Sun Dec 17 2017

...00z AVIATION UPDATE...

.SYNOPSIS...
Issued at 315 PM CST Sun Dec 17 2017

An area of light rain and drizzle continued into the mid afternoon
hours, along and south of Interstate 80. There were only a few
brief peaks at the sun across the area. Low clouds and light fog
lingered along the Highway 20 corridor, where temperatures were
held down in the mid to upper 30s. To the south, temps were in the
upper 30s to upper 40s.

&&

.SHORT TERM...(This Evening through Monday)
ISSUED AT 315 PM CST Sun Dec 17 2017

The main forecast issues in the short term period involve
fog/drizzle intensity and coverage, and the potential for some
patchy freezing drizzle north.

Only spotty light rain is expected into the early evening hours,
with mainly drizzle overnight into early Monday morning.
Visibility progs of several CAMS and the SREF focus on the north
and east for more fog development tonight. This is reasonable
given the northerly track of the weak surface wave and the
location of a weak trailing convergent axis. With winds becoming
W/SW, do not believe we will have any widespread dense fog.
Surface temperatures will be warm enough to keep precipitation as
drizzle most areas. The exception, based on the current forecast,
is the DeWitt area, north to Dubuque, and east through the Galena
and Freeport areas. Some light freezing drizzle is a possibility,
and might linger long enough to impact a portion of the Monday
morning commute in the north.

Any fog/drizzle should end by late Monday morning, followed by
increasing SW winds and another unseasonably mild day. Expect
highs in the low 40s northeast to low 50s southwest.

.LONG TERM...(Monday Night through Sunday)
ISSUED AT 315 PM CST Sun Dec 17 2017

Monday night through Wednesday...General flattened/westerly flow
regime acrs the mid CONUS this period, with embedded weather makers
moving well south and to the north of the area for a dry and
seasonably mild regime. Mon night-early Tue morning lows near or
above the normal highs for that time of year. Dry FROPA on Tue to
bring about a cooler night Tue night, along with somewhat cooler
daytime highs Wed with east to southeasterly sfc flow...but still
well above normal values.

Thursday and Friday...The latest suite of 12z medium range models
continue with a northwest trend of handling the storm system/main
ejecting sfc low out acrs the eastern plains and up toward the
western GRT LKS this period, with the GFS jumping to the furthest
northwest and eventually deeper/slower solutions by late Friday. But
the general idea/ensemble mean of the solutions suggest that Thu
could be an initially dry and mild day, with mainly rain and
sprinkles breaking out north of I80 during the afternoon. Thu night,
the weaker more progressive ECMWF suggests mainly light rain and
sprinkles or drizzle, with light amounts into Fri morning. Some post
wave wrap around cooling may produce a rain-snow mix in the northwest
toward daybreak. The slower GFS keeps the CWA in the warm sector of
pre-wave sfc trof into Fri morning, again with mainly liquid precip
at the sfc.

All this while these models have shifted the accumulating snow axis
from southeast SD, through MN and northeast WI. The Euro just has
some patchy light rain Fri morning then dry slots the area behind
departing wave, but the far east and southeastern CWA may have some
overrunning post-frontal rain into Fri evening. The 12z GFS
strengthens the low acrs the western CWA Fri before it shuttles off
into WI By Fri evening, producing a line of light to moderate rain
showers acrs much of the area before signs of a dry slot brings a
temporary precip reprieve by mid afternoon. The GFS does clip the
far northwestern CWA with wrap around light snow of a tenth or two
Fri evening, but blustery west to northwest winds will be the main
Fri night story if the GFS is correct on timing and depth of the
departing sfc low. Confidence still on the low side regarding this
end of week system, but the trends have been noted. For now will
continue with a rain becoming a rain-snow mix from northwest-to-
southeast Thu night into Fri morning, with a mixed precip scenario
hanging on while precip lasts into Fri. But the latest model trends
suggest it will be a mainly liquid precip event Thu into Fri for the
local area. Also feel that the post-frontal precip fields will push
east and southeast of the CWA by the time it top-down cools enough
to turn the western edge of those fields into wintry precip
production parameters during Friday night.

Saturday and Sunday...Longer range indications suggest a mainly dry
and colder day Saturday, as a large scale pattern flux acrs the
North American continent commences. An increasing arctic air mass
entrained long wave trof is fcst by the medium range models to
engulf much of the western half or more of the CONUS by late Sunday,
while a strong polar upper low with underlying -30C or colder H85
frigid pool looms just north of the border acrs the plains of south
central Canada. In the adjustment, the latest model runs align
southwest-to-northeast LLVL baroclinicity from the southern plains,
to the eastern GRT LKS by Sunday.

Both the 12z GFS and ECMWF now suggest a overrruning or elevated
frontogenetical snow event to funnel up along this thermal ribbon
Sunday into Sunday night. Much will depend on what moisture this
zone will have to utilize for snow production especially with more
significant precip bands further to the southeast from the lower MS
RVR Valley and up along/south of the OH RVR Valley. But the models
are showing a long duration snow event possibly of 24 hours that
could utilize high LSR`s for several inches of snow to eventually
pile up in bands somewhere in or near the area. The GFS may be too
wet, but it advertises higher end advisory or even winter storm
warning snow amounts by Monday morning acrs the northwest 2/3s of
the DVN CWA, but again that is over a long duration. The drier ECMWF
is a touch further southeast with it`s snow axis and has half the
snow accum amounts that the GFS does. There is still plenty of time
for the models to change this scenario, but moderate CHC POPs for
snow will need to be added to the fcst for Sunday and Christmas Eve.
An intense cold regime being advertised by the latest run medium
range models for the upper mid CONUS, lurking just past Christmas
with unbelievable sfc high pressure strengths into the northern
Rockies and northwest high plains.   ..12..

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday Evening)
ISSUED AT 534 PM CST Sun Dec 17 2017

Conditions to become predominately MVFR and IFR tonight in low
clouds and fog, with some light rain and drizzle mostly this
evening. Some pockets of LIFR/VLIFR conditions are possible, but
too much uncertainty on development and spatial extent precludes
from any mention at this time. Overnight, a developing southwest
wind and weak ridging aloft will likely shunt much of the drizzle
east of the terminals. This will be closely monitored as if the
drizzle should linger late tonight then it could bring the
possibility of freezing drizzle at CID/DBQ. Strengthening and
deepening southwest winds on Monday should entrain enough dry air
to allow for a decrease in stratus and return of VFR conditions
from late morning through mid afternoon.

&&

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

$$

SYNOPSIS...RP Kinney
SHORT TERM...RP Kinney
LONG TERM...12
AVIATION...McClure



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