Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 090257 CCA
AFDDVN

Area Forecast Discussion...CORRECTED
National Weather Service Quad Cities IA IL
857 PM CST Fri Dec 8 2017

.UPDATE...
Issued at 838 PM CST Fri Dec 8 2017

Snow showers diving quickly from MN into far northeast IA into
ARX cwa. ARX radar is offline and so that is why they have
appeared to dissipate, and also due to these returns largely
below beams of neighboring radar sites being at much greater
distances. These snow showers appear to be tracking a bit more
toward the south in line with some of the latest guidance on
digging PV anomaly and 850 mb frontogenetical forcing. Therefore,
I have expanded PoPs a bit further west/southwest with the evening
update. These snow showers will enter our northeast IA counties
roughly after 9 pm through 10 pm then continue sweeping SSE
exiting cwa by around 3 am, with bulk of snow showers occurring
along and east of line from Vinton to Iowa City to Macomb. These
snow showers will last less than an hour, thereby limiting
accumulations from a trace to less than 0.5 inch. Visibility may
be reduced to around 1 mile in a few locations in the stronger
snow showers.

After the snow showers depart the attention will turn to the
winds. Strong CAA in wake of departing cold front will aid in
steepening of low level lapse rates, with some of the latest
data showing around 40 kts or so sitting atop the mixed layer.
This could result in winds gusting potentially near advisory
criteria for a several hour period overnight. Flurries may also
persist into Saturday am, as any stratus will reside in deepening
dendritic growth zone.

&&

.SYNOPSIS...
Issued at 330 PM CST Fri Dec 8 2017

Current water vapor loop was indicating tonight`s clipper system
currently embedded in steep northwesterlies over the eastern Lake
Winnipeg region of Manitoba. Pre-system brisk southwesterly sfc
winds and more sunshine than earlier expected, helping afternoon
temps warm into the upper 30s to near 40 acrs much of the southern
2/3`s of the area. The High amplitude west coast ridge and mid CONUS
northwest flow pattern to continue over the weekend and into much of
next week...leaving the area prone to occasional clippers and weak
waves embedded in this flow pattern dropping over or adjacent to the
local area. Also an occasional light precip and cold regime.

&&

.SHORT TERM...(This Evening through Saturday)
ISSUED AT 330 PM CST Fri Dec 8 2017

Tonight...Suite of 12z run models in general agreement of suggesting
low to mid level low/upper trof with the upstream clipper system to
drop acrs the central into southern LK MI region by Sat morning. The
best synoptic scale lift combined with whatever saturation/ moisture
convergence it can utilize, to occur on it`s left flank and thus well
east of the local area.  But a decent wing of upward omegas sweeping
into the area tonight will utilize even marginal/meager moisture
dragged down with the system, along with rather high liquid to snow
ratios of 15:1 to near 20:1, to produce from a tenth of an inch, to
0.6 to 0.8 tenths of an inch of powdery snow along and generally
east of a line from Independence IA, down through the Quad Cities,
and to east of Galesburg IL. The higher amounts still expected to
occur in northwest IL from just west of Freeport IL, down to Kewanee
and Princeton IL.

Besides some lead flurries, for timing expect the clipper snows to
move into the northern CWA/HWY 20 corridor after 9 or 10 PM,
spreading southeastward toward Clinton IA and MT Carroll IL by
midnight, and points east of the Quad cities/eastern I80 corridor
through 2-3 AM. Most organized snows moving out of the CWA by 3-4
AM. Again most of these areas to get from a dusting to a half inch,
with an isolated 1 inch amount possible in the very far eastern and
northeastern CWA by 12z Sat morning. But with this system rolling
into and drawing upon such a dry airmass, low confidence on snow
amounts and hesitant to go with higher amounts that the lift of this
system supports. The other problem is with the expected post-frontal
northwestern wind switch and isallobaric surge up to 15 to 25 MPH
and gusts over 30 MPH, these winds will blow any powdery
accumulations around and make it hard to get a true measurement. In
fact, there may be a 2-3 hour window where winds approach wind
advisory criteria in spots. Late night cold air advection to make
for lows well down in the 20s, with even some upper teens possible
by 12z Sat.

Saturday...Cold blustery post-clipper day on tap, with not much temp
recovery from morning values making for highs in the mid 20s to
around 30 in most spots. Expect a morning mixture of clouds and sun
with even some lingering flurries out of stratocu clumps or rows
aligned in the wind fields, to clear for more sunshine by afternoon.
Morning single digit wind chills with northwest winds 15 to 25 MPH
and gusts over 30 into early afternoon, before they start to relax
toward late afternoon/evening lower sun angle period.   ..12..

.LONG TERM...(Saturday Night through next Friday)
ISSUED AT 330 PM CST Fri Dec 8 2017

The main forecast issues in the long term period continue to revolve
around timing and track of several clipper systems.

Saturday night into Sunday, a surface trough will push through the
region. There may be enough moisture to squeeze out a few flurries
early Sunday, but most areas should remain dry. Milder air from the
plains will overspread the area, with highs in the mid 30s to mid
40s, slightly above normal for the date.

Sunday night and Monday, a weak clipper tracking northeast of the
area may bring some light snow or rain at times, depending on
boundary layer temperatures. The far north and far east would be the
favored areas, but most locations will be dry.

Looking ahead, Tuesday will likely be the coldest day of the week,
with another cold air surge keeping highs mostly in the 20s.
Temperatures will gradually warm Wednesday through Friday. There
will likely be some periods of light snow/flurries, but trends
remain very difficult to pin down that far out. The 08/12z GFS is
the most active solution for the forecast area. Right now, we are
carrying low pops for Wednesday evening, as well as Thursday and
Friday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday Evening)
ISSUED AT 546 PM CST Fri Dec 8 2017

Clipper system to pass east of the terminals, as it moves quickly
through the western Great Lakes region into Saturday AM. Attendant
cold front will pass through the terminals late this evening and
overnight accompanied by flurries and a chance of snow showers.
DBQ will have greatest chance at seeing snow showers and potential
for light accumulation, with lower chances at CID and MLI and just
flurries for BRL.

Ceilings... VFR early evening, then becoming MVFR late evening and
overnight except maybe at BRL. Varying low VFR and MVFR sct-bkn
coverage Saturday AM with cold advection and surface heating, with
VFR expected Saturday afternoon.

Precipitation... Flurries and snow showers by late evening into
the early overnight (04z-09z). Best chance for snow showers is at
DBQ with lower chance at CID and MLI.

Visibility... VFR for much of the TAF period. Pockets of MVFR
(3-5SM) in snow showers 04z-09z, with potential for isolated
visibilities of around 1SM in the strongest snow showers.

Winds... southwest around 10-15 kts through mid evening. Becoming
gusty northwest late evening through much of Saturday at 15 to 25
kts, with period of gust potential to around 35 kts overnight and
Saturday AM.


&&

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

$$

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


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