Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 050539
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1139 PM CST Sun Dec 4 2016

...AVIATION UPDATE...

.UPDATE...
Issued at 950 PM CST Sun Dec 4 2016

The potential for dense fog is looking rather murky this evening
as stratus remains in place along with a sufficiently strong, and
generally unfavorable, low level SW flow at the surface. Latest
satellite fog channel images show the back edge of the stratus in
central IA roughly from Waterloo to Lamoni making very slow
eastward progress. So far, fog has been mainly confined to the far
east, closest to the weaker wind fields and lingering moisture
near the departing trough to the east, while areas of MVFR type
fog is widespread along and west of the clearing line in central
Iowa.

Will continue to monitor trends closely, as fog will be possible
in the enhanced radiational cooling behind the slowly eastward
spreading clearing line, especially where it reaches the deeper
fresh snow cover. Short term high res models and the SREF aviation
fog probs continue to target this area, especially over eastern
IA after midnight through mid morning Monday. Have adjusted fog
in the forecast for overnight, keeping patchy wording early, then
widespread late, but will hold off on mentioning dense fog until
trends suggest otherwise. Have also adjusted mins upward some as
the combination of light SW winds and widespread dewpoints in the
30 to 32 range over the entire region should keep temperatures
nearly steady through the night.

&&

.SYNOPSIS...
Issued at 309 PM CST Sun Dec 4 2016

A strong over lapping of mid level forcing and lower level FGEN
produced a warning criteria heavy snow event today over our central
CWA. Amounts within 40 miles of Interstate 80, mainly east of Iowa
City metro, saw totals of 6 to 10 inches. Outside of this band, to
the west, north, and south, 3 to 6 were common totals, with about 2
to 3 inches found in our far southwest.
ERVIN

&&

.SHORT TERM...(This Evening through Monday)
ISSUED AT 309 PM CST Sun Dec 4 2016

The snow event will end in the east around 3 PM, and I will end the
winter headlines at that time. With no wind or icing following up
this event, we can call it over.  That said, the big pile of
snowpack may continue to plague forecast challenges now for a few
days. Tonight, models and satellite suggest clearing by mid evening.
That is of low confidence, but with saturated low levels and a fresh
snow pack, dense fog and low clouds are both likely to develop later
tonight. I will not issue a new fog headline yet, in order to clear
up the winter storm, but one may be needed by mid evening.

Tomorrow, models are warm, but we have deep snow. That`s a problem,
and I have started by undercutting guidance highs and increasing
clouds. Given the event today, and the models not likely handling
the affects of it on the boundary layer, the next 24 hours is of
poor confidence, but going cooler than guidance usually pays off.
ERVIN

.LONG TERM...(Monday Night through next Sunday)
ISSUED AT 309 PM CST Sun Dec 4 2016

Monday night and Tuesday...Bulk of 12z runs roll out main sfc low
far enough to the southeast acrs the TN valley or southern OH RVR
Valley, that it`s def zone just barely clips the far southeastern
and eastern CWA late Mon night and Tue morning, or misses it
entirely. If it manages to make it acrs portions of the local area
by early Tue morning, thermal profiles support a wintry mix
potentially dynamically cooling to all wet snow...but amounts would
be light. Then a strong ridge of Canadian high pressure will nudge
it`s way down the plains and into the mid MS RVR Valley. Highs Tue
in the 30s, and high clouds streaming in off the next system acrs
the west central plains late Tue night will prevent lows from going
colder than the upper teens to lower 20s. Otherwise the fcst lows
for Tue night/early wed morning will not be cold enough.

Wednesday through Friday...Assessing the latest run/12z medium range
model runs, it appears a isentropic overrunning precip/snow event
possible along leading edge of the main arctic airmass intrusion
down into the plains and upper Midwest. The majority of the models
suggest the DVN will be in-between the phasing processes and thus
the precip zones this period. Only the 12z ECMWF brings snow acrs
the CWA from west-to-east Wed afternoon and especially Wed evening,
with 1-3 inches possible of finer/more powdery of a snow type by Thu
morning. For now will pivot low to moderate CHC POPs acrs the
southern into east central CWA Wed afternoon and into Wed night and
await more model consistency in handling the southern stream wave
energy interaction with upper trof axis further to the north.
Again, if the snow mainly misses us to the south, the main story for
the mid to late week period will be the arctic air mass producing
sharply colder conditions. Highs in the 20s or even teens, with lows
well down in the teens and even some single digits possible. Wind
chills in the single digits both above and just below zero for both
Thu and Fri mornings as well. Could be even colder if arctic surge
winds are stronger that what the models give them credit for.

Next Saturday and Sunday...longer range indications in the latest
medium range models suggest somewhat flattening mean steering flow
will usher acrs another wave, with it`s associated return
flow/moderating temp wing producing a band of snow somewhere acrs
the Midwest Sat into Sunday. But models continue to vary on how far
north or south this feature will be, as well as the amount of
moisture it will have to utilize to produce certain snow amounts.
And they will continue to do so for the next several days. So for
now, will go with low to moderate CHC POPs for mainly light snow
from late Sat into early Sunday.   ..12..

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday Night)
ISSUED AT 1139 PM CST Sun Dec 4 2016

Confidence in the potential for dense fog tonight is lower than
in the previous forecast cycle which is reflected in less
pessimistic conditions in the latest forecasts. Widespread IFR
conditions, primarily due to low ceilings, are likely to remain in
place overnight through at least the morning hours Monday. A
general trend of visibilities lowering through MVFR range, then dipping
into IFR for several hours due to fog remains in place, but there
is not enough confidence to mention VLIFR conditions. South winds
developing on Monday should lead to VFR conditions by afternoon,
but lingering stratus and MVFR conditions cannot be ruled out.

&&

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

$$

UPDATE...Sheets
SYNOPSIS...Ervin
SHORT TERM...Ervin
LONG TERM...12
AVIATION...Sheets



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