Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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970
FXUS63 KDVN 181743
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1143 AM CST Sun Feb 18 2018

...HYDROLOGY UPDATE AND 18Z AVIATION UPDATE...

.UPDATE...
Issued at 1022 AM CST Sun Feb 18 2018

Dense fog has dissipated as expected over all but portions of far
northwest Illinois, where Freeport was still reporting a
visibility below one quarter mile at 10 am. GOES 16 snow/fog RGB
channel showed the extent of this fog well and trends suggest it
should be dissipating there by 11 am. As it appears rather
localized and dissipating, have allowed the remaining Dense Fog
Advisory to expire.

Strong winds were developing this morning over central and
northeast Iowa in response to a tightening pressure gradient and
strong pressure falls over southern MN. Just north of the
forecast area, Olwein reported a wind gust over 50 mph around 930
am this morning. While this appeared to be an outlier, sustained
winds of 20 to 30 mph with gusts over 40 mph look likely today and
will be monitoring closely for a possible wind advisory over
eastern IA.

&&

.SYNOPSIS...
Issued at 304 AM CST Sun Feb 18 2018

06Z surface data has high pressure from the western Great Lakes into
the lower Mississippi Valley. A frontal boundary ran from eastern
Lake Superior into northern Nebraska. Dew points were in the 20s and
higher from the Great Lakes to the Gulf Coast. The exception was
eastern Iowa and northern Illinois were dew points were in the teens
across a new snow field. north of the frontal boundary, dew points
were in the single digits and teens.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 304 AM CST Sun Feb 18 2018

The relatively warm air over the new snow field has resulted in
patchy dense fog to develop. This patchy dense fog will continue
through sunrise and a dense fog advisory has been issued for parts
of the area.

After sunrise, the fog will burn off due to daytime heating and
increasing winds. Once the fog burns off dry and warmer conditions
will be seen through sunset.

Tonight, moisture will move into the area allowing the atmosphere to
saturate. Based on atmospheric profiles, dry conditions should be
seen across the area through midnight.

After midnight, saturation of the atmosphere is achieved as strong
forcing moves into the area. Rain will develop and overspread the
area with the best chances along and south of I-80. As forcing
increases further, some embedded thunderstorms may be possible in
the pre-dawn hours south of a Peru, IL to Ottumwa, IA line.

.LONG TERM...(Monday through Saturday)
ISSUED AT 304 AM CST Sun Feb 18 2018

Seasonably quite mild into Tuesday with increased likelihood of
significant and much needed rains and some embedded thunder Monday
and Monday night the main issues with widespread 1+ inch amounts.

Long term forecast confidence assessment...fair or average.  The
main issue continues to be precipitation amounts with a slow moving
cool front Monday into Tuesday.  Phasing issues with forcing remain
the main challenge.  Best forcing tools suggest some moderate to
locally low end heavy rain totals expected with most locations to
receive .5 to 1.5 inches.

Monday and Monday night...very minor changes made. Above normal
temperatures with highs in the lower 40s NW to 50s central, with
low/mid 60s southeast sections. Periods of rain and isolated to
scattered embedded thunder. Reasonable estimate of .5 to over 1 inch
amounts NW to SE over 24 hours. Lows mainly in the 30s with some
upper 20s NW in far NW quadrant Tuesday AM behind cold front mostly
40s and lower 50s southeast of front in SE 2/3 quadrant of area.

Tuesday...poor confidence on how forcing and cold air interplay.
Best trends indicate cold air coming in slightly behind best forcing
limiting freezing and frozen precipitation.  This may take another
24 hours to better quantify. The default freezing rain risk on NW
winds is more likely if occurs to be changing to a mix of sleet and
light snow due to strong CAA.  Expect falling temperatures during
the day to below freezing everywhere by evening on blustery NW
winds. Any significant precipitation should move southeast or area by
late PM. Tuesday night night to bring clearing skies and lows mostly
in the teens by morning NW to lower 20s far SE.

Wednesday and Thursday...mostly dry with some light snow possible as
a weak disturbance passes.  Highs 20s and 30s with lows teens and
20s.

Friday and Saturday...near seasonable with highs 30s and 40s and
lows teens and 20s. Chance of light rain and snow as weak
disturbances passes with any suggested amounts light.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday Afternoon)
ISSUED AT 1142 AM CST Sun Feb 18 2018

Strong south winds this afternoon will gust as high as 30 to 35
kts. Otherwise, conditions will remain VFR. Tonight, fog will
become an issue as winds diminish ahead of an approaching frontal
system. This front will also bring drizzle, then showers and
possible thunderstorms going into Monday morning. All sites have
been trended with IFR and then LIFR conditions due to lowering
ceilings and visibilities. The probability of thunderstorms
affecting any given site was too low to include in the current
forecasts. Confidence is high that conditions will remain IFR
through late Monday morning, but confidence is low on trends with
possible dense fog and associated LIFR or even VLIFR, which could
linger through 18z at many sites.

&&

.HYDROLOGY...
Issued at 1142 AM CST Sun Feb 18 2018

River flood watches have been issued for the Rock River at Joslin
and Moline due to forecasts of moderate flooding at both
locations. These forecasts are based on the 48 hour rainfall
potential of 1 to 1.50 inches. These forecasts and products will
change based on both forecast and observed rainfall. Please see the
latest river forecasts and watches for the most up to date
information.

&&

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

$$

UPDATE...Sheets
SYNOPSIS...08
SHORT TERM...08
LONG TERM...16
AVIATION...Sheets
HYDROLOGY...Cousins



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