Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 181621
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1121 AM CDT Wed Apr 18 2018

...HYDROLOGY UPDATE...

.UPDATE...
Issued at 1121 AM CDT Wed Apr 18 2018

Have updated the Winter Storm Warning and Winter Weather Advisories
to extend through this evening. This is to account for the
likelihood of lingering light snow and falling temperatures after
sunset, leading to hazardous travel conditions.

Overall trends with this system noted so far: The upper low,
north of DSM this morning, is trending a bit slower than expected.
A large dry slot east and south of this low will likely lead to a
lull in the precipitation over all but the Highway 20 corridor
through early afternoon. Forecasts have been updated for lower
precipitation amounts with further forecast refinements likely
through the afternoon

&&

.SYNOPSIS...
Issued at 330 AM CDT Wed Apr 18 2018

Sub-999 MB sfc low rolling east-northeastward acrs far northeastern
KS ATTM, with inverted trof jutting north of it up the MO RVR
valley. MSAS indicating strong pressure fall bullseyes to the lee of
these features in NW IA and toward the KS City areas. Very strong
elevated F-Gen band and H85 MB convergent zone driving a wing of
elevated thunderstorms acrs west central IA which are currently
prolific lightning producers. Current winter headlines to continue
for today with passing vigorous storm system, then dry and
moderating weather into the weekend.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 330 AM CDT Wed Apr 18 2018

Intense elevated Frontogenetical forced band/convergent H85 MB flow
zone, mentioned above driving wing of thunderstorms, being handled
by NAM40 lift tool which sweeps it acrs the northern third of the
DVN CWA this morning. In general this wing to enter the northwestern
CWA mainly along and north of Hwy 30 between 11-12z, and move east
along an north of Hwy 30 through late morning...weakening somewhat
along the way. Still lightning and at least small hail to be
produced by these storms. Some isolated storms possible further
south to I80 or even further south as well this morning, looking at
wing of 200-800 j/kg elevated plume of MUCAPES through 15-16z before
shifting east and waning. The incoming showers/rain to produce some
freezing rain at the sfc, with increasing easterly sfc flow and
lower WBZ feed. The icing may be more prevalent on wind blown
elevated surfaces, but current pavement temps indicated in the upper
20s to low 30s north of I80 suggest some untreated roads may become
ice glazed as well. See a quick transition to freezing rain-sleet
mix, with sleet becoming more prevalent and may accumulate some. But
then the ongoing very strong lift(Omegas over 20 microbars/sec) and
dynamical cooling machine to transition the column to mainly a wet
snow producer from west-to-east as the morning progresses...probably
faster than what the models give this process credit for.

Expect banded heavy snowfall swaths of convective nature developing
in ongoing warning areas, and northern advisory areas as well. 1-2+
inch per hour snowfall rates very possible under these bands as they
set up and then progress eastward. thunder snow possible this
morning thru midday north of I80. Assessing several parameters from
the latest run models, including HiRes solutions, the Hwy 20
corridor-especially from Dubuque on westward, still looks like the
prime area of strong forcing combined with earlier snow switch over
for higher accumulations of 4-7+ inches by mid afternoon. Most of
this to fall before 18z. The Hwy 20 corridor east of DBQ should
range from 2-5 inches by late afternoon. Sharp snow accums gradient
to the south toward I80, with less than an inch along that
interstate corridor. But there may be banded and very localized
higher swaths of snow acumms even in these more southern areas
toward I80. Some lingering light snow in this systems wrap around
after 00z through 03z with additional light accumulations possible
east of the MS RVR. All in All, will let the headlines ride as they
are, but the southern sections of both the Warning areas, and the
Advisory areas may be marginal for their respective threat hazards
(snow and ice glaze amounts).

.LONG TERM...(Thursday through next Tuesday)
ISSUED AT 330 AM CDT Wed Apr 18 2018

Quick look at the longer range, still looks mainly dry with
moderating temps back closer to normal into the weekend, as Rex
block and precip-making wave, rolls to the south of the area.
Temps may push 70 degrees by Monday in the south. A frontal system
may then bring the next chance of rain by Late Tuesday into next
Wednesday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday Morning)
ISSUED AT 645 AM CDT Wed Apr 18 2018

Scattered showers and some thunderstorms producing a wintry mix
of rain, freezing rain, sleet and snow will sweep through the
region through 16z this morning. As they do, conditions will
deteriorate rapidly to IFR/LIFR. Greatest impacts are still
expected at CID and the DBQ terminals where freezing rain/sleet
should quickly switch to snow within 1-2 hours after precipitation
onset, with snow then persisting through the afternoon before
tapering off into the early evening. Sleet and snow accumulations
of at least 1 to 2 inches are expected on pavement, with possibly
higher amounts at DBQ. At the MLI and BRL TAF sites, expect rain
and sleet for a few hours by mid morning before transitioning to
bouts of snow by afternoon with minor accumulation possible on
pavement at MLI during any bouts of heavier snow. Brisk easterly
sfc winds gusting to over 30 MPH at times will complicate
visibilities today during the falling precipitation. As the
wintry precip and snow exits the area tonight after mid evening,
CIGS may improve to high MVFR or low VFR overnight. Winds to go
northwest tonight behind the exiting system.


&&

.HYDROLOGY...
Issued at 1121 AM CDT Wed Apr 18 2018

Latest forecasts for the Cedar River at Conesville continue to
have the river reaching minor flood stage, just above 13 feet, by
midday Friday. This is driven by routed flow from snowmelt and
QPF. With QPF from the current system trending lower than
anticipated, confidence of the river reaching this level is low
and thus no warning has been issued. If later forecasts maintain
this crest, a flood watch may be needed.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Winter Weather Advisory until 1 AM CDT Thursday for Cedar-
     Clinton-Iowa-Johnson-Muscatine-Scott.

     Winter Storm Warning until 1 AM CDT Thursday for Benton-Buchanan-
     Delaware-Dubuque-Jackson-Jones-Linn.

IL...Winter Weather Advisory until 1 AM CDT Thursday for Carroll-Rock
     Island-Stephenson-Whiteside.

     Winter Storm Warning until 1 AM CDT Thursday for Jo Daviess.

MO...NONE.
&&

$$

UPDATE...Sheets
SYNOPSIS...12
SHORT TERM...12
LONG TERM...12
AVIATION...12/05
HYDROLOGY...Sheets



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