Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 231508
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1008 AM CDT FRI SEP 23 2016

.UPDATE...
Issued at 1008 AM CDT Fri Sep 23 2016

MCS that brought flash flooding last night to portions of eastern
IA has diminished this morning and the threat of heavy rain has
ended. Therefore, the flash flood watch has been allowed to expire.

An area of mostly light rain and a few embedded brief lightning
strikes continues from north of Des Moines to the Cedar Rapids/Iowa
City to Quad Cities area. This area of rain will continue to
diminish as the morning progresses. However, this is keeping
temperatures way cooler than expected and have already lowered
highs along the I-80 corridor.

Nearly stationary frontal boundary extends along Highway 34 with
extensive low stratus and some fog to the north. We will continue
to monitor rain and temperature trends and update some more if
needed. As the day progresses upper ridging/height rises should
push into the cwa suppressing any precipitation.

&&

.SYNOPSIS...
Issued at 348 AM CDT Fri Sep 23 2016

Stalled frontal boundary is draped near the I-80 corridor early
this morning. An upper level shortwave and wing of elevated
moisture transport was providing the focus for showers and storms
north of the boundary, arcing from portions of east central Iowa
into far northwest Iowa. Anomalously high precipitable water
values of near 2 inches along with repeated development was leading
to heavy rainfall and several reports of flash flooding,
especially from northern Buchanan through Delaware and into
southwest Dubuque counties where radar estimates 3-6 inches has
fallen within the past 12+ hours.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 348 AM CDT Fri Sep 23 2016

Shortwave looks to pass east of the area by mid morning with
attendant moisture transport gradually waning, thus anticipate
a gradual diminishing trend to the coverage and intensity of
the precipitation toward sunrise and through mid-morning.
Thereafter, will maintain low chances for a few showers and
possibly a storm through afternoon across the northern counties,
as the boundary attempts to edge northward ahead of ejecting
western trough and northward advancing surface low into the
Northern Plains. Temps today will be quite challenging as expect
a thermal gradient of 15-20+ degrees across the boundary, with
coolest readings far north of near 70 and warmest south with upper
80s, and the greatest bust potential from around Highway 30
through I-80.

An upper level ridge is shown to amplify over the region tonight
and the increasing subsidence will preclude any mention of
precipitation. Also, do not anticipate much movement to the front
with heights building aloft and boundary nearly parallel to
surface flow.

.LONG TERM...(Saturday through Thursday)
ISSUED AT 348 AM CDT Fri Sep 23 2016

Saturday night through Sunday will bring our next chance of wet
weather, as the western trough and attendant surface low
lift through the Northern Plains. This will usher in a front
that should be slowing but making steady eastward progress as
it runs into ridging and given that the forcing is pulling away
from the region. Overall, it would appear as though convection
forming to our west ahead of the front Saturday will gradually
weaken while slowly making inroads from west to east Saturday
night, with residual showers lingering on Sunday. East/southeast
winds Saturday not favorable for much of a warmup from that of
Friday, but still continued above normal with highs widespread in
the 80s. Sunday will be cooler with the front and attendant
showers limiting highs to the 70s, with a few 80s possible east
if enough solar insolation.

Next week, is a period of lower confidence as medium range models
offer varying solutions on the upper pattern. GFS rolls a closed
low slowly eastward across the region into midweek, which would
offer potential for some showers rotating down across the region.
Meanwhile, the ECMWF is progressive with the shortwave and keeps
the residual southwestern energy at bay with a rex block, which
would be a drier scenario. And then the Canadian model which
maintains the upper ridge along and east of the cwa and develops
a slow moving upper low ejecting from the southwest into the
central conus. This would offer some heavy rain potential, mainly
to our south/west within near continuous deep moisture transport
on eastern flank of the slow moving trough. Overall, forecast is
toward a blend of GFS and ECMWF and a drier scenario with some
potential for lingering showers early in the week. Temperatures
look to be near to below normal early to midweek, with highs in
the 60s/70s and lows in the 40s/50s, then it looks like moderating
temperatures late week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday Morning)
ISSUED AT 636 AM CDT Fri Sep 23 2016

Tricky aviation forecast with cigs and vsbys today. Heavy rain
across the area and upper level ridging will combine to produce
fog and low cloud issues. As the day warms, these clouds and fog
will lift and burn off. Expect visbys and cigs to drop again
tonight. Have trended lower in the forecast. Some guidance goes to
IFR. At this time, decided not to go any lower due to low
confidence.


&&

.HYDROLOGY...
Issued at 338 AM CDT Fri Sep 23 2016

Main changes to the hydro tonight is a flash flood event with 4 to 6
inches fallen in Maquoketa River Basin.  This has lead to fast rises
at Manchester.  A flood warning has been issued.  The conservative
side of the forecast has the site reaching 17.3 feet this afternoon
whereas the high end has it reaching 22.3 in the same period.  Will
monitor to see which forecast is right.

Previous Discussion.

Overview: Repeated rounds of very heavy rain have occurred over
northern Iowa, southeastern Minnesota, and western Wisconsin over
the past three days. A widespread 3 to 5 inches of rain has fallen
with isolated amounts of 6 to 9 inches. Additional heavy rainfall
is possible tonight and again Saturday night in the same general
area.

These extreme rainfall amounts will result in significant river
level rises downstream into eastern Iowa, including the
Mississippi River, into next week.

The majority of local river sites are now under either a flood
warning or flood watch as significant rises are forecast to begin
this weekend or early next week.

Warnings:

On the Cedar River, Iowa River, Wapsipinicon River (De Witt 4S),
and Mississippi River (sites Dubuque LD11 and Dubuque)

Confidence...Moderate on river level forecasts. Uncertainty arises
with respect to forecast crests, not on whether river levels will
exceed flood stage. How high rivers rise above moderate or major
flood stage is dependent on the downstream propagation of upstream
runoff. As new data is gathered over the coming days, forecast
confidence will likely increase.

Watches:

on the Mississippi River (all sites except Dubuque LD11 and
Dubuque) and Wapsipinicon River (Independence and Anamosa).

Confidence...Low to Moderate on river level forecasts because
river levels will depend on the amount of runoff that is routed
downstream through several tributary rivers. As new data is gathered
over the coming days, forecast confidence will likely increase.

&&

.DVN Watches/Warnings/Advisories...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

UPDATE...Haase
SYNOPSIS...McClure
SHORT TERM...McClure
LONG TERM...McClure
AVIATION...Gibbs
HYDROLOGY...Gibbs/Brooks/Uttech



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