Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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902
FXUS63 KDVN 292040
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
340 PM CDT Sat Apr 29 2017

.SYNOPSIS...
Issued at 340 PM CDT Sat Apr 29 2017

Vigorous upper level low over NM this afternoon was helping
transport anomalously high moisture northward from the Gulf of
Mexico into the central plains and lower to mid MS River Valley.
Where this was interacting with a nearly stationary surface low,
surface front and very strong baroclinic axis, heavy rain was
occurring from OK through AR, MO into western IL. This surface
boundary marked a sharp separation in airmasses. The local area
was cold and brisk with temperatures in the 40s and widespread
moderate to heavy rain, while south of the boundary, temperatures
were in the 70s and to lower 80s with dewpoints in the 70s across
AR into far se MO and southern IL, feeding into nearly stationary
flood producing storms over the OK/KS/MO/AR border region.

&&

.SHORT TERM...(This Evening through Sunday)
ISSUED AT 340 PM CDT Sat Apr 29 2017

The next several hours looks to be the period of greatest
potential for widespread heavy rainfall as strong moisture
transport interacts with a mid level vortex lifting through the
region. Widespread 1 to 1.5 or greater rainfall amounts will
likely be found over the southeast half to third of the forecast
area by evening, which will not likely be matched in coverage as
the system affect the region through the rest of the weekend.

Once this round exits to the east early this evening, there will
likely be a relative lull in the precipitation overnight as a
period of mid level subsidence migrates over especially the
northwest. Short range high res models and operational models
suggest additional showers, although not likely as strong, will
continue over the southeast third overnight. Overnight QPF was
kept around .25 northwest to .5 to .8 over the southeast. Otherwise,
brisk northeast winds of 20 to 30 mph will continue overnight
with temperatures holding in the 40s to perhaps a few upper 30s in
the far northwest by morning.

Sunday, the surface and upper lows begin to lift northeast,
sending the surface boundary northward and resulting in another
period of upper level lift under a difluent flow aloft. Latest
models continue to show an occluded front lifting north through
the area through the day with a suggestion of triple point or weak
low along this feature into eastern IA. This could lead to enough
shear with possibly several hundred J/KG MUCAPE that could act as
a focus for locally severe storms or even an isolated tornado,
where low level shear is maximized. SPC maintains a marginal risk
over the forecast area mainly south of highway 30. Heavy rainfall
will continue to be a threat Sunday as high PW is shown feeding
into the area until at least mid afternoon, which could result in
a flash flood threat, depending on rainfall coverage and amounts
from today and tonight. For now, will stay with general showers
and thunderstorm wording in the forecast and keep these threats
advertised in the hazardous weather outlook and web graphics. The
far southeast will tap into the warm side of this system, with
temperatures pushing into the 60s, while the far north and
northwest hold in the 40s through the day.

.LONG TERM...(Sunday Night through Saturday)
ISSUED AT 340 PM CDT Sat Apr 29 2017

Low risk for severe storms Sunday evening then a chilly rain to
start the month of May on Monday. After that, most of next week
appears dry with a slow warming trend to normal temperatures.

Sunday evening: Intense cyclone/closed upper low will track into
central IA with a dry punch pushing northeast into the dvn cwa. On
the edge of the dry slot, where the shear will be the strongest, at
least scattered thunderstorms should occur. The question will be how
much MUCAPE will be available and some of the models suggest around
1000 J/KG especially in our southern cwa. With the strong shear
there would certainly be the potential for rotating thunderstorms
producing large hail, damaging winds and a tornado or two early in
the evening. As of now, SPC continues with the marginal risk roughly
along and south of Highway 30. The window of opportunity for severe
shuts quickly by mid evening as the much drier/stable air rapidly
pushes northeast across the area. Once again, the severe threat is
conditional on the amount of instability.

Colder air wrapping around the cyclone will spread across the cwa
overnight with lows by Monday morning in the 40s.

Monday:  We will start the month of May on a cloudy, chilly and
windy note. Along with this will be at least scattered showers, as
the cyclone tracks into western Upper Michigan by evening. West
winds will be gusting to around 30 to possibly 35 mph which will
make it feel even colder.  Highs will be only in the upper 40s to
lower 50s, which is about 15 to 20 degrees below normal for May 1.

Tuesday through Saturday: Northwest flow aloft gradually transitions
to a split flow later in the week. No significant rain is expected
during this time frame to allow to dry out. Temperatures will
gradually moderate during the week so that by the end of the week
readings are back to near normal (in the mid to upper 60s).

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday Afternoon)
ISSUED AT 1251 PM CDT Sat Apr 29 2017

A strong storm system affecting the region will result in
gradually deteriorating conditions from south to north this
afternoon due to lowering ceilings and widespread rain. There will
also be brisk east to northeast surface winds as high as 20 to 30
kts this afternoon and evening. Overnight, widespread light rain
or drizzle will remain in place with IFR ceilings and light fog.
There is a low potential for thunderstorms from late afternoon
through the overnight, mainly at the BRL site, which is not
included in the forecast at this time. Rain showers and a few
thunderstorms will likely become more widespread as a warm front
associated with the low lifting out of the plains pushes
northward Sunday. For now, forecasts for the Sunday morning
period will remain broad brushed with continued IFR conditions.

&&

.HYDROLOGY...
Issued at 120 PM CDT Sat Apr 29 2017

Confidence has increased that periods of moderate to heavy rainfall
will occur this afternoon through Sunday evening across much of
the dvn hydrological service area. Therefore, we have issued
flood warnings for the Rock River from Como to Moline, and the
Mississippi River from Rock Island to Muscatine. Farther
downstream on the MS River, flood warnings remain in effect. A
flood warning has also been issued for the Wapsipinicon River at
De Witt.

Additionally, flood watches have been issued for the Green River at
Geneseo, the Iowa River at Wapello and Oakville, and the English
River at Kalona.

Widespread rain amounts of 1 to 3 inches can be expected and this
will push many tributaries and portions of the Mississippi River
into at least minor to moderate flood. At this time, the heaviest
rains (approaching 3 inches) should occur in our Illinois counties.

Depending on where the axis of heaviest rain occurs and the actual
amount of rain that falls, there is the potential for major flooding
on the lower Rock River from Como to Moline, and on the Mississippi
River from Keithsburg to Burlington during much of next week.
Interests along these rivers should take necessary precautions to
protect life and property.

There is still some uncertainty on the axis of the heaviest rain,
and those with interests along area rivers, streams and creeks
should check back for updates through the weekend. Haase

&&

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

$$

SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...Haase
AVIATION...Sheets
HYDROLOGY...Haase



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