Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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943
FXUS63 KDVN 230435
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1135 PM CDT Sat Jul 22 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 228 PM CDT Sat Jul 22 2017

Visible satellite imagery and sfc obs clearly showed the effects
of a southward moving cold front: widespread low to mid-level
clouds for locations north of highway 34 and light sfc winds with
a northerly component to the north of I-80.

Current heat indices are in the mid 90s to lower 100s roughly
south of a line from central Iowa County to Moline, IL to Kewanee,
IL. The highest values near or over 100 F are restricted to
locations along and south of highway 34 west of Galesburg, IL.

The Excessive Heat Warnings and Advisories are set to expire at 8
PM this evening.

&&

.SHORT TERM...(This Evening through Sunday)
ISSUED AT 228 PM CDT Sat Jul 22 2017

This Evening and Tonight

Flooding:

Areal flooding will continue across a good portion of the CWA,
approximately the NE half. The concern here, outside of river
flooding, is flooding on creeks and streams (and areas of
persistent or deep standing water) that may impact travel,
recreation, or structures such as small bridges or buildings.
Since widespread heavy rain is not expected tonight, areas of
standing water and levels on creeks and streams should fall
through this evening and tonight. The Areal flood warnings are in
effect until midnight. At this point, an extension of the warnings
are not likely, but will depend on the latest reports/observations.

Thunderstorm Chances:

There is a low chance for isolated storms across the S/SE CWA
late this afternoon and evening where the atmosphere is quite
unstable - MUCAPE values are over 4000 J/kg. Right now, thinking
most locations will stay dry and PoPs are at 20%. If a few storms
develop, strong wind gusts, brief heavy rainfall, and lightning
would be the primary threats.

There is also a reinforcing cold front, currently up in Minnesota
and Wisconsin, that is forecast to approach E Iowa/NW Illinois
tonight. Isolated storms are possible after 9 PM across the
northern CWA, which could reach central sections along I-80 after
midnight. The key message here is widespread coverage is not
expected and additional rainfall will not be enough to impact the
ongoing flooding situation.

Sunday

Thunderstorm Chances:

Mid-level height falls and the right entrance region of a 250mb
speed max, coupled with a hot and humid air mass with high temps
in the upper 80s to lower 90s, may provide sufficient lift for
scattered thunderstorm development during the early to mid
afternoon. The NAM12/3km Hi-res NAM/GFS/NMMB all develop
convection. The ECMWF/ARW have little to no convection. But given
the aforementioned setup, thinking it is more likely storms form.
The favored area is south of I-80. However, low-end PoPs exists
across the eastern two-thirds of the CWA. High instability will
from along and south of a nearly stationary front, which models
have positioned across the southern CWA. This is where the
greatest potential exists for a few severe storms - where MUCAPE
is forecast over 3000 J/kg and deep layer shear over 35 kts.

The Storm Prediction Center has a Marginal Risk for severe storms
SE of a Galena, IL to Washington, IA line. Could envision an
upgrade to a Slight Risk for parts of the forecast area (favored
south) if confidence on formation of convection increases. Stay
tuned to the latest forecasts tonight and tomorrow morning.

Heat Indices:

Peak values between 95 - 100 F are likely in the counties along
and south of highway 34 (west of Galesburg, IL) because sfc
dewpoints will still be in the ~mid 70s. Uttech

.LONG TERM...(Sunday Night through Saturday)
ISSUED AT 228 PM CDT Sat Jul 22 2017

Sunday night through Monday night, quiet and dry conditions will be
seen across the area as high pressure moves through the Midwest. Dew
points will drop Sunday night with much lower dew points across the
entire area Monday and Monday night. Temperatures will average
slightly below normal.

Tuesday on...

Quiet and dry conditions will continue on Tuesday as high pressure
moves into the Ohio Valley. Temperatures should be close to normal.

Late Tuesday night into Wednesday the next front drops down into the
area and eventually stalls. The model consensus has slight chance to
chance pops late Tuesday night into Wednesday. Temperatures should
be around or above normal.

Wednesday night through Thursday night the weather pattern becomes
active again. The key to the overall forecast is the location of the
stalled frontal boundary. Here the models disagree on the location
but suggest it will be somewhere over the area. As a result, the
model consensus has 40-60% pops across the CWA.

Friday through Saturday, models in agreement bringing an area of
high pressure into the Great Lakes area. Model consensus has temps
in the 80s and mainly dry conditions.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday Night)
ISSUED AT 1133 PM CDT Sat Jul 22 2017

Areas of MVFR cloud bases will slowly give way to fair skies as high
pressure moves in. VFR conditions will then prevail through Sunday
with cumulus clouds with light northwest winds of 5 to 10 MPH. Patchy
light MVFR ground fog is possible overnight with light northwest winds
and is included as a tempo group at each terminal since dewpoint depressions
are 2 degrees or less at 11 PM.

&&

.HYDROLOGY...
Issued at 228 PM CDT Sat Jul 22 2017

Maquoketa and Wapsipinicon Rivers... Major flooding is forecast
based on routed flow and significant rainfall past 24 hours.
Much of these basins received around 1.5 to 2.5 inches of rain,
with swaths of much higher amounts of 4+ inches focused near
Tripoli and the headwaters of the Wapsi and also over portions
of Clinton/Scott Co, and also near the headwaters of the
Maquoketa near Dundee and Backbone State Park and especially
just north of there into Clayton and Fayette Co. Confidence in
flooding is high, but somewhat low on crest heights as many
streams and creeks feeding into the Maquoketa Basin are
currently cresting or have already crested and falling, thus
concerns that based on input CFS, ground observations and trends
that most crests on the Maquoketa River may end up considerably
lower than currently forecast. Similar lower confidence on crest
heights exist on the Wapsi due to the long routing reach, and
unknowns with how much flow may be lost to off-channel storage
and to the wildlife management area below the TPLI4 gage. As
such, we have indicated 2 foot ranges with crests on the initial
warnings sent this morning.

Rock River... Rainfall amounts (2 to 5 inches) came in much
higher than forecast over lower portions of the basin therefore
while overall flooding category remains unchanged at major,
forecast crests were adjusted higher at Moline and Joslin to
near record levels. Confidence is moderate on forecast crest
heights. Some concern with the Mississippi River rising so fast
that crests on the lower Rock Basin could be prolonged and levels
slower to recede.

Pecatonica River... Major flooding is still forecast with a crest
in the next 12-18 hrs, although confidence somewhat low on reaching
Major category as flow trending lower than forecast at Martintown WI.

Mississippi River... Many forecast crests along the Mississippi River
have gone up today due to the recent significant rainfall. Many sites
have the potential for minor to moderate flooding from the Quad Cities
on southward to Gregory Landing. Due to L/D15 rising fast and forecast
to go above FS late in the day on Sunday, we opted to go straight
to warning as confidence high on flooding due the heavy rains falling
in the lower Rock Basin. Confidence is somewhat lower on crest heights
at L/D15 though, as will depend largely on responses on the lower Rock
Basin. Elsewhere mainly south of the Quad Cities, rises above FS on the
Mississippi River are not anticipated until next week. Therefore,
being several days or more out and low confidence on forecast crests
have opted for a flood watch to cover the rest of the Mississippi sites
shown to reach FS next week.

&&

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

$$

SYNOPSIS...Uttech
SHORT TERM...Uttech
LONG TERM...08/Uttech
AVIATION...Nichols
HYDROLOGY...McClure



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