Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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638
FXUS63 KDVN 301752
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1252 PM CDT TUE AUG 30 2016

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 334 AM CDT Tue Aug 30 2016

Another warm and humid night across the region with scattered, slow
moving thunderstorms producing heavy rainfall rates over the area.
Radar showed this activity clustered mainly in two areas as of 3 am:
one over NW IL from around Galesburg to Rockford, and the other just
west of the highway 218 corridor from Washington to Iowa City. This
elevated convection appeared to be due to weak impulses gliding
along the edge of the upper high over the mid MS River and Ohio
River valleys, interacting with elevated instability and high PW
lingering over the region. Additional storms were over southern MN,
drifting south with the help of a cold front that will drop south
through the forecast area tonight, bringing an end to what has been
a long stretch of humid and very wet weather over the area.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 334 AM CDT Tue Aug 30 2016

Forecast focus remains on scattered thunderstorms with heavy
rainfall, which will be likely over the area again today. Until the
cold front moves through tonight, persistence will be the rule with
a similar airmass and synoptic setup that has been over the region
the past several days. Will thus have another day with largely pulse
type, slow moving showers and thunderstorms capable of very heavy
rainfall rates and flash flooding where storms become nearly
stationary due to outflow boundary interactions. Also, there will
the potential for more funnel clouds where rapidly developing
convection occurs along boundary interactions. Have gone with high
chance to likely pops for today, with higher values in the south
where models depict greater coverage. With the more widespread
convection and cloud cover, temperatures should be slightly cooler
in most areas with highs in the lower to mid 80s. Dewpoints in the
lower 70s will however lead to another very humid day.

Tonight, the cold front, likely entering southern WI into NE IA in
the evening, will drop southeast through the area overnight, ending
the showers and thunderstorms from north to south. The cooler, drier
airmass that follows will lead to lows in the upper 50s in the far
north to the upper 60s in the far south.

.LONG TERM...(Wednesday through Monday)
ISSUED AT 334 AM CDT Tue Aug 30 2016

Main forecast concern for the long term are the chances for rain and
thunderstorms across the south on Wednesday afternoon and then a
return to the same pattern we have been in as of late, late in the
holiday weekend.

Guidance has sped the timing of fropa up for Wednesday.  Agreement
between all of the guidance is that the front will be through the
area by 18Z Wednesday.  As such, have split 12Z to 00Z Wednesday
POPs into 6 hour blocks with the first six seeing a slight chance
for rain and storms across the extreme southern CWA Some models
linger the moisture behind the front. Think this is overdone with
the almost due north flow. HIRES cams depict that with fropa, the
moisture is quickly ushered out of the area.

Wednesday night through Sunday morning look like a good glimpse of
early fall with no chances of rain, low humidity and highs in the
70s.  Canadian high pressure still forecast to settle across the
area and lead to more palatable weather.

Late in the weekend, Sunday and into next week, few changes from
previously forecasts.  SW flow begins and ushers in higher humidity
and higher PWATs.  Ripples in the flow will lead to showers and
thunderstorms.  Past this, it appears that the ECMWF and GFS predict
more of the same, boundary across the area with showers and
thunderstorms.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday Afternoon)
ISSUED AT 1245 PM CDT Tue Aug 30 2016

Area remains under a mantle of mostly mid and high clouds leftover from
convection the past 24 hours. Weak front is just moving into far NW
sections of the forecast area at noon and will pass across the region
by mid evening bringing drier air and fair skies into Wednesday. Trends
support having a tempo group of MVFR conditions at BRL in storms this
PM and a VCNTY wording at MLI terminal. Light and variable winds will
become northerly at 5 to 10 MPH tonight through Wednesday.

&&

.HYDROLOGY...
Issued at 334 AM CDT Tue Aug 30 2016

Overall changes from previous issuance are that the watch at
Keithsburg has been changed to a warning.  With a consistent
forecast and less than 24 hours until flood stage reached, decided
to go with a warning. Otherwise nothing has changed from previous
discussion.

Previous discussion is attached for brevity:

Issued at 300 PM CDT Mon Aug 29 2016

Flood Warnings: Steady river level rises the past several days have
increased confidence in reaching minor flood stage at three sites.
Flood Watches were upgraded to Flood Warnings at Conesville on the
Cedar River, and at Gladstone and Burlington on the Mississippi
River.

Flood Watches: Lower confidence exists for other sites where the
forecast to reach minor flood stage is beyond 24 hours.  Issued a
new Flood Watch for New Boston LD17, both on the Mississippi River.

On the Wapsipinicon River, decided not to issue a watch for Dewitt
because we are anticipating a river level fall to occur before the
rise that may bring it to minor flood stage toward the end of this
week. Uttech

&&

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

&&

$$

SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...Gibbs
AVIATION...Nichols
HYDROLOGY...Gibbs/Uttech



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