Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 281140 AAA
AFDDVN

Area Forecast Discussion...UPDATED
National Weather Service Quad Cities IA IL
640 AM CDT SUN AUG 28 2016

...12Z AVIATION UPDATE...

.SYNOPSIS...
Issued at 312 AM CDT Sun Aug 28 2016

Early this morning, widely isolated showers and thunderstorms were
occurring across the far southwest of the CWA. This showers were
slowly rippling to the east along a boundary that has served as
the weather producer across the area for the past few shifts.
These showers are not expected to expand much. To the north of
this diffuse boundary, sporadic dense fog has been reported. This
has been tempered by high clouds moving across the area. This fog
could expand and will continue to be monitored for possible
special weather statements or advisories. This diffuse boundary
will serve as the main weather producer again today across the
area.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 312 AM CDT Sun Aug 28 2016

Main forecast concern for the short term forecast are the chances
for showers and thunderstorms late this morning into the early
evening. Differences in models mean that there will be a chance
for showers and thunderstorms starting late in morning through the
early evening. Clearing skies should take hold tonight.

Large scale flow is the same between the models with a lack of
upper level forcing for ascent. Broad zonal to brief NW flow will
dominate the short term. Later today into the evening, weak upper
level ridging should lead to an end for showers and thunderstorms.
Initiation of these showers and thunderstorms will be driven by
mesoscale features. These features are weak convergence along the
boundary along with diurnal heating. Hires models start to
percolate storms and showers by the early afternoon. Model
soundings in the GFS show that initiation may occur in the late
morning whereas the NAM suggests the afternoon. Regardless, it
looks like the same as yesterday with slightly better coverage.

Showers and thunderstorms should weaken shortly after sunset. The
main impacts from these storms would be occasional lightning and
brief heavy rainfall. Some sites in Henry county, Illinois could
have seen 2 inches last night. These totals could be possible
again today in extremely isolated areas as the hires ARW supports
this notion as well. Otherwise, most of the areas will see less
than a quarter of an inch and more likely see no rain at all.

.LONG TERM...(Monday through Saturday)
ISSUED AT 312 AM CDT Sun Aug 28 2016

Weak flow at the surface and aloft, along with the lingering very
moist and moderately unstable airmass over the region, will require
at least daily chances for thunderstorms over especially southern
half of the area through Tuesday. Beyond, a building upper ridge out
west is expected to send Canadian high pressure into the Great
Lakes, providing a stretch of dry, slightly cooler and less humid
weather late in the week.

Monday and Tuesday: Main challenge is the low confidence potential
for what is largely airmass type showers and thunderstorms. Deep
moisture, shown by a lingering axis of seasonably high PW values and
surface dewpoints in the upper 60s to lower 70s, will lead to
periods of sufficiently high MUCAPES with occasional weak mid level
impulses triggering convection. This potential remains greatest
across the southern half of the forecasts area, where 30 to 40
percent pops are maintained, with mostly slight chances to the
north. Persistence will be a good rule for temperatures due to the
unchanging airmass, keeping highs in the lower to mid 80s and low in
the 60s.

Tuesday night into Wednesday, the developing ridge out west in the
amplifying upper level flow sends Canadian high pressure southeast
into the Great Lakes. This will be punctuated with a cold front
pushing south through the area around Tuesday night. Will maintain
widespread slight chances for thunderstorms with the fropa
overnight, then low chance pops across the far south on Wednesday.
Otherwise, lower dewpoints will follow with temperatures shown by
model blends reaching the upper 70s to lower 80s.

Thursday through Saturday: The area remains under the influence of
the surface ridge with a light low level easterly flow Thu and Fri
providing highs in the 70s, lows in the 50s and more comfortable
humidity levels. The high edges east by Saturday leading to a return
southerly flow and warmer temperatures. Slight chance pops,
mentioned by the consensus over the western half of the forecast
area, where the GFS develops convection due to the warm advection
and returning moisture, are likely overdone. The somewhat similar
GFS and ECMWF in these periods show more favorable conditions for
thunderstorms holding off toward next Sunday or even Sunday night.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday Morning)
ISSUED AT 640 AM CDT Sun Aug 28 2016

Sporadic LIFR fog still possible this morning at the TAF sites,
however confidence is very low, so took it out of the TAFs. Today
should be VFR with light winds across the area. Some airmass
thunderstorms are possible later today. However, due to the
isolated nature of these storms, have left them out. MVFR and IFR
fog return for tonight. Started to trend the TAFs that way, later
shifts will be able to fine tune them.

&&

.HYDROLOGY...
Issued at 1140 AM CDT Sat Aug 27 2016

Many area rivers are forecast to rise over the next week due to
recent heavy rainfall. The point with the main near term concern
is the Cedar River near Conesville. The new forecast has that
point rising above minor flood stage around midday Tuesday. The
river has yet to begin another rise, and would prefer to assess
trends before committing to a flood watch. We will be re-
evaluating tonight into Sunday. The Wapsipinicon River near DeWitt
is also forecast to rise above minor flood stage next Friday,
which is much too far out for a flood watch, especially given that
location`s history of under performing crests.

On the Mississippi River, the main change with the morning
forecast guidance was somewhat higher crests at some points. Three
locations are now forecast to go over flood stage in the next four
to 5 days: Keithsburg, Gladstone, and Burlington. Confidence
remains low, with another complicating factor being the
considerable uncertainty in additional QPF over the next several
days. Will hold off with any flood watches again today.

&&

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

$$

SYNOPSIS...Gibbs
SHORT TERM...Gibbs
LONG TERM...Sheets
AVIATION...Gibbs
HYDROLOGY...RPK



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