Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 261120 AAA
AFDDVN

Area Forecast Discussion...UPDATED
National Weather Service Quad Cities IA IL
620 AM CDT FRI AUG 26 2016

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 323 AM CDT Fri Aug 26 2016

Early this morning a surface front was stalled just south of the
area into northern MO and central IL. Showers and thunderstorms
were ongoing across this front as a couple of shortwaves
transversed the flow. Some of these showers and thunderstorms
were affected far southern portions of the CWA, this is expected
to continue. The position of this boundary and the convection this
morning are the main concerns for the short term forecast.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 323 AM CDT Fri Aug 26 2016

Main forecast concern for the short term forecast are the chances
for showers and thunderstorms throughout the day and into the
overnight tonight. SW H5 flow along with a deep wave across the
Dakotas will try to advect this boundary north as a warm front.
Ongoing convection this morning will likely reinforce this
boundary, leading to a slow north progression today. Residual
clouds today will also lead to a forecast concern in both the
highs for today and thunderstorm chances later in the afternoon as
instability may be slow to build.

Overall, large scale forcing and fields are consistent in the
model runs. SW flow, with a deep trof over the Dakotas, with
pockets of shortwaves in the SW flow to transverse the area today
and tonight. Main question comes in the mesoscale features. Two
major players in this are the warm front and how far it moves and
any cloud cover today and how that limits destabilization.

Current CAM runs has the band of rain across the south moving
north through the early AM then weakening. This could lead to OFBs
across the CWA. Any clearing and heating, could lead to pockets of
instability across the area. However, right now believe that the
cloud cover will be thick until later today. This will limit
instability north of the front.

Instability near the front may sneak into the southern CWA later
today, just in time for deep layer shear in the 40 kts realm. With
the warm front across the area, llvl shear will be maximized. This
and the deep layer shear may lead to a strong and even severe
storms across the area. The main threat would be damaging wind
and large hail. The SPC has the southern 2 tiers of counties in a
marginal risk. If instability can build, then there would be a
greater risk for severe weather. NCAR 3-km ensembles has the
Iowa/MO border painted with updraft helicity today, supporting the
threat for strong to severe storms as well.

PWATs across the southern CWA will be close to 2 inches, this
means that heavy rain may be possible. Do not think that flash
flooding will be a factor, as H85 flow is weak and therefore will
not support the same area seeing repeated rains.

At this time, there is high uncertainty with the specifics of this
event today and tonight. There will be storms and rain, the
question is will there be strong and even severe storms. Keep an
eye on any clearing and where the sfc boundary is this afternoon,
those are the keys to this forecast.

.LONG TERM...(Saturday through Thursday)
ISSUED AT 323 AM CDT Fri Aug 26 2016

An active weather pattern featuring warm, humid and periodic wet
weather appears likely to remain in place through the weekend into
at least early next week. Chances for rain look to diminish by late
week, with temperatures remaining near to above normal.

High pressure at 850 mb and 500 mb over the eastern and southeast
U.S. will continue to channel very moist air from the Gulf of
Mexico, with PW values in the range of 1.5 to nearly 2 inches on
southwesterlies into the region over the weekend. Saturday morning,
remnants from nocturnal convection and possible heavy rain along the
returning warm from will likely be waning over the area. By late
afternoon into evening, forcing ahead of the advancing upper trough
over the northern Rockies passing to our north will likely trigger
at least scattered showers and thunderstorms over especially the
northern half of the area, where likely pops are maintained. With
the returning warm front, dewpoints will return to the lower 70s and
temperatures should reach highs in the lower 80s and possibly
warmer, depending on how much clearing takes place between the
rounds of storms.

Saturday night, the potential for organized convection will
diminish, with possible isolated to scattered showers and
thunderstorms continuing along a suggested boundary over the south
overnight. Thunderstorm chances will continue across mainly the
south on Sunday along a depicted weak surface outflow boundary, edge
of high PW to the south, and assistance from at least weak vort
maxima along the edge of the main W-SW flow aloft. With little
airmass change, it will remain humid with temperatures again
reaching the lower to mid 80s.

Monday through Thursday: Daily chances for thunderstorms are
maintained, especially early in the week. A building upper level
ridge, leading to a possible omega block configuration shown by the
GFS and ECMWF toward next weekend would suggest continued near to
well above normal temperatures late week into next weekend. This
would also be less favorable for thunderstorms, which model
consensus lowers to only slight chances by Wed night into Thu.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday Morning)
ISSUED AT 618 AM CDT Fri Aug 26 2016

Active weather during the TAF period with overnight thunderstorms
and MVFR BR being the main aviation issues. Rain this morning will
give way to a mid cloud deck with winds light from east
southeast. A warm front will push north and lead to showers and
thunderstorms tonight. Timing of the storms is hard to nail down
after 00Z, so opted to go with a PROB30.

&&

.HYDROLOGY...
Issued at 323 AM CDT Fri Aug 26 2016

Rises continue on many area rivers due to runoff from recent heavy
rainfall. On the Cedar River, Conesville is currently forecast to
reach minor flood stage early next week. Minor flooding is also
forecast along the Mississippi River at Gladstone and Burlington
early next week. With this potential flooding still 5 or more days
away and below average forecast confidence, no watches or warnings
have been issued. The likelihood of flooding will increase at these
sites and perhaps others as well if heavy additional heavy rainfall
occurs in eastern Iowa and northwest Illinois over the next several
days.

&&

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

$$

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



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