Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 211746
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1246 PM CDT Wed Jun 21 2017

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 345 AM CDT Wed Jun 21 2017

At 3 am CDT, a frontal boundary extended across northern Missouri
through central Illinois. This front will return north as a warm
front today. A band of mid-level clouds extended from northwest
Iowa into west central Illinois during the early morning.
Temperatures were mainly in the 55 to 65 degree range.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 345 AM CDT Wed Jun 21 2017

The main forecast issues in the short term period are convective
trends and intensity today and tonight.

Elevated storms are starting to fire in northwest Iowa early this
morning and will continue to develop and push east into the
forecast area around mid morning per the various CAMS. Areas north
of Interstate 80 are favored for the highest shower and storm
coverage through the afternoon hours. Activity is expected to
remain elevated, with marginally severe hail the main threat in
the more intense updrafts. There will be sufficient deep layer
shear for organized storms, but relatively modest MU Cape should
limit the overall severe threat. The 06z 12km WRF is suggesting
potential for more surface based development farther south near
the warm front. This is a low probability scenario but cannot be
completely discounted. Afternoon temps will reach the 80s to
perhaps the low 90s in the far southwest. Dewpoints will be
highest in the far south, reaching the mid/upper 60s, with max
heat indices there up to 95 degrees.

Storms are expected to push northward out of the area this
evening, followed by a lull in storm activity ahead of an
approaching cold front. There is some potential for an MCS to
impact the far N/NW counties late tonight, accounting for the
latest SWODY1 slight risk area. Hail/damaging winds would be the
main threats.

.LONG TERM...(Thursday through Tuesday)
ISSUED AT 345 AM CDT Wed Jun 21 2017

Forecast focus on severe storms Thursday evening then below normal
temperatures this weekend.

Thursday and Thursday night: Models indicate thermal ridge over the
cwa with 850 mb temperatures of 20+C.  I have raised temperatures
from guidance with many locations pushing into the lower 90s. There
is the potential for some mid 90s but with dewpoints pushing into
the upper 60s and lower 70s this may be unachievable. Forecast
soundings indicate a cap in place for most of the day which should
suppress convection. This will allow for a moderately unstable
airmass to build through the day, and deep layer shear increasing to
about 40 knots by evening. This will support line segments and
supercells during the evening ahead of an advancing cold front and
large scale ascent, as a deepening upper level trough dives
southeast into the region.

All modes of severe weather (damaging winds 60-70+, large hail and a
few tornadoes) are all on the table. In addition, with PWAT`s
increasing to over 2 inches, torrential rainfall will accompany
these storms. Rainfall amounts of 2 to 3 inches not out of the
question, especially with any training (repeating) storms. The
severe threat should continue even after midnight in portions of our
cwa (as there will still be sufficient CAPE/shear), but storms will
end with the passage of the cold front. I have added severe wording
in the grids for Thursday night. Heat index readings will soar into
the mid to upper 90s during the afternoon/early evening, especially
south of Highway 30, but this should remain below heat advisory
criteria.

Friday and Friday night: Any lingering showers or storms should end
in our se cwa early Friday morning, otherwise cooler and drier on
northwest winds. Highs will range from the mid 70s to lower 80s with
lows in the 50s Friday night.

Saturday into Saturday night: GFS/NAM has the area firmly in a
northwest flow with dry and comfortable conditions. However, the
ECMWF has another wave coming in the zonal flow which produces a
widespread stratiform rain and a few embedded thunderstorms. Once
this wave passes the northwest flow takes over, meaning the ECMWF is
slower with this pattern change and confidence is low.

Sunday through Tuesday: Northwest flow with below normal
temperatures and comfortable humidity. However, embedded waves in
the flow may generate a few instability showers or storms in the
afternoon hours. This would be especially true in our northern and
eastern counties where a pocket of colder air aloft will reside with
the upper level trough anchored across the Great Lakes region. Low
confidence at this time with regard to timing and placement of these
individual waves, and which days will be more susceptible to
convection. Highs should be mainly in the 70s with lows in the 50s
for much of this time frame, except a tad warmer on Tuesday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday Afternoon)
ISSUED AT 1240 PM CDT Wed Jun 21 2017

On going showers and thunderstorms are the main forecast concern
in the beginning of the TAF period. SHRA and vcts will continue
to affect CID, DBQ, and MLI through 21 UTC. MVFR ceilings are
possible at those TAF sites in heavier rain but not likely.
Ceilings and visibilities will remain VFR through the TAF period.
Wind shear has been introduced at all of the TAF sites from 06 utc
to 12 utc tonight with lower winds at the surface and
strengthening winds aloft as a warm front lifts northward across
eastern Iowa.

&&

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

$$

SYNOPSIS...RP Kinney
SHORT TERM...RP Kinney
LONG TERM...Haase
AVIATION...Cousins



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