Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 180844
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
344 AM CDT Fri Aug 18 2017

.SYNOPSIS...
Issued at 334 AM CDT Fri Aug 18 2017

As of 300 AM...Pressure gradient on the backside of a sfc low
that is now located over the Eastern U.P. has shifted east of the
local area, allowing westerly winds to subside into the 5-10 kt
range. Otherwise, skies were clear over E Iowa/NW Illinois and
temps were in the lower to middle 60s.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 334 AM CDT Fri Aug 18 2017

Today

Digging trough in NW flow aloft will aid weak sfc low development
across W Iowa this morning, which is forecast to track through
southern Iowa into this evening. Temps will be the warmest along and
south of the low. Have mid 80s over the SW forecast area, but upper
70s across the northern tier of counties.

Thunderstorm Chances:

Forecast dewpoints in the lower to middle 60s aren`t overly
impressive. However, with the trough and associated height falls/
cooling aloft overspreading E Iowa/NW Illinois through the day,
SBCAPE is progged to peak in the 1500-2500 J/kg range.
Furthermore, 0-6 km bulk shear likely exceeding 35 kts will be
more than sufficient for organized storms. Given the environmental
CAPE-shear balance, a few severe storms are possible. The main
threats are damaging winds and lightning.

Analysis of the most recent runs of the convective allowing models
(06Z 3 km NAM and HRRR), a southward shift in the convection was
noted. Intuitively this makes sense, because further to the south
the storms tap into the higher sfc-based instability. If this
scenario plays out, a large portion of the CWA -- at least the
northern half if not more -- will receive little or no rainfall.
And scattered storms would stay mostly in the counties south of
I-80. Current PoPs are 30-50%, but may be able to adjust to fit
latest hi-res model solutions in future updates this morning.

Tonight

Mid-level heights quickly rise and sfc high pressure brings light
winds and cooler temps into upper 50s to lower 60s. Uttech

.LONG TERM...(Saturday through Thursday)
ISSUED AT 334 AM CDT Fri Aug 18 2017

Dry and seasonably warm weather is expected for the weekend, then a
series of shortwaves in an active zonal flow brings a good chance
for showers and thunderstorms Monday and Tuesday. A change to
northwest flow aloft and surface high pressure is indicated for
Wednesday through Friday, leading to a period of dry and cooler
weather.

Saturday, high pressure will provide sunshine and slightly lower
humidity levels for Saturday with highs again in a range from the
upper 70s north to mid 80s south. As the high slides east, warm
advection commences Sunday on south to southwest winds from the
surface through 850 mb. This should boost highs into the mid 80s to
near 90 with dewpoints returning to the 60s to around 70. Elevated
thetae advection and at least weak lift on the southern edge of
embedded waves passing to the north may lead to late night showers
and thunderstorms over the northwest and west, where low POPs are
mentioned Sunday night.

Monday and Tuesday, a series of mid level shortwaves will interact
with a feed of moisture from an open Gulf flow, leading to an
extended period of rain chances in the forecast. Blends from latest
model runs now place likely POPs in the Monday night and Tuesday
timeframe, but confidence is low in the critical timing of the upper
level shortwaves and location of surface boundaries, so these will
likely continue to undergo adjustments over the next several days.
Monday`s highs in the mid to upper 80s in the south may be too warm
if cloud cover is as extensive as the GFS suggests. The humid
airmass in place will hold overnight mins in the upper 60s to lower
70s. Tuesday, the overall consensus has the cold front dropping
south through the forecast area during the day resulting in lower
highs from around 80 to the mid 80s and high POPs.

High pressure then provides cooler, drier weather for Wednesday
through most of Friday, leading to overnight lows in the 50s to near
60 and daytime highs mainly in the 70s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday Night)
ISSUED AT 1154 PM CDT Thu Aug 17 2017

Skies will be clear overnight with VFR conditions and west winds of
5 to 10 mph through Friday morning. Winds will shift to the southwest
at 5 to 10 MPH by afternoon with increasing clouds ahead of the next
disturbance to arrive after 19/00Z. After 19/00Z areas of showers and
thunderstorms will arrive producing prob30 grouping until late evening
with MVFR conditions and then clearing skies with NW winds moving in.

&&

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

$$

SYNOPSIS...Uttech
SHORT TERM...Uttech
LONG TERM...Sheets
AVIATION...Nichols



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