Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 202019

Area Forecast Discussion
National Weather Service Quad Cities IA IL
319 PM CDT Sun Aug 20 2017


Issued at 312 PM CDT Sun Aug 20 2017

A band of shower continued to push northeast through the early
afternoon hours, and extended from far northeast Iowa into
northern Illinois. Strong thunderstorms persisted across areas
from southwest Iowa through northern Missouri. There were some
passing breaks in the cloud cover through early afternoon.
Temperatures varied widely, ranging from the low 70s to mid 80s.


.SHORT TERM...(This Evening through Monday)
ISSUED AT 312 PM CDT Sun Aug 20 2017

The main forecast issues in the short term period are
timing/coverage/intensity of showers and storms.

In the near term, there have been some breaks in the cloud cover
south, with latest HRRR runs developing storms near and south of
the Quad Cities during the late afternoon/evening. Have bumped up
pops slightly across the southeast, but not optimistic that
instability will rebound enough to overcome the capping in place,
as a cirrus shield is already moving into those areas.

Looking ahead, the various CAMS have been fairly persistent in
tracking an MCS into the forecast area late tonight into tomorrow
morning. SPC has upgraded the far western counties to a slight
risk, which looks to be the favored area for more intense storms
toward daybreak before an expected weakening trend with eastward
progress Monday morning. The main threats will be damaging wind
gusts around 60 mph, and large hail. Flash flooding potential is
not looking as favorable overnight/Monday morning, with the system
expected to be progressive enough to preclude significant hydro
issues. Later Monday into Tuesday may be a different story with
the second round.

After the morning showers and storms, there will likely be a lull,
ahead of redevelopment in the late afternoon/early evening.
Eclipse viewing is not expected to be ideal with cloud coverage of
50 to 70 percent from late morning through early afternoon. Parts
of eastern Iowa would be favored for initial storm development
during the late afternoon, with a threat for large hail/damaging
winds. Higher coverage of storms is expected later Monday night.

.LONG TERM...(Monday Night through Sunday)
ISSUED AT 312 PM CDT Sun Aug 20 2017

Widespread thunderstorms expected Monday night then a dry and
pleasant stretch of weather the remainder of the work week.

Monday night and Tuesday: Widespread showers and thunderstorms can
be expected Monday night, then ending Tuesday morning as the strong
cold front exits to the southeast. Strong forcing will be arriving
Monday evening as a significant upper level trough/height falls
dives southeast into the Midwest/Great Lakes. A strong cold front
will also be approaching from the northwest, which will be
interacting with PWAT`s over 2 inches, and an unstable airmass with
SBCAPES of 3000-4000 j/kg ahead of the front. Deep layer shear
increases during the evening as westerly mid level winds increase to
45 knots.  CAM`s models suggest this event to be mainly multicell
clusters/lines which would support damaging winds and large hail.
SPC has a slight risk of severe storms across much of the dvn cwa.
These storms will also produce torrential rainfall and 2-3 inches of
rain is possible especially where storms repeat over the same area.
I will continue to mention "heavy rainfall" in the grids for Monday

Tuesday night through Friday: Northwest flow aloft will be locked in
place for the remainder of the work week. A sprawling Canadian high
pressure will set-up-shop across the Midwest/Great Lakes providing
clear skies and comfortable temperatures. Highs in the 70s and lows
in the 50s will be common, which is at least several degrees below
normal for late August. I would not be surprised if lows dip into
the 40s at some locations for a couple of nights.

Next weekend: Zonal flow returns with a small chance for showers or
storms especially in our northern counties. However, the models keep
the bulk of the rain to the west and north of the cwa. Temperatures
should rebound to near normal readings.


.AVIATION...(For the 18Z TAFS through 18Z Monday Afternoon)
ISSUED AT 312 PM CDT Sun Aug 20 2017

Main challenge this TAF period is timing/intensity of storms
overnight into Monday morning. Based on the more persistent high
resolution models, have timed a 3 hour window for storms at each
site, between 09z and 14z Monday. Expecting mainly MVFR ceilings
and some IFR visibilities in the heavier rain. KCID/KDBQ are more
favored for strong/severe storms, with some weakening expected by
the times storms reach KMLI/KBRL.




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