Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 200958
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
458 AM CDT Sun Aug 20 2017

.UPDATE...
Issued at 455 AM CDT Sun Aug 20 2017

Expanded likely PoPs over the southwest and south. The MCS is now
along the I-35 corridor and steadily moving eastward. The Lamoni,
IA ASOS recorded a 53 mph wind gust at 4:43 AM as the storms
passed.

GOES-16 IR cloud top temps suggest a slight drop off in intensity
over the past hour. However, the 0-1 km bulk shear values of
20-30 kts shown on the SPC Mesoanalysis are balancing and
sustaining the cold pool/mesohigh. These values drop off to the
east into the DVN CWA, which should cause further weakening of the
system. Although gusty winds remain a threat in this weakening
phase.

&&

.SYNOPSIS...
Issued at 346 AM CDT Sun Aug 20 2017

As of 330 AM: An MCS was located from central Iowa down into NE
Kansas. Longwave IR imagery was impressive with a large spatial
area encompassed by cloud top temps colder than -65 C.

Radar loops clearly showed a developing comma head and MCV. Near
the apex of the complex of storms, individual cell motion was
moving to the NNE in a cyclonically rotating fashion - a testament
to the organization of this MCS. Over E Iowa/NE Illinois it was
quiet. Temperatures ranged from the lower 70s across the far south
to the upper 50s NE.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 346 AM CDT Sun Aug 20 2017

This Morning

Thunderstorm Chances:

Northern portion of an MCS, currently in SW Iowa, has been moving
straight to the east over the past few hours. Thinking it will
impact portions of the SW forecast area as early as 6-7 AM. The
best chances for storms are in the counties along and south of
highway 34 (west of Galesburg, IL). Although chances exist
further north - they are just lower. Across the south and
southwest, have 50-60% PoPs but will likely need to make upward
adjustments as the MCS gets closer.

Forward-propagating Corfidi vectors, the sfc-based instability
gradient, and 1000-500mb thickness isopleths all favor a SE
trajectory. However, due to the mature MCS structure and eastward
momentum, a slightly further north path is probable. The
strongest cells over the far south could produce strong wind gusts
within precip loaded downdrafts, but do not anticipate a
widespread damaging wind threat.

This Afternoon through Tonight

The Storm Prediction Center has included areas along and west of a
line from Delaware County to the Quad Cities in a marginal risk for
severe thunderstorms. The primary risks are localized flash flooding
from slow moving storms capable of producing 1-2"/hr rates due to
PWATs increasing to over 1.50 inches. A few of the strongest storms
could also produce strong winds.

Thunderstorm Chances:

Hi-res convective allowing models all redevelop convection either
this afternoon or this evening. There are typical placement and
coverage discrepancies, making for low forecast confidence.

The synoptic set up, characterized by reinvigorated WV transport
via a enhanced 925-850mb low-level jet from Texas up into Iowa,
favors this secondary round of storms during the evening or the
overnight. As mentioned, the exact location of the heaviest
rainfall is uncertain. The HRRR as been suggesting along the I-80
corridor and points to the south. The 06Z 3 km NAM is much later
and further north with convection not moving into the W/NW
forecast area until late tonight.

The second round of convection will likely focus along a newly
formed warm front. We`ll be monitoring where that sets up, and
should be able to refine the timing and placement of the heaviest
rain. Uttech

.LONG TERM...(Monday through Saturday)
ISSUED AT 346 AM CDT Sun Aug 20 2017

Main challenges center on the Monday and Monday night periods where
the focus remains on thunderstorms and also cloud cover critical for
eclipse viewing. Current indications continue to suggest a potential
for both severe storms and heavy rainfall, especially across the
north. Cooler and dryer weather then follows for mid week onward
after a cold front pushes through Tuesday.

Monday: An MCS that develops upstream overnight will likely be on-
going in the morning, most likely in a dissipating mode over eastern
IA and possibly extending into northwest IL. There remains
considerable model disagreement with details of how this system
breaks down and where the warm frontal boundary sets up through the
day. The NAM is a notable outlier holding a rain complex together
over the area tracking E-SE into MO and west central IL through
midday. Will follow a non-NAM blend scenario for now, with the
support of all models indicating a weak mid level shortwave ridge
showing a period of subsidence in the morning into early afternoon.
This may effectively cap additional convection, especially across
the south during the day, while new storms will be favored along a
boundary, which the majority of models have north of I-80 during the
afternoon.

With temperatures expected to reach the 80s and dewpoints
well into the upper 60s to lower 70s, this may allow SB CAPES to
push well above 3000 J/KG by afternoon to support at least isolated
severe storms along and north of the boundary. Shear is less
favorable during the day, but increases across especially the north
in the evening and overnight. Precipitable water values are shown
pushing 1.5 to near 2 inch during the day, which may lead to locally
heavy rainfall with storms. Considering the uncertainties in the
coverage of morning convection and only scattered afternoon activity
expected, POPS are kept in the chance category during the day.

Looking at Monday`s cloud cover: Confidence that there will be
sufficient clearing for viewing the solar eclipse is low, but there
will be the potential for a period of at least partial clearing
following the dissipating showers and thunderstorms in the morning.
This possibility is greatest south of I-80, where forecast cloud
cover is 50 to 60 percent for now.  This may not be resolved with
any confidence until a few hours ahead of the eclipse.

Monday night: Widespread showers and thunderstorms are likely,
mainly focused across the north in the evening, spreading southeast
ahead of the advancing cold front with an organized MCS overnight.
Severe storms with primarily a damaging wind threat will be possible
based on deep shear and high MUCAPES ahead of the boundary.
Additional support will be provide by a mid level shortwave and jet
level divergence. SPC has a day 2 slight risk over the entire area,
covering especially the overnight period in the DVN CWA. High
precipitable water due to lingering high surface dewpoints and
thetae convergence into the boundary on a 30 to 40 kt southwest
winds at 850 mb will keep a threat of heavy rainfall. This is
greatest along the highway 20 corridor, where local amounts of
several inches will be possible.

Tuesday, the front pushes southeast of the forecast area by
afternoon, driving out any lingering morning showers and isolated
storms early. North winds that follow Tuesday will advect a much
drier airmass into the area. High pressure then slowly migrates from
the upper MS River Valley to the Great Lakes Wed through Sat,
providing seasonably cool and dry conditions with highs in the 70s
and lows in the 50s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday Night)
ISSUED AT 1147 PM CDT Sat Aug 19 2017

Main challenge is timing of any precip chances during the period.

It would appear as though some increase in shower chances is
in the offing late tonight through Sunday morning with veering
LLJ ahead of an approaching shortwave. Chances appear better
heading west of the Mississippi River aft 10z through 18z.
Can`t also rule out a few storms developing after 18z Sunday
ahead of passing shortwave, then additional storm chances by
mid to late Sunday evening mainly north of I-80 with LLJ. Have
gone with VCSH wording at most sites during favored timing with
PROB30 mention at DBQ by afternoon where some potential to be in
closer proximity to vort max and better storm chances. Overall
though lower confidence on details of convection evolution and
timing exist.


&&

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

$$

UPDATE...Uttech
SYNOPSIS...Uttech
SHORT TERM...Uttech
LONG TERM...Sheets
AVIATION...McClure



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