Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
1157 PM CDT Mon Aug 6 2018

.SHORT TERM.../Tonight through Tuesday/
Issued at 322 PM CDT Mon Aug 6 2018

The old, weak surface boundary has reached our far
south/southeastern counties this afternoon, but the airmass is
very shallow with elevated instability lingering across much of
the area. Any diurnal thunderstorm development later this
afternoon should occur just southeast of our area, and the
imminent severe weather threat has moved off in that direction.
However, scattered thunderstorms are currently developing to our
west, across central Nebraska, and will advect eastward into our
southern counties later this evening and tonight. While shear/flow
fields are weak, the remaining instability justifies a marginal
severe weather threat, mainly in the form of isolated wind/hail,
along with locally heavy rain. This should be mostly confined to
areas south of I-80, but as the night progresses we may see widely
scattered showers/storms popping up further north in the still
somewhat juicy airmass. This will continue during the day on
Tuesday as diurnal heating, even muted by debris clouds and weak
cold air advection, provides sufficient impetus for scattered
shower/storm development at times. No severe weather is expected
on Tuesday, but POPs have been increased to account for this
scenario.

.LONG TERM.../Tuesday night through Monday/
Issued at 322 PM CDT Mon Aug 6 2018

From Tuesday night through Monday the forecast is very quiet
overall. We will remain beneath weak northwesterly steering flow
aloft through around Thursday or Friday, meaning only a slow
warming trend as the airmass in place gradually modifies,
resulting in an increase of a degree or two per day in
temperatures. The flow will also keep us pretty dry, with the only
real chance of rain coming on Thursday into Thursday night as a
weak surface trough drops across the area. Severe weather appears
unlikely with this feature given the lack of upper-level forcing
and only modest moisture/instability.

By Friday a large, deep thermal ridge will be established over the
western U.S., draping across the High Plains and placing Iowa
under its eastern periphery. This will aid the slow warming trend
later in the week and particularly next weekend and beyond.
Meanwhile a longwave trough, initially stretching from eastern
Canada down into the southeastern U.S., will retrograde as the
ridge builds northeastward, eventually closing off with the
resulting 500 mb low slowly approaching Iowa from the east even as
the ridge builds up across the Upper Midwest and into southern
Canada. Yesterday it appeared the low would only reach about the
Chicago area before slowly ejecting eastward, however, today the
GFS and EC are bringing it over roughly to the Mississippi River
valley around Sunday before kicking it eastward. The forecast for
the latter part of the weekend is thus in flux, both regarding
temperatures and the potential for a few showers/storms. For now
have not bit on this solution and maintained a dry forecast, but
we will be monitoring the evolution of model solutions in the
coming days. As of now the forecast from Friday night through
Monday is dry, with daily highs in the mid 80s to lower 90s.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1156 PM CDT Mon Aug 6 2018

Little has changed from previous thinking at 00z. The primary
concern will be convective trends, the fate of Siouxland and SE
Nebraska clusters in particular. For the time being have nothing
beyond VCSH in most likely times and locations versus a prolonged
overdone thunder inclusion. Will update as confidence increase.
Also have some mention of mainly MVFR fog north early this
morning. Confidence in VFR conditions increases several hours
after daybreak into Tue. Could be some additional precip Tue, but
confidence certainly not great enough to mention yet.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

SHORT TERM...Lee
LONG TERM...Lee
AVIATION...Small

Hazardous Weather Outlook
National Weather Service Des Moines IA
347 PM CDT Mon Aug 6 2018

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-072100-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun-
Webster-Hamilton-Hardin-Grundy-Black Hawk-Crawford-Carroll-Greene-
Boone-Story-Marshall-Tama-Audubon-Guthrie-Dallas-Polk-Jasper-
Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union-
Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
347 PM CDT Mon Aug 6 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

Rain showers and thunderstorms are forecast this evening. Stronger
thunderstorms will be confined to southern Iowa. There is a
marginal risk for severe weather in the south through this
evening. Local rainfall accumulations up to 1 inch possible with
some thunderstorms.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Scattered thunderstorms chances return Thursday and Friday.
Overall severe weather threat appears low at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not likely, though rainfall reports are
always appreciated.

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Special Weather Statement
National Weather Service Des Moines IA
258 AM CDT TUE AUG 7 2018

IAZ033-044-070845-
Sac IA-Crawford IA-
258 AM CDT TUE AUG 7 2018

...Strong Storms over Crawford and Sac Counties...

At 257 AM CDT, Doppler radar was tracking a strong thunderstorm 11
miles northwest of Denison, moving east at 25 mph.

Nickel size hail and winds in excess of 30 mph will be possible with
this storm.

Rainfall amounts up to one inch will be possible with this storm.

Locations impacted include...
Denison, Odebolt, Schleswig, Wall Lake, Charter Oak, Vail, Westside,
Kiron, Deloit, Ricketts and Denison Municipal Airport.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

Persons in campgrounds should consider seeking sturdy shelter until
this storm passes.

&&

LAT...LON 4221 9567 4221 9532 4234 9532 4234 9510
      4195 9509 4203 9567
TIME...MOT...LOC 0757Z 282DEG 22KT 4216 9545

MF

Special Weather Statement
National Weather Service Des Moines IA
329 AM CDT TUE AUG 7 2018

IAZ033-034-045-070915-
Calhoun IA-Sac IA-Carroll IA-
329 AM CDT TUE AUG 7 2018

...Strong Storm Moving into Calhoun, Sac, and Carroll Counties...

At 328 AM CDT, Doppler radar was tracking a strong thunderstorm 11
miles southwest of Black Hawk Lake, or 14 miles northeast of Denison,
moving east at 35 mph.

Nickel size hail and winds in excess of 40 mph will be possible with
this storm.

Rainfall amounts up to one inch will be possible with this storm.

Locations impacted include...
Carroll, Lake City, Glidden, Lake View, Black Hawk Lake, Wall Lake,
Arcadia, Breda, Auburn, Westside, Halbur, Lidderdale, Lanesboro,
Willey, Ralston, Yetter, Hobbs County Park, Arthur N Neu Airport,
Blackhawk Lake State Park and Wall Lake Municipal Airport.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

To report severe weather, contact your nearest law enforcement
agency. They will relay your report to the National Weather Service
office in Des Moines .

Persons in campgrounds should consider seeking sturdy shelter until
this storm passes.

&&

LAT...LON 4231 9464 4193 9463 4195 9509 4234 9510
TIME...MOT...LOC 0828Z 280DEG 30KT 4215 9514

MF