Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
654 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 00Z TAFS through 00Z Tuesday evening/
Issued at 653 PM CDT Mon Aug 6 2018

The primary concern through the period will be convective trends
and possibly minor fog north near KALO toward daybreak. A
prolonged period of showers and thunderstorms is anticipated
across at least southern IA through the night. Have an extended
mention of at least VCSH at both KDSM and KOTM with a period of
VCTS at KOTM as well. Stronger wording is certainly possible, but
will hold off on that until confidence increases.

&&

.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.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
136 PM CDT MON AUG 6 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0800 AM     HEAVY RAIN       1 S HUDSON              42.41N 92.46W
08/06/2018  E4.00 INCH       BLACK HAWK         IA   TRAINED SPOTTER

&&

BF

Special Weather Statement
National Weather Service Des Moines IA
623 PM CDT MON AUG 6 2018

IAZ082-070030-
Union IA-
623 PM CDT MON AUG 6 2018

...Hail and Strong Winds near Creston...

At 622 PM CDT, Doppler radar was tracking a strong thunderstorm over
Green Valley Lake, or over Creston, moving east at 25 mph.

Nickel size hail and wind gusts up to 50 mph will be possible with
this storm.

Locations impacted include...
Creston, Green Valley Lake, Afton, Lorimor, Cromwell, Thayer, Green
Valley Lake State Park and Creston Municipal Airport.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 4099 9447 4116 9447 4116 9401 4096 9402
TIME...MOT...LOC 2322Z 268DEG 21KT 4108 9438

Podrazik

Special Weather Statement
National Weather Service Des Moines IA
712 PM CDT MON AUG 6 2018

IAZ082-083-093-094-070045-
Clarke IA-Ringgold IA-Union IA-Decatur IA-
712 PM CDT MON AUG 6 2018

...Strong storm developing in southern Iowa...

At 711 PM CDT, Doppler radar was tracking a strong thunderstorm near
Sun Valley Lake, or 15 miles northeast of Mount Ayr, moving southeast
at 35 mph.

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

Locations impacted include...
Sun Valley Lake, Grand River, Ellston and Beaconsfield.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 4088 9421 4101 9413 4094 9380 4071 9385
TIME...MOT...LOC 0011Z 292DEG 29KT 4090 9409

Podrazik