Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
551 AM CDT Sat May 12 2018

.SHORT TERM.../Today through Tonight/
Issued at 440 AM CDT Sat May 12 2018

Convective trends will obviously remain the primary concern into
tonight with at least two rounds expected. Elevated convection has
been in progress since just before midnight along the I80 and/or
Highway 92 corridors. While the available CAPE rooted 1.5-2km is
near 2K j/kg with strong effective shear approaching 50kts, the
relatively weak 15-30kt effective layer inflow near the 305K isent
surface and lack of kinematic support of any significance seems
to be keeping the MCS mainly below severe limits. RAP 305K winds
and moisture convergence suggests this initial MCS will weaken and
exit toward daybreak as winds begin to veer.

Upstream  another MCS has developed across NE, which seems more
influenced by synoptic scale lift in the SW flow aloft. Neither
the parameterized or convection allowing models /CAMs/ seem to
have a good handle on this and appear underdone. The models do
depict another surge of warm/theta-e advection and kinematic
forcing into IA today however so another round of storms is
possible but likely a bit farther north. While the effective shear
will likely remain quite strong, the RAP MUCAPEs diminish into
the morning so the storms should mainly stay below severe limits.
The stronger storms may still produce small hail and brief heavy
rains however. NAEFS and ECMWF specific humidities and
precipitable water percentiles remain quite high into tonight so
rainfall rates of 1-2” per hour will be possible, but not
widespread.

The aforementioned MCS today may still be in progress into the
evening, but most CAMs and model solutions suggest another round
of nocturnal convection will not develop for us tonight. Have
tapered off the PoPs overnight, but this will have to be watched.
The warm advection regime will still be in place aloft, albeit
weaker than previous days and models still depict elevated areas
of MUCAPE south.

Temp wise, with persistent cloud cover and northeast winds
readings should rebound little today and generally be cool.
Outside of low 70s possible south toward the lingering warm front,
highs should only reach mid 50s to lower 60s in most locations.

.LONG TERM.../Sunday through Friday/
Issued at 440 AM CDT Sat May 12 2018

Our pattern will change somewhat in this period with our weak
upper level ridging and SW flow giving way to even weaker zonal
flow into midweek. Other then Monday Night into early Tuesday, the
forcing generally appears nondescript leading to intermittent low
end PoPs continuing through the period. While the models to not
show much, instability does increase quite a bit south late Sunday
so there would be a conditional threat of severe weather. PoPs do
increase Mon Night into Tuesday however when a weak northern
stream wave seems to increase both kinematic and thermodynamic
support. Instability tries to surge northward once again, but flow
aloft and associated shear diminishes somewhat which may keep the
severe potential down. Any of the convection through Monday Night
may produce locally heavy rains as well with similar specific
humidity and precipitable water anomalies.

A more summer-like regime is anticipated later into the week with
weak flow surface and aloft, along with somewhat warm and humid
conditions. Unorganized, isolated convection will be possible but
weak shear will reduce the severe potential. NAEFS and ECMWF
kinematic and moisture percentiles are unremarkable, leading to
lower potential for significant precipitation as well.

&&

.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 550 AM CDT Sat May 12 2018

Although there is a brief break from convection, widespread LIFR-
MVFR ceilings remain in place across all but southern IA and KOTM.
Additional showers and storms will cross the MO River into IA
later this morning however with the steady northeast wind also
likely bringing at least IFR conditions to KOTM. There is low
confidence in convective trends so have only included a near term
thunder mention at most likely locations and no mention of any
precip later. This does not mean precip or thunder won't occur,
but will be omitted until confidence increases. Little change in
conditions is anticipated into tonight with at least IFR likely
remaining in place.

&&

.HYDROLOGY...
Issued at 440 AM CDT Sat May 12 2018

A total of four flood watches and warnings remain in effect for
portions of the West Fork Des Moines and Cedar Rivers. Three day
QPF, while certainly not dry, is not too remarkable and should
have minimal impact on the current flooding situation being either
too weak or too far south and east. The airmass will remain
conducive for locally heavy rains however so there could still be
some further impacts depending on location. RFC contingency
forecasts continue to suggest northern Iowa, in particular the Des
Moines, Iowa and Cedar basins remain the most susceptible.

&&

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

&&

SHORT TERM...Small
LONG TERM...Small
AVIATION...Small
HYDROLOGY...Small

Hazardous Weather Outlook
National Weather Service Des Moines IA
443 AM CDT Sat May 12 2018

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-130945-
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-
443 AM CDT Sat May 12 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Scattered thunderstorms are expected throughout the day today and
into this evening. A few isolated severe storms are possible with
the main threat of large hail and damaging winds. Locally heavy
rain is also likely at times.

River flood warnings remain in effect for portions of the upper
reaches of the Des Moines River. Please refer to our website at
weather.gov/desmoines for further details.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

More storms are expected on Sunday afternoon into Monday, Monday
night into Tuesday, and periodically throughout the rest of the
work week. The severe weather potential remains low at this time,
but a few isolated severe storms cannot be ruled out. Heavy rain
is possible as well and may exacerbate the river flooding.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated, but reports are always
welcome and encouraged.

Podrazik

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
710 AM CDT SAT MAY 12 2018

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

0330 AM     HAIL             SAINT MARYS             41.31N 93.73W
05/12/2018  M1.00 INCH       WARREN             IA   PUBLIC

            REPORT VIA SOCIAL MEDIA. TIME ESTIMATED BY
            RADAR.

&&

Special Weather Statement
National Weather Service Des Moines IA
947 AM CDT SAT MAY 12 2018

IAZ086-095>097-121530-
Wapello IA-Appanoose IA-Wayne IA-Davis IA-
947 AM CDT SAT MAY 12 2018

...Small Hail, Frequent Lightning, Heavy Rain...

At 946 AM CDT, Doppler radar was tracking a strong thunderstorm 13
miles southeast of Corydon, moving east at 45 mph.

Dime size hail will be possible with this storm.

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

Locations impacted include...
Centerville, Bloomfield, Corydon, Lake Sundown, Rathbun Lake,
Seymour, Moravia, Moulton, Mystic, Cincinnati, Drakesville, Exline,
Promise City, Unionville, Numa, Rathbun, Plano, Udell, Honey Creek
State Park and Sharon Bluffs State Park.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4077 9336 4090 9278 4090 9264 4093 9264
      4096 9251 4060 9236 4058 9325
TIME...MOT...LOC 1446Z 259DEG 37KT 4065 9311

Curtis

Special Weather Statement
National Weather Service Des Moines IA
949 AM CDT SAT MAY 12 2018

IAZ083-084-094-095-121530-
Clarke IA-Wayne IA-Lucas IA-Decatur IA-
949 AM CDT SAT MAY 12 2018

...Small Hail and Frequent Lightning...

At 948 AM CDT, Doppler radar was tracking a strong thunderstorm 10
miles northeast of Leon, moving east at 35 mph.

Dime size hail will be possible with this storm.

Brief heavy rainfall will be possible with this storm, including
amounts around a half inch.

Locations impacted include...
Chariton, Rathbun Lake, Russell, Humeston, Van Wert, Lucas, Garden
Grove, Woodburn, Weldon, Derby, Millerton, Le Roy, Red Haw Lake State
Park and Chariton Municipal Airport.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4078 9380 4097 9388 4111 9310 4081 9310
TIME...MOT...LOC 1448Z 265DEG 31KT 4088 9367

Curtis