Severe Thunderstorm Warning 34, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
1151 PM CDT Thu May 23 2019

.DISCUSSION.../Tonight through Thursday/
Issued at 347 PM CDT Thu May 23 2019

The weather pattern will be active from tonight into at least the
middle of next week with episodes of thunderstorms that will pose
some severe risk and as mentioned in past days a growing concern
for flash flooding and probably some new river flooding.
Currently, GOES-East clean IR water vapor imagery shows an upper
level low in the western US trough spinning over the Intermountain
West. Meanwhile, upper level ridging is over the Southeast US and
extends into Iowa today providing for a relatively tranquil day.
However, clouds are already increasing over western Iowa from a
thunderstorms complex in central Nebraska and central Kansas per
GOES-East Day Cloud Phase Distinction RGB. As a shortwave trough
ejects out of the western US trough, this will help to lift a
surface boundary back into Iowa as a warm front tonight. Theta-e
advection along with weak instability and good deep layer shear
will help thunderstorms to develop. This may be enough for a few
isolated strong to perhaps severe storms. Heavy rainfall will also
be a risk as precipitable water values increase to between 1.5
and 2 inches. Factor in good warm cloud depths of 3500 to 4000m
and this will lead to efficient rain producing storms. Storms
should be progressive in nature with 850-300mb winds in excess of
40 knots. Still, the 23/00z NCAR ensemble has 1.5 to 2 inches of
rain from southwest into central Iowa with the 12z HREF
localized probability matched mean more confined over southern
Iowa with upper end values nearing 2.5 inches through midday
Friday. Deterministic models also point to an isolated area of 2.5
inches, but hard to pin down where exactly. Many of the high
resolution models along with WPC place the highest amounts over
southern Iowa and this is where 3 and 6 hour flash flood guidance
values are between 1.6 and 2.1 inches respectively. With this
rainfall, not planning to issue a flash flood watch. More on this
in the following paragraphs.

With the warm front lifting northeast of the state during the day
Friday, better instability is expected to develop over the area with
MLCAPEs topping around 2000 J/kg with more than sufficient deep
layer shear and mid-level lapse rates of 7 to perhaps 8C/km. Severe
risk will be there if storms are able to develop later in the
afternoon hours. With the low level jet strengthening and veering
into Iowa Friday night, this may help to induce further storm
development. With the severe risk waning in the evening hours,
attention will be more so on the flash flooding risk. With much of
the storm activity expected Friday night over the southeastern half
or so of the state, concern is that the rain that falls Friday night
will be over areas that have rain tonight (Thursday night). What
is also slightly better for a flash flooding setup besides
antecedent conditions at that point will be the 850-300mb flow
that will be more parallel to the boundary. This may allow for
repeated round of efficient rain producing storms given 1.5 to 2
inch precipitable water values and good warm cloud depths Friday
night into early Saturday. The ingredients for severe storms may
be the best on Saturday as the boundary remains over the state
with MLCAPE values between 2500 and 3000 J/kg and strong deep
layer shear of 50 to 60 knots. Heavy rain parameters are not
peaking as high with lower warm cloud depths and precipitable
water values. As the boundary slips south perhaps into northern
Missouri into Sunday and the weak ridging returns, this may give a
good portion of the state a break. However, southern Iowa should
be close enough to the boundary to warrant some chance of
thunderstorms.

Into Memorial Day, another shortwave trough will eject out of the
Western US trough with the overall trough beginning to slowly advance
eastward. This will cause the boundary once again to drift back
northward into the state and pose a risk for severe weather with
good thermodynamic and kinematic properties. Another round of storms
is expected later Tuesday as the parent upper trough moves onto the
Plains and crosses the region into early Wednesday. With Thursday
night and Friday night both looking like southern Iowa may be more
of a focus for rainfall along with southern Iowa being best
favored for rainfall through early next week, concern is that the
cumulative rainfall will lead to lower flash flood guidance and a
higher risk of flash flooding. Whether it is tomorrow or sometime
this weekend, thinking yesterday and today is that somewhere in
southern to perhaps central Iowa (depending on where rain falls)
will need a flash flood watch. Further, with WPC 7 day rainfall
forecast having 3 to 7 inches over Iowa with highest amounts over
southern Iowa, this rainfall should lead to additional river
flooding.

Large scale differences result after the trough passes the area
with two distinct camps--the ECMWF and non-ECMWF (GFS, FV3, CMC).
The ECMWF drops a northern stream trough into the northern states
with precipitation possible on Wednesday followed by cooler
weather while the non-ECMWF stay north of the US/Canadian border
and would be milder and drier.

&&

.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1151 PM CDT Thu May 23 2019

Showers and thunderstorms moving into TAF locations from the west.
Expect cigs to drop to MVFR with local IFR cigs in heavier showers.
MVFR vsbys will accompany tsra with local IFR vsbys in stronger
storms.  Latest models suggest a break in precip during the day
though isold shra may linger then redevelopment south and east aft
25/00Z.

&&

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

DISCUSSION...Ansorge
AVIATION...FAB

Hazardous Weather Outlook
National Weather Service Des Moines IA
421 AM CDT Fri May 24 2019

IAZ050-061-062-072>075-081>086-092>097-250930-
Tama-Jasper-Poweshiek-Madison-Warren-Marion-Mahaska-Adams-Union-
Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
421 AM CDT Fri May 24 2019

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

A Flash Flood Watch is in effect for this area. Please refer to
the National Weather Service website at weather.gov/desmoines for
further details.

Thunderstorms are expected throughout today. A few storms may be
strong to severe, with large hail and damaging winds the primary
threat. Heavy rainfall may lead to flash flooding and river
flooding.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

Outside of a brief break around Sunday, periodic thunderstorm
chances are expected into early next week, including Memorial
Day. Severe weather primarily in the form of large hail, damaging
wind gusts, and moderate to heavy rainfall will be possible at
times. Flash flooding and river flooding risks will increase
through the weekend.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed off and on through the Memorial Day
weekend, especially today, Saturday, and Monday.

IAZ004>007-015>017-023>028-033>039-044>049-057>060-070-071-250930-
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-Audubon-Guthrie-Dallas-Polk-Cass-Adair-
421 AM CDT Fri May 24 2019

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Thunderstorms are possible today, especially towards central and
to southeastern Iowa. A few storms may be strong to possibly
severe, with large hail and damaging winds the main threats. Heavy
rainfall may also increase flash flooding and river flooding
risks.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

Outside of a brief break around Sunday, periodic thunderstorm
chances are expected into early next week, including Memorial
Day. A few storms may be strong to possibly severe. Flash
flooding and river flooding risks will increase through the
weekend.

A River Flood Warning remains in effect for the West Fork Des
Moines River near Estherville for the foreseeable future. Please
refer to the National Weather Service website at
weather.gov/desmoines for further details.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed off and on through the Memorial Day
weekend, especially today, Saturday, and Monday.

Kotenberg

Special Weather Statement
National Weather Service Des Moines IA
107 AM CDT Fri May 24 2019

IAZ071-072-082-240630-
Adair IA-Union IA-Madison IA-
107 AM CDT Fri May 24 2019

...Line of Storms May Produce Dime to Nickel Size Hail and
Strong Winds..

At 107 AM CDT, Doppler radar was tracking a strong thunderstorm near
Green Valley Lake, or 8 miles north of Creston, moving northeast at
50 mph.

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

Locations impacted include...
Creston, Winterset, Green Valley Lake, Orient, Lorimor, Macksburg,
Green Valley Lake State Park, Creston Municipal Airport and
Winterset-Madison County Airport.

LAT...LON 4126 9444 4146 9400 4116 9397 4116 9401
      4110 9402 4098 9445
TIME...MOT...LOC 0607Z 239DEG 43KT 4118 9440

Kotenberg

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Special Weather Statement
National Weather Service Des Moines IA
234 AM CDT Fri May 24 2019

IAZ038-039-049-050-061-062-240800-
Black Hawk IA-Tama IA-Poweshiek IA-Grundy IA-Jasper IA-Marshall IA-
234 AM CDT Fri May 24 2019

...Line of Storms May Produce Strong Winds of 40 to 50 mph...

At 234 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from 7 miles northwest of Marshalltown to near Rock
Creek Lake. Movement was northeast at 55 mph.

Wind gusts up to 50 mph will be possible with these storms.

Locations impacted include...
Marshalltown, Toledo, Tama, Traer, Reinbeck, Conrad, Meskwaki Casino,
Rock Creek Lake, Gladbrook, Le Grand, Garwin, Gilman, Albion,
Chelsea, Montour, Laurel, Clutier, Elberon, Beaman and Haverhill.

LAT...LON 4192 9230 4173 9295 4212 9310 4245 9254
      4205 9230
TIME...MOT...LOC 0734Z 232DEG 51KT 4208 9304 4176 9288

Kotenberg

Special Weather Statement
National Weather Service Des Moines IA
307 AM CDT Fri May 24 2019

IAZ060-061-073-074-240845-
Marion IA-Jasper IA-Polk IA-Warren IA-
307 AM CDT Fri May 24 2019

...Line of Storms May Produce Brief Winds in Excesss of 40 mph...

At 305 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Iowa State Fairgrounds to 9 miles
southeast of Avenue Of The Saints Event Center. Movement was east at
40 mph.

Winds in excess of 40 mph will be possible with these storms.

Locations impacted include...
Des Moines, Newton, Indianola, Altoona, Pleasant Hill, Knoxville,
Carlisle, Bondurant, Mitchellville, Colfax, Monroe, Pleasantville,
Prairie City, Adventureland Amusement Park, Iowa Speedway, Prairie
Meadows, Iowa State Fairgrounds, Knoxville Raceway, Lake Red Rock and
Milo.

LAT...LON 4175 9292 4126 9301 4122 9363 4167 9355
TIME...MOT...LOC 0805Z 265DEG 34KT 4162 9352 4119 9369

Kotenberg

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
359 AM CDT FRI MAY 24 2019

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

0359 AM     HEAVY RAIN       9 SW CUMBERLAND         41.17N 94.97W
05/24/2019  M4.18 INCH       CASS               IA   PUBLIC

            MEASURED BY HOME WEATHER STATION.

0358 AM     HEAVY RAIN       6 NNE CARBON            41.13N 94.79W
05/24/2019  M4.22 INCH       ADAMS              IA   PUBLIC

            MEASURED BY HOME WEATHER STATION.

&&

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