Flash Flooding and Hail in Iowa, May 25 to 26, 2016

Area Forecast Discussion
National Weather Service Des Moines IA
343 AM CDT WED MAY 25 2016

.SHORT TERM.../Today/
Issued at 340 AM CDT Wed May 25 2016

Complex forecast once again today as uncertainty is present as to
the impacts of a couple different MCS's that will move across the
forecast area this morning.  Current mesoscale analysis is showing
that the current storms to impact the area are moving into a little
less favorable environment for then to become severe though small
hail and some wind gusts to between 40 and 50 mph are likely with
the stronger storms.  The storms from these complexes will move
across the forecast area through the morning hours...coincident with
a shortwave passing across central Iowa and will lay an outflow
boundary out somewhere across Iowa...likely across eastern or
southeastern Iowa...but this will be the area of concern for
potential convection later.

The remainder of todays forecast is difficult.  Uncertainty is great
regarding how much clearing and resulting destabilization will occur
today and if we do get sunny and destabilize...where the old outflow
boundary lies will be a pretty good focus for convection.
However...the models are handling the convection poorly and seems to
be ignoring an area of drying/subsidence moving in to the west
and north, and with time...central Iowa. Much like the past
couple of days...once this MCS dies/moves out I believe much of
the day will be dry and we will likely even see some sun. I have
trended pops with this scenario in mind and have generally dry or
slight chance conditions going for the afternoon. The short term
updates will be the best tool to handle how things evolve in the
smaller scale today.

.LONG TERM.../Tonight through Tuesday/
Issued at 340 AM CDT Wed May 25 2016

Forecast focus in the extended range remains on convective trends,
including potential for severe weather and locally heavy rainfall.
As has been discussed for the past several days, forecast
confidence is rather low due to daily mesoscale uncertainty.

Tonight: Forecast Confidence = Low to Medium

Complex forecast again tonight, much of which will be predicated on
what occurs today.  Current thinking is that convective overturning,
and subsidence associated with todays convection will limit
convective potential overnight. A weak upper trough is forecast to
track ewd across ND/MN however weak forcing and upper level height
falls should remain well north of this area. In addition,
orientation of the nocturnal LLJ does not appear favorable for
ascent as the exit region is progged to be south and east of Iowa.
As such, have trimmed PoPs considerably over many areas tonight.
Several of the CAM solutions do indicate the potential for
isolated/scattered convection developing across northern Missouri,
or perhaps southern Iowa. Left a chance of thunder in these
locales. Lows tonight should remain mild in the 60s. Several
guidance products are suggesting the possibility of patchy fog or
haze as well /perhaps 1-3SM/ - but will leave out explicit mention
in the forecast for now.

Thursday and Thursday Night: Forecast Confidence = Low to Medium

Thursday afternoon and Thursday night appear to have the highest
"conditional" risk of severe weather. A surface low is expected to
deepen over western KS with a frontal boundary extending
northeastward into Iowa.  A warm and moist airmass is expected to be
in place over Iowa by Thursday afternoon with highs in the 80s and
sfc dewpoints in the mid/upper 60s.  Quick note - the GFS/NAM have
been overdoing dewpoints recently - so cut back their forecast
values which are currently in the in the low/mid 70s.  In any event,
expect moderate to strong instability to develop by afternoon with
MLCAPE values of 1500-3000+ J/kg/.  While the wind profiles will not
be extreme, expecting roughly 35-45kts of deep layer shear by
Thursday afternoon which would support multicellular and
supercellular storm structures. The combination of stronger upper
forcing associated with an approaching shortwave /PV anomaly/ and
convergence along the surface boundary should initiate storms
over the region by late afternoon/early evening. There is still
uncertainty exactly where storms will initiate, and this will need
to be fine tuned in the future. All modes of severe weather appear
possible into Thursday night. The current SPC Day 2 Outlook has
the entire region in a Slight Risk, which is reasonable for now.
PWAT values will also increase to 1-1.5 inches as moisture pools
over the area, and this will likely lead to areas of heavy
rainfall into Thursday night if an MCS does indeed form.

Friday through Tuesday: Forecast Confidence = Low

Unfortunately, the unsettled weather pattern appears to persist into
the Memorial Day holiday weekend.  Periods of showers and
thunderstorms are likely on Friday and Saturday as the stronger
upper shortwave slowly pivots northward across the region.  Details
become more nebulous on Sunday through Tuesday but at least a chance
for precip will continue.  Temperatures should remain mild, with
readings in the 70s and 80s.

&&

.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1206 AM CDT Wed May 25 2016

Delayed the onset of precipitation and took out mention of MVFR
ceilings as lower confidence with the duration of thunderstorms.
Still anticipating a break towards the late morning Wednesday.
Could see some redevelopment during the evening across northern
Iowa, but lower confidence in mentioning thunder at OTM and DSM.

&&

.DMX Watches/Warnings/Advisories...
None.
&&

SHORT TERM...FAB
LONG TERM...Fowle
AVIATION...Podrazik

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
1142 PM CDT TUE MAY 24 2016

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-260445-
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-
1142 PM CDT TUE MAY 24 2016

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA.

.DAY ONE...TONIGHT

SCATTERED TO WIDESPREAD THUNDERSTORMS ARE EXPECTED DURING THE
OVERNIGHT HOURS TOWARDS THE EARLY MORNING WEDNESDAY. A FEW STORMS
COULD BE STRONG TO SEVERE AND PRODUCE LARGE HAIL AND STRONG WINDS
ALONG WITH LOCALLY HEAVY RAINFALL.

.DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY

PERIODS OF THUNDERSTORMS WILL CONTINUE THROUGH SATURDAY. SOME OF
THESE STORMS MAY BE STRONG TO SEVERE...PRIMARILY ON WEDNESDAY AND
THURSDAY DURING THE AFTERNOON AND EVENING HOURS. ALL MODES OF
SEVERE WEATHER WILL BE POSSIBLE...WITH LARGE HAIL AND STRONG WINDS
THE MOST LIKELY. HEAVY RAINFALL IS ALSO POSSIBLE AND COULD LEAD TO
LOCALIZED FLOODING.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION MAY BE NEEDED AT TIMES...ESPECIALLY WEDNESDAY
AND THURSDAY.

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE DES MOINES IA
310 AM CDT WED MAY 25 2016

IAZ033-044>046-057>059-070-071-081-250915-
GUTHRIE IA-AUDUBON IA-CASS IA-CARROLL IA-DALLAS IA-CRAWFORD IA-
SAC IA-ADAIR IA-ADAMS IA-GREENE IA-
310 AM CDT WED MAY 25 2016

...THUNDERSTORMS WITH STRONG WIND GUSTS AND SMALL HAIL...

AT 309 AM CDT...DOPPLER RADAR WAS TRACKING A LINE OF STRONG
THUNDERSTORMS ALONG A LINE EXTENDING FROM NEAR DEFIANCE TO 10 MILES
NORTH OF LAKE ICARIA...AND MOVING NORTHEAST AT 35 MPH.

DIME SIZE HAIL AND WINDS IN EXCESS OF 40 MPH WILL BE POSSIBLE WITH
THESE STORMS.

LOCATIONS IMPACTED INCLUDE...
CARROLL...ATLANTIC...AUDUBON...GREENFIELD...GUTHRIE CENTER...
STUART...MANNING...COON RAPIDS...GLIDDEN...LAKE VIEW...PANORA...
BLACK HAWK LAKE...LAKE PANORAMA...ANITA...EXIRA...REDFIELD...
WALL LAKE...ADAIR...MANILLA AND FONTANELLE.

LAT...LON 4236 9504 4154 9414 4150 9418 4150 9424
      4145 9424 4109 9464 4150 9509 4151 9504
      4160 9504 4160 9509 4186 9509 4186 9538
TIME...MOT...LOC 0809Z 212DEG 32KT 4181 9531 4121 9466

BOKSA

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE DES MOINES IA
408 AM CDT WED MAY 25 2016

IAZ059-060-071>074-082>085-093>095-251000-
MADISON IA-UNION IA-LUCAS IA-POLK IA-WAYNE IA-MARION IA-ADAIR IA-
CLARKE IA-RINGGOLD IA-MONROE IA-DECATUR IA-WARREN IA-DALLAS IA-
408 AM CDT WED MAY 25 2016

...THUNDERSTORMS WITH GUSTY WINDS AND SMALL HAIL MOVING NORTHEAST...

AT 406 AM CDT...DOPPLER RADAR WAS TRACKING A LINE OF STRONG
THUNDERSTORMS ALONG A LINE EXTENDING FROM 6 MILES WEST OF WINTERSET
TO 10 MILES NORTHEAST OF OSCEOLA TO 7 MILES SOUTH OF CHARITON...AND
MOVING NORTHEAST AT 35 MPH.

DIME SIZE HAIL AND WINDS IN EXCESS OF 40 MPH WILL BE POSSIBLE WITH
THESE STORMS.

LOCATIONS IMPACTED INCLUDE...
DES MOINES...WEST DES MOINES...URBANDALE...JOHNSTON...CLIVE...
INDIANOLA...WAUKEE...NORWALK...PLEASANT HILL...GRIMES...CRESTON...
KNOXVILLE...WINTERSET...OSCEOLA...CHARITON...LEON...
WINDSOR HEIGHTS...CARLISLE...LAMONI AND PLEASANTVILLE.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

FREQUENT CLOUD TO GROUND LIGHTNING IS OCCURRING WITH THESE STORMS.
LIGHTNING CAN STRIKE 15 MILES AWAY FROM A THUNDERSTORM. SEEK A SAFE
SHELTER INSIDE A BUILDING OR VEHICLE.

&&

LAT...LON 4065 9407 4101 9447 4110 9447 4171 9384
      4156 9333 4151 9333 4151 9322 4116 9292
      4058 9355 4058 9381
TIME...MOT...LOC 0906Z 214DEG 38KT 4136 9413 4112 9361 4092 9326

MF