Flash Flood and Hail in Iowa, September 15 to 16, 2016

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
353 AM CDT Thu Sep 15 2016

.SHORT TERM.../Today/
Issued at 349 AM CDT Thu Sep 15 2016

High pressure to push off to the East today and although the high
has been pushing in dry air...we will saturate and see precip today.
The models however...even the hi-res models...are having a tough
time resolving just where initially and how quickly this will
happen. However...it's clear that NW Iowa and locations to the North
and West are already seeing saturation as a stratus deck has been
evident there since the beginning of the shift and now FSD is
reporting light rain and some fog.

The hi-res models that have picked up on this the best are the ARW
and NMM and as such I've modeled sky condition and pop trends along
those models solutions.  I've increased pops this morning across the
Northwest and decreased them across the South central as a result.
By late morning into the early afternoon theta-e advection and
forcing move into the Western third of so of the state so I have
increased pops accordingly in these areas.  The better forcing
outruns the better instability and shear so while we will see some
decent qpf amounts in the afternoon...the better chance for
strong/severe storms should remain to the West of the forecast
area...perhaps eclipsing the far Western portion of the forecast
area by later this afternoon.  Some areas may see heavy rainfall
later in the afternoon but at this point I'm not thinking flash
flooding since it's been some time since the last rainfall and PWATs
are under 1.75 inches.  This may not be the case in later periods
since precip is expected to be continuing through Friday but I am
not considering any headlines for today.  I did nudge highs down a
degree or so today from the expected thicker cloud cover.

.LONG TERM.../Tonight through Wednesday/
Issued at 349 AM CDT Thu Sep 15 2016

The primary concerns through the period will remain precip chances
into Friday, and then another potential window into the middle of
next week. At onset, remnants of current NE convection are expected
to persist into the immediate MO River Valley driven by weak to
moderate large scale forcing with contributions of diffluence aloft,
DPVA and theta-e advection. This will likely persist into the night
across IA in some form but remain fairly unfocused as IA is
essentially in the warm sector with little baroclinicity. Soundings
suggest it will be only slightly elevated, likely around 500m per
soundings, and driven by 20-30kts of inflow as shown along the 300K
isent surface. It is expected to remain sub-severe however as
instability should be relatively weak with minimal effective shear.
Additional more potent surface based convection may develop across
NE during peak heating but end up with a similar fate per the
reasoning above. Additional convection may develop into Friday
along and ahead of a rather unorganized frontal boundary, but
synoptic scale forcing contributions look weaker this far south
and mainly kinematic/mid based.

Rainfall may be more of a concern than severe. Precipitable water
and warm cloud depth values are not extreme versus climatology, but
there will be a more anomalous supply of 0-1km moisture which could
certainly boost rainfall amounts.  The mean wind changes little Thu
Night into Fri and remains weak with only around 20kts of flow so
training is certainly possible. This could result in isolated 1-3”
amounts possible before the precip diminishes NW-SE late Fri. The
00z GFS has lingered precip later into Fri Night and even early
Sat morning south with stronger forcing moving out of the Plains
but this is an outlier versus other solutions and is new from
previous runs so has been left out for now but is something to
watch.

After muggy weather tonight and Friday, fair weather and more
seasonal temperature and humidity levels are the offing for the
weekend as high pressure settles into Iowa behind the departing
Great Lakes upper low. The question will turn to when to introduce
precip again. The GFS and ECMWF have come into somewhat better
agreement since yesterday with a weak MO Valley short wave Sunday
followed by central Plains ridging into Tuesday. Forcing with the
wave is weak or neutral keeping the forecast dry. A weak wave will
top the ridge late Monday however with a front just dropping into
Iowa resulting in some brief low end PoPs by then, but appreciable
chances do not return to the forecast until around Wednesday when
heights lower ahead of the NW CONUS trough resulting in a fairly
strong surge of theta-e advection into the MO Valley. This will
also bring another bout of late summer humidities and slightly
above normal temperatures.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1150 PM CDT Wed Sep 14 2016

Large surface high pressure continues to provide dry air over the
forecast area and looks to ward off the MVFR stratus until mid to
late morning Thursday to mainly FOD and DSM. The latest HRRR has a
good handle on the ceiling trends and keeps ALO, OTM, and MCW in
VFR until the afternoon. Introduced MVFR visibility and ceilings
toward the end of the forecast period with the scattered showers
and thunderstorms expected over much of central Iowa.

&&

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

&&

SHORT TERM...FAB
LONG TERM...Small
AVIATION...Podrazik

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE DES MOINES IA
431 AM CDT THU SEP 15 2016

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-161100-
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-
431 AM CDT THU SEP 15 2016

THIS HAZARDOUS WEATHER OUTLOOK IS FOR MUCH OF CENTRAL IOWA.

.DAY ONE...TODAY AND TONIGHT

THUNDERSTORMS CHANCES WILL INCREASE WEST TO EAST TODAY BEFORE
BECOMING LIKELY AREA WIDE TONIGHT. LOCALLY HEAVY RAINS...AND
POTENTIALLY ISOLATED FLASH FLOODING...ARE POSSIBLE. THE SEVERE
WEATHER CHANCES ARE LOW.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

CHANCES FOR THUNDERSTORMS ARE ALSO IN THE FORECAST
FRIDAY...NORTHEAST LATE MONDAY...AND STATEWIDE TUESDAY NIGHT AND
WEDNESDAY. THE POTENTIAL FOR SEVERE WEATHER OR HEAVY RAIN IS LOW
AT THIS TIME.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION FOR SEVERE WEATHER IS UNLIKELY THROUGH THE
PERIOD...BUT HEAVY RAIN AND FLOODING REPORTS MAY BE NECESSARY INTO
TONIGHT.

SMALL