Flash Flood and Hail in Iowa, June 18 to 19, 2018

Area Forecast Discussion
National Weather Service Des Moines IA
354 AM CDT Mon Jun 18 2018

.SHORT TERM.../Today through Tonight/
Issued at 346 AM CDT Mon Jun 18 2018

Boundary has entered northern Iowa this morning with non severe
convection across the far north. This will continue to gradually
decrease in coverage and intensity through daybreak as low level
jet continues to veer with convergence decreasing. With the
boundary remaining in northern Iowa today, the southern half of
the state will once again be quite warm with highs into the low
90s with low 70 dewpoints. Heat advisory will remain in place and
in fact added Tama and Marshall counties back in as they will also
see index values over 100 once again. This will be the last day
of the more intense heat and the heat headlines as temperatures
will be cooler the rest of the week.

CAMs output suggest thunderstorms development increasing around 2-
3pm this afternoon along the highway 20 corridor near the boundary.
Deep layer shear is rather weak despite some decent CAPE across
the state and this will limit overall severe weather
organization. While a few severe storms with large hail seem
likely with initial development, the threat for a widespread
severe day seems minimal. Bigger concern will be shifting to a
heavy rainfall/flash flood threat. There will be ample moisture
with PWATs of around 2" or even slightly higher along with deep
warm cloudy layers in excess of 12kft by late afternoon into
tonight. Training of cells along the nearly stationary boundary
seems likely with efficient rainfall producing high rates. WPC has
highlighted the area with a slight outlook for later today into
tonight and with some of the basins from Pocahontas county down
toward Story and Jasper having been hammered last Thursday night,
there will be real concern for flash flooding. A flash flood watch
will likely be needed for some locations for late today/tonight
for this potential. The convection will shift slightly southward
overnight as the outflow eventually moves southward and have
trended POPs in that direction overnight.

.LONG TERM.../Tuesday through Sunday/
Issued at 346 AM CDT Mon Jun 18 2018

The active weather pattern persists through much of the week work
and then confidence is increasing with finally a dry period by
Friday night into Saturday. Models continue to be in decent
agreement with the wet, cool, and cloudy pattern throughout the
week and utilized a blend of the ECMWF/GFS.

Tuesday into Friday...thunderstorm activity looks to be ongoing or
redeveloping over central to northern Iowa during the morning
Tuesday as a shortwave cuts across the state. The GFS is certainly
a bit stronger with the forcing, but the NAM/ECMWF do suggest a
shortwave to move through at the same time frame b/t 12-18z
Tuesday. The shortwave looks to quickly move east during the
afternoon with some lingering showers/storms possible past 18z.
The main concern remained focused on the isolated heavy rain
threat in the morning as the severe weather threat looks minimal
with weak instability and shear available. Likely to see the
northeast sections of the CWA see a break in the precipitation
Tuesday night into Wednesday morning as the focus shifts south
with the frontal boundary stalling across southern Iowa.

By early Wednesday, a surface low pressure develops and slowly
tracks east along the Kansas and Nebraska border with the GFS
slightly deeper with the low by 12z Wednesday. This low remains
stalled and even looks to retrograde back northwest as a strong
surface high pressure (over the Great Lakes) blocks its eastward
movement until Friday when the high finally shifts east. This low
provides a long duration period of rain across Iowa beginning
Wednesday afternoon through early Friday. The highest pops and QPF
look to be across the west to south past 18z Wednesday into
Wednesday night before shifting northeast across the state
Thursday. Precipitable water values remain around 2 inches for
Wednesday into Thursday and warm layer cloud depths continue to be
around 13-14kft. Certainly likely to see some efficient rain
producers past 18z Wednesday to 00z Friday and the potential for
some high rainfall totals during this time frame is highly likely.

Friday night through Sunday...finally some low pops and even dry
weather is in store Friday night into Saturday as the stubborn low
shifts east into the Ohio Valley.  However, the GFS brings another
surface low and boundary to set up across the CWA from west to
east by Sunday morning suggesting additional heavy rain. The
ECMWF depicts more of an open wave is further south, so low
confidence with pops late in the extended period.

&&

.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1150 PM CDT Sun Jun 17 2018

Official TAFs have VFR conditions throughout this period. However,
challenges remain on impacts of storms later tonight over northern
Iowa. Better chance for showers and storms with MVFR or perhaps IFR
conditions will be tonight as a cold front sinks into the state with
focus on FOD and MCW. They may also impact ALO, but this is a lower
chance. Lastly, a period of low level wind shear is possible tonight
across northern and central Iowa.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 7 PM CDT this evening for IAZ049-050-057>062-
070>075-081>086-092>097.

&&

SHORT TERM...Cogil
LONG TERM...Podrazik
AVIATION...Zogg

Hazardous Weather Outlook
National Weather Service Des Moines IA
447 AM CDT Mon Jun 18 2018

IAZ049-050-057>062-070>075-081>086-092>097-191000-
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-
447 AM CDT Mon Jun 18 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

A heat advisory is in effect for the area. Please refer to the
National Weather Service website at weather.gov/desmoines for
further details.

Thunderstorms are possible this evening with the main concern of
heavy rain at times. A few isolated strong to severe storms may
produce hail and damaging wind.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Periodic thunderstorms remain forecast Tuesday through early
Friday. Although the severe weather potential is low, a few
isolated strong to severe storms cannot be ruled out at times with
the threat of large hail and damaging wind. The main concern is
heavy rain this week as significant rainfall totals are possible
by the end of the week, leading to the potential for river
flooding as well as localized flash flooding.

.SPOTTER INFORMATION STATEMENT...

Spotters might be needed at times tonight through early Friday.

IAZ004>007-015>017-023>028-033>039-044>048-191000-
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-
447 AM CDT Mon Jun 18 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Thunderstorms are forecast to develop late this afternoon and
continue into tonight. Heavy rain is likely along with isolated
strong to severe storms. Hail and damaging winds look to be the
severe threat today. Localized flash flooding is possible as well
tonight.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Periodic thunderstorms remain forecast Tuesday through early
Friday. Although the severe weather potential is low, a few
isolated strong to severe storms cannot be ruled out at times with
the threat of large hail and damaging wind. The main concern is
heavy rain this week as significant rainfall totals are possible
by the end of the week, leading to the potential for river
flooding as well as localized flash flooding.

.SPOTTER INFORMATION STATEMENT...

Spotters might be needed at times beginning this afternoon and
continuing through early Friday.

Podrazik