Hail and Damaging Winds in Iowa, May 1, 2018

Area Forecast Discussion
National Weather Service Des Moines IA
346 AM CDT Tue May 1 2018

.SHORT TERM.../Today through Tonight/
Issued at 346 AM CDT Tue May 1 2018

This period will signify the beginning of an unsettled period as a
frontal boundary settles toward the state. Currently, axis of low
level jet and moisture transport is veering into the state.
Convection is being fed along this axis and should continue to
propagate east over the next several hours as the flow continues
to veer. Therefore, isolated to scattered thunderstorms will push
through the forecast area this morning before passing east by late
morning. There will likely be a period of several hours from late
morning into much of the afternoon with little convection for
much of the forecast area with a nice cap in place. However,
weakening of the warm layer aloft will occur in northern Iowa by
early afternoon to mid afternoon with some scattered convection
developing, mainly north of Highway 20. Farther south,
temperatures should be quite warm into the afternoon with little
convection although clouds may limit overall potential with most
places in the upper 70s to lower 80s.

As peak heating approaches, convection will become more robust near
the boundary in northern Iowa with large hail becoming possible by
late afternoon. This convection will remain largely along a north of
a Denison to Waterloo line with additional convection developing in
Nebraska/Kansas and sweeping east this evening. This convection
will likely advance east through the forecast area during the
evening into the early hours of Wednesday as a progressive system.
Large hail and damaging winds will be the main threat, although
an isolated tornado is still possible mainly early this evening as
the convection enters the western forecast area from Nebraska in
the enhanced outlook area. Outflow will drop into southern Iowa
and may act as a new convergent area later tonight with storms
continuing in far southern Iowa into Wednesday morning, primarily
with heavy rain although a severe storm would still be possible.

.LONG TERM.../Wednesday through Monday/
Issued at 346 AM CDT Tue May 1 2018

GOES-East mid-level water vapor imagery early this morning shows the
cyclonic spin dropping into northern portions of California and
Nevada. The IR band shows an area of clouds associated with a jet
streak moving down from British Columbia into the Pacific Northwest,
which will be a component of our Thursday's weather. In the
meantime by early Wednesday morning, trough of low pressure with a
positive tilt will have dug out in part due to this jet streak
over the western US with most models indicating a 500mb cutoff low
centered near Las Vegas. This will place Iowa under southwesterly
flow with mid-level shortwave troughs moving through this flow
Wednesday and Thursday toward the state. At the surface, a
stationary front or slowly moving southeastward cold front will be
over the region and the primary focus for convective initiation
each day.

By Wednesday morning, this front will be draped from northeastern
Iowa into west central or southwest Iowa. Ongoing precipitation
from Tuesday night should wane before the next shortwave crosses
the region. MLCAPE values of 2500 to 3500 J/kg are expected by
Wednesday afternoon with 0-6km bulk shear of 40 to 50 knots ahead
of the front over southern Iowa with higher values along the
front. A capping layer between 700 and 800mb as shown in both NAM
and GFS soundings will delay the more vigorous convection until
late afternoon into the evening and mainly along the front where
sufficient lift will be able to overcome the cap. GFS soundings
appear to be contaminated at 00z by convection so uncontaminated
00z NAM soundings show plenty of CAPE in the -10 to -30C layer.
This would support a large hail threat. In addition, NAM soundings
show dry air above 800mb so may be able to get some strong to
severe wind gust entrained in thunderstorm downdrafts leading to a
damaging wind concern. Hodographs at 00z Thursday are more linear
so thinking is tornado threat will be a lesser concern compared
to hail and wind. Precipitable water values of 1.25 to 1.5 inches
will be common over southern Iowa with good moisture transport
into the state, which will bring much needed rainfall to those
areas.

By Thursday, the front sinks a bit farther south into the state as
the western US trough begins to lift northeastward over the Rockies.
In addition, the previously mentioned jet streak noted in the GOES-
East imagery will be coming towards the region. Models are
indicating the front becoming oriented more east to west or east-
northeast to west-southwest. While MLCAPEs in the warm sector may be
a bit lower than Wednesday, bulk shear values will likely be in
excess of 60 knots in the mid to late afternoon. Still likely
looking at hail and wind as main concerns despite weaker model
CAPE. However, there's a bit more directional shear in the low
levels aiding more curvature in the hodographs. This could lead to
a better chance for a tornado compared to Wednesday toward the
Missouri border, but still a lot of variability with mesoscale
features three days.

For this entire period of stormy weather from Tuesday through
Friday morning, total rainfall from global and mesoscale models
along with WPC guidance shows heaviest rainfall over central to
southern Iowa. Fairly high confidence in much of this part of the
state receiving at least 1 to 2 inches. Also think a few places in
southern Iowa will end up with between 3 to 4 inches, though more
variability in the placement of this given convective nature. Will
be monitoring how the rain falls in the coming day or two and
whether this may lead to a localized flash flooding threat by
Thursday.

Showers and thunderstorms should end across central Iowa by Friday
morning as the trough axis pushes east of Iowa. With subsidence and
northwest flow spreading over the state, expect a much quieter
weekend. Still may be a chance for a few showers or thunderstorms
late Saturday into Sunday as a shortwave propagates through the
northwest flow. Temperatures over northern Iowa in the upper 60s
to around 70 degrees will be near seasonal levels with central and
southern Iowa having temperatures 5 to perhaps 10 degrees above
normal in the 70s.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1101 PM CDT Mon Apr 30 2018

Storms moving from SD and NE into IA overnight. Starting to lose
their intensity, so have mainly kept VCSH in the TAFS overnight.
Chances for storms again into by Tuesday afternoon/evening.
Southerly winds through the period. Still strong low level winds
aloft with some low level wind shear possible through 12z.

&&

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

SHORT TERM...Cogil
LONG TERM...Ansorge
AVIATION...Beerends

Hazardous Weather Outlook
National Weather Service Des Moines IA
812 PM CDT Mon Apr 30 2018

IAZ004>007-015>017-023>028-033>039-044>050-011200-
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-
812 PM CDT Mon Apr 30 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

Isolated non severe thunderstorms are possible late tonight.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Thunderstorms are possible Tuesday into Thursday. There is the
potential for isolated severe storms along with locally heavy
rain, mainly during the late afternoon into evening hours each
day. The main threat of any strong to severe storms will be large
hail and damaging winds.

River flooding due to snow melt is expected to continue
along the upper reaches of the Des Moines River Basin. Flood
Warnings remain in effect for these areas. Please refer to the
National Weather Service website at weather.gov/desmoines for
further details.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will be a possibility this week.

IAZ057>062-070>075-081>086-092>097-011200-
Audubon-Guthrie-Dallas-Polk-Jasper-Poweshiek-Cass-Adair-Madison-
Warren-Marion-Mahaska-Adams-Union-Clarke-Lucas-Monroe-Wapello-
Taylor-Ringgold-Decatur-Wayne-Appanoose-Davis-
812 PM CDT Mon Apr 30 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

An isolated non severe thunderstorms is possible late tonight.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Thunderstorms are expected on Tuesday with some severe storms
likely. The severe threat include large hail, damaging winds, and
even a few isolated tornadoes possible.

Additional thunderstorms are expected on Wednesday afternoon and
evening with another threat of a few severe storms. Once again,
large hail and damaging winds will be the main threat. An isolated
tornado is also possible with the initial development in the
afternoon.

Thursday will present another threat of thunderstorms across
central and southern Iowa. Additional severe storms will be
possible during this time.

Localized heavy rain will be possible each day.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is likely to occur at times this week.

MJB