Hail and Damaging Winds in Iowa, May 16 to 17, 2019

Area Forecast Discussion
National Weather Service Des Moines IA
647 AM CDT Thu May 16 2019

.SHORT TERM.../Today through Tonight/
Issued at 353 AM CDT Thu May 16 2019

Confidence: Medium

Return of active weather pattern on the doorstep.  Subjective H850
analysis at 00z showed wave just to the west of Iowa with attendant
warm fronts stretching from northwest MN southeast toward La Crosse
and secondary weak low over South Dakota with another trough/warm
front extending south-southeast into eastern Nebraska at 04z. The
northern stream wave continues to develop an area of showers and
thunderstorms in the zone of warm air advection...while the southern
stream activity over central Nebraska weakened by 05z. A new area of
isolated thunderstorms has now developed near Sioux Falls in an area
of higher sfc dew points and better instability aloft. This area was
possibly triggered by a gravity wave from decaying earlier
convection. For the remainder of the overnight into early Thursday
morning; scattered convection will continue to drift across northern
Iowa exiting the north between 15z and 17z this morning.  As the
northern stream wave continues to move east today, the trailing cool
frontal boundary will become more east-west oriented over central
Iowa by 00z. Prior to the arrival of the front, H850 readings over
the south will remain about 19-20C during the afternoon hours with
westerly winds aloft while north of the boundary, H850 temperatures
will drop to about 16-17C with northerly winds at the surface and
aloft. With sufficient mixing south of the boundary, afternoon highs
will reach the mid to upper 80s with a spot or two reaching 90 F.
Somewhat depending on cloud cover, the warmer highs are expected
over the southwest. The Day1 convective outlook remains nearly
identical from yesterday with the better threat for any severe
weather over northern Iowa and mainly late tonight into early Friday
morning. There is some uncertainty regarding the exact initiation
time of the convection this evening, though it appears that there
may be some convective development in the very late afternoon/early
evening over the east. After 00z there may be a quick ramping of
convection just along the boundary over Central Iowa with the
activity dying down aft 04z. Then as stronger warm air advection
develops during the late evening/overnight, a potential MCS may
develop over northeast Nebraska and migrate east along the warm
frontal zone. The early period may be the more likely time to see
some large hail with the most unstable conditions, though there may
be a second push of more isolated hailers later in the evening and
overnight hours farther north as the warm advection driven MCS moves
east over the region into early Friday morning.  Most of the models
continue to convect through about 18z Friday, with lesser activity
in the afternoon as the front remains nearly stationary over
southern Iowa and a stronger Western Plains low moves into southwest
Nebraska/northwest Kansas by 00z Saturday. With rain cooled
conditions over northern Iowa Friday, highs are likely to be held
back into the 60s to near 70.  In the south, temperatures should
again climb into the upper 70s to lower 80s for the afternoon hours.
General rainfall totals range from about a quarter inch on the far
southern edge of the expected precipitation shield to about three
quarters to an inch from the northwest to the east central counties
from today through Friday. The likelihood that heavier totals will
occur is pretty certain; especially with a nearly stationary
boundary in place over the area.  Some upward adjustments to precip
amounts are therefore expected for localized areas as more certainty
to initiation time and frontal position occur later today.

.LONG TERM.../Friday through Wednesday/
Issued at 353 AM CDT Thu May 16 2019

Stalled frontal boundary somewhere across southern Iowa will begin
to buckle/lift north Friday. The reason is that a deep trough out
west will begin to lift out of the Rockies and into the Plains
then become negatively tilted. The surface reflection of this
will be a strengthening surface low that will develop over the
Plains and lift into Iowa. As it does, that stalled cold front
will lift and become a warm front as the surface low lifts
northward into either northwest Iowa or eastern South
Dakota...depending on which model you believe. Either way, western
Iowa will see warmer temps, increasing moisture and more
instability. By peak heating, there will be impressive shear along
and north of the warm front that will develop however this may
very well be in Minnesota instead of Iowa. Thus the severe
potential is still in question but we will definitely see
storms...some of which may be strong to severe. The better chances
will be north and west of our forecast area but we will not go
without seeing at least a few severe storms. The frontal location
will be key. Further south and Iowa will see more severe weather.

Beyond Friday the negatively tilted upper trough will take its
time passing through the area. In the mean time, surface lows will
develop and ride up along the boundary reinforcing them and
bringing periods of heavy rainfall and some chance for severe
storms but timing, location and threats are very dependent on
where the front and low will be and as such, will be very
difficult to pinpoint at this time. Will make a general statement
in products that periodic chances for severe storms will exist as
long as the upper trough is near/over Iowa and surface lows keep
riding up into the state. The heaviest rainfall looks to arrive
late Saturday into Sunday. This system clears the area late Sunday
night into Monday bringing a brief break to storms and rain Monday
but a similar system will lift out of the Rockies again Monday
night and Tuesday. Models are all over the place with where the
surface low and frontal boundaries will set up so while additional
storms are a reasonable certainty...timing and location are
unclear. Both models hint at a further south development of the
surface low and if that is the case then our severe potential will
be lower...except for maybe far southern Iowa. It will also mean
that we will be cooler next week. While we are forecasting 80s to
around 90 thorugh Saturday, 60s to lower 70s are what is forecast
early to mid next week.

&&

.AVIATION.../For the 12Z TAFS through 12Z Friday morning/
Issued at 647 AM CDT Thu May 16 2019

Thunderstorms continue to move northeast of the area early today.
Frontal boundary dropping south this afternoon will become focus
for convective development into the evening hours near US 30 to
I80 corridor. Late afternoon instability will increase along the
boundary with mid to late evening warm air advection increasing
aloft across the boundary. Result will be a period of storms from
01-04z near KDSM/KALO with additional storms overnight through 12z
along and north of I80...affecting all sites at times. Confidence
limited on timing of convection and will cover with VCTS for now.
Cigs are expected to drop to MVFR behind the boundary north sites
KFOD/KALO/KMCW aft 10z. Some mixing today will promote SW winds 12
to 18 kts KDSM/KOTM. /rev

&&

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

SHORT TERM...REV
LONG TERM...FAB
AVIATION...REV

Hazardous Weather Outlook
National Weather Service Des Moines IA
344 AM CDT Thu May 16 2019

IAZ004-170845-
Emmet-
344 AM CDT Thu May 16 2019

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

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

Isolated non-severe thunderstorms are possible this morning.
Additional storms are expected late afternoon and tonight. There
is a slight risk for severe storms during this period with large
hail and damaging wind the threats. Locally heavy rainfall is also
possible.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

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

Periods of thunderstorms are forecast Friday through Sunday then
again Tuesday through Wednesday. Severe storms will be possible
but timing and location as well as specific threats will be better
determined in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed periodically from Thursday evening through
Wednesday.

IAZ005>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-170845-
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-
344 AM CDT Thu May 16 2019

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Isolated non-severe thunderstorms are possible this morning.
Additional storms are expected late afternoon and tonight. There
is a slight risk for severe storms mainly north of highway 30 during
this period with large hail and damaging wind the threats.
Locally heavy rainfall is also possible.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Periods of thunderstorms are forecast Friday through Sunday then
again Tuesday through Wednesday. Severe storms will be possible
but timing and location as well as specific threats will be better
determined in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed periodically from Thursday evening through
Wednesday.

FAB