Tornado and Hail in Iowa, May 29, 2018

Area Forecast Discussion
National Weather Service Des Moines IA
1230 PM CDT Tue May 29 2018

.SHORT TERM.../Today through Tonight/
Issued at 355 AM CDT Tue May 29 2018

Currently have an area of weak high pressure situated across the
state with a developing area of low pressure off the west stretching
from north to south along the front range of the Rockies. This area
of low pressure is associated with an upper level trough that is
moving eastward through the central Rockies. Also have Alberto
moving through southern AL, with it expected to continue to track WNW
into the IL/IN/KY areas toward 12z Wednesday, with
influence/moisture advection expected to push all the way into the
eastern half of Iowa by late tonight. The Rockies upper trough also
to move ENE through the period moving across the northern Plains by
late tonight, with southerly flow off the Gulf stretching into Iowa
as well. With moisture from both these system expected to merge into
Iowa, precipitation chances will be increasing through the period,
with the greatest chances coming tonight.

A series of weak impulses to rotate around the main parent upper
trough across the Rockies, with the initial wave moving through
KS/NE currently and helping with storm developing across both
states. Some storms already developing along a lead push of moisture
in the SE NE/ERN KS region. This activity mainly to lift north and
ever slightly east through the morning and could push into the far
western reaches of the forecast area by mid morning. Otherwise
better chances for storms and convective initially into the afternoon
with increasing moisture and weak WAA. A strong shortwave rounding
the base of the trough to approach the state toward evening into
tonight and will allow for an increase in coverage/intensity of
storms. Most storms the past few days have been diurnally driven and
weakened/dissipated with sunset, however with this wave moving in
and the increasing moisture through the night expect decent
thunderstorm chances to continue past sunset. NAM shows an MCS
developing across eastern NE and moving into the forecast area
around midnight tonight, with a larger slug of moisture/activity
moving into the east around the sate time from Alberto. Therefore
have the highest pops across the area during that time. Overall
shear parameters fairly weak, but should be sufficient with the
large amount of CAPE expected that storms could produce some
isolated large hail. Also DCAPE expected to be fairly large
especially across the western half of the forecast area that
damaging wind gusts will be a potential as well. Overall expect at
least a few severe storms across the area potentially by late
afternoon and into the evening hours.

As for temperatures today, with the precip potential and expecting
increasing cloud cover have highs topping out into the upper 80s to
low 90s across the state.

.LONG TERM.../Wednesday through Monday/
Issued at 355 AM CDT Tue May 29 2018

.../Wednesday through Saturday Night/ Confidence: Medium

Active weather pattern will set the stage for several periods of
thunderstorms this period.  Timing is generally good between the
models. Early Wednesday morning some leftover convection may still
linger over the east. Meanwhile...remnants of Alberto and an
approaching H500 trough over eastern South Dakota/Western Iowa will
continue to influence the afternoon/evening weather with increasing
coverage of thunderstorms in the late morning through mid evening
hours. Airmass will still be unstable enough for perhaps a few
strong storms tomorrow...though wind fields again not all that
impressive though mechanical forcing may make up for any lack of
strong winds aloft. Each run of the GFS/Euro has brought Alberto
back to the west slightly...and some phasing Wednesday afternoon at
H500 will result in the more widespread afternoon convection here.
Highs Wednesday will remain warm but with clouds and more rain
chances will likely only reach the mid to upper 80s across the
region.  As the system pulls east Wednesday night the showers and
thunderstorms will end from central through eastern areas. In the
wake of the midweek system a weak ridge of high pressure and
building heights aloft will result in warmer H850 temperatures by
Thursday afternoon. Once again models suggest that readings aloft
will warm to near 18C northeast to 22C southwest. This will likely
push afternoon highs into the mid to upper 80s north to the lower
90s central and south...that after overnight lows early Thursday in
the 60s.  The next system to affect Iowa and the Central Plains will
be entering the PacNW late Thursday afternoon. A sharp short wave
will drive quickly toward Iowa by Friday night into Saturday...with
sfc low pressure developing south of the region over Kansas into
Missouri. Though there remain some differences between the Euro and
GFS with regard to timing/duration of thunder chances...both suggest
a possible MCS developing over Nebraska Friday night and tracking
east southeast into Iowa overnight into early Saturday. Ahead of the
system rather warm and unstable conditions will remain Friday. This
may result in another period of stronger thunderstorms Friday night
into early Saturday. Saturday becomes a bit more difficult to
project and muddled due to the potential of a larger mesoscale
feature Friday night. Some consistency in the GFS/Euro regarding
timing of the trough passage Saturday suggests that most of the
convection will be east of I35 by noon. If that occurs...the
likelihood of any stronger storms Saturday will be nearly east of
our area. Along with that a more refreshing airmass (compared to
hot) will be arriving with cooler afternoon highs Saturday in the
70s to mid 80s as northwest flow returns aloft. Saturday night will
follow with comfortable conditions and lows in the 50s and 60s.

.../Sunday through Tuesday/ Confidence: Medium to High

By Sunday westerly winds aloft will return with temperatures
gradually warming up once again over the next several days. A large
trough will be entering the Pacific NW again with a weak short wave
ejecting east across the Plains Monday into Tuesday. Sometime Monday
through Tuesday a weak trough/boundary may be lingering near Iowa
and may be the trigger mechanism for showers and thunderstorms.
Neither the GFS nor the Euro has a great handle on the timing...
though both are similar with little emphasis on organized rainfall
chances. Highs will recover from the 70s/lower 80s Sunday to the
lower to upper 80s Tuesday.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1230 PM CDT Tue May 29 2018

Scattered thunderstorms possible again later this afternoon and
evening. The main risk with these storms will be local microbursts
and thunderstorm outflow winds. The predominant wind will be from
the southeast today then gradually transitioning to southwest on
Wednesday. Any MVFR or lower cigs and vsbys will be in vicinity of
thunderstorms otherwise VFR conditions are forecast.

&&

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

SHORT TERM...Beerends
LONG TERM...REV
AVIATION...Donavon

Hazardous Weather Outlook
National Weather Service Des Moines IA
314 PM CDT Tue May 29 2018

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Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun-
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Boone-Story-Marshall-Tama-Audubon-Guthrie-Dallas-Polk-Jasper-
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Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
314 PM CDT Tue May 29 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

Scattered thunderstorms are expected this afternoon through the
evening, and at times overnight. Some of the storms this afternoon
and evening may be severe, mainly west of Interstate 35, with
strong winds and large hail the main threats.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Thunderstorm chances will persist on Wednesday, then return from
Friday night into Saturday. Severe weather may be possible at
times.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed this afternoon and evening.