Hail and Damaging Winds in Iowa, August 4 to 6, 2018

Area Forecast Discussion
National Weather Service Des Moines IA
936 PM CDT Sat Aug 4 2018

.UPDATE...
Issued at 922 PM CDT Sat Aug 4 2018

Rest of tonight...clouds continue to dissipate from west to east
this evening with any remaining showers confined to far eastern
Iowa. Temperatures are relatively comfortable with readings
generally in the 70s as of 9pm. Overall - a rather pleasant
evening.

Keeping an eye on a couple areas of thunderstorm activity...the
first is over SD with a second area over west-central Nebraska.
These scattered storms continue to drift slowly e/sewd. The latest
RAP/HRRR/ESRL HRRR guidance is not to excited about this activity
making it into Iowa...per weak upper forcing and a relatively
poor orientation and strength of the nocturnal LLJ. Will keep
token PoPs over the north and west in case activity shows a rapid
uptick but not too concerned.

Still looking like a hot and breezy day on Sunday with readings
well into the 90s across central and southern Iowa. Storm
potential still there in the afternoon and evening but will defer
to midshift for a thorough examination of all the overnight
guidance.

Fowle

&&

.SHORT TERM.../Tonight through Sunday/
Issued at 348 PM CDT Sat Aug 4 2018

A band of light to moderate showers and thick clouds has been
slow to progress from west to east across the forecast area today.
While measurable rain has been confined to northern Iowa for the
most part, instability has been limited to the extent that no
additional convection is anticipated in wake of this activity.
Other than the showers currently departing our northeastern
counties in the next few hours, the area should remain dry
through midnight. Meanwhile, thunderstorms will develop across
central Minnesota later this evening and rocket eastward into
Wisconsin. Some high-resolution short-range models depict an
outflow boundary surging southward from these storms toward the
Iowa border, with associated storms possibly reaching our far
northern counties after midnight. Such storms could carry a
marginal severe weather threat, but this is highly conditional on
the development of the parent boundary and the degree to which
destabilization occurs by that time. The result is only a low-end
severe weather outlook right up by the border, as outlined by SPC
products. Other than these far northern counties, we should stay
quiet overnight.

On Sunday Iowa will reside beneath quick zonal flow aloft, through
which a subtle shortwave impulse will approach in the afternoon.
Between temperature/cloud differences from rain today, the
possible remnant outflow boundary from Minnesota convection
discussed above, and modest frontogenetical forcing ahead of the
approaching impulse, we should see the development of a weak
effective warm front somewhere across northern Iowa during the day
on Sunday. This will serve as a focus for fairly robust
thunderstorm development in the afternoon/evening as broad forcing
ahead of the impulse overcomes any capping and initiates
convection. There is some uncertainty regarding the location of
the resulting thunderstorms, but somewhere across the northern
half of the state is likely. Parameter space indicates a risk of
severe wind/hail, and SPC has upgraded our far northern counties
to a slight risk accordingly. Meanwhile, south of the front and
storm area, much warmer air will advect into southern and central
Iowa aided by mostly clear skies and decent mixing. This will
bring temperatures up into the mid-90s from around I-80 southward.
However, dewpoints will likely fall into the low to mid-60s,
resulting in heat index values only a few degrees above
temperatures. Therefore, no heat headlines have been considered.

.LONG TERM.../Sunday night through Saturday/
Issued at 348 PM CDT Sat Aug 4 2018

On Sunday night, a broader 500 mb trough will approach from the
west and bring stronger, larger-scale forcing for ascent across
much of the region. The lingering quasi-synoptic boundary
discussed in the short term section above, reinforced by
afternoon/evening convection, will likely serve as a focus
mechanism for additional thunderstorms overnight, especially after
midnight as the nocturnal low-level jet impinges upon the
convective zone. This will result in some severe weather threat in
the form of wind/hail, but will also bring a heavy rain threat
across our northern counties given the high atmospheric moisture
content and potential for robust updraft development/training
along the boundary. WPC has issued a slight risk of excessive
precipitation which appears warranted. However, with that said,
recent dry weather has resulted in a large amount of capacity,
especially with the flatter topography of northern Iowa, and it
would take a lot to produce significant flooding. The only
exception would be in urban areas, where rapid rainfall rates
could theoretically cause some issues late Sunday night.

On Monday the 500 mb flow will begin to turn more northwesterly,
pushing a large high pressure area down over the Upper Midwest and
slowly shoving the lingering synoptic boundary south of our area.
While this occurs, remnant thunderstorms from overnight convection
will continue to affect parts of the forecast area in the morning
hours, and if the boundary has not exited our area by late
afternoon additional diurnal convection could clip our
south/southeast late in the day. Any such storms would carry a low
threat of severe weather, however, this is highly conditional as
the location of the boundary and amount of debris clouds and
destabilization will be heavily dependent on multiple rounds of
convection earlier in the forecast period.

Once the boundary finally pushes south of Iowa late Monday or
Monday night, a quieter and drier weather regime will return to
the area for the remainder of the 7-day forecast. Light
northwesterly flow aloft will moderate and slow any warming trend,
and while instability may be sufficient for a few pop-up
showers/storms from time to time, no organized storms or severe
weather are expected. Despite the influence of a northwesterly
flow aloft, airmass modification will allow for slow warming, with
temperatures ticking up a degree or two each day from around
mid-week onward. By Friday and Saturday this should bring highs
into the mid-80s north to lower 90s south, give or take.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 631 PM CDT Sat Aug 4 2018

VFR conditions will persist through the overnight and into much
of Sunday. A few light showers will push east of ALO by sunset,
with just residual high cirrus clouds in place tonight. Still
watching the potential for storms developing across southern
Minnesota to approach far northern Iowa after midnight but
confidence in reaching MCW/FOD is too low to include in the TAFs
at this time. Breezy southerly winds from 15 to 25 kts will
decrease after sunset. VFR conditions with breezy southerly wind
expected again on Sunday. The best potential for storms will be
across northern IA Sunday afternoon, and introduced VCTS for now
due to uncertainty in timing and spatial coverage.

&&

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

UPDATE...Fowle
SHORT TERM...Lee
LONG TERM...Lee
AVIATION...Fowle

Hazardous Weather Outlook
National Weather Service Des Moines IA
316 PM CDT Sat Aug 4 2018

IAZ004>007-015>017-051115-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
316 PM CDT Sat Aug 4 2018

This Hazardous Weather Outlook is for portions of northern Iowa.

.DAY ONE...This afternoon and tonight

There will be minimal chances for thunderstorms through tonight
with a marginal risk of severe weather along the Minnesota border.
If any severe storms develop, the primary threats would be large
hail and damaging wind.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Thunderstorm chances increase Sunday afternoon and especially
Sunday Night when there will be a slight risk of severe weather.
Large hail, damaging wind and locally heavy rains are all
possible into early Monday morning. Chances for non severe storms
will then linger through the rest of the day.

There will be other very spotty and infrequent chances for storms
later in the week with the severe weather potential yet to be
determined.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed Sunday afternoon through early Monday
morning.

IAZ023>028-033>039-044>050-057>062-070>075-081>086-092>097-051115-
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-
316 PM CDT Sat Aug 4 2018

This Hazardous Weather Outlook is for portions of central and
southern Iowa.

.DAY ONE...This afternoon and tonight

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Thunderstorm chances increase Sunday afternoon north and into
central Iowa Sunday Night when there will be a slight risk of
severe weather. Large hail, damaging wind and locally heavy rains
are all possible into early Monday morning.

Chances for non severe storms will develop over southern Iowa late
Sunday Night and then linger area wide through the rest of the
day.

There will be other very spotty and infrequent chances for storms
later in the week with the severe weather potential yet to be
determined.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed briefly Sunday afternoon through early
Monday morning.

Small