Hail and Damaging Winds in Iowa, July 14 to 15, 2020

Area Forecast Discussion
National Weather Service Des Moines IA
323 AM CDT Tue Jul 14 2020

.DISCUSSION.../Today through Monday/
Issued at 322 AM CDT Tue Jul 14 2020

The primary weather concerns through the period will be severe
potential this afternoon and evening, followed by heavy rain into
the night, and what appears to a forthcoming Heat Advisory event
Saturday.

Of immediate concern, what's left of an MCS is expected to cross
northern IA early this morning with additional convection possible
in the same area later this morning. Convection continues to
persist across SD driven by 850mb moisture transport and
convergence into the NE/SD border, and additional kinematic
support from lift ahead of the MT trough and embedded PV anomaly.
Although MUCAPEs remain 3K J/kg ahead of this feature, shear is
marginal and not oriented favorably so it should remain below
severe limits.

This could very well impact afternoon severe potential however,
with lingering weak precip and debris cloudiness possibly reducing
and/or delaying insolation somewhat, especially north, which has
been hinted at in recent HRRR runs. Confidence isn't quite there
to back off too much on the severe potential however, so still
expect what was a good consensus among 00z HREF members developing
peak heating convection along the frontal boundary into central
IA. Most solutions depict moderate+ instability with 2-3K J/kg
MLCAPEs, but deep shear is somewhat marginal with around 30kts.
Thus expect some severe threat, mainly wind and/or hail, with
favorable shear orientation suggesting brief discrete cells before
evolving into multicell clusters into the evening. Although a
brief tornado cannot be ruled out, the LCLs and 0-1km kinematic
fields are not overly supportive.

The multicell clusters could very well evolve into heavy rain
into the night however with anomalously high warm cloud depths to
4km+, and 2+" precipitable waters. Effective inflow isn't too
strong, 20-30kts along the 305K isent surface, but it is
persistent with fairly weak mean winds. All this parameter space
is quite conducive for efficient rains and recent convection
allowing models reflect this as well with 1-3" common over some
portions of central or southern IA, and max amounts 3-6". This
would normally trigger a Flash Flood Watch, but with confidence in
location and areal extent low, and recent dry conditions and
thirsty crops suggesting the flash flood threat may be diminished
somewhat. Much of central and southern IA has only received <=25%
of normal rainfall in the past 14 days, some spots down to 5% or
less. Zero river forecast points even to Action Stage is another
sign of holding capacity limiting flash flood potential. Will
certainly include it in messaging this morning, but have opted to
not issue a headline until confidence increases in lieu of what
would be rather large geographic coverage to cover potential
outcomes.

Some forcing may linger into Wednesday supporting additional
convection but neither heavy rains or severe weather are
anticipated. The primary story will become increasing temperatures
to end the week as heights build and SSW flow increases by
Saturday. With favorable mixing, high temps may reach the mid 90s
and if recent rains materialize, evapotranspiration may still
support dewpoints well into the 70s boosting peak heat indices to
105F in many locations. Would not be surprised to see a spot pop
an 80 dewpoint.

Likely not the same extent, but Sunday looks rather warm too
before a frontal boundary slides into the state introducing
additional convection into the night. Deep shear is weak, but the
GFS suggests extreme instability may overcome those deficiencies
keeping all modes of severe weather on the table to at least some
degree. EC Ensemble absolute moisture parameters are approaching
climatological extremes as well suggesting heavy rains possible
into the night after the severe threat wanes. Seasonal conditions
heading into late July are then anticipated to end the period with
unremarkable temperature and precip trends for the time being.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1152 PM CDT Mon Jul 13 2020

General trend for a slower progression of showers/storms
continues, yielding mentions of VCTS and/or prevailing TSRA
ramping up around/after 21z at KFOD/KMCW/KALO, 00z at KDSM, and
03z at KOTM. Leading into storms, VFR will prevail with southerly
winds gusting to around 20kts as mixing increases. Generally kept
MVFR (vis) mentions with TSRA, but can expect IFR/LIFR conditions
during worst conditions.

&&

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

&&

DISCUSSION...Small
AVIATION...Curtis

Hazardous Weather Outlook
National Weather Service Des Moines IA
501 AM CDT Tue Jul 14 2020

IAZ005>007-016-017-024>028-034>039-045>050-057>062-070>075-
081>086-092>097-151015-
Kossuth-Winnebago-Worth-Hancock-Cerro Gordo-Humboldt-Wright-
Franklin-Butler-Bremer-Calhoun-Webster-Hamilton-Hardin-Grundy-
Black Hawk-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-
501 AM CDT Tue Jul 14 2020

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

There is a slight risk of severe storms primarily this afternoon
and evening but storms may linger overnight. Large hail, damaging
wind and heavy rainfall are the main threats. Tornadoes are a
secondary threat.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Hot and humid conditions will exist this weekend. Heat index
values may be in the 100 to 105 degree range. Additional
thunderstorms are forecast Sunday evening. Some storms may be
severe but the specific threats and timing of which will be better
determined in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon and evening.

IAZ004-015-023-033-044-151015-
Emmet-Palo Alto-Pocahontas-Sac-Crawford-
501 AM CDT Tue Jul 14 2020

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Thunderstorms are possible today...especially this afternoon and
evening. There is a marginal risk of severe weather with large
hail, damaging wind and heavy rainfall all possible.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Hot and humid conditions are forecast this weekend. Heat index
values may be in the 100 to 105 degree range. Additional
thunderstorms are possible Sunday night. Some storms may be severe
but the specific threats and timing of which will be better
determined in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon and evening.

FAB