Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
627 PM CDT Thu Aug 5 2021

...Updated Aviation...

.DISCUSSION.../Tonight through Thursday/
Issued at 335 PM CDT Thu Aug 5 2021

The primary messaging highlights through the period include...

* Several periods of convection possible through at least the end
  of the week, most active this weekend
* Heat and humidity return, peaking Monday

The immediate concern will be precip trends upstream along the
MN/SD border where convective initiation has occurred along a
surface pressure trough across MN into SD, aided somewhat by the
trailing end of a lower tropospheric short wave along the
MN/Canada border. Inhibition continues to diminish in a 1500-2000
j/kg MLCAPE environment. Deep shear and mid level lapse rates are
fairly weak reducing hail potential, but the lack of deeper
moisture and increased DCAPEs may lead to a few storms with gusty
winds north. Storm coverage and intensity should diminish with
loss of heating.

What's left of the aforementioned boundary will linger across
central or northern IA Friday, and models do hint at isolated
convection again later in the day. While models do suggest MLCAPEs
of 2000+ j/kg building into the afternoon, this may be more
elevated rather than surface based and driven by the remnants of
the current Saskatchewan and Dakotas short waves. Inhibition and
the lack of low level convergence appear to hold back anything
rooted lower. Later into Friday Night, the baroclinic zone and low
level jet pushes through the Siouxland area into MN, and may
trigger low end nocturnal convection north or northeast with the
effective shear/instability parameter space not conductive for
much severe potential.

Looking into the weekend, increasing heat and low level moisture
will continue to build greater instability into IA with MLCAPEs
~3000 j/kg. The severe potential may have decreased a bit, with
the surface based genesis zone to our west, and unimpressive deep
shear downstream into IA as heating is lost. The more favorable
low level kinematics appear to have pushed farther north into the
Upper MS Valley as well, closer to the triple point, but the
increasing low level jet and 0-1km shear/SRH could still support
some QLCS tornado potential. Otherwise, there may still be an
appreciable MCS into the night as the PV anomaly now entering
northern CA reaches the MO Valley. This will produce plenty of
mid-level QG forcing and support with most models suggesting
convection from at least MO northeastward. Strong moisture
transport driven in a 40kt low level jet will continue to feed
convection overnight, and with precipitable water values around
two inches, and warm cloud depths to at least 4km, heavy,
efficient rainfall is possible. Several EPS moisture parameters
are also well through the 90th percentiles. Neither ensemble or
deterministic QPF output is particularly alarming at this point,
but it will need to be monitored with favorable heavy rain
parameter space and SW-NE mean winds suggesting possible training
ahead of the forcing. Convection may linger into Sunday along and
ahead of the upper system as well.

The next highlight will be Monday with heat and humidity building
into the MO Valley with a brief upper level short wave ridge.
With highs and dewpoints 90+/70+ respectively, heat indices will
be 100-105 central and south with another round of Heat Advisories
in play. For Tuesday and beyond, confidence is low through the
end of the period with considerable discrepancies between the
ECMWF, GFS, and CMC regarding the northern mean trough evolution,
magnitude, and timing. For now this leads to seasonal warmth and
humidity levels in the forecast, as well as intermittent low end
chances for convection until confidence in a particular solution
increases.

&&

.AVIATION.../For the 00Z TAFS through 00Z Friday evening/
Issued at 627 PM CDT Thu Aug 5 2021

VFR conditions currently prevail for most of the area. A few
thunderstorms are possible in northern Iowa later this evening into
tonight. Additional isolated to scattered storms are possible
through the day Friday. Uncertainties regarding spatial coverage and
the potential for impacts to any particular terminal precludes
any mention of TSRA in the TAF at this time. Pockets of MVFR fog
are possible around KMCW and KALO toward early Friday morning. Any
visby restrictions should quickly improve after sunrise.

&&

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

DISCUSSION...Small
AVIATION...Martin

Hazardous Weather Outlook
National Weather Service Des Moines IA
319 PM CDT Thu Aug 5 2021

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-062030-
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-Audubon-Guthrie-Dallas-Polk-Jasper-
Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union-
Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
319 PM CDT Thu Aug 5 2021

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

A low chance of severe weather is possible through tonight.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

There are small chances for an isolated storm Friday afternoon
northeast. Slight risk for severe storms Saturday afternoon and
evening. Then additional low risk storms through Sunday into the
next week.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed Saturday night.

Special Weather Statement
National Weather Service Des Moines IA
1011 PM CDT Thu Aug 5 2021

IAZ005-015-023-024-033-034-060415-
Palo Alto IA-Humboldt IA-Kossuth IA-Sac IA-Calhoun IA-Pocahontas IA-
1011 PM CDT Thu Aug 5 2021

...Strong thunderstorms will impact portions of northwestern Webster,
Pocahontas, northeastern Sac, southwestern Kossuth, western Calhoun,
western Humboldt and Palo Alto Counties through 1115 PM CDT...

At 1010 PM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near High Lake to Truesdale. Movement was
southeast at 25 mph.

HAZARD...Winds in excess of 45 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

Strong thunderstorms will be near...
  Five Island Lake around 1020 PM CDT.

Other locations in the path of these storms include Manson, North
Twin Lake and South Twin Lake.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

Brief heavy rainfall is also occurring with these storms and may
lead to localized ponding of water and low visibility.

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4294 9411 4264 9439 4264 9444 4258 9444
      4225 9475 4256 9514 4256 9492 4324 9491
TIME...MOT...LOC 0310Z 298DEG 22KT 4327 9468 4272 9516

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...40 MPH

REV