Damaging Winds in Iowa, August 5, 2019

Area Forecast Discussion
National Weather Service Des Moines IA
637 PM CDT Mon Aug 5 2019

.DISCUSSION.../Tonight through Monday/
Issued at 337 PM CDT Mon Aug 5 2019

Synoptic Overview:

Mid to upper-level flow over the upper Midwest will remain
northwesterly for much of this week, sandwiched between troughing
over the mid Atlantic and east coast with amplified ridging over
the western CONUS. A few short-wave troughs will slip through the
mid-level flow brining periodic rain shower and thunderstorm chances
this week. Temperatures to remain near seasonable in this large-
scale pattern.

Today and Tonight:

Severe weather potential will remain in place late this afternoon
through late this evening, especially across the north-central and
northeastern parts of the CWA where SPC SWODY1 enhanced risk is.
Slight Risk for severe weather extends to the southern and
southeastern portions of the CWA. Best moisture transport remains
across north-central and northeastern Iowa this afternoon ahead of
short-wave trough and associated cold front across the eastern
Dakota's and Minnesota. GOES-16 visible imagery has been showing
decent clearing after this morning's weak convection across northern
Iowa. CAMs continue to depict axis of greatest instability from EST
up through MSP, with SBCAPE in excess of 4000 J/kg. HREF mean SBCAPE
across the rest of the north and northeast CWA depicts SBCAPE
between 2500 and 3500 J/kg. In conjunction with thermodynamic
instability, HRRR and RAP soundings also depicting healthy low-level
lapse rates near 8.5 K/km and decent lapse rates through the mid-
levels. Thus anticipate healthy updrafts early on which may be
strong enough to present large hail threat. Effective bulk-shear 25-
30 knots should allow for decent organization of thunderstorms early
this evening, especially north of U.S. Hwy. 30. Low-level shear is
not overly impressive, a large factor into the limited tornado
threat with storms tonight. Dewpoint depressions values will be
quite high prior to the onset of convection. Combine this with
considerable low-level lapse rates, expect the potential for
damaging wind gusts with thunderstorms, with a few downbursts
favorable in this environment. There is still some discrepancy in
CAM runs this afternoon regarding the convective mode. ESRL HRRR and
NMM have attempted to develop bow-like features later this evening
into the overnight hours, which would align with the threat for
damaging wind gusts. Through the 16z runs, the HRRR was not nearly
as excited about bowing features along the cold front this evening.
However, CAMs are in decent agreement in highlighting areas north of
U.S. Highway 30 for active severe weather. The uncertainty with the
southward extent of severe weather this evening lies in how well any
cold-pool is maintained within the thunderstorm complex. If the
shear environment ahead of the boundary remains in place, there may
be enough balance to continue the severe weather threat as it
propagates southward. However, if this is not realized, it may just
lead to a widespread moderate rain showers south of U.S. Highway 30
to the Missouri state line. The only considerable change to the
forecast for thunderstorms tonight has been to delay the onset of
initiation a few hours. For Iowa, the bulk of the activity is not
expected until after 00 UTC. However a cumulus field is producing
precipitation as of 20z in northwestern Iowa. Given a favorable
environment, there is potential for this initial convection to reach
severe limits, thus have issued Severe Thunderstorm Watch 554 for
counties north of U.S. Highway 30 until 04 UTC tonight.

With respect to rainfall totals, thunderstorms will be rather
efficient this evening with PWATs around 1.75 inches. The flow
should be strong enough where the storms will progress at a decent
pace across the CWA. Looking at a widespread half to three-quarters
of an inch of rain, with up to 1.25 inches locally possible. With
rather dry antecedent conditions, not all too concerned with
flooding or flash flooding at this time. However, if the rainfall
begins to outperform the forecast, this may need to be adjusted. For
now, will hold off on any flooding headlines.

Extended Forecast:

Conditions dry out by Tuesday afternoon under northwesterly flow
with no short-wave perturbations in sight at this time as surface
high pressure slides in underneath. Next short-wave feature swings a
vort max through Iowa Wednesday morning brining rain showers
primarily to central and western Iowa during the morning and
afternoon hours. Mid to upper-level flow becomes relatively benign
toward the middle end of the work week through Sunday. Temperatures
in this pattern will remain seasonable in the mid to upper 80s for
the majority of this week. Substantial precipitation chances then
return to the forecast Sunday Night into Monday as another short-
wave trough works through the northwesterly flow over the upper-

&&

.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 637 PM CDT Mon Aug 5 2019

Surface front will slide through the state overnight with
scattered thunderstorms in vicinity of the boundary. Conditions
will be mostly VFR, but MVFR to IFR in and around thunderstorms.
Some areas of MVFR to IFR fog will also be possible around
daybreak with winds becoming light and variable along with
residual moisture from this evenings rainfall in surface layer.
Mainly VFR conditions on Tuesday with north to northwest winds.

&&

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

&&

DISCUSSION...Krull
AVIATION...Cogil

Hazardous Weather Outlook
National Weather Service Des Moines IA
351 PM CDT Mon Aug 5 2019

IAZ004>007-015>017-023>028-034>039-047>050-062100-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Calhoun-Webster-
Hamilton-Hardin-Grundy-Black Hawk-Boone-Story-Marshall-Tama-
351 PM CDT Mon Aug 5 2019

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

An enhanced risk for severe thunderstorms is forecast tonight
where damaging winds and large hail look to be the main threat.
Locally heavy rain is also possible.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Thunderstorms chances are forecast Wednesday night into Thursday.
An isolated severe storms are possible with hail and damaging wind
threat.

.SPOTTER INFORMATION STATEMENT...

Spotters are likely to be needed at times tonight and maybe again
Wednesday into Thursday.

IAZ033-044>046-057>062-070>075-081>086-092>097-062100-
Sac-Crawford-Carroll-Greene-Audubon-Guthrie-Dallas-Polk-Jasper-
Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union-
Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
351 PM CDT Mon Aug 5 2019

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

A slight risk for severe thunderstorms is forecast late tonight
where strong winds and hail look to be the main threat. Locally
heavy rain is also possible.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

Thunderstorms chances are forecast Wednesday night into Thursday.
An isolated severe storms are possible with hail and damaging wind
threat.

.SPOTTER INFORMATION STATEMENT...

Spotters are likely to be needed at times tonight and maybe again
Wednesday into Thursday.

KLP

Special Weather Statement
National Weather Service Des Moines IA
719 PM CDT Mon Aug 5 2019

IAZ007-017-060045-
Cerro Gordo IA-Worth IA-
719 PM CDT Mon Aug 5 2019

...Strong wind gusts possible...

At 718 PM CDT, Doppler radar was tracking a strong thunderstorm 7
miles southeast of Rice Lake, or 12 miles southwest of Northwood,
moving east at 45 mph.

Pea size hail and winds in excess of 40 mph will be possible with
this storm. Personal weather stations recently recorded wind gusts
to around or at 40 mph near Fertile and Joice.

Locations impacted include...
Manly, Rice Lake, Plymouth, Fertile, Kensett, Grafton, Hanlontown,
Joice, Rock Falls, Bolan and Rice Lake State Park.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

A Severe Thunderstorm Watch remains in effect until 1100 PM CDT for
north central Iowa.

LAT...LON 4318 9303 4322 9350 4341 9350 4342 9303
TIME...MOT...LOC 0018Z 278DEG 39KT 4331 9339

Ansorge