Severe Thunderstorm Warning 16, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
652 PM CDT Thu May 16 2019

.DISCUSSION.../Tonight through Thursday/
Issued at 407 PM CDT Thu May 16 2019

*/ Highlights...

 - Details of severe weather potential across sections of Iowa the
next few evenings/overnights continuing to come into clearer focus.
 - To some extent, all modes of severe weather on the table.
 - Heavy rainfall/flood/flash flood potential amplifying.

Synopsis...

GOES-R imagery picking up on well-defined, well-advertised upper low
spinning off the California coastline. Run-to-run consistency of
this feature can be spanned back over a week ago, so confidence has
been high with this track/timing of the evolution of this system...
by Saturday, it should be over the Rockies, and by Sunday morning-
afternoon, it looks to push through Iowa/Minnesota, making it to the
Great Lakes Region by around 00z Mon.

On the heels of this system, another upper low looks to push through
the Great Plains Region Tuesday through Wednesday... keeping the
pattern active for Iowa into the extended.

This evening-Tonight Severe Potential...

Location... Focusing mechanism for this evening/tonight will be
boundary slated to be positioned near Interstate-80 to Highway 30.
Model consensus generally pretty locked in at this point with
location of boundary. 12z and 18z NAM data overdoing sfc moisture,
so am discarding its data. Significant CAPE values of 2500+ J/KG
slated to focus along the boundary. PWATs of around 1.5+ inches
pooling along boundary.

Timing... 00z to 06z looks to be a good broadbrush. More
specifically, 02z to 04z Fri looks to be close to the maximum/peak
of this event. At 06z and after lapse rates, etc. diminishes. Am
expecting rainfall/thunder to continue after 06z Fri, but intensity
and severe potential likely to end.

Mode... Ultimately, am thinking a line of isolated storms, with
pockets of moderate to heavy rainfall appears likely. Large hail
the primary threat, with an isolated, short-lived, weak tornado or
two appearing possible. Wind potential interesting... outflow
boundaries dropping generally southward seems plausible. Local
enhancement possible with any intersecting outflow boundaries.
Will have to keep an eye on how far south and wind surges
associated with outflow boundaries make it.

Friday Evening Severe Potential...

The boundary migrates northward throughout the day as the event's
single sfc low propagates into eastern/NE Nebraska. Overall, Friday
morning/day should be quiet in terms of severe potential. Models
converging on gulf moisture surge arriving in SW to western Iowa
around 21z Friday. This will begin the severe weather threat for
Friday. CAPE values again exceeding 2500 J/KG. Differential heating
amplification could be present as SW to western IA will likely see
more pockets of sun than northern/NW Iowa. Large hail the primary
threat, with damaging winds, and an isolated tornado threat as well.

Friday Night Heavy Rainfall Potential...

Severe threat begins to transition to heavy rainfall threat from
around 03z-09z Sat especially in northwestern Iowa. Set-up
actually looking very impressive as +4 sigma moisture advection
makes it into this part of the state, aided by a 50kt 850mb jet.
PWAT values look to near 1.75 inches, with 0-6km mixing ratios off
the charts. Low- level moisture flux convergence supporting
precip enhancement as well. Corfidi vectors become <5 kts,
oriented towards the south, suggesting areas of back-building
worth watching as training storms could lead to flooding. Also
worth noting that crops, particularly in northern Iowa, behind
schedule... so flooding potential certainly higher than would
normally be this time of year. WPC has a 'Slight Risk' for
excessive rainfall potential in NW Iowa during this time- frame...
but cannot rule out need to upgrade to a moderate excessive
rainfall risk in tonight's forecast update.

Saturday Severe Potential...

Iowa looks to be in the warm sector much of Saturday, as our warm
front is slated to be near I-90 in Minnesota. The initial sfc low
seems to weaken over NW Iowa during the day Saturday. A "boundary"
oriented NNE to SSW associated with the original warm front/sfc
low tries to push eastward across Iowa. With the low weakening,
this boundary weakens as well. The best Gulf moisture is into
Illinois by Saturday evening. Some dynamical forcing present as
850mb moisture flux convergence along the "boundary". Ultimately,
am thinking central to eastern Iowa targeted area for potential
severe weather, with large hail and strong winds main threats. Am
not overly-excited about severe threat, but certainly not none.

Long Term...

Again, active pattern continues as several large upper lows come
onshore from the Pacific. 500mb anticyclonic flow in the SE CONUS
will support keeping Iowa in SW flow.

&&

.AVIATION.../For the 00Z TAFS through 00Z Friday evening/
Issued at 650 PM CDT Thu May 16 2019

Mainly VFR conditions are anticipated tonight with scattered
showers and thunderstorms. A few stronger storms may produce MVFR
or less, but timing and location confidence is too low for mention
until short term trends note otherwise. The frontal boundary
currently bisecting Iowa at 00z is expected to move little through
Friday with at least IFR stratus developing to the north for much
of the day.

&&

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

&&

DISCUSSION...Kotenberg
AVIATION...Small

Hazardous Weather Outlook
National Weather Service Des Moines IA
404 PM CDT Thu May 16 2019

IAZ023>028-033>039-044>050-057>062-070>075-172115-
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-
404 PM CDT Thu May 16 2019

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

Thunderstorms are expected to develop tonight with a few severe
storms possible with large hail and damaging wind the primary
threats. An isolated tornado and locally heavy rainfall are also
possible.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Periods of thunderstorms are forecast Friday through Sunday then
again Tuesday through Wednesday. Severe storms and locally heavy
rainfall will be possible but timing and location as well as
specific threats will be better determined in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed periodically from Thursday evening through
Wednesday.

IAZ081>086-092>097-172115-
Adams-Union-Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-
Wayne-Appanoose-Davis-
404 PM CDT Thu May 16 2019

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

Isolated thunderstorms are possible tonight. A severe storm may
occur.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Periods of thunderstorms are forecast Friday through Sunday then
again Tuesday through Wednesday. Severe storms and locally heavy
rainfall will be possible but timing and location as well as
specific threats will be better determined in the coming days.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed periodically through Wednesday.

IAZ004>007-015>017-172115-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
404 PM CDT Thu May 16 2019

This Hazardous Weather Outlook is for portions of north central
Iowa.

.DAY ONE...This afternoon and tonight

Thunderstorms are possible tonight. An isolated severe storm and locally
heavy rainfall is also possible.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Periods of thunderstorms are forecast Friday through Sunday then
again Tuesday through Wednesday. Severe storms will be possible
but timing and location as well as specific threats will be better
determined in the coming days. In addition, locally heavy rainfall
may occur, especially Friday night.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed periodically during the period.

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