Flash Flooding and Flooding in Iowa, May 23 to 24, 2019

Area Forecast Discussion
National Weather Service Des Moines IA
614 AM CDT Thu May 23 2019

.DISCUSSION.../Today through Wednesday/
Issued at 413 AM CDT Thu May 23 2019

There is little change in the primary weather messaging this
morning with our pattern remaining active for an extended period.
Outside of what looks to be break today and then again Sunday,
there will be off and on showers and thunderstorms through the
holiday weekend and into the middle of next week. Although there
are differences with frontal positions and pinning down details
with each potential MCS, the GFS, ECMWF and their associated
ensembles are in good agreement with the progression of our long
wave pattern at least starting with upper level ridging building
into the MS Valley today, and then a return to SW flow aloft into
the weekend before this cycle repeats itself Sunday and into the
middle of next week before the long wave trough axis finally
reaches IA in some form by Wednesday. This will include multiple
instances of severe weather potential to some degree, and a
incremental increase in flooding potential with each event as rain
accumulate. The good news however is that the latest RFC
contingency forecasts from yesterday, and precip through 12z
Monday, suggest only isolated potential for more extensive minor
river flooding through that period.

At onset today fair weather and seasonal temperatures are
anticipated, but this will quickly change into tonight. The
current frontal boundary across OK, MO and IL will move northward
again as the ridge builds, washing out to some degree, but still
allowing at least weak instability to return. The associated
warm/theta-e advection will produce nocturnal showers and storms
to some degree, although confidence is not the best with varied
solutions among the parameterized and non-parameterized model
depictions in terms of areal coverage. Regardless the severe
potential seems to be on the low end with limited instability and
better effective shear pushing north. Some moderate to locally
heavy rains are possible however with 30kt 925-850MB flow and
associated moisture convergence focused into IA in seasonal
absolute moisture content and warm cloud depths.

The weak surface low associated with this progression will move
north through the central Plains and dissipate somewhat as it
leaves the warm front just to our northeast by late Friday. This
will leave a much more favorable thermodynamic environment into IA
with MLCAPEs to around 2K j/kg and easily favorable deep shear
for organized severe weather. The question will be how the trigger
materializes as the aforementioned low progression will leave a
thermal boundary in place into northwest IA, but little surface
convergence in broad southwest surface flow. This may be initially
be inefficient for surface based development, but the maturing
low level jet and subtle lift in the southwest flow is expected
to blossom a line of convection across central or southeast IA.
The shear may not be favorable into the night, but with
increasing precipitation efficiency parameters, weak Corfidi
vectors and the mean wind parallel to the boundary the training,
heavy rain potential will increase leading to WPC noting a
Moderate Risk of excessive rains.

The front should retreat north again by Saturday, and although the
heavy rain potential seems somewhat less favorable, the severe
potential may increase with MLCAPEs to 3K j/kg and strong deep
shear. Both the GFS and ECMWF depict the boundary over northern IA
during peak heating so the Day 3 SPC outlook probabilities may
eventually increase to the north. This convection will likely
linger into the night before exiting to the south and east for a
welcome break by Sunday. Details for the remainder of the period
will be more sketchy, but the nocturnal warm advection, frontal
surface peak heating cycle looks to repeat into early next week
with the Monday/Monday Night looking like a peak in intensity due
to more favorable instability and shear parameter space. The
potential for active convection persists into Tuesday and
Wednesday as well, but confidence is low. As mentioned above the
model larger mass fields are in good agreement, but short wave
details and associated surface reflections are not resulting in
anything from a more active ECMWF into the middle of next week
down to the more tranquil GFS pushing the boundary off to the
south and east.

&&

.AVIATION.../For the 12Z TAFS through 12Z Friday morning/
Issued at 612 AM CDT Thu May 23 2019

VFR conditions are anticipated through much of the period, but
precipitation and lowering and thickening cloudiness is
anticipated tonight. Conditions should remain VFR during the
evening, but will likely transition to MVFR in spots overnight.
Confidence is not great enough for inclusion of thunder at this
lead time.

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

&&

DISCUSSION...Small
AVIATION...Small

Hazardous Weather Outlook
National Weather Service Des Moines IA
530 AM CDT Thu May 23 2019

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-241100-
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-
530 AM CDT Thu May 23 2019

This Hazardous Weather Outlook is for much of central Iowa.

.DAY ONE...Today and tonight

Chances for thunderstorms return tonight with moderate to locally
heavy rains being the primary threat.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Outside of a brief break around Sunday, periodic thunderstorm
chances are forecast Friday, through the Memorial Day weekend,
and into the start of the next work week. Severe weather, mainly
large hail and damaging winds, and locally heavy potentially
flooding rains will be possible at times.

A River Flood Warning also remains in effect for the West
Fork Des Moines River near Estherville for the foreseeable future.
Please refer to the National Weather Service website at
weather.gov/desmoines for further details.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed off and on through the Memorial Day
weekend, especially Friday, Saturday, and Monday.

Small