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Area forecast discussion 
National Weather Service Des Moines Iowa
332 PM CDT Thursday Aug 27 2015

Short term /tonight/...
issued at 332 PM CDT Thursday Aug 27 2015

Primary concern is incoming upper level shortwave and the
timing...extent...and magnitude of precipitation this
evening/overnight and Onward through Friday. Large scale
models...NAM/GFS/Euro...have all been consistent in placing northern
Iowa/southern Minnesota under the gun for heavy precipitation...while the higher res
models...nmm/arw...were hot on central Iowa prior to moving north in
the most recent runs. With the models in better agreement...have
kept the bulls eye north of Ames. Forcing is of no question with
this system with a strong warm air advection/Theta-E push...associated vorticity
with the approach of the surface low...and an extra boost from the
low level jet this evening and overnight. An initial push of showers/storms
may affect northern Iowa this evening as a weaker warm air advection/Theta-E push
moves through ahead of the main show around 3-6z. The main show will
begin to push into the County Warning Area around 6-9zz. With hi-res models noting
some weak cape in places overnight...precipitation will likely be mostly
convective and elevated initially with dry air remaining in the
lowest 3km. Precipitable waters in the 1.5-2.0 inch range give further concern for
heavy precipitation...and in all...would not be surprised to be see rather
widespread 2 inch reports across the northwest and northern thirds
of the state and even down towards Ames with locally heavy rainfall
towards 3+ inches. Given that...flash flooding and rivers/streams
may be of concern in a few areas. Recently dry weather and healthy
crops will likely curb most non-urban flash flooding issues...but
excess runoff from heavy precipitation may move the main concerns to small
streams and rivers.

Long term /Friday through Thursday/...
issued at 332 PM CDT Thursday Aug 27 2015

Focus essentially will be on the first 12-18 hours of the
forecast as system crosses the state. At beginning of the
period...strong moisture transport will be ongoing into the state
on low level jet of 55kts ahead of surface/850mb low moving into western
Iowa. This will be impacting a baroclinic zone across northern
Iowa and supported in the middle level by decent kinematic forcing.
This will support widespread rainfall with embedded convective
elements although overall instability will be quite weak. Precipitable waters
generally 1.75-2.00 inches and generally a deep warm cloud layer
with warm rain processes. Anticipate moderate to heavy rainfall to
be ongoing and translating eastward with the upper system during
the morning. Quantitative precipitation forecast in the morning will generally be around an inch
across the north in the morning on top of what the receive
overnight. By afternoon...the primary forcing will be shifting
eastward with heavier precipitation in the north passing into
Wisconsin. However...instability farther to the south and
southwest will increase during the afternoon near the surface low
and may see some strong convection across the southeast third of
the forecast area by afternoon. This threat will come to an end
across the southeast on Friday evening with drier conditions later
Friday night and even some clearing across central Iowa. This may
allow for fog formation...especially west of i35 overnight as
winds weaken and low level will be moist from the recent rainfall.

Thereafter...upper ridging builds into the region and remains
generally in place for the remainder of the long term. This will
keep conditions dry with temperatures that will make it feel like
Summer once again.

&&

Aviation...28/00z
issued at 647 PM CDT Thursday Aug 27 2015

MVFR conditions with local IFR conditions expected after 06z through
much of at least Friday morning as upper level shortwave and
attendant surface low lift across Iowa. Surface flow will be S to southeast at
10-15kts ahead of the low then switch around to the to the north or
northwest across western taf locations very late in the forecast period.

&&

Hydrology...
issued at 332 PM CDT Thursday Aug 27 2015

Moderate to heavy rainfall event expected over the next 36 hours is
expected to result in significant within-bank rises on many area
rivers. The greatest rises are expected within middle to upper
portions of the Des Moines...skunk...Cedar and Iowa river basins. At
this time...using quantitative precipitation forecast for the full event does not bring any river
forecast points to flood stage. Some locations do rise within 2 to 3
feet of flood stage however. Antecedent soil conditions are expected
to help mitigate significant runoff issues.

There will be a flash flood risk however with this event...
especially in urban areas well as other flood-prone locations. At
this time...extent of flash flooding is not expected to be
widespread enough to warrant a Flash Flood Watch. Weather trends
will continue be monitored...however.

&&

Dmx watches/warnings/advisories...none.

&&

$$

Short term...Curtis
long term...cogil
aviation...fab
hydrology...zogg

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