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Area forecast discussion 
National Weather Service Des Moines Iowa
347 am CDT Wednesday Aug 27 2014

Short term /today/...
issued at 317 am CDT Wednesday Aug 27 2014

High pressure to the north of Iowa is doing its best to limit
instability over the state this morning. Despite this...a couple
areas of convection have developed. The first is over central Iowa
along the Highway 30 and Interstate 80 corridor where weak convection
has developed along the 700 mb moisture advection axis. This activity
should gradually shift to the east as the advection through this
area departs. Another area is over far southwest Iowa where the 850 mb
moisture advection is occurring. This area should be the focus region
for precipitation through the morning.

A short wave across eastern Colorado will lift into central Nebraska
by middle afternoon and begin to spread some kinematic forcing into
west central and southwest Iowa along with increasing upper level
divergence from the right entrance region of the upper jet. This in
addition to the moisture transport intensifying into the region will
result in increasing thunderstorm chances. Will keep the current
Flash Flood Watch in place through 15z. Still enough threat for
heavy rain through this period...especially on the southern edge to
keep in place. Dry conditions are expected to prevail much of the
day over the central and northeast portions of the forecast area.
Temperatures will be similar to Tuesday though potential for
considerable cloudiness over the southwest may hold temperatures
down a bit in that area.

Long term /tonight through Tuesday/...
issued at 317 am CDT Wednesday Aug 27 2014

Primarily wet pattern will continue to affect central Iowa
through much of extended. Upper level low will begin to push
eastward into Four Corners region through the day today which will
begin to help push surface low across western Kansas to the east.
However...this system will be very slowly moving with low lifting
into Iowa for late in the period. With slow moving
system...periods of precipitation will linger...with heavy
rainfall remaining possible. Main period of concern will be
tonight as strong shortwave pushes east over boundary...bringing
best chance for heavy rainfall. Models have once again begun to
slow down progression of this system...and given slower start to
overnight precipitation Tuesday night...have backed off on probability of precipitation
for beginning of period. Best forcing will begin to push into
southern and western Iowa near 00z...though low level jet remains further
west into NE...helping to keep probability of precipitation west. Precipitation will
spread into Iowa...with best chances coming after 06z as low level jet
begins to push into west central Iowa. Models have
remained consistent with pulling jet into west central and
northern Iowa through the overnight hours. Therefore have kept
best chances to the west and north...near going...though models
continue to bring heaviest precipitation further north...into
southern Minnesota through the day Thursday...and have cut back
slightly on southern extent of probability of precipitation through the day Thursday.

Frontal boundary will remain draped into Iowa for the overnight
Thursday and will slowly push east during the day Friday. This
will lead to additional chances for rainfall...again with the
potential for heavy rainfall...overnight Thursday as upper low
pushes east. However...timing of this system and best chances for
precipitation remains uncertain with NAM more
aggressive...bringing wave through Thursday night and other models
a bit slower...resulting in best chances for precipitation into
the day Friday. Have therefore kept chance probability of precipitation mentioned for
these periods. System will slowly push out Friday...with
additional lingering precipitation possible Saturday. Saturday
night will see brief break in precipitation chances before a
large upper level low pressure system pushes east across
Canada...with frontal boundary dropping south into Iowa for the
end of the weekend and into the end of the period.

Best chances for heavy rain will come tonight with ample moisture
around...continuing to increase through the overnight. Precipitable water values
will near 2 inches into early Thursday morning. This is seasonally
very high. In addition...good warm cloud depths and strong forcing
will allow for efficient rainfall producing storms. Flow will be
generally along the boundary with relatively slow storm motions
which will allow for slow moving storms and possibly back building
along boundary. Given relatively wet conditions across much of
Iowa...will likely need a Flash Flood Watch across portions of the
north and west. However...have opted not to issue at this time to allow
for more fine tuning with timing and placement of heaviest

Temperatures will be generally steady through the
period...however precipitation trends will make temperatures
forecasting difficult. Impacts from precipitation at times will
likely allow for much cooler temperatures than going.
However...have kept temperatures a bit warmer in the overnight
with clod cover expected...though again local influences from
storms may allow for cooler lows.


issued at 1147 PM CDT Tuesday Aug 26 2014

850mb moisture gradient is on the fringes of Iowa and should keep
the bulk of convection away from the taf sites through morning.
Generally will see VFR conditions across the area overnight into
Wednesday with MVFR to IFR conditions confined to in and around
convection. Winds will be light and variable but become east to
southeast on Wednesday.


issued at 317 am CDT Wednesday Aug 27 2014

Chances for heavy rainfall will remain through the beginning of
the extended...with the best chances coming tonight across the
west and north and again Thursday night. Given recent heavy rains
across much of Iowa...flash flood guidance remains near or below
two inches an hour. Heavy rainfall and training of storms will
allow for precipitation rates very near to flash flood guidance
across affected areas. A Flash Flood Watch may be needed for
portions of the County Warning Area tonight and again Thursday night.

In addition...heavy rainfall will lead to rises on local streams.
Current stream flow is fairly low...though rapid rainfall may
allow for significant run off which will help to rise streams
quickly. Low end flooding may be possible along rivers...with
smaller streams seeing greater rises if affected by heavy


Dmx watches/warnings/advisories...
Flash Flood Watch until 10 am CDT this morning for Audubon-



Short term...donavon
long term...awb

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