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Area forecast discussion 
National Weather Service Des Moines Iowa
338 PM CDT Sat Sep 24 2016

Short term.../this evening through Sunday/
issued at 337 PM CDT Sat Sep 24 2016

Thunderstorms have developed over western Iowa. These storms are
moving from south to north that is resulting in some training of
storms and locally heavy rainfall with radar estimates over 1.5
inches in some areas across Crawford County the main surface
boundary is still farther west across the eastern Dakotas and
Nebraska but will be moving slowly east through the overnight.
Moisture advection will continue overnight and will begin to
saturate profiles with pwats again rising to around 2 inches. Deep
layer shear values are modest in the 25 to 35 kt range and should
support multicell clusters as opposed to more organized storms and
as profiles saturate the potential for large hail will diminish with
decreasing parcel acceleration potential aloft. There is the
potential for cold pool development leading to a few damaging wind
gusts with favorable 0-3 km Theta-E differences.

The saturating profiles helping creating deep warm cloud depths and
the aforementioned pwats that are 225 to 250 percent above seasonal
normals, will lead to efficient rain producing storms. Locally heavy
rainfall will again be a concern with localized amounts over 2
inches possible. This may generate additional flash flooding
especially in areas hit hard over the past several days. Will issue
a Flash Flood Watch to match mpx/arx over the far northeast and in
the region that already has a tremendous amount of standing water.
Local emergency managers reports several roadways have just reopened
over the past 24 hrs and additional heavy rainfall will likely cause
renewed flash flooding.

Long term.../Sunday night through Saturday/
issued at 337 PM CDT Sat Sep 24 2016

Forecast concerns will mainly be centered around heavy rain/flooding
from overnight convection and lingering showers/storms through
Sunday morning. A strong cold front should be through the forecast
area by 18z Sunday with waning instability and shear so the threat
for strong storms will be minimal. However any rainfall over the
northeast will be problematic as numerous river and areal flood
warnings continue. Showers of moderate intensity will continue over
the east through early to mid afternoon before the upper trough
finally pushes through. There will be strong cold advection with
this trough and colds may be slower to clear than models are
suggesting so as a result I have lowered temps and gone below
guidance for both Sunday and Monday. We will also have some pretty
stiff winds on Monday making it feel quite fall-like...despite more
sunshine.

For Tuesday the trough that passes to our east begins to develop
into a deep closed low over the Great Lakes in response to a
building ridge over The Rockies and the west. It is this feature
that the models do not handle well going forward. With a deepening
low to our east and a building ridge to our west...that will put the
region in a deep northerly flow so while I went more with the model
blends on day 4...this could be too warm as well and we may not see
temps out of the 60s again.

For Wednesday into late week the western ridge works it's way over
to the upper Midwest and temps moderate into the 70's. The whole
time though the forecast is dry so some relief is in site for the
flooded portions of the state. Then for Friday and Saturday the
Euro takes the low that develops over the Great Lakes and
retrogrades it back into the Ohio Valley while the GFS pushes it off
to the southeast. The extended is still dry but the implications of
the Euro would suggest at least some threat of precip over the east
later next weekend. Both models develop a deep trough over the west
in the Friday/Saturday time frame and both try to advect some
moisture back into the plains with precip chances...mainly west of
the state. All this is going to be interesting to watch how the
models resolve but sensible weather-wise for US...it will still be
dry with seasonal temperatures in the 70s and lows in the 50s.

&&

Aviation.../for the 12z tafs through 12z Sunday morning/
issued at 637 am CDT Sat Sep 24 2016

Low stratus and areas of fog are expected early in the taf period
at most locations with moisture trapped under a frontal inversion.
This should begin to mix out as winds increase today ahead of
approaching system. However, prior to mixing will see IFR
conditions at most locations improving to VFR by 18z. Showers and
thunderstorms are expected to cross central Iowa tonight with a
frontal boundary with MVFR to IFR conditions in and around the
storms.

&&

Dmx watches/warnings/advisories...
Flash Flood Watch from 10 PM CDT this evening through Sunday
morning for iaz006-007-016-017-025>028.

&&

$$

Short term...donavon
long term...fab
aviation...cogil

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