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Area forecast discussion 
National Weather Service Des Moines Iowa
630 am CDT Thursday Aug 21 2014

Short term /today/...
issued at 409 am CDT Thursday Aug 21 2014

Thunderstorms developing across northwest Iowa and southern
Minnesota will affect our far northern counties early this morning
with a low risk of severe weather and localized flooding...but will
exit the forecast area around or shortly after 12z. Otherwise
isolated thunderstorms will remain possible across central Iowa both
from early morning development overhead and from the cluster of
storms moving in from central Nebraska...which should fade as they
enter Iowa later in the morning. This leads to considerable
uncertainty regarding the degree to which storms will or will not be
able to survive in the late morning and afternoon...which will have
a significant effect on our temperatures later in the day. We saw
yesterday what can happen if the precipitation/clouds linger into the
afternoon as temperatures were nearly 10 degrees cooler than
anticipated in some areas. For now have lingered isolated
thunderstorm wording through around midday across central
Iowa...lowered high temperatures a bit as it appears residual
convective debris clouds will once again be an issue at least to
some extent...and do not feel that heat headlines are warranted in
this scenario. Toward this evening storms will again be
possible...but believe we will remain capped off with precipitation
likelier after dark and into the overnight period once again.

Long term /tonight through Wednesday/...
issued at 409 am CDT Thursday Aug 21 2014

Precipitation and temperature trends will be the primary concerns through the
entire period with pattern remaining quite active. Models remain in
decent agreement on the larger scale into early next week with
fairly high amplitude western trough to Ohio Valley/eastern Great Lakes ridge.
Short wave currently pushing through Dakotas/Minnesota and driving mesoscale convective system from
northern Iowa into Minnesota will also eventually sag surface reflection and
boundary into Iowa by tonight. There is some potential for surface
based convection to fire along the low level convergence and
continue into the evening as Theta-E advection and moisture
transport intersects the boundary. It quickly veers toward
daybreak however...so it should begin to wane and dissipate into
Friday morning. Looking for things to turn more active toward 00z
and into the night however. Surface based convection may re-fire
near the warm front from NE into Iowa. Western trough is expected to be
just progressive enough to increase synoptic scale lift and
thermodynamic forcing to fuel mesoscale convective system into the night as warm front
again lifts northward. Precipitation chances may still linger through the
day...but gradually lift south to north. Chances will remain in
the forecast...mainly northwest third attendant to NE/South Dakota surface
front...then increase into Sun night west to east as surface based
convection oozes into Iowa later in the night fueled by an uptick in
kinematic forcing associated with short wave lifting through the
northern plains.

Forecast confidence decreases into next week however as GFS/Gem are
still more progressive lifting long wave trough from the northern
rockies into the northern plains while the European model (ecmwf) has changed somewhat
leaving cutoff remnants over The Rockies. Have not changed
forecast too much due to this uncertainty. Models suggest probability of precipitation to
some degree through Wednesday for different reasons...European model (ecmwf) warm
advection vs GFS lingering weaker precipitation behind the trough.

The airmass through early next week will not change much and remain
favorable for locally heavy rain with elevated precipitable waters and warm cloud
depths. As mentioned in previous discussions...flash flood watches
may eventually be needed but not until confidence in convective
trends over favorable high soil moisture areas increases. Severe
potential will remain there as well but with no particular time or
location to focus on as of yet...although surface based development
late sun seems somewhat more favorable with better phased
instability...shear and forcing.

Heat indices will approach advisory criteria through the
weekend...and headlines could eventually be needed if this occurs
repetitively for several days...but convection and associated debris
could certainly affect things and keeps confidence low enough to
preclude any headlines at the moment.

&&

Aviation...21/12z
issued at 630 am CDT Thursday Aug 21 2014

Widely scattered thunderstorms and rain will persist for the next several hours...primarily
affecting mcw...then diminish by midday with very low probs
thereafter precluding taf mention. Expect VFR conditions to
prevail for most of the day...then tonight after dark stratus/fog
should develop especially at mcw/alo. At this range confidence in
magnitude and timing is low but have advertised IFR visibilities at those
terminals. Additional thunderstorms are also possible overnight...but
uncertainty is high in that regard so no mention has been made in
the 12z tafs. This will be closely monitored today and tonight.

&&

Dmx watches/warnings/advisories...none.

&&

$$

Short term...Lee
long term...small
aviation...Lee

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