Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
627 PM CDT Tuesday Aug 4 2015
Short term /tonight/...
issued at 337 PM CDT Tuesday Aug 4 2015
A relatively quiet night is in store with a surface high pressure
area sliding across the area and moving off to the east. A middle
level shortwave will approach from the west...but the broad ascent
on its forward flank will not arrive in earnest until Wednesday
and there is little ambient instability to work with in any event.
Nevertheless...decent available moisture combined with the
development of the nocturnal low level jet may be sufficient to
kick off scattered showers and thunderstorms across our southern
counties overnight...and have maintained chance probability of precipitation in that area
but slowed down the progression northward. In any event...no
significant impacts are anticipated. Radiative cooling will be
more mitigated tonight by convective debris clouds spreading
across the area...except in the north where skies are mostly clear
and dewpoints remain lower. Have dropped overnight low
temperatures by a couple of degrees up around Waterloo and Mason
City as a result.
Long term /Wednesday through Tuesday/...
issued at 337 PM CDT Tuesday Aug 4 2015
Main forecast concern was focused on temperatures and periodic
thunderstorms chances through the end of the work week and into
the weekend. Overall...a fairly active weather pattern but low
confidence in any widespread severe weather potential.
Plus...timing of weak upper level shortwaves riding the ridge
through the weekend remains problematic. Thus...leaned toward a
blend of GFS/European model (ecmwf) during the forecast period.
Wednesday through Friday...zonal flow transitions to northwest
flow by the end of the work week with a couple 500mb vorticity maxes
cutting across the region. The first pulse of energy will slide
across southwest Iowa near a weak stationary boundary draped
across northern Missouri/southwest Iowa Wednesday. Expecting some
scattered showers/T-storms through the morning tomorrow before
shortwave exits the state by early Wednesday evening. Kinematic
forcing is weak and disorganized and there is limited instability
present...hence not anticipating any severe weather Wednesday.
Another shortwave tracks across the Dakotas into Minnesota late
Thursday night into Friday affecting mainly northern Iowa. This
will bring the best potential for rain...and severe
weather...albeit low threat during the extended. There is slightly
stronger forcing and Theta-E advection that tracks across northern
Iowa Thursday night...and have mention of likely probability of precipitation over
this area during the aforementioned time frame. A marginal severe
threat with the lack of significant large scaled
forcing...instability and shear present. One change to
note...winds look slightly stronger than previous forecast and
increased winds Friday afternoon as decent mixing develops. Winds
atop the mixed layer near 30 knots and thus also tweaked maximum
temperatures up slightly and lowered dew points due to the mixing.
Saturday into Tuesday...upper level ridge remains over the
southwest and keeps Iowa well into the northwest flow aloft. Low
confidence in timing of any shortwave riding the ridge this
weekend with GFS/European model (ecmwf) depicting anywhere from 2 to 4 weak
systems to affect the state. The strongest pulse looks to be in
the south early Sunday morning...and then a decent cold
front/trough move across the region by early Monday.
issued at 624 PM CDT Tuesday Aug 4 2015
Expect VFR conditions at sites through period. May see rain showers or
thunderstorms and rain at sites after 06z...with the best chances at kotm...kdsm and
kfod. However...do not have enough confidence to include mention
at this time...with only vcsh mention at kdsm near 12z. Winds will be
generally variable...becoming southeast through the day Wednesday.
Short term...jl Aug 15