Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
1144 PM CDT sun Aug 30 2015
..updated for 31/06z aviation discussion...
Short term /tonight/...
issued at 315 PM CDT sun Aug 30 2015
Conditions remain sluggish over the area and only a little change is
expected overnight tonight. Though low pressure is gradually moving
our way overnight through morning...the surface gradient is likely to
relax enough for stratus and some fog to redevelop across the area.
Complicating matters is a slow northwesterly drift of low level
moisture at this time. Areas of fog will likely develop again after
midnight with the better chances of some locally dense fog across
the east/southeast. Across the west slightly stronger winds should
tip the balance tonight in favor of patchy fog with lesser chances
of dense fog. With gradually warming thickness overnight it becomes
a balancing act between the increase in low level moisture and
increasing thickness...as to whether more widespread dense fog
develops. For now...will stick with mention in grids and let the
evening/middle shift better determine if headlines are needed.
Otherwise...the partial breaks in the clouds over the southeast and
northwest are likely to fill in again overnight. Lows will be a few
degrees warmer than last night across the board and remain in the
low to middle 60s. Looking ahead to Monday night...conditions remain
favorable for another round of patchy fog with locally dense fog a
Long term /Monday through Sunday/...
issued at 315 PM CDT sun Aug 30 2015
Thoughts on the upper level ridging across the central and eastern
U.S. Have not changed much. Models continue to depict ridging being
rather broad through middle-week before overall amplification at the
end of the week and into the weekend as a trough digs into the
Pacific northwest. In the meantime...a couple of minor concerns
regarding precipitation as a few perturbations move through.
Monday to Thursday morning...a weak trough will eject out of the
northern rockies tonight/Monday morning and quickly move through the
northern plains...compacting the ridge a bit and keeping northwest
Iowa on the edge of the SW upper level flow and in line to be
clipped by a couple of passing short waves. The first wave of any
consequence for the County Warning Area looks to move through overnight Tuesday into
Wednesday morning followed by another Wednesday night into Thursday
morning. Both instances have relatively weak forcing...vorticity maximum and
Theta-E advection in the former and q-vector/ageostrophic and isentropic
in the latter...but certainly not weak enough to rule out slight to
chance probability of precipitation.
Thursday afternoon to Sunday...by the end of the week the ridge
should amplify as a trough digs into the Pacific northwest. As a
result...temperatures appear to be warmest of the long term with the
southerly flow...best shot at clear skies...and ample mixing.
Currently have central/southern Iowa topping out int he upper 80s
but would not be surprised to see a 90 or two creep into the
forecast over the next couple of days if the smattering of probability of precipitation the
models have clear out. At the very end of the long range...ridging
will break down as the trough over the western U.S. Begins to trek
eastward. Surface low pressure and an attendant cold front will push
through late Sunday into Monday...bringing with it the best chances
for widespread precipitation for the forecast period.
issued at 1144 PM CDT sun Aug 30 2015
Main concern will be fog and stratus potential. Expect at least MVFR
conditions overnight...with many locations IFR or lower. Kfod and
kmcw will see the lower visibilities initially...with lower visibilities migrating
southeastward toward the kotm and possibly kdsm terminals. Clouds
moving in from the west may mitigate the lower visibilities across western
Iowa. During the day on Monday...VFR flight rules expected.