Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
708 PM CDT sun Aug 2 2015
..updated for 03/00z aviation forecast discussion...
Short term /tonight/...
issued at 255 PM CDT sun Aug 2 2015
A cool front will cross the forecast area from northwest to
southeast this evening...with an associated wind shift and falling
temperatures and dewpoints as the boundary passes through concurrent
with sunset. The airmass ahead of the front is very unstable and
there is a possibility of thunderstorm development late this
afternoon and evening. However...mesoanalysis indicates warm
temperatures aloft are suppressing convective initiation thus far as
confirmed by a distinct lack of cumulus development in satellite
imagery. High resolution models are picking up on this and generally
developing few if any storms across our area...with most activity
remaining over Wisconsin and far eastern Iowa through the evening.
Have maintained isolated thunderstorms ahead of the boundary in the
outgoing forecast...as if they do initiate the coverage should be
spotty. However...despite the low probability and coverage any
storms that do form have the potential to be strong to severe given
the degree of instability present...with hail and wind being the
threats. Once the boundary and any storms sweep out to the
south/southeast early tonight cooler and quiet weather should
prevail into tomorrow morning.
Long term /Monday through Sunday/...
issued at 255 PM CDT sun Aug 2 2015
Active weather pattern will set up for extended after a brief
break in precipitation chances. Boundary pushing south through
Iowa today will continue to push south into Missouri...keeping
conditions dry early Monday. Models have begun to come into good
agreement with much of period...though NAM appears to be most
aggressive and precipitation heavy...and have therefore trended
towards GFS/European model (ecmwf) blend. Boundary lingering in the Missouri River
valley will push east into Iowa late Monday...bringing chances for
precipitation to the southwest. This system will push
east. However...expect precipitation trends to continue to push
northeast as wave lifts into Iowa through the day Tuesday setting
up prolonged periods of precipitation chances as northwesterly
flow persists across area as ridge across west begins to slowly
shift east. Generally expect to see only periods of
precipitation...though timing of waves is difficult at this time. Best
chance for heavy rain will come Tuesday night across the south as
low pushes east. Best moisture transport will be very near Iowa/MO
boarder and may push south into Missouri. Currently models
indicate precipitable water values near 1.6 inches in the south...which is
relatively high for August. This will be followed by another round
of uncertain timing on precipitation chances with large upper low
pushing across Canada finally breaking down ridge and pushing
east by end of week...bringing a break in precipitation chances
for the end of the period.
With northwesterly flow aloft temperatures will be cool for the
period...general below normal for early August. Do not have high
confidence in such cool temperatures...given uncertainties in
timing of precipitation chances in the extended...and have bumped
temperatures up a few degrees...especially early in the period.
However...feel highs in the 80s is a good trend for much of
period...with cooler temperatures possible late in period.
issued at 708 PM CDT sun Aug 2 2015
Rain showers/thunderstorms and rain will accompany a cold front in southern Iowa as it slides
southward this evening. Some brief MVFR/IFR visibilities/ceilings are possible
in the stronger storms. Behind the front...winds will be northwest.
VFR flight rules are expected during the balance of the taf period.