Area forecast discussion
National Weather Service Des Moines Iowa
638 am CDT Tuesday may 21 2013
Short term.../today /...
issued at 257 am CDT Tuesday may 21 2013
Upper low near Aberdeen South Dakota will drift very gradually to
the southeast today. Subsidence is expected to be in place across
southern Iowa for much of the day with some lift farther north near
Minnesota. However...moisture is more limited than previous days and
the threat of precipitation looks rather minimal through the
daytime. Clouds associated with comma head of system will likely
be in place across much of the northwest third to half of the
forecast area during the day. In addition...cold advection will
persist through the day as 850mb drop to around +5c by 00z this
evening and anticipate temperatures to be quite cool in the
northwest today and have generally undercut guidance. Deeper
saturation associated with the deformation zone will arrive in the
northwest toward the end of the period but not in time for precipitation
threat until tonight.
Long term.../tonight through Sunday/...
issued at 257 am CDT Tuesday may 21 2013
The large upper low pressure system that has plagued the central
U.S. For the past day or two will finally begin to move eastward
tonight...moving over Iowa on Wednesday and then off to the east
thereafter. As a result a large area of clouds and light rain
showers will move into our County Warning Area late tonight and persist for most
of Wednesday before tapering off Wednesday night. While quantitative precipitation forecast is
quite light...rainfall appears fairly probable and have increased
probability of precipitation a bit on Wednesday...but held off on likelies for now due to
the expected patchy nature of the precipitation. A little elevated
instability will support the possibility of a few thunderstorms
mainly on Wednesday afternoon...but the overall scenario is not
conducive for any severe weather threat.
As the system and light rain depart to the east on Wednesday
night/Thursday morning a large high pressure ridge will build down
into the region bringing cool and dry weather for the remainder of
the work week. By Friday night the axis of a weak 500 mb ridge
will move overhead...and after this the forecast becomes more
uncertain. Behind this ridge axis lurk several weak middle-level
impulses that will provide the impetus for shower/thunderstorm
development at times this weekend and into early next
week...concurrent with a return of roughly southerly low-level
flow as the aforementioned surface high moves away. This will
support intermittent rainfall chances across the area...however
the timing and location of these chances is very uncertain due to
the weak nature of the forcing systems. In addition...it appears
likely that a surface low pressure area will develop in the Lee of
The Rockies and extend a warm front eastward across the
region...providing an enhanced and favored location for showers
and storms during the ensuing period. The GFS and Gem both develop
this feature across Iowa but the European model (ecmwf) is consistently forcing it
further south across Missouri...which would result in a largely
dry forecast for central Iowa. Given these competing factors have
stuck with several periods of chance probability of precipitation in the extended...but
this will need to be refined as the scenario comes into better
focus over the next couple of days.
&&
Aviation...21/12z
issued at 635 am CDT Tuesday may 21 2013
Upper low over South Dakota will drift slowly southeast during the
forecast period with extensive cloudiness across Iowa. Will see
MVFR ceilings across the north and west this morning but these
should improve to VFR with daytime heating. Surface winds will be
from the west-southwest during the day behind a cold front currently passing
through the state. Ceilings will fall overnight as upper low
enters the state and deeper saturation occurs as deformation zone
sweeps into Iowa. Will see MVFR ceilings become widespread across
the northwest affecting kfod and kmcw with the threat of some
scattered showers toward Wednesday morning. VFR conditions will
linger across the south and east overnight.
&&
Dmx watches/warnings/advisories...none.
&&
$$
Short term...cogil
long term...Lee
aviation...cogil
