Area forecast discussion
National Weather Service Des Moines Iowa
353 PM CDT Wednesday may 22 2013
Short term.../tonight/
issued at 353 PM CDT Wednesday may 22 2013
Upper low is moving very slowly to the east tonight and as a result
will likely see light showers across the north this evening and far
east much of the night. Still expecting some light showers or
sprinkles elsewhere as well but these will be much lighter and much
more hit and miss than north and east. The nam12 shows another wave
of Theta-E advection pushing south across the County Warning Area this evening but
forcing is weak if there at all. This should provide for a brief
period of better shower activity across the north through middle
evening before everything finally pushes east overnight. With
expansive cloud shield in place and low ceilings in place...I would
expect the lower clouds to linger for a fair part of the night with
clearing expected across the north late. Temperatures will need to be
monitored if we clear out a little quicker as the airmass in place
is pretty cool for late may. Temperatures could fall quicker and farther
with a faster clearing sky.
Long term.../Thursday through Wednesday/
issued at 353 PM CDT Wednesday may 22 2013
Primary concern will be convective trends and precipitation from the
weekend right on through the middle of next week. Period will begin
with a few days of fair weather however as current system exits and
Iowa remains under the influence of upper level ridge. Progression of
ridge and introduction of prolonged intermittent warm/Theta-E
advection will begin Friday night however and last from the Memorial
Day weekend right into the middle of next week. With heights
gradually lowering in persistent west-southwest flow between slow moving western
trough and departing MS/Ohio Valley ridge...this will place Iowa into
what looks to be a nocturnal maximum pattern with soundings showing
only minimal potential for surface based convection.
Difficult to provide too many details yet...but Sat night into Sun
morning is one period of concern with regard to severe weather and
heavy rains with low level jet and 300k inflow right into western Iowa.
GFS elevated convective available potential energy approaching 1500 j/kg with seasonally high precipitable waters
and warm cloud depths up to 12kft will provide favorable
environment for efficient rainfall. Storm movement will not be
much either with mean wind 20kts or less.
Do not expect mcss every night but pattern really changes little
into the middle of next week with 850 mb flow remaining focused from
Southern Plains into Iowa. Thus have kept prolonged period with chance
wording and...unfortunately...little confidence in when to
highlight particular periods with higher or lower probability of precipitation at this
point. Elevated convective available potential energy are not as impressive as Sat night for
following nights...but stagnant pattern will keep potential for at
least weak mesoscale convective system activity in the forecast. Cumulative rainfall will
keep things wet with little hydrologic recovery and at least some
potential for additional flooding.
&&
Aviation...22/18z
issued at 1239 PM CDT Wednesday may 22 2013
An upper level low will continue to move east today with bands of
IFR conditions and light showers passing across the taf locations.
After 23/15z the low will shift far enough east that conditions will
become VFR from west to east. Surface wind will be west to northwest at 15g25kts
diminishing after 02z to 10-15kts and becoming north then increasing
again after 23/12z.
&&
Dmx watches/warnings/advisories...none.
&&
$$
Short term...fab
long term...small
aviation...fab
