Area forecast discussion
National Weather Service Des Moines Iowa
636 PM CDT Friday may 24 2013
Short term.../tonight/
issued at 400 PM CDT Friday may 24 2013
Surface high pressure will remain in place over the Great Lakes as a
south-southwest flow aloft sets up a few thousand feet aloft and
aims into Iowa. This will result in the thermal gradient and zone of
frontogenesis setting up from northwest to southeast across Iowa. Leading edge of
warm advection is producing isolated to scattered thunder from the
Missouri River north into the Dakota/Minnesota border region. This
will continue to dribble east and eventually turn southeast across
north and northeast Iowa. Around midnight the southwesterly
low-level jet will increase and more widespread thunderstorms are
expected to develop on the back side of this region...most likely
over north central Iowa and then expanding east and south during the
overnight hours. Shear and instability are not favorable for severe
weather...but conditions are somewhat favorable for heavy rainfall
due to fairly slow storm motion to the east and possible training of
storms. Pinpointing the location of heavier rain is difficult due to
elevated nature of storms...but again north central into east
central Iowa would be most likely to see over 1 inch of rain
overnight. Short term runoff problems in areas of high rain rates
would be the most likely flood threat tonight since we have not seen
rainfall in several days.
Long term.../Saturday through Friday/
issued at 400 PM CDT Friday may 24 2013
The primary concern through this period will remain convective
trends and heavy rains...and eventually better severe potential into
next week. Evaluation will change little from as outlier in above
short term discussion with details hard to come by. Main theme will
be nocturnal/warm advection maximum with better chances during the
nighttime and early morning hours. Expect mesoscale convective system to be ongoing at
beginning of the period with Theta-E advection still focused right
into central Iowa. Typical diurnal decrease in probability of precipitation is anticipated
later in the day until the next round of overnight convection cycles
up. Theta-E advection Sat night does not appear to be as strong as
what may occur tonight...but what occurs may be more conductive for
heavy rains. Precipitation potential placement...warm cloud depths
and precipitable water will all either appreciably increase or be
more entrenched into Iowa with mean wind even a tad weaker. This
could certainly result in locally heavy rains and efficient rainfall
producers. Do not expect widespread rainfall with any particular
mesoscale convective system...but cumulative effects from night after night will likely
produce widespread one to two inch amounts by Monday with locally
higher amounts possible. Temperature forecasts will be problematic
this weekend too due to uncertainty with regard to
mixing...climatology vs models southeasterly flow/stratus signal. Generally
played a middle of the Road approaching but less convection could
certainly end up with higher temperatures.
Airmass will definitely become more Summer-like into next week with
persistent 1km southerly fetch boosting dewpoints into the 60s.
This may also increase our severe and surface based potential
setting up somewhere in the vicinity of Iowa...whose latitudinal
extent will be difficult to pinpoint due to repetitive convective
outflow. There may be somewhat of a lull around Wednesday as warm sector
becomes better entrenched into Iowa and warm advection zone lifts
northward. However shortly after this time long wave trough will
finally advance into the plains increasing kinematic forcing. This
may increase severe threat although surface features and associated
convergence zones still look to be off to our north and
west...especially if the European model (ecmwf) verifies.
&&
Aviation...25/00z
issued at 635 PM CDT Friday may 24 2013
Band of high based showers and thunderstorms will move through the
area early this evening. Expect a break with VFR conditions
prevailing before ceilings gradually lower and thunderstorms become
more widespread late tonight into Saturday morning. Ceilings will drop
to MVFR to LIFR at times. Expect visibilities to drop to IFR or less also
at times during periods of intense rainfall. Precipitation chances
will be lower after 18z Saturday but potential is there for low
ceilings to linger. Breezy southeast winds can be expected through the
period.
&&
Dmx watches/warnings/advisories...none.
&&
$$
Short term...jungbluth
long term...small
aviation...donavon
