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Area forecast discussion 
National Weather Service Des Moines Iowa
344 am CDT sun may 3 2015

Short term /today/...
issued at 341 am CDT sun may 3 2015

Two areas of thunderstorms across the County Warning Area at this time...the first on the
leading edge of 850 mb Theta-E advection across the northeastern County Warning Area
slowing moving east-northeast and should push out of the County Warning Area in the next few
hours. The other associated with weak shortwave energy and area of
warm air advection across the southwest. This area has been steadily decreasing in
intensity over time...but has produced some higher wind gusts across
the far western County Warning Area. As the upper level wave pushes east this
morning...expect this area of convection to push eastward through
the southern County Warning Area through middle morning. NAM tries to invigorate it a
bit with some expansion northward with daylight toward middle/late
morning. Additionally possibility of the energy responsible for
convection across eastern/NE Kansas to lift northeastward toward the
southern County Warning Area this afternoon. With strong warm air advection today...and southerly
flow decent warming expected. This this will bring cape values into
the 1500-3000 j/kg range across the County Warning Area by late afternoon. Push of
Theta-E expected into the southern County Warning Area by middle/late afternoon as
well...so could see some redevelopment with the energy lifting
north. Otherwise greatest chance for thunderstorm development will
be across the north late this afternoon into the early evening as a
cold front sags southward into the state. Have delayed storm chances
as looks like convective initially along/ahead of the front expected
toward 21-00z across the far north. Decent cape expected...with more
marginal shear...but still could see some isolated to scattered
severe storms across the north by early evening.

Long term /tonight through Saturday/...
issued at 341 am CDT sun may 3 2015

Precipitation trends will remain the concern through the entire forecast
period as there is no sign of an appreciable break in the coming
week. Convection should be either in progress or in the process of
initiation at onset as frontal boundary enters northwestern portions of Iowa.
MLCAPES will be 2-3k j/kg to start with minimal cinh and adequate
convergence to kick things off. Would not be surprised to see a few
pulse severe storms considering the degree of instability...however
soundings along and ahead of the front depict fairly unidirectional
winds and seasonally weak middle level flow. This will keep the
effective shear down and likely result if a fairly quick decrease in
intensity through the evening. 300/305k inflow above the inversion
and base of elevated instability weakens and becomes parallel to the
front so convection may still continue into the night as the front
sags south but decrease in coverage. Expect re-generation into Monday
however as deep convergence zone stalls for the time being over the
state and forcing increases once again as weak waves traverse the
southern edge of US/Canadian westerlies. Convection will likely persist
into Monday night as well as baroclinic zone and Theta-E advection
begins to lift northward due to subtle middle MS valley ridging. There
is some severe potential SW as noted in Storm Prediction Center day 2 outlook southwest
as models depict some surface based potential along the boundary.
Middle level winds do not increase much and MLCAPES are not too
impressive for this time of year...but 0-6km shear may increase
sufficiently in the proximity of the boundary to offset thermal
deficiencies.

Could see a brief break by later Tuesday as baroclinic zone and
associated thermodynamic forcing lifts to our north with surface
warm front laying across northern Iowa. This will result in quite the
thermal contrast with highs from the lower 80s south to near 70
north. Warm front should push north out of Iowa by Tuesday night with
attention turning to rockies trough upstream. Although uniform
warm sector surface flow will be in place with little convergence
Wednesday...weak lobe of forcing will increase probability of precipitation once again Wednesday and
Wednesday night. Warm advection on nose of Central Plains thermal ridge
may also aid nocturnal development. Weak frontal boundary reaches
the siouxland area by late Thursday and remains somewhere in the
vicinity of Iowa through the end of the period keeping relatively
high probability of precipitation in the forecast for an extended time. Severe potential
in the extended does not appear overly concerning with shear and
instability not phasing to high levels too often.

&&

Aviation...03/06z
issued at 1152 PM CDT Sat may 2 2015

Interesting forecast tonight as hi-res models have picked up on middle
level instability in response to a weak wave pushing out of Nebraska
and across Iowa tonight. Thunderstorms have developed in this area
and will continue to trek east in a narrow corridor overnight...
mainly affecting kfod and Kalo. After 21z a front will slowly sag
south across taf locations bringing thunderstorms chances increasing
across northern tafs by early to middle evening and approaching kdsm by
06z.

&&

Dmx watches/warnings/advisories...none.

&&

$$

Short term...beerends
long term...small
aviation...fab

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