Scientific Forecaster Discussion

Return to Local Conditions & Forecast
Area forecast discussion 
National Weather Service Des Moines Iowa
621 PM CDT Thursday Apr 17 2014

Short term /tonight/...
issued at 303 PM CDT Thursday Apr 17 2014

A surface high pressure ridge along the Missouri River will transit
the state tonight with the axis positioned just east of the County Warning Area by
sunrise on Friday. Northwesterly winds will lessen as the ridge
approaches and then veer to the east and southeast on the backside
of the ridge. Lowered min temperatures a couple of degrees below guidance
across the northeast with the ridge axis directly overhead between
06 and 12z...though clouds approaching after 09z may keep temperatures
warmer than forecast. These clouds are tied to a region of Theta-E
advection on the leading edge of the returning warm air.
However...the best thermodynamic forcing is further to the northwest
over southwest Minnesota and eastern South Dakota. Removed the slight chance probability of precipitation
that was originally in the forecast for the northwest County Warning Area with the forcing
further northwest than previously prognosticated and substantial wedge of dry air
below 5-10 kft as noted on the slb and form NAM bufr soundings.

Long term /Friday through Thursday/...
issued at 303 PM CDT Thursday Apr 17 2014

Forecast challenges in extended revolve around precipitation
chances near weekend. For near term...ridge will build in for
Friday and Saturday ahead of low pressure system.
However...shortwave will push across northern Iowa for Friday
morning/afternoon. Have left probability of precipitation out for now with very limited
moisture in low levels. However...may see a few scattered showers
or sprinkles early. With warm air advection and southerly flow under ridge
temperatures will warm into Friday and Saturday.

Frontal boundary will drop southeast across Iowa Saturday night.
Models continue to slow progression of this boundary...and have
also cut back on timing of precipitation. However...thunderstorms
are possible Saturday afternoon across the northwest ahead of
the boundary with decent instability and forcing near boundary.
Boundary will slowly push secondary low in central
Kansas lifts northeast. Models are currently in decent agreement
with this set up...though are not in good agreement with timing or
track of southern low. However...general thinking is that low
lifting northeast will limit southern push of frontal
boundary...with prolonged periods of precipitation across norther
County Warning Area. Models remain aggressive with amount of moisture pulled
north...with precipitable water values near or above 1 inch for Saturday night
and Sunday. have some concerns about amount of
moisture...and feel models are too aggressive and will need to be
monitored for heavy rain potential. In addition...instability is
sufficient for thunderstorms...indicating sb cape values below
1000 j/kg late Saturday afternoon and again Sunday afternoon. Will
need to monitor for severe potential.

Boundary will eventually push southeast of area by late Monday...with a
return to ridge Tuesday and Wednesday...again allow temperatures
to warm. Large area of low pressure expected to push across
northern rockies late in the week. Models are not in good
agreement in placement of this low...with European model (ecmwf) further
north...and GFS trending towards this more northern solution.
However...models indicate secondary surface low again lifting NE
into eastern NE/Iowa by late Thursday...which could lead to
precipitation chances again. Have included precipitation chances
beginning Wednesday and persisting through end of period.
However...given uncertainties...may be too aggressive with precipitation
onset Wednesday.


issued at 621 PM CDT Thursday Apr 17 2014

VFR conditions expected throughout the taf period...with some
high clouds expected to stream through the northern County Warning Area tonight.
The high clouds may push into kdsm and kotm late tonight into
Friday. Otherwise winds to become light and variable overnight as
they shift around to the southeast Friday.


Dmx watches/warnings/advisories...none.



Short term...skow
long term...awb