Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
700 am CDT Friday Aug 29 2014
Short term /today/...
issued at 329 am CDT Friday Aug 29 2014
The forecast is beginning to sound like a broken record but today
the upper trough finally begins to shift east across the upper
Midwest. The trough axis is prognosticated to be over the Missouri River
valley around 18z and slowly progress across Iowa through tonight.
A surface low over the Sioux Falls area will lift off to the
northeast today which will drag a cold front across central Iowa
during peak heating. Unfortunately that will mean more
thunderstorm development this afternoon into the evening.
Convective available potential energy are forecast to be between 1500 and 2500 so we will be quite
unstable today with quite a bit of forcing moving across the
forecast area associated with the front. While shear is not great
it should be enough for a few strong to severe storms to develop.
Heavy rainfall is once again likely especially with the strongest
storms. However the heaviest rainfall should be across northern
Iowa where they can take a bit more rainfall. We will have the added
concern this afternoon for some storms capable of producing large
hail and damaging wind in addition to the heavy rainfall potential.
Morning convection should lift off to the northeast and diminish
before re-development this afternoon. There is some concern about
the stratus that has been prevalent across the north and east but
once the sun is up the low clouds should lift and there should be
enough breaks in the clouds to allow sufficient heating by the
afternoon to destabilize the airmass. As for temperatures...they should be
a few degrees warmer across the southern two thirds or so of the
forecast area. We will be in warm advection ahead of the frontal
boundary and should break out of the clouds a little earlier today
across that area. Northern Iowa will still be somewhat cooler with
highs in the middle to upper 70s.
Long term /tonight through Thursday/...
issued at 329 am CDT Friday Aug 29 2014
Persistent upper level low will continue to pull eastward away
from Iowa through the beginning of the period...however...one last
round of precipitation will be possible with this system tonight
before it finally pushes east. As frontal boundary pushes east
across Iowa overnight tonight...precipitation chances will remain
possible...and another round of heavy rainfall will also be
possible. Forcing is slightly less with this system...though given
boundary pushing through and moisture environment...expect
precipitation...especially across the east where forcing is best.
Behind system there will be a brief dry period Saturday and most
of Sunday. However...this will be short lived as another low
pressure system pushes east across southern Canada/northern US.
This will drag a frontal boundary into Iowa beginning late Sunday.
Ahead of the system...southerly flow will help to keep moisture in
place...and thunderstorms may again bring heavy rainfall Sunday
and Sunday night...with precipitable water values again over 1.5
inches...almost 2 inches in places. In addition...instability will
be high ahead of boundary...with cape values of 2000 j/kg or
higher...with high enough shear to support thunderstorm
development. This may allow for thunderstorms which develop to
become severe. However...much will depend on the timing of the
system...with models continuing a slightly slower trend...which
may allow for most robust convection to be west of area.
However...given diurnal heating...feel confident in keeping
thunderstorm chances in the northwest late Sunday. System will
continue to push east overnight and may linger into Monday
High pressure will push in behind system...however models attempt
to hang boundary up across northern MO which may allow for
precipitation chances to linger into southern Iowa through the
early week. Overall...conditions will be mostly dry until another
low pressure system pushes east across the northern US late in the
period. Models have very poor agreement on timing and placement of
this system...and therefore have only kept slight and chance probability of precipitation
Temperatures will be warmer Sunday with southerly flow and warm air advection
ahead of boundary...however temperatures will cool behind
boundary. Temperatures will warm again late in period.
issued at 700 am CDT Friday Aug 29 2014
Morning fog/stratus/precip will all be lifting by 15z with VFR
conditions through much of the afternoon. A cold front will move
across the forecast area this afternoon and evening. MVFR
conditions will occur with storms and heavy rainfall is again likely
with storms. Strong wind is also possible with some of the stronger
storms late afternoon into this evening. Local IFR conditions cannot be
ruled out. After 08z reduced visibilities in fog will prevail...especially
across the north and east.
issued at 315 PM CDT Thursday Aug 28 2014
Several additional episodes of rain are expected through Saturday
morning and again another Sunday afternoon through Monday morning.
At this time expected rainfall amounts through Saturday morning with
each event should be light to moderate. This will limit the likelihood
of flash flooding but still have some effect on rises on small stream
and area rivers. There is a greater risk of more substantial rainfall
over portions of the forecast area Sunday evening. This time period
will be monitored for both short term flooding and river flooding
over the upcoming Holiday weekend.