Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
643 PM CDT sun Oct 4 2015
Short term /tonight/...
issued at 305 PM CDT sun Oct 4 2015
Primary concerns tonight will be clouds and associated temperature trends.
Stratus advanced west to east through much of the forecast area
early this morning but has retreated back eastward somewhat by
afternoon...seeming to stop when it reached deeper cloud shield. WRF
arw...rap and hrrr had a decent handle on cloud trends last
night...and still appear to this afternoon...so will be followed
into tonight. This suggests clouds will move little and then begin
moving back north and west in weak 0-1km flow as it shifts from NE
to southeast by later this evening. Clouds should reach all but far northwest
sections toward sunrise and then expect stratus to stall by that
time with 0-1km mean wind dropping to near zero.
Temperatures will fall little under current thicker cloud cover so have
mins less than five degrees from current observation there. Farther north
and west diurnal range may be greater with a period of radiational
cooling during the evening...but confidence is lower in these areas
due to uncertainty regarding duration of clear skies.
Long term /Monday through Sunday/...
issued at 305 PM CDT sun Oct 4 2015
Main concern for the beginning of the week and extended will be
temperatures as Iowa remain generally under the influence of high
pressure and weak zonal flow aloft. While conditions will be
dry...sufficient low and middle level moisture has arrived today to
keep clouds across state...and models indicate this will persist
through the beginning of the week. Clouds will help to limit
warming...and have therefore continued trend of slightly lower
highs than going. However...with 850mb temperatures continuing to warm
by middle week...will see warming trend to temperatures...and if clouds
clear with mixing may be too cool...especially in the west.
Therefore have kept temperatures near warmest guidance. Overnight
lows will similarly be affected by clouds...may be too warm given
amount of cooling the past nights...though have favored continued
clouds and warmer temperatures.
Best chances for precipitation will come middle week and low pressure
system across SW US begins to push east as wave drops south from
Canada. Models remain in good agreement with timing and placement
with system...though are a bit slower with onset of precipitation
Wednesday. Expect precipitation to start late Wednesday afternoon
with best chances coming overnight Wednesday with system pushing
south. System will push south quickly...with high pressure
building in again behind. This will bring a return to dry weather
for the end of the period. Temperatures will be coolest with high
Friday...though only slightly below normal. Warm air advection will return behind
retreating high and temperatures will warm again for weekend.
issued at 643 PM CDT sun Oct 4 2015
Low stratus clouds continue to pose a forecast challenge...as
they have eroded across much of the area this afternoon but
linger across much of southern and eastern Iowa. As cloud layer
steering flow veers around tonight then GOES calm...and given low
dewpoint depressions at this time and the trends observed last
night...it is expected that the cloud deck will once again
expand/advance northwestward across much of the area after dark.
This will result in MVFR ceilings and possibly light br at most or all
of the terminals by the overnight hours...and lasting well into or
through Monday. Have taken best attempt at timing the onset of low
ceilings...but amendments may be necessary through this evening.