Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
1125 PM CDT sun Mar 16 2014
Short term /tonight/...
issued at 342 PM CDT sun Mar 16 2014
Little of concern tonight. Long wave trough is exiting the state
with only remnant lingering south-southwest-north-northeast axis of stratocu on backside
of low level cyclonic flow...which is expected to dissipate
toward sunset or shortly thereafter. Strong jet topping western Continental U.S.
Ridge does have a fairly strong short wave on its heals
however...now moving through Saskatchewan/Manitoba. This has
induced a decent axis of warm/Theta-E advection from the Dakotas
into NE...and widespread surface pressure falls through the
region. This thermodynamic lift will cross Iowa tonight but remain
moisture starved so expect nothing more than increasing middle
clouds...and mainly north. Generally stayed above MOS guidance for
lows based on consensus of raw model temperatures...which makes some
sense considering increasing warm advection into the night.
Long term /Monday through Sunday/...
issued at 342 PM CDT sun Mar 16 2014
Forecast concern was mainly focused on maximum temperatures Monday and with
system Tuesday into Wednesday. Models continue to be stubborn
with their respective tracks of the next system across the
region...as the European model (ecmwf) remains the further south and slightly
slower solution than the GFS/NAM/sref. However...the 12z European model (ecmwf) and
GFS are in a little better agreement than previous runs so leaned
toward a blend of GFS/ECMWF. Regardless...fairly strong upper low
to move across the state Tuesday night into Wednesday morning.
Monday...south to southwest flow and good mixing to about 900mb
or slightly higher puts temperatures back into the middle to upper 50s over
the central to southwest. Much of the snow has already melted or
will quickly melt and should not have much of an impact on temperatures
tomorrow. So leaned toward increasing maximum temperatures 2-4 degrees across
the forecast area which was significantly above the warmest
guidance and still maybe a bit too cool on maximum temperatures Monday as
models seem to be undoing the mixing potential.
Tuesday into Wednesday...500mb low enters western Iowa Tuesday
morning and some middle-level Theta-E advection and warm air advection out ahead the
low may spawn some light rain showers and possibly freezing rain
across the west to north Tuesday morning. Low confidence with a
prolonged period of freezing rain across northern Iowa Tuesday
morning with the warm air advection at the surface. However...kept mention at this time
as still seems reasonable across the north given the wet bulb
temperatures and ambient temperatures remain below freezing through 15z Tuesday.
The deeper moisture moves into northern Iowa by around 00z
Wednesday or even later into Tuesday evening as the strongest
large scale forcing moves across northern during this time. Main
concern is the very strong winds developing late Tuesday night
into Wednesday in conjunction with the falling snow potential.
Some forcing within the dendritic layer...but not ideal for
significant snow rates. Only have about 0.5 to near 2 inches at this time
as the cold air advection at the surface lags behind the better forcing for snow
growth. Winds atop the mixed layer range around 30 to 40 knots as
there is a good tropopause fold digging into western Iowa Tuesday night.
Increased winds across the north and there is a potential for
visibility restrictions depending on timing of rain switching to
snow. Strong cold air advection continues through Wednesday and European model (ecmwf) and even
GFS hinting on a second shortwave to cut across the region and may
bring additional precipitation Wednesday afternoon and possibly evening.
Thursday through Sunday...another weak system looks to bring some
snow/rain Thursday night into Friday. European model (ecmwf) is faster than GFS at this time
and leaned closer to the European model (ecmwf) as the latest GFS not consistent
from previous runs. High pressure moves into the region by the
issued at 1125 PM CDT sun Mar 16 2014
Little change from previous package. Winds increase overnight from
west to east as warm air advection increases with Lee side trough
advancing east and gradient strengthening. Main focus for any
upglide precipitation between 10-15z Monday should be north of the Iowa/Minnesota
border...so have left northern taf dry. Middle level clouds give way
to higher ceilings after 17z but prior to that overnight will see lower
ceilings to broken/ovc080-110 over most of the forecast area. By 22-00z
persistent southeast/south wind to continue with ambient flow near
or above 12kt. /Rev