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Area forecast discussion
National Weather Service Sioux Falls South Dakota
1105 PM CDT Tuesday Apr 15 2014
Short term...(this evening through wednesday)
issued at 352 PM CDT Tuesday Apr 15 2014
Southerly flow increasing across the area from west to east and
expect Stout flow to remain across the forecast area through the
overnight. With the warm but dry air across the area...have kept
forecast lows quite mild but have shaved off a few degrees from the
previous forecast. Still above most guidance in the upper 30s and
lower 40s. Expect lows across southwest Minnesota early...with
rising temperature much of the night as warm front lifts north.
Strong cold front is expected to work through the area on
Wednesday. Expect to see falling temperatures behind the front. Am
weakly unstable behind the front which may lead to some scattered
showers across northwest Iowa. Expect this precipitation to mainly
be rain...but further to the north along the Highway 14 corridor
temperature profiles get cold enough to support snow. This
precipitation further to the north is more synoptically driven
following the 700 mb frontogenetic band shift from west to east
along and north of I-90. Upper level divergence q quickly shifts east
through the afternoon hours...so expect profiles to start drying out
from west to east through the middle and late afternoon hours. As for
snowfall accumulations...have shifted the heaviest snowfall back to
the west slightly towards the Brookings area where an inch or two of
snowfall is possible. Have mention of snowfall further to the
south...but no accumulations in the grids as warmer soil
temperatures should limit accumulation potential.
Long term...(wednesday night through tuesday)
issued at 352 PM CDT Tuesday Apr 15 2014
Low pressure will continue to exit the area Wednesday evening.
Probably will see a little light snow or rain linger across portions
of southwest Minnesota and northwest Iowa in the evening...although
not expecting much more than an additional coating to few tenths of
accumulation. Models seem to be a little quicker to scour out the
low level moisture Thursday night...and latest indications are that
either the moisture will be too shallow...or that clouds dissipate
altogether later Thursday night...thus removed the freezing drizzle
and flurry mention after midnight. Although will need to continue to
Models already begin to diverge Thursday into Friday. European model (ecmwf)
and Gem are quicker building in the ridging aloft...with the NAM and
GFS stronger with northern stream wave and thus more suppressed with
the ridge. Based on recent performance...and the fact that run to run
consistency has increased with the European model (ecmwf) and Gem...will lean towards
the international model solution. So while the NAM and GFS show some
precipitation Thursday into Friday...believe the weaker wave and
further north frontal boundary will prevent US from seeing much in
the way of precipitation. Thus decided to remove probability of precipitation for
now...although not out of the question we see a few light showers
Thursday night into Friday somewhere in the County Warning Area. This could either be
in the form of rain or snow...but even if it did occur amounts should
be minimal. Thursday will see highs in the 40s...so even with partly
to mostly sunny skies it will be a chilly one. In going with the
European model (ecmwf) and Gem solutions as mentioned above...went ahead and warmed
highs several degrees on Friday into the 50s and 60s.
Our next chance of precipitation will come on Saturday into Saturday
night...as an upper wave and cold front pushes across the area.
Looks like decent moisture will be advected north ahead of the
front...with precipitable water values 1 to 2 Standard deviations above normal.
Forcing should be enough to thus get some showers out of this system
Saturday into Saturday night. We do get some elevated instability
into extreme southeast South Dakota and northwest Iowa...so could
very well see some thunder there. With instability and shear both
weak...severe weather is not anticipated. For now will just keep the
chance probability of precipitation...with some uncertainty on coverage of shower
activity...especially given the lack of instability...but if model
consistency continues to grow will eventually need to raise these
probability of precipitation. Did warm highs Saturday...as looks like we should warm ahead
of the front into the 50s and 60s. In fact am probably not warm
enough in our east...where would not be too surprised if we see some
70s. But given uncertainty in frontal timing and cloud and shower
coverage...felt trending up was good enough for now.
Sunday into Tuesday looks dry...warm and mostly sunny. Did bump
highs up some over consensus for each day with 60s and 70s expected.
So should be a pleasant end of the weekend and early next week.
Based on forecast 925 mb temperatures...may even need to nudge highs
up a bit more as we get closer. Kept Tuesday dry for now...but as a
strong upper trough moves from the West Coast into the
plains...would expect the shower and thunderstorm threat to increase
from later Tuesday into middle week. But being that far out...timing
and details are hard to pin down at this point.
Aviation...(for the 06 tafs through 06z Wednesday evening)
issued at 1105 PM CDT Tuesday Apr 15 2014
Gusty southeast winds will become west-northwesterly as low pressure
moves through the region tonight and Wednesday. Low level jet across
the southeast half of the forecast area will result in potential for
low level wind shear at ksux/kfsd overnight.
VFR conditions are expected ahead of the low pressure...and as temperatures
surface and aloft cool behind the system Wednesday....MVFR ceilings
will develop. Periods of MVFR visibility also likely in rain/snow
showers...particularly near and north of I-90 corridor. Surface
temperatures holding just above freezing should minimize any snow
accumulation or icing on runways/tarmacs. Gradual improvement to VFR
expected after 17/00z.