Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
104 PM CDT Monday Mar 17 2014
Short term...(today and tonight)
issued at 429 am CDT Monday Mar 17 2014
A fairly cloud day as an abundance of high level cloudiness should
be present most of the day. Regardless still anticipating a nice
day. May need to deal with a few sprinkles around the Marshall area
very early this morning but any precipitation that develops with the
weak wave moving into Minnesota should remain mainly northeast of
the County Warning Area. As for high temperatures will aim for 50 to 55 in southwest
Minnesota to 70 to 75 in south central South Dakota. The fire
weather threat looks a little lower today over the Missouri River
valley as winds now look to remain more in check. The stronger winds
likely to be mainly east of Yankton and relative humidity values not
nearly as conducive in these locations.
Late tonight the models have continued the slightly faster movement
of the system. This brings cooler air into the western County Warning Area more
quickly so will need to lower lows a bit down the James River
Valley. Just ahead of the system the models sporadically want to
produce a little shower activity across parts of the County Warning Area from about
0z through 12z but not really sure just where this will come from.
While some convection not impossible over the western High Plains as
the cold front aloft swings out around 0z to 3z the forcing weakens
as it approaches the County Warning Area. Then later in the night from 6z to 12z
some indications that precipitation could occur in southwest
Minnesota but an analysis of the soundings suggests the only level
with convective potential would be at or above 700mb. If this is the
case that boundary is much farther north than our County Warning Area. The 850mb
front is pretty formidable in southwest Minnesota but dry as a Bond
so no real potential for precipitation production there. So...in the
end swing a 20 pop from southwest to northeast across the County Warning Area
through the night but not expecting much. The main upper level
forcing crashes into the western County Warning Area from about 8z to 12z and
soundings suggest a quick change from rain to snow as the
precipitation develops. So started snow a little sooner and
increased amounts from Chamberlain to Huron before 12z
Tuesday...mainly around an inch possible. Still leaning towards
higher end of guidance on temperatures east of the James River
Valley with upper 30s to lower 40s expected. The western edge will
be a little iffy as the models have been trending just a bit quicker
with this wave the past day or two.
Long term...(tuesday through sunday)
issued at 429 am CDT Monday Mar 17 2014
Evolution to the potential winter storm will continue quickly on
Tuesday morning as frontal boundary and low pressure settles
gradually eastward...as upper trough continues to dig into the
Central Plains. Fairly warm temperatures surface and aloft will continue
to the southeast half of the area to start...with some indication
of very elevated weakly convectively unstable layer for the
northeast third of the area. While acknowledge the potential for a
few showers and even that there could be some sleet for a brief
time in passing showers...any more concentrated precipitation should lie
along to the north where frontogenetic axis is located. With a
decent indication of instability over and south of the frontal
boundary... should get some very good banding developing during
the event. Some hints in model evolution that could end up with a
concentration of precipitation along more northern and elevated
frontogenetic boundary with fairly distinct lack of lower level
moisture initially...then after cold front aloft/pv wraps
northward by early afternoon...development of a secondary
frontogenetic band further to the southeast at lower elevations...
also supported by the introduction of some increased moisture with
Overall...precipitation should become much more widespread through
the northwest half of the County Warning Area during the morning...as temperatures
collapse to around 30 in the snow band. Accumulation rates
difficult as dendritic growth depth is fairly shallow and will
have some pretty good shear for precipitation to fall through initially.
Almost convective instability will be a wild card for sure...but
will vary between about a 9 to 1 snow liquid ratio to perhaps as
high as 15 or 16 to 1 if decent band and aggregation can develop.
Likely northerly winds of 30 to 35 knots atop the mixed layer...
so will become fairly windy. With lack of snowcover and a fairly
wet snowfall for at least part of the event...visibility reduction
will be primarily due to falling rate and not so much with blowing.
Likely that will find some periods of a quarter mile or less
visibility in heart of the band. Overall fit would seem to be more
of a winter storm than a blizzard given the precursor conditions.
Issued a Winter Storm Watch generally for areas with at least 5
inches accumulation forecast at this time...north of i90 and east
of the James Valley.
Some lingering snowfall on Tuesday evening mainly SW Minnesota and northwest Iowa...
and will see those winds slowly decrease as the main area of low
pressure moves from the Kansas City area into Iowa.
On Wednesday...trailing energy in the northern stream drops across
the area. Temperatures very cold aloft...and have continued to mention a
few rain or snow showers across the area. Temperatures may not end up
quite as warm...depending on the final snowfall distribution
across the area.
Pretty much with active short term weather did not take an indepth
look into the extended forecast for Thursday through Sunday period.
Approach of another wave in zonal west/northwest flow will induce
return flow and send warm front northward into the County Warning Area...with some
diversity on how far. Increased northeast to southwest gradient a
bit for both the overall thermal pattern and likelihood of needing
some additional snow melt for that day. Clipper wave passes by
on Friday...with thermal diversity between solutions again making
it hard to nail down precipitation type potential. European model (ecmwf) becomes much
more amplified and would lead to a decent clipper snowfall across
the northeast...while more mixed type and lighter amounts
southwest. Consistent behind the system with much colder air
working southward through the weekend...and perhaps a skiddish
chance for some light flakes from time to time.
Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 1245 PM CDT Monday Mar 17 2014
VFR conditions expected across the area...with a storm system
bringing deteriorating conditions later tonight into tomorrow. As the low
pressure systems moves to our south...expect IFR to MVFR stratus to
move into the Highway 14 corridor and south central South Dakota
around 08z...impacting khon. Rain and snow will also move into the
area after 08z...and increase in intensity and push southeast through
18z. After a generally brief period of rain...most of the precipitation
is expected to fall as snow. Expect a wider area of light to moderate
snow...with a potentially smaller heavy band. Still large uncertainty
on where this sets up tomorrow morning. As of now...the highest
chance appears to be somewhere between Huron and Sioux Falls
northeast into southwest Minnesota...although confidence is low. Due
to the low confidence will keep khon with IFR visibilities...and kfsd
with MVFR. However locations in this heavier band could very well see
a several hour period of LIFR visibilities...so future shifts will
monitor model and radar trends and make adjustments as needed. The
most probable time frame for this potentially heavier snow is between
12z and 00z.
South Dakota...Winter Storm Watch from Tuesday morning through Tuesday evening
Minnesota...Winter Storm Watch from Tuesday morning through Tuesday evening