Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Aberdeen South Dakota
611 PM CST Sat Feb 13 2016
issued at 557 PM CST Sat Feb 13 2016
Updated aviation discussion follows. Also made a couple of updates
to the weather grids to introduce thunder mention to the freezing
rain/sleet and snow that is continuing to be forecast across the
Winter Weather Advisory area. Cloud to ground lightning has been
consistently being generated on the positive vorticity advection side of this potent upper
wave over the past 6 hours...including an uptick in lightning
activity to over 30 cloud-to-ground lightning strikes last hour. Will continue
to monitor trends this evening to see if/when any forecast zones
may need to be upgraded to a warning for heavy snow. Latest hrrr
output seems to suggest this County Warning Area will be missed by the most
intense convectively-influenced snow overnight. Stay tuned.
Short term...(this evening through sunday)
issued at 235 PM CST Sat Feb 13 2016
Fairly active short term forecast for the area as two features
will combine to produce moderate to occasionally heavy snowfall
over the County Warning Area. The strong shortwave that will slide southeast
across southwest South Dakota is still located in central Montana
at this time...and is leading to a couple areas of precipitation
in eastern Montana (one along the warm front and another tied to
the shortwave and cold front). Closer to the area...seeing
precipitation just to the west of the Missouri River that is
slowly moving east-southeast. This precipitation is due to the
initial area of middle-level warm air advection ahead of the warm
front stretching south-southeast through far western South Dakota.
Webcams show that the precipitation towards Haakon/Stanley
counties is in the form of rain and with surface temperatures in
that area around or above freezing...Don/T think there is too much
freezing rain occurring at this point. Farther north...rap
analysis soundings indicate the warm nose aloft weakens which
should transition the precipitation to snow the farther north you head.
Over the rest of the area...clouds have been lowering with the
increasing low level moisture and departing surface ridge.
Temperatures are still fairly cold over northeast South Dakota...where 2pm
temperatures range from the upper single digits at k8d3 to 13 at
kabr and then 17 at kmbg.
Models continuing to struggle with the track/evolution of the
shortwave in Montana...which leads to some uncertainty in the
forecast snowfall over north.C./N.E. South Dakota this evening into
the overnight hours. Latest runs of the rap and 12z global
models have trended the track of the strongest forcing a little
farther to the southwest and in some cases only barely brushing
southwestern South Dakota. While that best forcing slides southwest of
much of the area...still have some support from the upper trough
and especially from the warm air advection / isentropic ascent
along and ahead of that trough through the early overnight
hours. Fairly strong lift with the warm air advection (and seen in the 700mb
omega) with favorable 850-500mb q-vector divergence.
Therefore...expect to see a fairly quick hitting (4-7hr period)
area of precipitation moving through the area...especially this
evening into the early overnight hours. Models have trended quantitative precipitation forecast
amounts to the north with the most recent runs...some even
showing 12hr quantitative precipitation forecast amounts exceeding 0.6in between
Mobridge/Aberdeen. That potential (likely somewhat convectively
driven with 750-500mb lapse rates around 7 c/km) will need to be
watched through the evening and if it sets up as shown with some
of the higher res runs...may need to upgrade a few counties to
warnings. With that farther north trend...expect a little less
impact for the southwestern County Warning Area...but still think the potential
for freezing rain through the first part of the evening before
transitioning to snow for a brief period before ending. That
northward shift should keep snowfall amounts below 2 inches from
Pierre and southwest (where some areas will stay mainly rain).
As for snowfall amounts over the north central and
northeast...have a general 3-6 inches with snow ratios
increasing from around 10-1 around the Missouri River to the
12-14 to 1 range over the northeast. Over the northeast...a good
portion of the lift is at or above the dgz...which will likely
lead to more columns in the snow process and more potential for
compacting (along with aggregation as the snow falls through the
cloud layer below the dgz). Thus...didn/T want to go much higher
than the previously mentioned values ... ratios could
briefly increase if the forcing aligns a little lower and in the
dgz. With the higher quantitative precipitation forecast and ratios...have near warning snowfall
amounts (5.5-6in) over McPherson/Edmunds/Brown/Marshall/day
counties. With the variability in the model runs today...didn/T
have enough confidence to upgrade to a warning at this point and
the evening shift will need to monitor for a possible upgrade. In
addition...with the snow occurring late this evening and during
the early overnight hours...thought the impacts would be lower
than if it occurred during the day.
One other item to mention will be the gusty south-southeast
winds initially this evening ahead of the trough (gusts to
25-30mph). That will lead to patchy/areas of blowing snow and
will continue to mention that in the forecast/WSW.
For the rest of the period...the snow will diminish from west to
east late tonight and through the day on Sunday (largely over
the eastern half/third of the cwa). A broad surface trough will
be over the area and limit the winds and in turn any blowing snow
concerns. In addition...with the warm air over the
area behind the warm front that moved through...highs will rise
into the middle/upper 20s over the far east to the lower 40s along
and west of the Missouri River.
Long term...(sunday night through saturday)
issued at 235 PM CST Sat Feb 13 2016
Low temperature forecast for Sunday night is a bit tricky. Area will still
be in a relatively mild air mass with 925mb temperatures around 0c to +2c
and a light west to southwest surface wind. Although...will have to keep
in mind the fresh coat of snow that will be over much of the area.
There will be some degree of cloud cover as well...so trying to
figure out which variables have the biggest impact on temperatures is
Will then be watching the next piece of shortwave energy which will
be dropping southeast through the area Monday night. Have increased
probability of precipitation for this event as models are in pretty good agreement on timing
and placement of quantitative precipitation forecast. Again...could be dealing with mixed precipitation
issues as there will be warm air in place.
Yet another wave moves through Tuesday night but is looking rather
weak at this point. Superblend gave slight chance probability of precipitation and this
looks good for the time being. Only change here was to remove the
slight chances they initial gave on Tuesday and held things off to
Tuesday night per model data.
Temperatures throughout the period stay on the mild side...especially by
Thursday when models try to bring in some very mild air. 925mb temperatures
around +20c show up in the models over southwest and south central
South Dakota. Superblend temperatures have readings as warm as the 60s over the
southwest County Warning Area on Thursday.
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 557 PM CST Sat Feb 13 2016
Flight category will be bouncing in and out of MVFR/IFR range for
ceilings and visbies with freezing rain and snow /perhaps even some
thunder too/ probable at the kpir terminal now through midnight.
Kmbg...kabr and Katy will probably just see light to moderate snow
overnight into Sunday morning. Breezy conditions at kmbg and kpir
will subside within the next several hours...but at
kabr/Katy...the breezy to occasionally windy southerly winds will
persist into Sunday morning before backing down.
South Dakota...Winter Weather Advisory until 6 am CST /5 am MST/ Sunday for sdz003-
Winter Weather Advisory until noon CST Sunday for sdz007-008-011-
Winter Weather Advisory until 9 am CST Sunday for sdz005-006-010-
Minnesota...Winter Weather Advisory until noon CST Sunday for mnz039-046.