Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
549 PM CST sun Nov 29 2015
Short term...(this evening through Monday afternoon)
issued at 319 PM CST sun Nov 29 2015
Long duration snow event currently sitting on the door step. Water
vapor shows elongated upper level low extending from the western
daktoas through the Desert Southwest. As the southern branch of the
jet intensifies...southern portion of the upper level low is
expected to pivot into the plains. At the surface ahead of the
low...very saturated atmosphere along and south of I-80...working
Short term high res models have suggested the potential for freezing
drizzle developing ahead of the main upper low. Forecast soundings
indicate deep saturation of near 4-5k feet with lift developing at
the top of the saturated layer. Dendritic layer remains above the
saturated layer...thus making it difficult to introduce ice into the
column. As warm air advection increases expect column to continue
to saturate and eventually change over to snow. Could be some sleet
mixed in across northwest Iowa prior to 12z Monday...but as the lift
increases the atmosphere cools sufficiently to result in snow by 12z
Strong positive vorticity advection is expected throughout the day
as short waves rotate around the upper low. With system effectively
cut off from jet...expect slow movement to the east. One of the main
vorticity lobes is expected to pivot toward northeast Nebraska late in
the day on Monday. Immediately ahead of this wave...there is some
negative epv* along and east of I-29 in the 700-650 mb level
suggesting potential for increased snowfall rates through the
afternoon and evening hours.
The upper wave pivots through the forecast area through the evening
hours with the upper low camped across the forecast area through the
overnight. As the low slowly works east...expect lift and snowfall
rates to decrease...however depth of the dendritic layer increases
resulting in more efficient atmosphere for snow production.
Overall result appears to be heavy snow through at least Tuesday
night. Have elected to issue a Winter Storm Warning along and east
of I-29 for heavy snow. Further to the west...snowfall amounts look
to be less...but still will be fairly impressive. Given the duration
of the event...used 8 inch criteria for determining western edge of
warning. Expect snow ratios to start out fairly low in this event
around 10-12:1...and increase to closer to 14-15:1 as temperatures
As for temperatures during the event...should see very little in
terms of fluctuation with near surface saturation...and kept diurnal
range very small.
Long term...(monday evening through sunday)
issued at 319 PM CST sun Nov 29 2015
As this snowstorm slowly moves away Tuesday night...still expecting
considerable wrap around light snows through Tuesday evening.
Soundings show a very saturated environment...with enough lift to
continue to produce light accumulating snow through midnight
Tuesday. Have bumped up probability of precipitation...and these may go higher in the
future. Winds will also increase considerably Tuesday night into
Wednesday. Have not introduced blowing snow...but it is possible by
the end of this storm.
Beyond Tuesday...mid-lvl ridging will take over by Wednesday with a
quick turn around to southerly winds. Temperatures with the fresh
snow will remain cooler than normal.
A very progressive northern stream will continue through much of
next week and into next weekend. Models continue the trend of
cutting off energy over the southwestern United States...and slowly
moving these waves through the middle-section of the country. As of
right now...the track of one such wave next Monday-Tuesday...would
appear to be to the south of the local area.
Aviation...(for the 00z tafs through 00z Monday evening)
issued at 543 PM CST sun Nov 29 2015
Conditions will deteriorate overnight from south to north with
widespread IFR conditions expected after about 9z to 12z. Some
light freezing drizzle will be possible during the overnight hours
into tomorrow morning...with snow becomeing widespread during the
late morning and afternoon hours. Winter weather warnings and
advisories are in effect for the entire area.
South Dakota...Winter Weather Advisory from 3 am Monday to 6 PM CST Tuesday for
Winter Storm Warning from midnight tonight to midnight CST
Tuesday night for sdz038>040-054>056-061-062-066-067-070-
Minnesota...Winter Storm Warning from midnight tonight to midnight CST
Tuesday night for mnz071-072-080-081-089-090-097-098.
Iowa...Winter Storm Warning from midnight tonight to midnight CST
Tuesday night for iaz001>003-012>014-020>022-031-032.
NE...Winter Storm Warning from midnight tonight to midnight CST
Tuesday night for nez013-014.