NWS Sioux Falls, SD wrote its last area forecast discussion before the first event at Jan 15, 9:15 PM CST, 2 hours 45 minutes earlier.
Area forecast discussion, Jan 15, 9:15 PM CST, 2 hours 45 minutes before
Area Forecast Discussion
National Weather Service Sioux Falls SD
915 PM CST Sun Jan 15 2017
.UPDATE...
Issued at 915 PM CST Sun Jan 15 2017
18Z NAM/GFS holding onto drier mid-level air for the Sioux Falls
vicinity through much of the day Monday. Also showing marginally
warmer temperatures/delayed cooling in the deep warm layer aloft
in areas east of Sioux Falls, specifically in northwest Iowa/east
of Highway 60. Brief look at 16/00Z NAM appears to continue this
trend. Have made some adjustments to the precipitation forecast
in this direction this evening, resulting in slightly higher ice
and lower snow accumulations across the advisory/warning areas.
Changes not significant enough to affect headline distribution at
this time, though it does appear that Sioux Falls proper may be
on the northwestern edge of any significant icing threat with
little if any snowfall anticipated based on these latest model
runs.
&&
.SHORT TERM...(This evening through Monday)
Issued at 349 PM CST Sun Jan 15 2017
Complicated weather pattern creating many forecast concerns for
tonight and Monday. Closed low over the Texas panhandle will
continue to lift north northeast over the next 24 hours, arriving in
southwest Kansas by daybreak Monday, then reaching the Iowa and
Missouri border late Monday afternoon. This progression continues
with the slower trend seen on the midnight shift, and will delay
precipitation moving into our southern border/Missouri Valley area
until around midnight tonight. High resolution models in particular
are pointing towards a very sharp cut off line for accumulating
precipitation and no precipitation, so have tightened up the western
and northern gradient pops quite a bit. The trend is also towards a
more southerly track, and therefore expect precipitation chances to
taper off quickly to the northwest of a Yankton to Sioux Falls to
Windom line.
Thermal profiles remain tricky, with a deep above freezing layer
aloft building across the region ahead of the precipitation onset.
The NAM thermal profiles are a little warmer and in good support
DMX/OAX soundings. While sub freezing temperatures are expected at
the surface, this layer will be too shallow for precipitation
refreeze into sleet, so tried to limit any sleet mention to isolated
or a chance. The main precipitation will be light but persistent
freezing rain.
Light freezing rain lifts northward across far southeast SD,
northeast NE, and northwest IA late tonight, finally expanding into
areas along and east of a Yankton to Sioux Falls to Windom line
around daybreak/morning rush hour. Initial saturation aloft may be
slow Deformation band slowly lifts across this region through the
day, slowly shifting slightly eastward in the afternoon.
Temperatures aloft start cooling late morning and afternoon from
west to east, and will allow the light freezing rain to switch to
sleet and then snow. The focus of the icing threat will be across
northwest Iowa where greatest QPF and freezing rain duration will
be. Expect around a quarter inch of icing, or locally up to a third
of an inch, in the Sioux City, Cherokee, and Spencer areas. The ice
storm warning remains in effect in this area.
Elsewhere, for areas along and east of the YKN/FSD/MWM line
mentioned above, a light glaze to two tenths of an inch of ice is
expected, along with light snow up to 1.5 inches. The current winter
weather advisory remains in effect, primarily for the icing
potential. The remaining winter storm watch will be converted to a
winter weather advisory. Debated whether to drop Minnehaha (SD),
Murray (MN), and Cottonwood (MN) counties from headlines as they
will be located right on the fringe of the precipitation, but there
is enough of a concern that a light glaze of icing, even an
hundredth, could create a huge mess on the roadways during rush
hour, so opted to go with an advisory.
.LONG TERM...(Monday night through Sunday)
Issued at 349 PM CST Sun Jan 15 2017
...corrected a typo in last paragraph...
By Monday evening, the northern stream upper trough extends from
eastern ND to central SD helping to exit the strong southern stream
trough into southern Iowa. As isentropic lift wanes, moisture begins
to rapidly dry out in the mid levels on the north side of the
precipitation shield which will translate into an extremely sharp
cut-off of the precip generally north of Highway 60. In fact the
CAMs, such as the NMM and ARW totally dry. It is doubtful by this
time that there will be much in the way of a light freezing drizzle
threat due to the lack of lift. And with the dry mid levels, it is
difficult to ascertain the exact precip type for these areas if
precip does fall. But at this time, the temperatures aloft look cool
enough that ice nuclei should be introduced at the top of the cloud
layer to warrant a chance for mainly light snow.
Now along and south of Highway 60 including the ice storm warning
area, total saturation still exists through the mid levels early
Monday evening. Using the warmer NAM sounding instead of the cooler
GFS, there may be a brief window for a sleet threat around Spencer,
Storm Lake and Cherokee very early in the evening. But the pure
icing threat appears to be gone. The soundings transition rapidly
over to all light snow by 02Z or 03Z Monday evening, but note that
as the northern stream trough dives southeastward, even our
southeast zones begin to lose their mid level moisture between 03Z
and 06Z late Monday evening. Therefore all locations in the CWA will
likely finish up with less than an inch of snowfall, except for the
southern sections of southwest MN who may receive an inch or so.
That said, there could be a sleet mess of around a half inch in the
Spencer and Storm Lake areas Monday evening.
A Pacific air mass will then dominate the area beginning by
Wednesday. With a south to southwest flow of air in place,
temperatures will likely be quite mild for this time of year helping
to melt the ice throughout the southeast quarter of our forecast
area. In the closer term on Tuesday, blended in some cooler
consensus raw model values including the ECMWF for northwest IA and
immediate adjacent areas due to ice and sleet. This scenario
produced highs in the upper 20s to near 30 for Tuesday for our
southeast zones. But then the warmer air begins in earnest on
Wednesday and the entire forecast area will see a lot of upper 30s
and 40s Wednesday and Thursday. It will then cool off Friday through
Sunday but highs will still be above normal as the next upper trough
moves northward into this area. The leading short wave early in the
weekend is actually part of a long wave troughing pattern throughout
the western half of the United States and eastern Pacific, so its
mild aloft also and very sketchy on the details for timing of
precipitation. Depending on timing, any light precip that far out
will be either light freezing rain, rain or light snow whether its
night time below freezing or day time above freezing. But in general
that air mass next weekend looks warmer then the one we are dealing
with for Monday.
&&
.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 915 PM CST Sun Jan 15 2017
Aviation concerns will focus on the incoming winter storm slated
to affect mainly the southeast half of the forecast area, near and
south/east of KFSD, through Monday evening. MVFR-IFR ceilings will
gradually spread north through the early morning hours Monday,
accompanied by an increasing threat of FZRA/FZDZ across much of
northwest IA, far southeast SD, and portions of southwest MN late
tonight through Monday. A glaze of ice possible around KFSD, with
much more significant icing expected to the south and east toward
KSUX/KSPW.
Temperatures aloft will gradually cool through the afternoon and
evening, and the freezing rain/drizzle may mix with or change to
light snow before ending Monday night. However, ice accumulation
expected to be a more significant impact to aviation through this
TAF period.
&&
.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Weather Advisory from 3 AM Monday to midnight CST Monday
night for SDZ070-071.
Winter Weather Advisory from 6 AM Monday to midnight CST Monday
night for SDZ062-066>069.
MN...Winter Weather Advisory from 6 AM Monday to midnight CST Monday
night for MNZ080-081-089-090-098.
IA...Winter Weather Advisory from 3 AM Monday to midnight CST Monday
night for IAZ001>003-012-013-020.
Ice Storm Warning from midnight tonight to midnight CST Monday
night for IAZ014-021-022-031-032.
NE...Winter Weather Advisory from 3 AM Monday to midnight CST Monday
night for NEZ013.
Ice Storm Warning from midnight tonight to midnight CST Monday
night for NEZ014.
&&
UPDATE...JH
SHORT TERM...
LONG TERM...MJ
AVIATION...