Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 212354
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
554 PM CST Tue Feb 21 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 425 PM CST Tue Feb 21 2017

Still looking at a very weak mid level short wave scooting across
the area tonight. Most of it is north of the forecast area,
generally in ND and northern MN, but the southern reaches of the
wave with some mid level convergence does make it as far south as
our northern zones north of I 90. With abundant 700-650mb
frontogenesis and mid level moisture, have some sprinkles mentioned
for locations north of I 90 after evening and overnight tonight. But
heading southward, the mid levels become increasingly drier.
Therefore there should be no threat for even trace amounts of rain
south of I 90. With light winds, bias corrected raw model
temperatures looked reasonable for lows, as they cooled off the low
lying locations and kept more mild readings on exposed terrain. In
general, lows will average in the upper 30s to mid 40s which is
warmer than our average highs for this time of year.

Wednesday will be our last very mild day. Behind the aforementioned
short wave, there will be a wind shift to a northwesterly direction
with wind speeds becoming rather breezy by midday for most
locations. 925-850mb temperatures do begin to decrease ever so
slightly along and north of I 90 during the afternoon hours.
Therefore not expecting Wednesday to be as warm as today with highs
ranging from the mid 50s in east central SD, to the upper 60s around
the Sioux City area.

.Fire Weather...

The Grassland Fire Danger Index will reach the very high category
again Wednesday afternoon generally along and south of a line from
Brule county, to Bon Homme county in South Dakota. Wind speeds will
average 15 to 30 mph, with minimum humidity values falling to 30 to
35 percent in the afternoon.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 425 PM CST Tue Feb 21 2017

The main story in the long term continues to be the increasing
likelihood of a major winter storm affecting the area by the end of
the week.

As a lead into that, Wednesday night will still remain relatively
quite with increasing clouds out ahead of the system. It will be a
cooler night in a cold air advection regime with lows running upper
20s to mid 30s.

On Thursday an upper level trough begins to lift out of the Rockies
as a surface low deepens over the western Central Plains. With
increasing mid level frontogenesis across our CWA, precipitation is
expected to develop across the area through the day. Models are in
agreement with sub 0C 850 temperatures at that point, so looking at
either rain or snow in terms of precipitation type dependent on
surface temps. Any snow accumulation during the day on Thursday
should be limited with highs mid 30s to lower 40s, though 2 to 3
inches will be possible over south central SD where temps will
remain colder.

Things then become more interesting on Thursday night and Friday as
the western upper level trough moves into the Plains and the
aforementioned surface low continues to deepen over the Central
Plains and moves to the northeast. Models are still having
differences in upper and surface features. Discounted the 12Z NAM
altogether with it being an outlier in offering a much faster
solution than the other models. Both the 12Z GFS and ECMWF have
shifted solutions just a bit farther north, with the ECMWF still
remaining farther to the south than the other solutions. Did not go
as far south with the QPF amounts as the ECMWF with it again be an
outlier, but with a continuing northward trend with each run, am
becoming more confident in the heavier snow band through the heart
of our CWA.

Any rain/snow on Thursday afternoon will change to all snow by
Thursday evening and then continue into Friday afternoon. At this
point, the highest risk of snow greater than 6 inches will reside
from the lower James/Missouri River Valleys northeastward into east
central SD, southwestern MN, and our northern IA zones. In this area
there is a risk of 6 to 10 inches of snow by Friday afternoon. Areas
through the upper James River Valley will receive less with lower
QPF amounts over that area. Locales through the Sioux City to Storm
Lake corridor are also expected to receive less with the change over
occurring later on Thursday evening, and the possibility of dry
slotting later on Thursday night into early Friday. In addition to
the snow, northerly winds will pick up friday as mixed layer winds
increase to 40 to 45 kts. This will result in blowing snow
resulting in even greater impacts to the area.

Based on all the above, and in coordination with surrounding
offices, issued a Winter Storm Watch for Thursday night and Friday
for areas expecting to receive around 6 inches or greater. Had
considered the possibility of a Blizzard Watch with expected winds,
but with coordination from other offices and until greater
confidence is gained on wind speeds, elected to stay with the above
watch.

With the focus on the nearer term storm, made little change to the
remainder of the extended period. With expected snow cover
temperatures will be closer to normal through the beginning of next
week, with little chance for precipitation.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 548 PM CST Tue Feb 21 2017

VFR conditions are expected through the TAF period.

&&

.FIRE WEATHER...
Issued at 413 AM CST Tue Feb 21 2017

A drying of the airmass today will occur with a better westerly
component to winds. Diurnal mixing will allow humidity levels to
drop to 20 to 25 percent by early to mid afternoon through the
Missouri River corridor, with increasing westerly winds at a gusty
15 to 30 mph. Conditions should stay shy of critical with only an
hour or two of lowest humidity, and winds remaining generally
below 25 mph, but enough to reach a very high rangeland fire
danger through the Missouri River Corridor eastward through
northwest Iowa. While it will be cooler on Wednesday, there will
still be potential for a very high fire danger toward Lower Brule
locations with humidity 25 to 35 percent and northwest winds 15 to
25 mph.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Watch from Thursday evening through Friday
     afternoon for SDZ040-050-054>071.

MN...Winter Storm Watch from Thursday evening through Friday
     afternoon for MNZ071-072-080-081-089-090-097-098.

IA...Winter Storm Watch from Thursday evening through Friday
     afternoon for IAZ001>003-012>014-020>022.

NE...Winter Storm Watch from Thursday evening through Friday
     afternoon for NEZ013.

&&

$$

SHORT TERM...MJ
LONG TERM...JM
AVIATION...
FIRE WEATHER...MJ



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