Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 211141

Area Forecast Discussion
National Weather Service Sioux Falls SD
541 AM CST Sat Jan 21 2017

.SHORT TERM...(Today and Tonight)
Issued at 343 AM CST Sat Jan 21 2017

Upper low clearly spinning slowly northeast across the region early
this morning. At the surface, moist southerly flow persisting for
most of the area as well. However, a weak cool front has pushed into
our western CWA, with a marked change from light south-southeast
flow ahead, to westerly flow behind. Drier/cooler air accompanies
the wind shift, with temperatures in the 20s across central SD and
still in the mid-upper 30s in the east. Earlier this morning, we
expanded the dense fog advisory westward, to just east of the James
River Valley, but this boundary projected to slide east across the
region through early this afternoon, and expect improvement in areas
which have been plagued with persistent very low stratus and fog the
past couple of days, as winds switch around to the west.

As far as precipitation chances, satellite view showing fair amount
of higher cloud over our forecast area as of 09z, which has allowed
for some areas of measurable light rain across the area through the
overnight hours. Depth of the moisture aloft wanes through the day,
though, as the upper trough shifts northeast of the area, so the
potential for additional measurable precipitation should be limited
to the morning hours for most of the area. Far northeast portions of
the forecast area, generally northeast of Brookings to Jackson,
remain in more of a cyclonic flow, providing weak lift all the way
through tonight. This area also most likely to see more persistent
stratus with limited moisture aloft, so would expect precipitation
to transition back to patchy drizzle this afternoon through tonight.
By late tonight, temperatures are likely to dip below freezing, so
may have to watch for spotty icing in these far northeast areas of
the CWA.

.LONG TERM...(Sunday through Friday)
Issued at 343 AM CST Sat Jan 21 2017

At the tail end of a final short wave passage Sunday morning, there
is a vort filament moving across our northern zones. With moisture
saturated through the lowest kilometer under dry mid levels, believe
there is a risk of freezing drizzle or drizzle in southwest MN
during the morning hours. The CAMs are picking up on a hundredth
of an inch, especially from near Marshall to Windom, so we will
have to watch this area for some early day light icing. But if
this does occur, it will not last long as temperatures rise to
above freezing by mid morning. It should be dry, but mostly cloudy
elsewhere, minus our far southwest zones who will see clearing.
With a brisk northwest breeze, do not believe fog will be too much
of a threat on Sunday, and highs mix into the 30s.

Not a lot happening Sunday night and Monday as an upper ridge of
high pressure moves slowly across the middle part of the country.
Then all eyes turn to the next upper trough impacting our weather
mainly Tuesday and Tuesday night. The GFS, ECMWF and GEM Global were
in very good agreement on the 12Z run. However for the current 00Z
run, they have gone back to their long term trend of the GFS being
the quickest, the GEM global a full 8 to 12 hours slower, and the
ECMWF being in the middle. Notably, the GEFS mean also shows a
solution which is a bit slower then the operational GFS, meaning it
is not too much different then the operational ECMWF. Now all this
said it does not make a huge difference in the pops. This is because
the upper low is rather broad and somewhat positively tilted and
sheared. Therefore the precip trend follows suit, in showing a broad
pattern and at this time, frontogenetic banding looks minimal. That
could always change however as once the system gets closer, the
higher resolution models could pick up on some mesoscale lifting
mechanisms that the global models are not showing yet. At this time,
it still looks like a general range of 3 to 6 inches of snow across
the forecast area. And again, these amounts could be more here and
there if mesoscale banding begins to develop.

However if one puts a little more stock in the ECMWF, it does make a
difference on temperatures as the ECMWF is a bit cooler. For Monday
night, Tuesday and Tuesday night, continued the trend of blending
raw model consensus temperatures, plus the ECMWF bias corrected
readings into the superblend. The superblend looks a bit too cool
Monday night, and too warm on Tuesday. So the blend used had the
affect of minimizing the temperature diurnal spread due to
precipitation and cloud cover. The all blend may not be wet bulbing
temperatures downward enough on Tuesday. Therefore given the 925-
850mb temperatures, the precipitation should be primarily snow
except for Yankton, to Sioux City/Le Mars, and eastward to Storm
Lake who could see a rain/snow mix on Tuesday and all snow Tuesday

The system will exit on Wednesday with some lingering light snow.
The main item for the rest of the week is there is an absence of
arctic air coming down behind this system. Although forecast
temperatures may be a little too mild, the readings looks to be
pretty close to seasonal normals. A northwest flow of air will
prevail and it could be breezy each day.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 541 AM CST Sat Jan 21 2017

LIFR conditions persist across much of the region this morning,
though pockets of VFR ceilings/visibility found west of I-29,
behind a weak frontal boundary shifting winds from southerly to
westerly. Boundary will swing northeast through our northwest
Iowa/southwest Minnesota counties by this evening, and expect some
improvement to at least MVFR ceilings/VFR visibility as it moves
through. Exception will be northeast of a KBKX-KLYV-KSPW line,
where low level moisture and IFR conditions will remain much more


SD...Dense Fog Advisory until 10 AM CST this morning for SDZ039-040-

MN...Dense Fog Advisory until 10 AM CST this morning for MNZ071-072-

IA...Dense Fog Advisory until 10 AM CST this morning for IAZ001>003-

NE...Dense Fog Advisory until 10 AM CST this morning for NEZ013-014.



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