Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 191008

Area Forecast Discussion
National Weather Service Sioux Falls SD
408 AM CST Fri Jan 19 2018

.SHORT TERM...(Today and Tonight)
Issued at 408 AM CST Fri Jan 19 2018

A weakening closed low evident on WV imagery off the BC coast will
continue to fill as the parent trough moves onto the west coast. Our
region will sit under west to southwest flow aloft allowing for
unseasonably warm temperatures. 925 mb temperatures in the +4 to +11
C range will lead to widespread 40s to mid 50s across the region, or
about 20 to 25 degrees above normal. Shaved a few degrees off
temperatures given the dense cirrus shield expected through much of
the day. A weak cold front will slide through tonight with low
temperatures generally in the 20s. With the anticipated snow melt
this afternoon, will need to keep an eye on any fog potential early

.LONG TERM...(Saturday through Thursday)
Issued at 408 AM CST Fri Jan 19 2018

On Saturday, the various deterministic models are beginning to come
around in better agreement with the on shore timing of the strong
upper trough. By 00Z Sunday, the center of the trough axis is still
west of the four corners area. This forecast area is marked by a
light northerly flow of air at the surface with plenty of mid and
upper level moisture. But the air mass continue to be warm, with
925mb temperatures generally in the +2 to +4C range, which should
give above normal highs in the upper 30s to lower 40s.

Saturday night, the GFS has slowed down the eastward progression of
the upper low similar to the ECMWF and CMC models, and is near the
four corners area by 12Z Sunday. In effect, this slows down the pops
in our southeast zones late Saturday night for any freezing drizzle
threat, with now only slight chance pops in northwest IA and
adjacent areas for light freezing drizzle or very light snow Sunday
morning. Those low pops may too be lowered even more for Sunday
morning in the future if the eastward progression slows down
anymore. Lows Saturday night are largely in the mid 20s and are
problematic. There will be plenty of cloud cover helping to keep
lows mild for this time of year, however the surface flow veers to
the northeast ushering in a relatively dry low level air mass
initially. Therefore kept these consensus model lows and did not
alter them warmer.

On Sunday and Sunday night, there is now consensus in moving the
upper low across northern KS and southern NE, with much better
consensus with the speed of the system also. The upper low then
continues a track toward the mid Mississippi valley by late Monday
where some subtle timing and placement differences then show up. The
forecast area is in the right rear quad of a jet streak over the
Great Lakes on Sunday which will aid in the beginning of broad
ascent. However as the upper level features lift across the central
plains Sunday night and Monday, the southern branch of the jet does
not quite curl up northward which does not give the CWA a strong
left exit region of the southern jet stream. What we do have however
is strong low to mid level frontogenesis which aids in rapid
saturation beginning late Sunday and continues through Sunday night
and Monday morning. Mid level frontogenesis is right now, maximized
along an axis from Tyndall SD to Brookings SD and Marshall MN, with
the max low level frontogenesis further east in northwest IA. There
is also some less stable per the 600mb EPV* but not overly unstable.
Lastly, a strong trowal maximized in the 295-300K layer exists over
southwest MN and extreme northwest IA through the entire day on
Monday. The 00Z NAM appears to be too far west with its maximum QPF
placement, putting it along the 600mb frontogenesis across our
western zones where little in the way of instability exists. So that
scenario was discounted. In fact the 06Z NAM shifted its max QPF
eastward. For now, blended the CMC, GFS and ECMWF which were much
closer in placing the higher QPF amounts in southwest MN, southeast
SD, northeast NE and parts of northwest IA minus our far southeast
zones around the Storm Lake vicinity. It is these areas that are on
the unstable side of the mid level frontogenesis. When one looks at
the last several runs combined of the ECMWF, the GFS and CMC, and
even the German standard on the Internet, more often than not, the
max QPF band has been placed along an axis from Yankton to
Vermillion SD, northeastward through southeast SD and into southwest
MN. Therefore wanted to continue that trend with this forecast.

That said, some of the 00Z NAM got into the consensus, therefore
believe that our northwest zones along and west of a Chamberlain to
Kingsbury county line could be too high with amounts. Therefore did
not include those areas in a winter storm watch. In our far
southeast, we also left them out of a winter storm watch for now
with snow amounts being held down by warm air aloft. The CMC is the
warmest aloft as it is slightly further north with its low level
features. What that does do however is give them a potential for
more accumulating ice Sunday night around Spencer and Storm Lake.
Elsewhere, a winter storm watch is warranted at this time, extending
it into Monday, a rare watch beyond 72 hours. The watch area could
receive 6 inches or more of snow, coupled with burly winds averaging
in the 25 to 35 mph range. To increase winds, blended consensus mos
values into the superblend speeds which were too low. Thought about
putting blizzard wording in the watch, however the cyclogenesis does
not appear to strengthen with this storm as it tracks through the
plains. It does not weaken either, it just stays steady state.
Without the jet curling northward and aiding really sharp ascent and
curvature, strong surface cyclogenesis may be tough to realize.
Therefore for now, left blizzard wording out of the watch but if
things change, we will have plenty of time to refine it.

One thing to watch for in future forecasts is any southeastward
adjustment with the track of the low. I think the timing will not
alter much, so now its down to placement. But again, I fall back to
the overall consensus of the last several runs of several models
described above. The rest of the week looks fairly benign, with
temperatures hovering several degrees either side of normal. Of
course if deep snow cover occurs, we may need to shave a few degrees
of these readings.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1023 PM CST Thu Jan 18 2018

VFR conditions are expected through the TAF period.


SD...Winter Storm Watch from Sunday morning through Monday morning
     for SDZ050-053-054-058>060-063>065-068.

     Winter Storm Watch from Sunday afternoon through Monday morning
     for SDZ040-055-056-061-062-066-067-069>071.

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

IA...Winter Storm Watch from Sunday afternoon through Monday morning
     for IAZ020-031.

     Winter Storm Watch from Sunday evening through Monday afternoon
     for IAZ001>003-012-013.

NE...Winter Storm Watch from Sunday afternoon through Monday morning
     for NEZ013-014.



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