Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 202352

Area Forecast Discussion
National Weather Service Sioux Falls SD
552 PM CST Mon Feb 20 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 341 PM CST Mon Feb 20 2017

Record highs returning to the area!

Gusty northwest winds developing behind a cold front moving through
the area.  These winds will settle down with sunset this evening.
Skies will clear through the overnight, providing good radiative
conditions.  This will allow temperatures to fall, and will cancel
out all the record warm lows we had this morning.

Could see some patchy fog develop tonight as ridge axis moves in and
surface winds go light. Expect the fog to be relatively shallow and
burn off relatively quickly on Tuesday.

Southwesterly flow will develop on Tuesday drawing mild air into the
region.  Raised forecast highs close to BCCONS_Raw, which is near
the warmest guidance. Should be one of the last warm days with
cooler air moving in late in the week.

.LONG TERM...(Tuesday night through Monday)
Issued at 341 PM CST Mon Feb 20 2017

After a very unseasonably mild night Tuesday night, Wednesday will
be our last warm day for what appears to be quite a while. Short
wave energy moves into the central Dakotas by late afternoon, with a
band of strong mid level frontogenesis noted along some 700mb
baroclinicity in our northern zones during the morning hours.
Therefore kept a chance of sprinkles along highway 14 for Wednesday
morning, as a lack of moisture below 700mb should keep bonafide
light rain at bay. Despite some lower level moisture increasing in
the 850-800mb layer during the afternoon hours in our northern
zones, the sprinkle threat could end as the frontogenesis slips
further southeast into increasingly drier air. Temperatures
Wednesday are a little tricky. Surface winds shift to the north and
northwest across the forecast area during the afternoon hours. The
various deterministic models show some subtle cold air advection
filtering in generally along and north of I 90 evidenced in the 925-
850mb thermal layer, coupled with the aforementioned mid clouds.
Therefore kept highs in our north along highway 14 in the upper 50s
to near 60, not too far from superblended highs. But elsewhere,
mixed in some warmer raw bias corrected values which should give
record highs for Sioux City and Spencer Iowa.

All eyes then turn to the storm system which will affect the region
late this week, most notably in the Thursday night through Friday
night time frame. Cooler air advects into the area on Thursday, as
isentropic lift increases to the north of a central plains warm
front located in southern KS, all the while deepening the moisture
profile. Because of this, lowered highs a category from superblended
highs, as temperatures should not warm a whole lot from Thursday
morning lows. POPS will correspondingly increase from southwest to
northeast as Thursday progresses, with likely or categorical pops
warranted generally along and south of I 90 by afternoon for mainly
light rain.

By 12Z Friday, there is extreme model spread with the handling of
this system giving low confidence. For example looking just at
surface low pressure, solutions range anywhere from the UKMET
placement between Wichita and Dodge City KS, to where MO/IA/IL meet
per the ECMWF solution. The GFS and GEM Global solutions have been
the most consistent at least in terms of placement with the surface
and upper air features, and they both have the surface low placed in
extreme southeast NE and near St. Joseph MO at 12Z Friday. At any
rate, the UKMET appears to be a slow and southern outlier, and for
now, favored the middle of the road solutions involving the GFS and
GEM Global. The GFS/GEFS spaghetti plots are generally in good
agreement through Friday night, other then the usual couple of
outliers which the operational GFS does not follow. So the ensembles
provided little help as to whether follow any of the foreign
solutions, as the GFS ensembles clustered around each other. The GEM
Global solutions still shows what would really be an out-right
blizzard for much of our forecast area. The GFS being a bit further
south has slightly less wind, and of course less snow accumulation.
So at this point, accumulating snow is still a threat Thursday night
through Friday night, with highs on Friday mainly in the lower 30s.
So the message is still extremely uncertain at this point with this
much disparity between the solutions.

The GFS shows another short wave moving eastward late Saturday night
and Sunday, mainly affecting areas along and south of I 90. However
this is not backed up by the ECMWF and GEM Global so only chance
pops are warranted. One thing is certain, temperatures will be much
closer to normal this upcoming weekend.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 549 PM CST Mon Feb 20 2017

Primarily VFR conditions expected through the TAF period. Winds
will rapidly decrease this evening, becoming light and variable
late tonight. Models suggest that shallow patchy fog will be
possible, so will leave mvfr fog at TAF sites through 12 or 13z.


Issued at 341 PM CST Mon Feb 20 2017

The Grassland Fire Danger Index could reach the very high category
generally in our Missouri River valley zones Tuesday and Wednesday
afternoon. Minimum afternoon humidity values will range from about
25 to 35 percent, with wind speeds averaging 15 to 25 mph.




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