Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 291707

Area Forecast Discussion
National Weather Service Sioux Falls SD
1207 PM CDT Sat Apr 29 2017

.SHORT TERM...(Today through Monday afternoon)
Issued at 330 AM CDT Sat Apr 29 2017

Primary concerns in the short term portions of the forecast are
focused around the incoming storm system centered over the Four
Corners region early this morning. Mid-level moisture continues to
stream northward through the Plains tonight, with mid-level clouds
slowly beginning to pull northward as we move into daybreak.

That said, dry conditions will prevail for much of the daytime hours
of Saturday. Temperatures will climb into the upper 40s to lower 50s
in most locations.  By the afternoon however, expecting to see broad
isentropic lift spread northward, with increasing upper diffluence
and a gradual uptick in shower activity may begin over the
southern portions of the CWA by the late afternoon hours.

Moisture surges northward overnight as upper low begins to eject
eastward into the Southern Plains.  Persistent and broad isentropic
lift centered between the 295-300k sfc will produce a large area of
light to occasionally moderate rain spreading northeast. Have
some concerns about the impact of convection over the lower
Missouri River valley Saturday evening and night robbing a bit of
the moisture and holding QPF values down especially north of I-90.

With a bit of westward trend in models over the past 24 hours,
pronounced dry slot may rotate back into western and northwestern
Iowa by mid-day Sunday. Convection may begin to blossom on the edge
of this boundary Sunday morning over eastern Nebraska and western
Iowa, pivoting northward into the CWA by the afternoon. The westward
trend in the upper low also pushes QPF well west towards central SD
by late afternoon and lowers overall QPF over Iowa slightly. I
have extended PoPs westward to account for the change and
realigned the QPF axis slightly further west as well. GFS/NAM
also showing a layer of very elevated instability through Sunday
afternoon as rain pivots northward so have introduced an isolated
thunder risk.

Guidance is really focusing in on the temperature profile in the
lowest 1000 ft over the past 24 hours, raising confidence on the
development of snow overnight into Monday morning.  Regarding snow,
1 or 2 degrees and a more intense localized precipitation rate will
make a significant difference within this setup.  The area of focus
for snow has shifted slightly westward, along a Lake Andes to
Brookings to Marshall line.  Blended SLR tools are yielding average
rates around 8:1 at times, but could see these tick a bit higher in
the more intense rates. That said, have painted a broad area of
0.5" to 3" of snow through the CWA, with slightly higher totals
within the higher elevation areas in the Missouri River valley and
over the Buffalo Ridge. Still too soon to tell, but I wouldn`t be
surprised to see totals break the 5" range along the ridge.

The other portion of the forecast that could be fine tuned further
is the loss of mid-level moisture in NW Iowa Sunday night.  A light
rain could turn into a persistent moderate drizzle. With warmer air
aloft moving into NW Iowa, it may prevent any snow in those areas.

This system will slowly pivot eastward on Monday, with snow/rain
lingering through mid morning. Temperatures quickly warm by mid-day
and by the afternoon many areas could be well into the 40s or 50s.
Any snow that falls should have a short lifespan.

.LONG TERM...(Monday evening through Friday)
Issued at 330 AM CDT Sat Apr 29 2017

As the large H5 low continues to move east-northeast out of our
area, there will be a short break in precip chances on Mon night
into Tues afternoon. By Tues night, another short wave drops south
and a weak frontal boundary passes through which could kick off
some showers late Tues night into Wednesday. Moisture isn`t that
deep so have left pops in the low chance/slight chance cat.

Wed night through Saturday will be dry as H5 ridging really starts
to strengthen over the southwest CONUS.  GFS/ECM begin to diverge in
the strength and position of this ridging and the deepening and
position of a deepening low somewhere between the Great Lakes and
the deep south on Friday.  The main impact of these differences will
be how warm we will get next weekend. The more pronounced ridging
seen in the ECM will lend itself to warmer temps than the flatter
GFS. With these differences, have not made changes to blended
output for high temps on Fri/Sat although there is a possibility
that the upper 60s and low 70s could be too cool.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1202 PM CDT Sat Apr 29 2017

VFR conditions through this evening with northeasterly winds
gusting 20 to 25 kts. Rain will develop this evening then spread
northward tonight, with ceilings lowering into the MVFR range
at KSUX and KFSD after 06Z. Northeasterly winds will again pick up
toward Sunday morning, gusting 25 to 30 kts.




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