Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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773
FXUS63 KFSD 281103
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
603 AM CDT Fri Apr 28 2017

.SHORT TERM...(Today and Tonight)
Issued at 435 AM CDT Fri Apr 28 2017

Mid-level trough pivoting through the Central Plains today will cast
considerable cloud cover northward for Friday.  Latest high
resolution guidance continues to maintain a large corridor of rain,
but rotates this band of rain just south of the CWA. Soundings also
indicate a very dry mid-level atmosphere along the I-90 corridor,
thanks to an influx of dry air from the northeast. Thus, have
discounted the GFS and trended the forecast closer to the
HRRR/RAP/EC which suggest the greatest rain chances along the very
southern portions of the CWA in the Missouri River valley.
Temperatures with increased cloud cover will after from the upper
40s to lower 50s.

Dry northeasterly flow will erode precipitation later this evening,
but cloud cover should remain fairly substancial in most areas.  The
increased cloud cover should hold temperatures a bit warmer into
Saturday morning, with the coolest readings near Huron.

.LONG TERM...(Saturday through Thursday)
Issued at 435 AM CDT Fri Apr 28 2017

Main concern for the mid-range portion of the forecast deals with
the large Spring storm for the weekend and Monday.  Gross features
with the storm have come around to be fairly consistent between
various operational solutions (other than slightly colder NAM
solution overall), while ensembles continue to suggest that subtle
features including critical rain/snow temperatures will continue to
drive uncertainty in the forecast. Very little attention will be
given to the forecast beyond the Monday storm, other than to
slightly modify temperatures for the expected snowfall yet for
Tuesday. Spoilers.

Wave will continue to dig through the southern Rockies on Saturday,
spinning its way across AZ and NM through the day. As flow
downstream ridges more, will see increasing support for developing
mid-level precipitation with elevated warm advection pushing into
southern areas toward midday, and slowly expanding northward through
the afternoon. May be a bit quick spreading precipitation northward
on Saturday, but should at least get some light showers as far north
as near the Highway 20 corridor, but the northeast flow will
continue to challenge the advance of precip with influx of drier low-
level air.  Advance of precipitation will continue on Saturday night
and Sunday as inverted trough begins to develop northward into the
CWA and main frontogenetic axis spirals north and west into
southeast SD by Sunday evening. The far western CWA on west side of
inverted trough could easily remain precipitation free through
Sunday, with precipitation becoming more widespread and concentrated
near the mid level trough. PV advection from the south later in the
day will likely begin to really enhance precipitation coverage
toward the lower Missouri River valley, which will progress north
and westward Sunday evening into southeast SD as dry slot pushes
toward northwest Iowa. While have not put in any the grids at this
point, would not be surprised to have a small thunder risk translate
into at least NW IA Sunday night.

For now, Sunday and early Sunday evening look to remain just warm
enough so that rain will fall in all locations, with potential
exception of some early day mix toward parts of southwest MN and the
Iowa Great Lakes.  Eventually on Sunday night, that lowest 50 hPa
which was protecting from snow aloft will begin to cool with wet
bulbing on northeast flow along with the onset of diurnal cooling
should help to kick start a transition to snow. It is likely that
elevation will play a great role on snow accumulation, as every
hundred feet gained will remove that much of the warming in what
will be a thermal environment which will be dancing on the rain/snow
boundary. Combine this with even a degree or two Celsius error and
this could wreak havoc with precip type and timing of changeover.
The location of the maximum precipitation rates overnight which set
up along the trowal airstream frontogenesis axis will likely have
diabatic impacts that help drive a changeover as well. At this time,
the corridor of this cooler wedge will likely extend along an axis
from KMMl/KBKX area toward KYKN for much of Sunday night.

Another source of uncertainty comes along with any convection which
develops Saturday night into Sunday along the lower level warm front
across Missouri and Illinois. If this gets too significant, there
may be an impact on reorienting or redistributing the frontogenesis
and trowal airflow. Similar cases have brought an eastward shift to
the QPF axis. Something to watch.

At this time, the forecast likely reflects a bit too wide an area of
potential accumulating snowfall with the uncertainty in temps. A
band of 1-4 inches of snow would be most probable, but with a small
temp change the difference between almost no snowfall accumulation
and potentially several inches, will have to be watched.  Most
likely area for a more significant snow at this time would focus
mainly along the Buffalo Ridge and perhaps the higher terrain west
of Brookings. Those with travel plans Sunday night into Monday
should keep aware of the potential for accumulating snowfall.

Made some modifications off initial blends to both hourly temp
trends and overall highs/lows from Saturday through Tuesday. Tried
to account for increased clouds/precip Saturday knocking temps back
a bit across northwest Iowa, then holding up temps somewhat in the
moist environment through Sunday, with minimal diurnal change. Away
from clouds/precip on Saturday through much of southeast SD and into
southwest MN, likely to find upper 40s to mid 50s with mixing of
northeast wind despite the clouds.  Coolest temps by Sunday night
favoring the snow band/terrain areas into the lower 30s, but mainly
mid 30s elsewhere. Temps will struggle Monday as low really wraps up
to the east through Monday evening.  Snowcover should keep some 30s
for highs in higher terrain over southwest MN, but outside areas
with more significant snow could see potential mixing as warm as the
mid 50s again for areas west of the James River. Winds too low in
blends in general from Sunday night through Monday night, and
blended with stronger MOSGUIDE and even nudged up a bit more to
account for 30-35 knots topping the mixed layer on Monday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 558 AM CDT Fri Apr 28 2017

Clouds gradually thicken through the morning hours, with an
overcast mid-level deck in place by late morning. Northern edge of
rain may just clip Sioux City later this morning, bringing
ceilings down into the MVFR to perhaps even IFR category briefly.
Light rain may linger into the afternoon hours before VFR
conditions move back into the area in the evening. Further north,
VFR conditions expected through the TAF.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Dux
LONG TERM...Chapman
AVIATION...Dux



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