Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 171659
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1159 AM CDT Mon Apr 17 2017

.SHORT TERM...(Today and Tonight)
Issued at 326 AM CDT Mon Apr 17 2017

Mid level short wave, best seen by using QG forcing in the 700-500mb
layer, will exit our far eastern zones between 12z and 15z this
morning. Therefore the current mid level clouds and isolated
light showers or sprinkles in our east will exit with it leaving a
dry day. Surface high pressure to the north will shift off to the
east turning the winds around to a southeast direction, and they
will become quite brisk this afternoon across our west. Boosted wind
speeds throughout the forecast area this afternoon, as some dry air
wraps down around the high, however certainly not as dry as
yesterday. And given the latest 850mb temperatures, highs are little
changed from yesterdays forecast with a lot of 60s in store, perhaps
lower 70s from Yankton to Sioux City.

Tonight, a strong short wave will move rapidly eastward across the
northern plains. QG forcing in the mid levels is focused to the
north of our forecast area, with a trailing southern edge of it
streaming down into Nebraska. Thetae advection is pretty strong
tonight as expected, with a low level jet increasing to 30 to 40
knots. By 12z Tuesday, the thetae advection is just east of the I 29
corridor, so the wave is moving quite quickly. This will hold rain
amounts down, with a widespread 0.10-0.33 expected. But generally
high pops are still warranted most locations. The lightest amounts
of rain and smallest pops are confined to our edges, near
Chamberlain in central SD, and from Spencer to Storm Lake Iowa. This
is because the moisture profile is not very deep yet at Chamberlain,
and the dynamics of the wave are still a bit west of Spencer and
Storm Lake by 12Z Tuesday. Elevated instability in terms of computed
CAPE and lifted index values are strong enough to warrant a mention
of TSRA. But with elevated CAPE values of 100 to 400 J/kg, and
lifted index values averaging around -1C or -2C, there is not much
of a severe weather potential. In addition, soundings are pretty
stable in the lowest 1.5-2.0km and mid level lapse rates are not
overly unstable due to the saturated profile from 700-500mb.
Therefore mixed layer CAPEs are next to nothing. It will be quite
windy tonight, with some of the aforementioned low level jet coming
down to the surface. Therefore southerly winds of 20 to 30 mph will
be quite common, and thus raised low temperatures a smidge due to
the wind and mild air.

.LONG TERM...(Tuesday through Sunday)
Issued at 326 AM CDT Mon Apr 17 2017

For Tuesday - Wednesday night, the primary forecast concern is the
potential for rain beginning Tuesday night and ending Wednesday
evening.

On Tuesday, all models show the upper level wave and accompanying
front moving from far eastern South Dakota at 12Z to southeastern
Minnesota by 00Z. There is still weak instability in place so there
could be a few claps of thunder over southwestern Minnesota and
northwestern Iowa during the morning. However, MUCAPE is less than
200 J/kg so severe weather is not expected. As the upper wave moves
into the Great Lakes during the day, the forcing for ascent will
decrease and any rain should move south and east of the area by
early afternoon. Skies will clear from west to east during the day
and there is not any cold air behind the front. So high should rise
back into the mid 60s to lower 70s. Wind should actually decrease
during the afternoon as the surface pressure gradient weakens.

A more significant system will move toward the area on Tuesday night
and Wednesday. A strong low level jet will bring moisture back into
southern South Dakota overnight just south of the 700 mb front which
will be near I-90. A jet streak will be moving across North Dakota
with central South Dakota in the right entrance region of the jet
overnight. While there could be an isolated shower in south central
South Dakota prior to midnight, the rain is most likely to be after
midnight west of the James River. 700-500 mb lapse rates are less
than 7 C/km and soundings show a near moist adiabatic lapse rate.
For that reason, not expecting any thunder - just light to moderate
rainfall. This rain could be as far east as I-29 by sunrise on
Wednesday. With increasing clouds and easterly flow at the surface,
lows will remain in the 40s.

Wednesday will be cloudy and breezy with a cold rain over most of
the area. The jet streak near the Canadian border will only slowly
move east as a second jet streak moves from Colorado into the
central Plains. This should result in an expansion of the
precipitation through the morning and into the afternoon. At some
point later Wednesday afternoon and evening, storms look to develop
near or south of I-80 in southeast Nebraska or southern Iowa. This
should eventually result in a decrease in the more stratiform
precipitation that will be over southeastern South Dakota,
southwestern Minnesota, and northwestern Iowa. There is evidence of
limited elevated instability in northwest Iowa. The instability is
limited with less than 500 J/kg of MUCAPE so severe weather is not
anticipated. Overall with the strong dynamics, expecting an average
of a third to two-thirds of an inch of rain with the heaviest
rainfall most likely in northwestern Iowa. With the clouds and rain
much of the day, highs on Wednesday will be 15 to 20 degrees colder
than Tuesday - from the upper 40s to mid 50s. The rain will
gradually come to an end on Wednesday night. Skies will be slow to
clear though and this will keep lows in the upper 30s.

For the extended...Thursday through Sunday...expecting a quieter
weather pattern. The Canadian GEM is an outlier solution bringing an
upper low into the eastern Plains by Saturday. The GFS and ECMWF
both keep this wave farther south digging it into the southern
Plains Friday and then slowly moving east on Saturday. The GFS does
bring some moisture up to Highway 20 late Friday night and Saturday.
There is enough uncertainty to keep a 20 to 30 percent chance for
rain Friday night and Saturday. Otherwise, temperatures look to be
near normal into the weekend. The ECMWF and GFS both hint at warmer
temperatures by Sunday. For now kept highs in the low to mid 60s but
the potential is there for temperatures at or above 70.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1154 AM CDT Mon Apr 17 2017

Easterly winds will transition to southerly and pick up by this
evening, gusting 20 to 25 kts overnight into Tuesday. With a low
level jet developing tonight, there will be a period of low level
wind shear at KFSD and KSUX after 09Z. In addition, will have
showers develop overnight, with isolated thunderstorms possible.

&&

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

$$

SHORT TERM...MJ
LONG TERM...Schumacher
AVIATION...JM



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