Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 172352

Area Forecast Discussion
National Weather Service Sioux Falls SD
652 PM CDT Mon Apr 17 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 247 PM CDT Mon Apr 17 2017

An upper level shortwave presently sliding through Montana this
afternoon will track across the Northern Plains overnight, with the
better upper level energy remaining to the north of our area. Even
so, still expecting showers to develop across our CWA tonight with
increasing mid level theta e advection and a 50 to 60 kt low level
jet which will ramp up in the evening. Still cannot rule out some
isolated thunderstorms with this activity with model computed
elevated CAPE of 200-300 J/KG, though obviously would not expect any
severe storms. With a surface low tracking across northern SD
overnight and an increasing gradient across the area, will see
southerly winds really increase during the overnight hours, and this
will impact temperatures with a very mild night as lows only drop
into the lower to mid 50s.

By Tuesday morning the aforementioned upper level low will pull off
into the western Great Lakes region, while the surface low just to
our north pulls a frontal boundary across the CWA during the
morning. Out ahead of this, there could still be lingering showers
and isolated thunderstorms over portions of southwestern MN and
northwestern IA in the morning, before coming to an end by early
afternoon. As the front pulls through the area and drier air filters
into the region, there will be decreasing clouds with highs back
into the mid 60s to lower 70s.

.LONG TERM...(Tuesday night through Monday)
Issued at 247 PM CDT Mon Apr 17 2017

Mid and upper level moisture will increase on Tuesday night as the
right entrance region of a quick moving upper level jet spreads
north. Still expect rain to develop after about 6z along a mid level
frontal band, close to 700mb, with the better chances apparent over
central SD towards points north of Interstate 90 in SD.

After this it looks like sometime from mid to late morning interest
will then transition farther south, to areas south of Interstate 90
and especially towards the Missouri River. During the morning hours
the 925mb to 750mb layer will be a bit dry but moisture is expected
to quickly shift north to the aforementioned areas. Frontal forcing
becomes pretty strong by early afternoon and instability across
northern Nebraska looks to increase along and south of the east-west
oriented boundary. A look around at the soundings across much of
Nebraska hints that anything from about 850mb and lower will be
capped so the focus turns to the boundary and instability from about
800mb to 750mb. Both the Nam and GFS are pretty agreeable on this
layer as the best possible layer for the weakest capping. Elevated
CAPE values look to climb to about 500-1000 J/kg with the strongest
focus south of Interstate 90 into northwest Iowa. Strong 7-9 C lapse
rates in the 700mb to 500mb layer nose north towards the Missouri
River leaving the gradient along the Missouri River. Overall the
severe weather threat looks low with the potentially fairly low
instability values, however the 2km to 6km shear running about 20
to 30 knots so if a stronger updraft can get going would suspect
that somewhere from about Dixon Nebraska to Spirit Lake IA and
points south quarter to half dollar sized hail would be possible.
The focus for showers and thunderstorms will shift east of the area
on Wednesday evening bringing a dry and seasonally cool Thursday.

Overall still looking like Thursday into the weekend will be dry
with only very small chances for showers along the Missouri River
into parts of northwest Iowa. Temperatures seasonally mild with
highs from 55 to 65 and lows from 35 to 45.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 652 PM CDT Mon Apr 17 2017

Increasing southerly winds overnight and isolated thunder threat
will be primary aviation concerns through this TAF period. A 45 to
50kt low level jet will develop through the evening, resulting in
a period of non-convective low level wind shear across the region
from late evening into early Tuesday morning. Occasional surface
wind gusts near 25kt are also possible through the overnight/early
morning hours.

Primarily VFR ceilings/visibility will dominate as the SHRA/TSRA
move through, although a period of MVFR ceilings and/or brief
visibility reductions will be possible.

The LLWS and band of SHRA/TSRA will be ahead of a cool front,
which will move into far western portions of the region around
KHON/K9V9 by 09Z. The front will then progress eastward toward
I-29 after daybreak Tuesday, and exit the eastern forecast area
late Tuesday morning. Lighter westerly winds and VFR conditions
will prevail following the frontal passage.




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