Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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851
FXUS63 KFSD 171135
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
635 AM CDT Thu Aug 17 2017

.SHORT TERM...(Today and Tonight)
Issued at 345 AM CDT Thu Aug 17 2017

Strong surface and upper level low positioned just northeast of the
forecast area, near Gaylord Minnesota as of 08Z. Deep northwest flow
behind the departing low will bring gusty conditions to the region
today, especially in eastern areas, whereas locations west of I-29
will begin to see influence of surface ridging by mid afternoon. May
see a few lingering light showers in the far northeast early in the
day. However, the ridging and associated subsidence behind the low
should bring mostly sunny skies to most areas by mid afternoon, with
temperatures climbing much closer to mid-August normals. Highs
expected to range from mid 70s in southwest Minnesota, to lower 80s
along and west of the James River Valley.

Tonight, models in fairly good agreement showing a decent shortwave
tracking into south central South Dakota by 06Z, then continuing
east across the forecast area through the latter half of the night.
Low level jet impinging on elevated warm front near the Missouri
River Valley should focus greater rain potential in our southern
counties, though broad lift ahead of the wave may lead to scattered
light showers even north of I-90. Instability overnight is rather
weak, and largely focused across the western CWA, so will have more
limited thunder mention near and east of I-29. Light winds and clear
skies in the evening will allow for a rather quick temperature drop,
especially in the east, where these conditions will linger later
into the night. Readings should level off during the overnight as
clouds increase, with lows in the upper 50s to near 60 expected most
areas.

.LONG TERM...(Friday through Wednesday)
Issued at 345 AM CDT Thu Aug 17 2017

On Friday, a cold front will move through the region.  Showers and
thunderstorms will be possible with the passage of the front, but
there appears to be a fairly stout cap in place at least across the
western half of the forecast area.  Have pops concentrated east of
I-29, but believe the greatest chance for precipitation will be
across the eastern 1/3 of the forecast area east of a line from
Marshall MN to east of Sioux City IA.

Saturday looks to be a relatively pleasant with surface high pressure
across the region.  As this system works into the Mississippi River
Valley, southerly winds will increase.  Models are hinting at the
potential for a few elevated storms Sunday morning along the
Missouri River, but the chances are conditional on the amount of
moisture in 800-700 mb layer.

More significant chance of precipitation is expected to impact the
area Sunday evening as the leading wave in a short wave train moves
through the area.  Very steep mid level lapse rates leads to 2000-
3000 J/kg of surface based CAPE late in the afternoon. Bulk shear
values are very model independent at this point, but there is a
broad consensus that 30-40 knots of 0-6 km.  This supports the day 4
slight risk across much of the area.

Have the potential for additional thunderstorms on Monday as
additional waves (albeit weaker) train through the area. Model
soundings indicate a fairly sizable cap across the area on Monday,
so hopefully convection and clouds hold off for viewing of solar
eclipse.

Tuesday through Thursday the upper level ridge tries to strengthen
across the desert southwest.  This results in more amplified
northwest flow with a series of weak disturbances passing through.
For now, left skittish pops in the blend as is until there is better
agreement amongst the various models.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 635 AM CDT Thu Aug 17 2017

Lingering MVFR ceilings near and east of I-29 will gradually shift
east through the day, as the low pressure system responsible for
the clouds and yesterday`s rain slides east. Gusty northwest winds
will also be prevalent across eastern portions of the region, with
gusts of 25-30kt likely over portions of southwest Minnesota and
northwest Iowa at times through this afternoon.

High pressure will build eastward behind the departing low today,
bringing VFR conditions through the remainder of the period. An
upper level disturbance may trigger some showers or isolated
thunderstorms from late this evening into the overnight hours, but
bases will be high and significant visibility reductions are not
expected.

&&

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

$$

SHORT TERM...JH
LONG TERM...BT
AVIATION...JH



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