Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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741
FXUS63 KFSD 291729
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1229 PM CDT Mon May 29 2017

.SHORT TERM...(Today and Tonight)
Issued at 423 AM CDT Mon May 29 2017

A breezy and cool Memorial Day is on tap for the region, as
northwest flow ushers in cold air advection across the northern
Plains. Main upper low will continue to wobble near Lake Superior,
however a weak shortwave lobe and vort max drops southward through
Minnesota today. Stratocumulus clouds will be on the increase,
likely blanketing all areas along and east of the James River by
midday. Isolated to scattered showers will be possible late morning
through the early evening within this deck, with the best chances
across southwest Minnesota and east central South Dakota. Activity
will be very hit-and-miss "popcorn" showers. Model soundings show
some pretty weak elevated instability, but should be deep enough for
some isolated thunderstorm mention. Northwest winds will again be
breezy at the surface around 15 to 25 mph with gusts to 35 mph. With
continued dry low levels, cannot rule out some even higher gusts in
showers or thunderstorms, but severe weather is not expected. Highs
today will be in the 60s.

Showers will dissipate in the early evening, with clouds decreasing
overnight. Breezy winds will likewise decrease below 10 mph from the
west northwest. With the cool airmass, temperatures will drop to
unseasonably cool readings in the mid to upper 40s.

.LONG TERM...(Tuesday through Sunday)
Issued at 423 AM CDT Mon May 29 2017

Influence of deep trough to the northeast will persist on Tuesday
with a repeat of the early day increase in northwest wind to reach a
gusty 15 to 30 mph, an increase in diurnal cloudiness especially
east of I-29, and temperatures likely very close to those from today
from lower to mid 60s in southwest MN to near 70 in the Missouri
River valley. Although another subtle lobe wraps around the vortex,
the main impact will remain east of the CWA during the afternoon,
but perhaps not far enough away to prevent just a few sprinkles at
times roughly east of a KTKC to KSPW line.

Under a sprawling surface ridge on Tuesday night, light winds and
mainly clear skies should yield a chilly night for the end of May,
with perhaps a few locations slipping just below 40, but mainly 40
to 45 degrees which continues to be aligned with the colder subset
of guidance.  As the ridge aloft begins to displace the upper-level
cyclonic flow slightly eastward over the northern Plains on
Wednesday, the surface ridge will slip east and bring a start to
warming temperatures. While winds will be lighter, they may get a
bit stronger out toward lower Brule areas during the afternoon. The
low-level jet will strengthen on Wednesday night as start to see
warm advection develop with overspread of steeper 700-500 hPa lapse
rates.  Main lower level baroclinicity remains tied up closer to I-
80 and could see a smaller MCS develop around central NE which
spreads east/southeast overnight and early Thursday. Question
remains how much activity will develop north of this, where there is
a bit more dynamical forcing on edge of stronger westerlies. Would
expect to see at least a broken band of elevated showers/thunder to
at least I-90, and especially a likely chance through the lower
Missouri River valley by later night as LLJ begins to veer more with
jet sliding to the southeast of the area.

Upper-level ridging will continue to build into the area through
Thursday, and models similar in suggesting that the lower level
boundary along with increasing instability will build northward
toward the area through the day on Thursday and into Thursday
evening. Larger scale lift forcing a bit more difficult to come by,
but it is likely that a continued chance for showers and
thunderstorms, especially toward northwest Iowa, will be in play as
flow converges some around lingering boundary from Thursday morning
activity.  While flow is not terribly strong reaching into the mid
levels, there will be a strong degree of directional shear from
southeast to northwest which remains fairly distributed.
Shear/instability parameters would at least suggest some caution as
to an isolated severe threat toward the evening, but likely that GFS
is greatly overdone with 3-4KJ/kg instability.

By Friday, model solutions are only as good as their ability to
predict the location of prior convection in terms of organizing
boundary positions and potential for destabilization.  There
continues to be some fairly significant differences which result
from subtle differences in the larger scale, with ECMWF barreling
energy into the ridge on Friday and Friday night, driving a wave
into the northwest flow and resulting in a more southward position
of boundary focusing the convective threat through Saturday - fairly
near the heart of the forecast area.  GFS is a bit slower with
initial energy, and less inclined to punch through ridge, keeping a
more northern solution to boundary location through Saturday, and
pushing a stronger cold front across from west to east.  The
deviations are even greater by Sunday with solutions varying from a
strong upper ridge through the Dakotas in the ECMWF to another
amplifying wave diving through Minnesota in the GFS. Will keep the
threat for thunderstorms around from later Friday through Saturday
evening, but keeping open the possibility that warmer temps aloft
could attempt to cap off development across the area until at least
later Friday night when better dynamical forcing will begin to
encroach on the area.  In terms of temperatures, there is certainly
potential for temps to be even warmer than expected on Friday, with
a gradual transition back to near normal over the weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1227 PM CDT Mon May 29 2017

Some upper end MVFR ceilings are possible in the Brookings SD and
southwest MN areas through mid afternoon. But otherwise it is
anticipated that the rest of the area will be VFR through midday
Tuesday. Northwest winds will frequently gust around 25 knots or a
little higher this afternoon.

&&

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

$$

SHORT TERM...
LONG TERM...Chapman
AVIATION...MJ



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