Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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526
FXUS63 KFSD 202113
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
413 PM CDT Sat May 20 2017

.SHORT TERM...(This evening through Sunday)
Issued at 413 PM CDT Sat May 20 2017

Low pressure center is presently located over west central IA this
afternoon, with a dry slot pushing into our northwestern IA zones at
this time. Back to the north and west, scattered showers continue
across the area. The low pressure will slide to the northeast
overnight, becoming situated over western Wisconsin by Sunday
morning. For this evening, the shower threat will continue across
the area, focused mainly through the James River Valley and Highway
14 corridor in association with a trowal wrapping around to the
north and west of the low. While there have not been any lightning
strikes for several hours in our area, did leave in isolated thunder
in portions of northwestern IA and southwestern MN through the
evening with weak instability (CAPE values of 100 to 200 j/kg) over
that area.

Later tonight, will see diminishing shower chances from west to east
as the low pulls away, though clouds will hold over most of the
area. Thermal profiles support a little snow possibly mixing in with
any lingering rain over the central Highway 14 corridor after 06Z
tonight, though surface temperatures will remain in the upper 30s
over that area so not looking at any accumulation. Northwesterly
winds will stay in the 10 to 15 kt range tonight, and lows will run
in the upper 30s to lower 40s.

Clouds will be slow to clear on Sunday, especially in the east, and
cannot rule out isolated shower chances remaining in our far
southwestern MN zones as the upper level low spins over the Upper
Mississippi Valley. Gusty northwest winds will remain through the
day, and while we will have warming temperatures, readings will be
below normal with highs only lower 50s over southwestern MN where
clouds will persist, to as warm as the mid 60s through the James
River Valley where some sun is expected in the afternoon.

.LONG TERM...(Sunday night through Saturday)
Issued at 413 PM CDT Sat May 20 2017

Deep trough circulating across Minnesota as it wraps up Sunday
night.  While is does dry out at lower levels as flow turns
southwest and westerly, indications are that there could be more mid-
level clouds than expected as weak lobe rotates through southwest MN
during the evening, and maybe even enough for passing couple of
sprinkles or light showers.

Main precipitation chance in intermediate range begins Monday, as
more coherent lobe swings around the deep Minnesota trough across
the Dakotas and Nebraska.  Most solutions are showing at least a
small degree of splitting to dynamics, but with weak stability
there should be a good response to the most minimal forcing, and
supportive of likely PoPs over much of the area during the afternoon.
Still potential for 500-800 J/kg MLCAPE, with weaker shear largely
in a linear mode. Not out of the question that parameters could be
briefly stronger near convergence boundary as it pushes across
northwest Iowa and extreme northeast Nebraska during the afternoon,
but probably not enough for more than the most minimal threat for a
stronger storm.

Wave takes with it the precipitation chance during the evening, and
should remain fairly quiet other than increasing northerly winds
Monday night.  Final strong lobe will wrap southward on Tuesday,
with impacts to be largely felt east of the James River, as
precipitation coverage is enhanced along with the diurnal cycle.
Instability does not look strong, but could see an isolated rumble
around northwest Iowa and adjacent areas from midday through the
afternoon.

Moisture wrapping around the upper low should again bring a diurnal
cloud increase to areas around northwest Iowa on Wednesday, but
enough subsidence with folding upper ridge to keep a lid on shower
development this far north/west.  As upper ridge axis shifts across
the area toward mid to late week, Thursday should be the most
pleasant day of the week with temperatures moderating back to
near/above normal, perhaps diminished in quality a bit by breezy
southerly winds.

Upper ridge begins to be knocked down by wave toward the end of the
week, with EC quicker than Canadian/GFS. Regardless, should see a
push of steeper mid-level lapse rates along with wave sliding up the
ridge, for a threat for elevated showers/thunderstorms pushing
northeast across the area, followed by additional precipitation
changes around a frontal passage around Saturday. Temperatures
Friday into Saturday should remain near/above normal.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1157 AM CDT Sat May 20 2017

IFR/MVFR ceilings will dominate through the TAF period. Scattered
showers, with some MVFR visibilities, will continue across the
area into this evening, then taper off overnight. Northwesterly
winds will pick up on Sunday morning, gusting 20 to 25 kt at
times.


&&

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

$$

SHORT TERM...JM
LONG TERM...Chapman
AVIATION...JM



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