Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 211104

Area Forecast Discussion
National Weather Service Sioux Falls SD
604 AM CDT SAT MAY 21 2016

.SHORT TERM...(Today and Tonight)
Issued at 356 AM CDT Sat May 21 2016

Expecting a pleasant day across the region today, though becoming a
little breezy in areas west of Interstate 29 this afternoon. Areas
west of the James River Valley will also see more abundant cloud
cover due to combination of high clouds streaming into the area from
the northwest, and lower clouds pushing north through the western
Plains. Meanwhile surface ridge which had shifted east toward the
Upper Mississippi Valley and western Great Lakes the past couple of
days will drift back to the west a bit today. This will allow for
lighter winds and mostly sunny skies along/east of Interstate 29.
Will also see another day of lower dew points as the southeasterly
flow out of the ridge reinforces the low level dry air. Adjusted
afternoon dew points downward toward mid-low end of model guidance,
but refrained from going quite as low as RAP suggests. Currently
have afternoon humidity levels dropping to less than 30 percent in
parts of southwest Minnesota/northwest Iowa, and again this could be
lower if RAP dew points pan out. But in either case, do not expect
significant fire weather issues due to the light winds and greened
up fuels across the area.

Surface ridge slides back to the east tonight, resulting in modest
southerly flow and mild temperatures overnight. Thicker cloud cover
will expand eastward as well, though still expected to remain west
of Interstate 29 through daybreak Sunday. Some models spitting out
some light QPF in south central South Dakota late tonight, and with
low level jet increasing through central portions of Nebraska and
South Dakota and mid level wave lifting through the western Dakotas
overnight, cannot rule out some ACCAS development. However, better
forcing looks to remain west of our forecast area and will keep pops
below measurable through 12z Sunday.

.LONG TERM...(Sunday through Friday)
Issued at 356 AM CDT Sat May 21 2016

Depth of the increasing moisture is quite tricky on Sunday.  The GFS
suggests shallower moisture return than the NAM, which impacts
stratus potential.  Through collaboration, have kept temperatures on
the cooler side of guidance on Sunday based on the expectation of
stratus becoming trapped below the elevated inversion making it
difficult to warm.  Appears that capping inversion will be enough to
limit convective potential until the trough approaches from the west
Sunday night.  Have removed pops during the Sunday afternoon period.
Models have slowed trough ever so slightly, so have slowed onset of
pops across central South Dakota.  Models suggest PWAT values prior
to convection is around 1.1-1.2 inches, which is around 75 percent
of the moving average.  Given usable shear for elevated storms is
parallel to front, could see some training of storms depending on
the speed of the front. Not a lot of directional shear with limited
speed shear, so would word severe threat as conditional at best.
Models have slowed the progression of the front eastward monday
morning, and have increased pops east of i-29 Monday.

Pattern remains active through the week as flow turns southwest
ahead of a latitudinal trough across the west.  Flow aloft remains
riddled with shortwaves, so have pops nearly every period.  Model
consensus still remains low, but still eying Tuesday for one of the
times of greater pops, and then again Thursday night into Friday.
Models are handling the waves a bit differently late in the week,
with the gfs suggesting more of a short wave train from Thursday
night into Friday, while the ECMWF has a more distinct wave on
Friday. It appears to be a fairly active week with scattered
convection-some of which may be severe; however, PWAT values hover
near the median range much of the week, so flash flooding threat
appears to be fairly low.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 603 AM CDT Sat May 21 2016

VFR conditions will prevail over most of the forecast area through
this TAF period, though periods of MVFR ceilings will be possible
in south central South Dakota. Occasional gusts to around 25kt are
possible near and west of the James River Valley this afternoon.


.FSD Watches/Warnings/Advisories...


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