Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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338
FXUS63 KFSD 151201
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
701 AM CDT Fri Sep 15 2017

.SHORT TERM...(Today and Tonight)
Issued at 419 AM CDT Fri Sep 15 2017

Complex forecast for the next 24 hours to say the least, covering
issues from fog/stratus, to a large gradient in temperatures, to
periods of convective threat as a boundary wanders back and forth
across the area.

A southwest flow aloft continues across the area this morning, but
at low levels, a frontal boundary continues to wedge slowly
southward, located from KMWM to KYKN at 08z.  Behind this front is a
large area of stratus and fog pushing southward, and most hi-res
solutions indicate this will settle across the I-90 corridor through
the early morning hours, before the increasing southerly flow begins
to force a northward migration of the feature through the afternoon.
Would not be shocked to even find a little drizzle especially around
areas where terrain supports shallow lifting, considering airmass
aloft remains somewhat drier. Clouds will be a bit tough to shake as
boundary lifts northward, and could remain fairly cloudy toward KHON
through much of the day.

Secondarily for the morning, a fairly strong MCV has been pushing
northward across central NE, and will spread convection quickly into
south central SD. While the primary convection will focus along the
core which really progresses quickly north along areas from the
James Valley westward through 15z, further east will find shearing
vort axis shifting eastward, and could induce a lesser coverage of
elevated showers/thunderstorms, which also should begin to taper off
by later morning lifting northeast, as indicated elevated mixed
layer builds across the area and gradient in theta-e advection
pushes off to the north/east.

Much of the day is expected to be quiet under this capping, with a
small threat for a rogue shower/storm with persistence of weaker
theta-e advections, especially north and west of the retreating warm
frontal boundary. Next issue to consider is where this boundary will
reside by later in the afternoon, as this will not only serve to
focus the greater shear, but also pool moisture for a bit better
instability. NAM to a greater degree suggests that there could be a
bit of drier air which works up through the I-29 corridor during the
day and enhance the effective cap, perhaps an early indication with
slightly lower dewpoint values NE KS into SE NE.  It may take the
greater and deeper lift forcing with PV advection associated with
stronger wave currently moving through CO/NM toward early evening to
initiate convection given the capping in place, and this will occur
along the location of the boundary toward the James Valley and east
central SD.  MLCAPE is perhaps at most 1000-1500 J/kg, and will be
dealing with potential cap. Behind the front, elevated parcels
around 800 hPa are less capped, and even than probably do well to
hold around 1000 J/kg.  Effective shear 25-35 knots, with a largely
unidirectional profile above the near-surface layer. Any severe
threat at any location would seem to be of a fairly short duration,
given the shorter diurnal support of September, the marginal severe
parameters, and tendency for storms to amalgamate fairly quickly
with the stronger large scale forcing during the evening. Still,
there is an appreciable quantity of shear which is concentrated in
the lowest couple of km given the low-level jet, so a non-zero
tornado threat between the James River valley and I-29 early evening.
A fairly decent DCAPE along with the greater low-weighted shear, so
an eventual transition to some gusty bowing segments is favored.

Temps today will depend on the degree of cloudiness, but with
challenge to clearing around the mid-James valley, did lower there
by several degrees to the mid 70s, while some near 90 degree
readings will return south of the warm front across northwest Iowa.
Tonight, frontal zone will settle into southwest MN and northwest
IA, and should see wide ranging temps from lower 50s toward south
central SD, to mid 60s in parts of northwest Iowa.

.LONG TERM...(Saturday through Thursday)
Issued at 419 AM CDT Fri Sep 15 2017

Models coming into reasonable agreement that a frontal boundary will
be pushed southeast by Friday night`s overnight convection. This
boundary may fall near or just southeast of the forecast area by mid-
day Saturday. Leaving areas to the north and west stuck under a
colder and much cloudier regime.  The positioning of the front will
be key to any convective chances later in the afternoon, which
appear to focus just outside the forecast area. Very possible to see
a 55 to 85 degree gradient over the CWA.

High pressure sweeps southeast by Sunday, providing a very chilly
second half to the weekend. This high pressure area will only linger
briefly as a pattern evolution begins to push warmer air back
eastward by Sunday night and Monday.

Transitioning upper flow as long wave troughing develops in the
western CONUS will lead to a period of southwesterly flow aloft on
Monday. The returning warm air advection could be strong enough to
produce scattered showers and storms Monday into Monday night.

The first in a series of sweeping frontal boundary will move into
the Plains by the middle of the week.  Return flow may be too
limited in duration to provide a higher thunderstorm risk on
Wednesday, but it will pull warmer air northward. Have limited PoPs
Wednesday with better focus further north of the forecast area.

As the upper trough over the western CONUS begins to reload during
the second half of the week, a much more substancial thunderstorm
risk may develop within the Thursday/Friday timeframe.  Still
considerable differences within extended forecast guidance for the
second half of next week, so timing of any rain will need to be
flexible.

Temperatures, given the model differences, typical late September
influences of stratus, and weakening diurnal insolation trends
should trend above normal, but the extent is a bit more uncertain.
Would prefer the warmer EC guidance.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 657 AM CDT Fri Sep 15 2017

Complex aviation forecast for the next 24 hours. For this morning,
LIFR stratus which has dropped southward to around Interstate 90
will begin to lift back northward today as warm front pushes back
north and west. KHON area will struggle to break out of lower
ceilings until early afternoon. South winds will become quite
gusty south of the boundary by midday including KSUX. Low-level
jet remains strong into the evening, and this will produce a few
hours of LLWS immediately after dark. Convection timing is a huge
challenge, outside the obvious threat early this morning at KHON
with the tail of the MCV pushing through, which could swing
through KFSD around mid morning. Additional storms could develop
toward this evening along the boundary, and these would be any
that may be severe with hail/wind threat. Likelihood of at least
MVFR/IFR stratus again later tonight behind the boundary as it
pushes southeast.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...Dux
AVIATION...Chapman



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