Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 181142
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
642 AM CDT Fri Aug 18 2017

.SHORT TERM...(Today and Tonight)
Issued at 355 AM CDT Fri Aug 18 2017

Pesky convective chances will litter portions of the area through
this evening and are the primary focus of the short term portion of
the forecast this morning.

Water vapor loop indicates the main participants in the convective
threat today, first a distinct leading wave pushing across far
eastern South Dakota in the wee hours, ahead of the main wave
digging through central North Dakota.  The leading wave is
associated with the leading elevated push of elevated instability.
However, as zone evolves into a more stable and drier airmass to the
east, the precipitation production efficiency of the theta-e
advection pulse has dropped through the overnight hours as
precipitation has shifted into southwest MN and northwest IA.
Rather, the secondary area of showers and isolated thunderstorms
back through the mid-James River valley was more closely associated
with the PV advection with wave, and has taken over as a more
coherent band of scattered showers and isolated thunderstorms, and
should continue that way through early this morning pressing toward
southwest MN.

Once precipitation exits the area along with the forcing with this
leading wave by around mid-morning, there should be several hours of
quiet weather, likely with some increase in capped cumulus
development as diurnal heating commences. This will also begin to
increase the potential instability by the time the secondary wave
treks southeast toward western MN by mid afternoon. With potential
for 1000-1500 J/kg MLCAPE, the main focus for any additional
afternoon development will be a narrow convergence along the
advancing frontal boundary, mainly areas north/east of Sioux Falls
toward the upper level support from jet streak, as further south
this support decreases fairly rapidly. Shear is pretty top heavy
with the stronger flow aloft, and flow in the lower levels remain
fairly light. The low-level jet will have veered fairly well
following the leading wave, so shear becomes increasingly
unidirectional as well. Parameters suggest mainly multicell support,
perhaps marginal to obtain some weakly splitting cells. The
instability profile is supportive of hail being a bit thicker aloft,
so would not be shocked to see a couple of marginally severe storms
wander through southwest MN toward the IA Great Lakes area from mid
afternoon through very early evening.

While mixing behind the boundary will be decent this afternoon from
the James Valley westward, those areas heading eastward from there
will be much less mixed and will see winds dwindle considerably this
evening. With light winds, clearing skies, and both expected and
recent precipitation leading to a moist boundary layer, expect that
most areas near/east of I-29 will see formation of at least areas of
fog under the surface ridge late tonight into early Saturday.

Temps today will be warmest toward south central SD with development
of northwest winds and decent afternoon mixing, where some mid to
upper 80s are likely. Further eastward, the recent moist stretch
along with impacts from convection and location ahead of the
advancing frontal boundary should lead to a bit of lid toward the
east, mainly limiting in the upper 70s.  Tonight should find quite
effective radiative conditions with moisture likely condensing out
well into fog and/or a heavy dew. Temps will fall into the 50s.

.LONG TERM...(Saturday through Thursday)
Issued at 355 AM CDT Fri Aug 18 2017

Mid to long range portion of the forecast highlighted by a couple of
decent chances for thunderstorms early in the week, followed by a
return to somewhat cooler and drier conditions for the mid-late week
period.

Saturday should be the better day of the weekend, with plenty of sun
after any early morning fog dissipates. Southerly flow increases a
bit in areas west of I-29, which should help push temperatures into
the upper 80s-near 90, while eastern areas remain in the upper 70s
to lower 80s.

Elevated warm front lifts into the area Saturday night, accompanied
by some modest elevated instability. Moisture profiles still not
overly impressive, ironically a little better on the NAM, which is
the only model not producing QPF in our area, as it has a stronger
wave to our south which interrupts the flow and results in weaker
warm advection north of I-80. Will hang onto some low pops near and
south of I-90 Saturday night into Sunday morning.

Sunday late afternoon/night will see the threat for thunderstorms
increasing, as the warm front lifts north through the forecast area
and a mid level wave tracks into the region. Models not in great
agreement on the location of the boundary by late afternoon, and
thus confidence on location of development not very high at this
point. Despite the uncertainty in location, confidence slightly
higher on potential for isolated strong to severe storms. Strongest
winds aloft are displaced well north of the area, leading to weaker
deep layer shear values across our forecast area. However, low level
shear would be locally enhanced near the boundary, and this combined
with steepening mid-level lapse rates and CAPE values increasing to
2000+ J/kg would support the development of severe storms, most
likely during the evening hours as model soundings indicate a fairly
stout capping inversion across the area much of the afternoon.

Models now all indicating a stronger wave swinging across the region
by late Monday and Monday night, but late enough in the day that we
should see partly cloudy skies for the eclipse in most areas. Wave
will be accompanied by the cold front which will bring a return to
below normal temperatures later in the week. Timing/track of this
wave pretty similar on most recent model runs, leading to higher
pops in the consensus blend. 18/00Z ECMWF is a little faster than
GFS/Canadian, but really just shifts focus from northern CWA into
our southern areas, so will stick with fairly high pops for now.
Upper level jet does sag a little farther southward than we see
on Sunday night, but still far enough north that strongest shear
remains north of Highway 14. Similar to Sunday night, though, enough
instability and low level shear to support isolated strong to severe
storms.

Cold front should push south of the area by Tuesday morning, with
dry and cooler conditions then prevailing through the remainder of
the week. Exception could be near the Missouri River Valley, as
the Canadian model indicates some weak warm advection across that
area Wednesday night, and both the GFS and Canadian the same for
Thursday night. Will carry some low pops in our far south, but
confidence is fairly low at this point.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 641 AM CDT Fri Aug 18 2017

Generally VFR showers with an isolated threat for thunderstorms
will gradually come to an end early this morning generally
northeast of a KBKX to KSPW line. By late morning, diurnal heating
will likely create an area of generally scattered capped cumulus
could develop over much of southeast SD toward northwest IA, but
at this point any MVFR level ceiling would be brief and fairly
unlikely.

The largest aviation concern across the area will come in the
form of redeveloping thunderstorms by mid afternoon in that same
area mentioned above, which could be strong to possibly severe
with hail and damaging wind. After precip ends early this evening,
light winds and a moist boundary layer will bring the risk for
areas of fog near/east of Interstate 29. Some of this fog will
likely reach IFR visibility around KFSD and KSUX by late night.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...JH
AVIATION...Chapman


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