Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 132002
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
302 PM CDT Sun Aug 13 2017

.SHORT TERM...(This evening through Monday)
Issued at 301 PM CDT Sun Aug 13 2017

Potent upper level low continues to trudge eastward near the SD and
ND in the eastern Dakotas this afternoon. Meanwhile a pair of
boundaries remain draped across the area, with a warm front lifting
across southeast SD into southwest MN and cold front extending
southwestward across south central SD. Expect these areas to become
the focus for additional convective development into the overnight
hours. Storms should initiate along the cold front/trough in south
central/east central SD late this afternoon and track east
southeast, towards the I-29 corridor this evening and points
eastward late evening into the early morning hours.

Models suggest that MLCAPE values increase to 1000 to 2000 j/kg
across southeast SD this afternoon, with modest mid level lapse
rates developing. Bulk shear also looks to increase later this
afternoon to 30 to 40 kt with a veering profile. With strong forcing
associated with the trough and the frontal boundaries advancing
across the area, expect a few strong to severe storms late afternoon
and evening. Severe potential decreases after midnight, as the
activity moves further east away from the I-29 corridor into a more
stable environment. LCLs are pretty low, and with models hinting at
some weak low level helicity values nosing into the Missouri Valley
in far southeast SD and adjacent areas, cannot rule out an isolated
tornadic threat if supercells do manage to develop, but the overall
threat is low. Otherwise, the main severe threat will be large hail
up to golf ball size and a lesser threat for wind gusts to 65 mph.

The uncertainty factor is of course the persistent cloud cover
across the bulk of the forecast area, which has limited heating so
far today. Some scattered showers and thunderstorms moving through
the area late this morning and this afternoon may have also
contributed to stabilizing conditions perhaps more than models
suggest, leading to further uncertainty. All high-res models however
are showing storm redevelopment, so will stay the course with
scattered convection and possible severe weather remaining possible
later this afternoon and evening.


.LONG TERM...(Monday night through Sunday)
Issued at 301 PM CDT Sun Aug 13 2017

Focus through the middle to long range of the forecast will be on
the next chance of rain, which could impact the region as early as
Monday night, but more likely Tuesday into Tuesday night. Models not
showing great agreement on how things will initially develop, but
all show the general idea of a a mid-level wave triggering in
northern Nebraska by Monday evening, near the 850mb warm front. This
activity could lift northward into the area through Monday night as
the warm front lifts north, ahead of a stronger wave which will move
into the Plains through Tuesday/Tuesday night. With the exception of
the GFS, most models indicate relatively weak instability/shear with
the Monday night activity, so severe threat appears low.

How storms evolve Tuesday/Tuesday night is even more in question, as
Monday night activity will certainly have an influence in the form
of lingering boundaries/ability to destabilize throughout the day.
Upper level jet remains north of the area until late Tuesday night,
which should limit deep layer shear. GFS again an outlier regarding
temperatures Tuesday, showing much greater surface heating than the
other available models. This, of course leads to greater instability
by late Tuesday throughout the entire CWA, where Canadian/NAM limit
this to just southern parts of the area. In either case, with the
weaker shear and uncertainty in stability, think SPC day 3 Marginal
outlook still looks reasonable.

Moving ahead to Tuesday night, upper trough continues to progress
east across the Dakotas, dragging a north-south cool front eastward
across the forecast area through early Wednesday. South-southwest
low level jet ahead of the front will maintain a good inflow of
moisture, with model precipitable water values nearing 2 inches by
late Tuesday afternoon. Rainfall could become fairly efficient, with
warm cloud depths exceeding 10kft. WPC has introduced a slight risk
for excessive rainfall for this period, and given pockets of heavier
rain which we have seen of late, this also seems reasonable.

Activity with this system should push east of the area Wednesday
morning, possibly lingering in our far east into the afternoon.
Beyond this, not seeing any strong/consistent signals for organized
rain chances, though will carry some spotty pops as various waves
track across the region. Temperatures look to remain seasonable to
maybe slightly shy of normal.


&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1228 PM CDT Sun Aug 13 2017

Several concerns through the next 24 hours. Isolated to Scattered
showers and thunderstorms will track across the region late
afternoon through the overnight hours. Activity will begin in
south central SD late afternoon and track southeast through the
region. A few strong to severe storms may contain hail and strong
winds. MVFR ceilings will lift to low end VFR later this
afternoon, but will likely drop back to MVFR in shower and
thunderstorm activity. Low clouds and patchy fog will become more
widespread late tonight into Monday morning.


&&

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

$$

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



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