Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 182221
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
521 PM CDT THU AUG 18 2016

.SHORT TERM...(This evening through Friday)
Issued at 310 PM CDT Thu Aug 18 2016

A small cluster of storms continues to track southeast across the
Minnesota River valley this afternoon.  Trailing outflow boundary
has surged southwestward towards the Tri-State area. Concerns
through the overnight hours are focused on severe weather risks.

This afternoon/evening:  A frontal boundary is cutting eastward
across the CWA as we approach 3pm, stretching from south of Winner,
NE, near Sioux Falls, SD and then toward Estherville, IA. South of
this boundary, the atmosphere remains considerably capped, with
convective temperature into the middle 90s in locations of eastern
SD, southwest MN, and northwest IA. To the north and northwest of
this boundary, considerable ACCAS field across the region indicative
of steepening mid-lvl lapse rates and increasing instability aloft.
Only location of congested CU remains centered over Cherry CO
Nebraska in the immediate vicinity of the frontal boundary in an
area of modest sfc convergence.

As we move into the evening hours, upper dynamics begin to increase
as mid-lvl energy ejects northeast out of the Central Rockies.
Should start to see a gradual uptick reflectivity around and after
00z in the immediate post-frontal airmass. One area to watch will be
north central Nebraska and adjacent areas to Gregory county where
soundings continue to suggest a loosely capped frontal environment.
Elevated storms may continue to develop and track parallel on the
northern side of the frontal boundary through midnight. Generally
followed the ARW/NMM and HOP-WRF guidance. The highest risk will
reside along and north of a Lake Andes to Mitchell to Madison and
Marshall line. While shear is on the marginal side, steepening mid-
lvl lapse rates should allow large hail development. A loosely
capped environment could promote damaging wind gusts even in the
post frontal regime.

Friday: A rather unsettled and much cooler day is anticipated for
Friday.  By daybreak the boundary should be lingering over the
extreme southeastern CWA. A myriad of shortwave energy will be
crossing the region during the day, and would anticipate several
bands of showers and embedded thunderstorms. Soundings do indicate
the continued presence of 1000-1200 J/KG MUCAPE, and while mid-lvl
lapse rates and shear are marginal couldn`t rule out some small hail
in afternoon storms. The greatest risk of severe storms will be over
the extreme southeastern CWA from Storm Lake to Spencer and Sioux
City where a higher potential for surface based activity is
possible.  Temperatures on Friday may be nearly 20 degrees cooler
than those of today.

.LONG TERM...(Friday night through Thursday)
Issued at 310 PM CDT Thu Aug 18 2016

NAM again looks to be a slow outlier among the various models for
timing of the surface boundary/upper trough Friday night, and thus
have generally favored blend of GFS/GEM/ECMWF for timing/trends as
precipitation exits the region. That said, with faster movement of
low level boundary to the southeast, models now showing a little bit
of separation between any lingering convection in northwest Iowa,
and an area of mainly showers west of I-29 with a trailing wave
sliding down the back side of the trough Friday evening.

This swings through by early Saturday, and aside from the potential
for isolated diurnally-driven showers/weak storms in east-central
South Dakota/southwest Minnesota Saturday afternoon, we are looking
at a dry but cool weekend. Should see the coolest readings Saturday
into Saturday night, with single digits 850mb temps and only partial
sunshine yielding highs a few degrees either side of 70, followed by
crisp nighttime lows in the upper 40s-near 50. More sunshine Sunday
will allow for slight warming, with highs in the 70s for most areas.

Next week will begin dry and warmer as the upper trough swings into
the Great Lakes and a weak upper ridge shifts into the Plains. This
ridge strengthens as it moves east into the eastern CONUS, allowing
another decent upper trough to slide out of the Rockies into the
Plains Tuesday night/Wednesday. Not nearly the temperature contrast
with this system as with the current boundary, but should bring an
increased chance of thunderstorms to the region Tuesday night into
Wednesday, followed by a slight cool down again for late next week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 516 PM CDT Thu Aug 18 2016

Thunderstorms are expected to develop from south central SD into
areas from the Interstate 90 corridor northward in the evening.
These storms will continue through much of the night, before
diminishing some on Friday morning. Additional development is
expected over most of the area during the day on Friday. Reduced
conditions will be found in heavier storms which may affect TAF
sites.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...Dux
LONG TERM...JH
AVIATION...JM



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