Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 231149
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
649 AM CDT TUE AUG 23 2016

.SHORT TERM...(Today and Tonight)
Issued at 431 AM CDT Tue Aug 23 2016

Deep trough pushing east of the Canadian Rockies early this morning,
with ridge across the northern plains shifting eastward, with a
series of subtle waves in southwest flow back through the southern
Rockies.  Strong low-level jet of 40 to 50 knots aligned from the
southern high plains into southwest Minnesota.  This has helped to
fuel terrain enhanced winds around the Buffalo ridge overnight, with
wind gusts of 30 to 40 mph. Fact that inversion is not as sharp is
likely keeping stronger winds from occurring. Strongest 925 hPa
winds start to drop between 09z-14z based on numerous model
profiles, so look for winds to even out some around sunrise.

Otherwise, a significant short term challenge today in determining
convective timing and location, and rarely has there been so diverse
a set of solutions between CAMS and lower resolution models, and
even individual members of each set.  Often when this occurs, will
settle back toward larger scale features to drive forecast timing
and location.  There is currently a cluster of thunderstorms across
southern Nebraska along with the leading wave across the high
plains.  CAMS lock on to this and push northeast toward the KSUX
area by around 16-17z. Would expect this area to weaken some with
the trend in low-level jet, but some degree of convection and
accompanying cloud debris likely to work northeast toward northwest
Iowa by midday.

Some models pick up on leading convection/wave and make it the
dominant mode with very quick progression to lead precip threat,
while others treat with less credibility and wait for trailing
development in favor of secondary wave currently located across
southeast Colorado into the Oklahoma panhandle.  Would tend to agree
more with the latter, so have allowed progression of pops more
quickly to the northeast, but will go with secondary development
across eastern Nebraska/northwest Iowa starting around 20-21z when
forcing with secondary wave will begin to increase.

Severe threat is a bit more muddled given the likelihood of leading
clouds and some precipitation impacting degree of destabilization in
and around northwest Iowa.  Unlikely to see temps past the mid 80s
toward the lower Missouri valley, but will be far more potential
instability (and capping) to the west between this area and
approaching cold front. Shear continues to be a sticking point,
weighted considerably in the 0-3km layer, and very little above with
weaker mid level flow.  Strength and orientation of shear vectors
would suggest a fairly low likelihood for brief supercell
development, and more likely multicell storms which evolve toward
more linear structures.  There will be a nominal hail and wind
threat, with brief rotating updrafts capable of hail up to ping-pong
ball size and wind gusts to 65 mph.  While tornado threat is non-
zero, especially around KSUX area, hail and damaging wind would be a
more significant threat.

Finally, concern is present for potential heavy rainfall and flash
flooding.  Precipitable water values will likely reach 1.5 to 1.75
inches across the area during the afternoon, safely on the higher
side of normal values for mid to late August. Much of northwest Iowa
and northeast Nebraska locations have received fairly little rainfall
over the last 4-5 days, which has bought some storage in the upper
profile. Storm motion will likely take more organized activity more
slowly to the east/southeast, leaving the low level jet ahead of
cold front capable of feeding into and regenerating on the southwest
flank in northwest Iowa. However, the overall confidence level on
location of heavier rainfall, or that heavy rainfall will be
widespread enough to warrant issuance of a flash flood watch is just
not present at this time. May need to treat it more like a
convective watch, and issued if needed once more confidence can be
gained in storm timing and mode, especially if trends shift toward a
WRF-ARW or ECMWF-type solution.

Another area likely to see at least scattered thunderstorms will be
along the cold frontal boundary pushing into areas west of the James
River by very late afternoon. A few solutions keep this area totally
capped to development, but enough support a much weaker cap toward
evening. With temps into the upper 80s to lower 90s, enough
potential CAPE and even DCAPE with drier air aloft to have a small
hail/damaging wind threat with these storms west of I-29.
This area will have a much more diurnal lifeline compared
to the more warm-frontal development toward northwest Iowa. As
activity pushes eastward during the evening, would expect weakening
of storms along with a decrease in coverage. There is actually a
minimum in larger scale forcing through the period along a
KMML/KFSD/KYKN line, and have reflected this with slightly lower
chances for precipitation along this corridor.

Most of any lingering convection should be dwindling across
southwest Minnesota and northwest Iowa after 09z.

.LONG TERM...(Wednesday through Monday)
Issued at 431 AM CDT Tue Aug 23 2016

Much quieter weather is expected for Wednesday and Thursday across
the area.  With the exception of the NAM...which continues to be a
slow outlier...all models show the precipitation and front southeast
of Storm Lake, Iowa by 12Z. This front continues to slowly move
southeast during the day and by the time the next wave arrives, any
convective development is much closer to I-80. Therefore have
removed the mention of precipitation for Wednesday morning. It will
be a fairly well mixed day as northwesterly wind bring drier air
into the area. This will allow dew points to drop into the upper 40s
and 50s by late afternoon. With some sun and mixing, highs will be
in the upper 70s to lower 80s.  On Wednesday night, winds will drop
off fairly rapidly on Wednesday night and, with the dry air and
mostly clear skies, lows will be in the 50s.

Thursday will be another well-mixed day but because it will be much
cooler aloft, highs will only rise into the 70s with the warmest
readings in the Missouri Valley. Although the surface gradient will
be rather lax, the efficient mixing will bring high winds to the
surface with winds gusting around 20 mph from mid morning through
late afternoon. As the boundary layer decouples on Thursday night,
winds will rapidly diminish with high pressure building in. Except
for high clouds moving into central South Dakota during the night,
skies will be mostly clear. Lows on Thursday night will be in the
lower to mid 50s.

In the extended, Friday through Monday, all models show an active
weather pattern with two primary waves - one moving through Friday
night through Saturday night and the second Sunday night through
Monday. Despite the synoptic scale similarities, fairly significant
differences remain in the mesoscale which impact where convection
will develop with both systems. The Canadian regional shows the
potential for heavy rainfall from southeast South Dakota into
southwest Minnesota. The GFS generally keeps the heaviest rainfall
south of Highway 20 while the ECMWF has generally lighter rainfall
than the other two models south of I90. So confidence is pretty high
that over the 4 day period many areas east of the James River will
see at least some rainfall. However, given the uncertainty of which
system will produce rainfall in a particular location, generally
kept precipitation chances from 30 to 50 percent.

The uncertainty in rainfall also leads to a lot of uncertainty with
temperatures, especially on Saturday and Monday. The duration and
location of rainfall will play a big role in determining how much
sun locations receive so generally kept highs near climatology.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 649 AM CDT Tue Aug 23 2016

A continued weakening of the low-level jet early this morning
will end low-level wind shear conditions shortly after 12z. Main
aviation concern will then be convection through tonight. KSUX
terminal is most likely of the three to be impacted by convective
threat today. Leading area of elevated thunderstorms will push
into the KSUX area by around 17z. Additional thunderstorms are
likely to develop between 20-22z and move across the area through
early evening. A severe storm is possible with large hail and
damaging wind. Outside of storms, conditions are expected to
remain VFR, but will likely see field of flat cumulus develop by
early afternoon north of the convective area with ceiling heights
remaining just on the high side of MVFR. A stray storm will be
possible near surface front very late afternoon and early evening
near KHON, and any storms along this front are expected to weaken
before reaching KFSD.

&&

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

$$

SHORT TERM...Chapman
LONG TERM...Schumacher
AVIATION...Chapman



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