Hail and Damaging Winds in South Dakota, August 1, 2017

Area Forecast Discussion
National Weather Service Sioux Falls SD
1253 PM CDT Tue Aug 1 2017

.SHORT TERM...(Today and Tonight)
Issued at 313 AM CDT Tue Aug 1 2017

A weak boundary/wind shift is still on tap to drop across the area
during the day, though any significant forcing and deeper moisture
is absent with the system. As the boundary settles to the south by
afternoon, may get enough low level convergence and instability,
primarily south of Interstate 90, to trigger isolated to scattered
thunderstorms. The better chances will exist over south central SD
with slightly better mid and upper level support as a weak shortwave
slides into western and central SD during that time frame. Although
afternoon CAPE values will be running around 1500 J/KG in our south,
shear is fairly pitiful at 15 to 20 kts, so thinking that the severe
potential is low. If an isolated severe storm were to occur, it
would probably be over south central SD where the shear is slightly
more enhanced. Soundings over that that area do exhibit an inverted
V, so strong winds would most likely be the greatest threat. High
temperatures today may be a notch higher than yesterday, running
mainly mid to upper 80s.

For tonight, any thunderstorm activity in our south will wane fairly
quickly in the evening as the low level convergence zone continues
to slide to the south and instability decreases. Skies will be clear
to partly cloudy early in the night, then will see some increase in
clouds from the west later in the night out ahead of the next more
significant system to affect the area. Lows will again be upper 50s
to lower 60s.

.LONG TERM...(Wednesday through Monday)
Issued at 313 AM CDT Tue Aug 1 2017

On Wednesday, models are in good agreement with both the large scale
and mesoscale features in the mid and upper levels. A leading short
wave develops and moves into central SD by 18Z Wednesday ahead of
the main, cold upper trough which is just beginning to move into
northwest ND at that same time. It's this first short wave which
moves across the forecast area Wednesday afternoon and evening that
will be responsible for convection chances. At the surface, there
are still discrepancies in frontal timing. The GFS remains the
fastest, with the cold front through the entire CWA by 06Z Thursday.
The ECMWF does not push the cold front through the entire area until
12Z Thursday, with the NAM in between the two. For now, compromised
with the NAM on the wind shift timing, noting that it could be a few
hours either way. It all depends on how the model interacts the
surface with its upper air. The GFS and NAM prefer to move the cold
front through more with the leading short wave, while the ECMWF
waits for the main upper trough to swing through later Wednesday
night to shift the wind. Believe in this case, the surface cold
front will follow suit with the leading short wave and convection,
so the ECMWF may bit a bit slow. At any rate, there is still some
severe potential across the forecast area Wednesday and Wednesday
evening as the advancing cold front produces a focus mechanism in
the low levels. 850-700mb frontogenesis is strong on all models,
with strong QG forcing in the mid to upper levels moving squarely
through the area. Wind shear is impressive with this system as long
as the cold front does not swing through too fast. 500mb winds are
averaging 40 to 50 knots Wednesday and Wednesday evening, with 700mb
temperatures cooling to just +4 to +6C by 00Z Thursday. Low level
lapse rates are very unstable, in fact dry adiabatic in the lowest
1.5km, giving the soundings a bit of an inverted V look to them with
ample mid level moisture. The inverted V soundings are more
pronounced west of I 29 as opposed to east. High pops are warranted,
advancing west to east Wednesday afternoon and night, with a lot of
the beneficial rainfall likely in the post frontal stratiform
regime. Concerning temperatures, highs continue to be rather tricky
on Wednesday as it largely depends on mid level cloud cover and of
course, rain. Continued to use superblend.

This is not the case Thursday through Sunday. As the cold upper
trough swings through Wednesday night, a long wave trough carves out
through the eastern United States. Any temperature which has bias
correction in it is way too warm with this pattern shift. Therefore
moderated superblend highs lower for these four days given the ECMWF
and GFS forecast 850mb temperatures. Blended in conventional
guidances such as consensus all and raw to cool off superblend some.
With self destructive sunshine through the day on Thursday under
breezy conditions and an upper cyclonic flow, a few showers may
linger along and east of the James River which will cool highs even
more. Highs will only be in the mid 60s in southwest MN, to mid 70s
in south central SD, cool enough that these highs will place
locations in the top 5 (or very near it) for record cool highs for
August 3rd. Temperatures will moderate some, but still way below
normal Friday through Sunday. The ECMWF and GFS are in very good
agreement in bringing another short wave through Friday night and
Saturday morning reinforcing the chilly air a bit, and giving very
good chances for rainfall.

Temperatures will moderate further next Monday and Tuesday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1252 PM CDT Tue Aug 1 2017

Scattered convection will slide through areas mainly west of the
mid-James River valley at early afternoon, and become isolated
with additional development near a slowly southward sinking
frontal boundary by early to mid afternoon mainly near/south of
I-90. Much of the night and morning hours should be quiet. There
is an outside chance with meek flushing of the moist near-surface
air mass that some spotty fog could develop in lower areas near
KSUX. Did not place any convection into the KHON TAF toward the
end of the forecast time with uncertainty in timing, but prospect
for convection will be increasing near the end of the period.

&&

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

SHORT TERM...JM
LONG TERM...MJ
AVIATION...Chapman

Hazardous Weather Outlook
National Weather Service Sioux Falls SD
420 AM CDT Tue Aug 1 2017

SDZ050-057-058-063-064-020930-
Gregory-Brule-Aurora-Charles Mix-Douglas-
420 AM CDT Tue Aug 1 2017

This Hazardous Weather Outlook is for portions of south central
South Dakota.

.DAY ONE...Today and Tonight

An isolated severe storm is possible late this afternoon and early
evening. The main threat would be straight lines around 60 mph.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

A few storms could be severe Wednesday afternoon. The primary
threat would be hail up to the size of half dollars and wind gusts
around 70 mph.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will likely not be needed today and tonight.

SDZ038-039-052>055-059>061-065-068-069-020930-
Beadle-Kingsbury-Jerauld-Sanborn-Miner-Lake-Davison-Hanson-McCook-
Hutchinson-Bon Homme-Yankton-
420 AM CDT Tue Aug 1 2017

This Hazardous Weather Outlook is for portions of central South
Dakota, east central South Dakota, and southeast South Dakota.

.DAY ONE...Today and Tonight

Isolated thunderstorms could occur this afternoon and evening,
mainly south of Interstate 90. Any storm development is not
expected to be severe.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

A few storms could be severe from mid Wednesday afternoon through
early Wednesday evening. The primary threats are hail up to the
size of half dollars and 70 mph winds.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today and tonight.

IAZ001>003-012>014-020>022-031-032-MNZ071-072-080-081-089-090-097-
098-NEZ013-014-SDZ040-056-062-066-067-070-071-020930-
Lyon IA-Osceola-Dickinson-Sioux-O'Brien-Clay IA-Plymouth-Cherokee-
Buena Vista-Woodbury-Ida-Lincoln MN-Lyon MN-Murray-Cottonwood-
Nobles-Jackson-Pipestone-Rock-Dixon-Dakota-Brookings-Moody-
Minnehaha-Turner-Lincoln SD-Clay SD-Union-
420 AM CDT Tue Aug 1 2017

This Hazardous Weather Outlook is for portions of northwest Iowa,
west central Iowa, southwest Minnesota, northeast Nebraska, east
central South Dakota, and southeast South Dakota.

.DAY ONE...Today and Tonight

Isolated storms may occur late this afternoon and evening, mainly
along and south of Interstate 90. Storm activity is not expected
to be severe.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

A few storms may be severe late Wednesday afternoon and evening.
The primary threats are quarter sized hail and winds up to 60 mph.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today and tonight.