017 
FXUS63 KFSD 011753
AFDFSD

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