Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 111744

Area Forecast Discussion
National Weather Service Sioux Falls SD
1244 PM CDT Fri Aug 11 2017

.SHORT TERM...(Today and Tonight)
Issued at 352 AM CDT Fri Aug 11 2017

Surface high pressure will result in another pleasant day across the
region.  After some very patchy early morning fog mainly across
northwest Iowa,  expect some fair weather cumulus to develop during
the day. With 850 hpa temperatures in the lower teens, expect most
places to warm to 75-80 degrees.

Shortwave approaches from the northwest late tonight.  Low level
moisture is limited initially.  Mid level moisture increases
throughout the overnight, but with mid level laps rates of moist
adiabatic, instability is pretty limited.  With increasing mid level
clouds, kept temperatures across the west warmer than locations
further to the east.  Have concentrated pops after 08Z Saturday as
the bulk of the forcing appears to be focused to the north and west
through 12Z Saturday.

.LONG TERM...(Saturday through Thursday)
Issued at 352 AM CDT Fri Aug 11 2017

The primary concern for Saturday through Thursday are the chance for
rain this weekend.

Overall there is good agreement between all models on the overall
evolution of the synoptic scale through Monday.  All models show a
fairly intense upper level wave moving from Alberta into the
northwestern South Dakota by Saturday morning and into southeastern
South Dakota by Saturday night and then slowly move through by
Sunday night. They also agree that the midlevel lapse rates will be
moist adiabatic at best meaning that there will be little
instability for much of the weekend. Moisture will also be rather
slow to move into the area.

The only real difference in model solution is on the mesoscale which
impacts where and when showers and thunderstorms will develop. With
the strongest forcing for ascent and best warm air advection
generally north of I-90 this is where the greatest threat for
precipitation will be. The lowest risk will be across northwestern
Iowa. A weak lead wave will move into central South Dakota by
Saturday morning. This may help showers that develop over western
South Dakota to move toward Huron, Chamberlain and Lake Andes during
the morning. At this time, it appears at least the southern portion
of this activity will dissipate through the morning making it
unlikely showers will be able to move east of the James River along
and south of I90. Farther north where there is better thermal
advection, showers are more likely to persist and could event reach
Brookings by early afternoon. While clouds will limit daytime
heating, the weak instability and approach of the main wave are
expected to result in new development from Huron down to Lake Andes
during the afternoon and then move east through the evening. What
little instability that exists will gradually dissipate after
sunset. What that means is where forcing for ascent is weakest south
of I-90, showers are most likely to dissipate as they approach I-29
North of I90 where both thermal advection and stronger forcing
ascent will exist, showers and isolated storms should not only
persist but may expand in coverage as they move toward I-29. These
showers should only slowly dissipate as they move east of I29 after
midnight. Therefore have 50-70 percent PoP north of I90 with less
than 30 percent over much of northwestern Iowa. With ample cloud
cover on Saturday, highs will only be in the 70s and if rain is more
widespread around Huron and Brookings there is a chance highs may
not even reach 70. Lows on Saturday night will be in the upper 50s.

On Sunday, the presence of the upper wave will likely keep showers
going - especially north of I90 and east of the James River. It is
likely coverage will decrease some by late morning and then more
showers and thunderstorms may redevelop during the afternoon with
the best chance along Highway 14 in southwestern Minnesota. One area
to watch will be west of I-29.  While forcing is much weaker there
on Sunday afternoon, there could be some sun during the late morning
and afternoon which will result in CAPEs approaching 1000 J/kg and
with no cap in place that would be enough to result in thunderstorms
developing during the mid to late afternoon. Any storms that develop
should move east during the evening and since they will be somewhat
surface based, should decrease in coverage after sunset. The NAM
does shows convection increasing in coverage Sunday night. No other
model shows this and with synoptic scale forcing for ascent non-
existent, have disregarded this solution. Highs on Sunday will range
from around 70 in southwestern Minnesota and east central South
Dakota to the mid 70s along and west of the James River. Lows Sunday
night will be in the mid to upper 50s.

In the extended, Monday through Thursday, model agreement really
decreases. There is an overall tendency toward warmer temperatures
with the potential for above normal temperatures by the end of next
week. How we evolve to warmer temperatures is very uncertain. All
models agree on a wave passing through the area during the middle of
the week but the ECMWF is 12-24 h faster than the GFS and Canadian.
This uncertainty is really tied to how the different models handle
the breakdown of the Hudson Bay low during the week and the
development of a western Canada trough.  As is typical, the GFS
breaks this down faster and then amplifies the western Canadian trough
while the ECMWF is slower to break down the Hudson Bay low and is
not nearly as deep with the western trough. At this point, there is
not a strong signal as to which model is correct so trended a bit
closer to the GFS since it at least is similar to the Canadian for
the mid week trough. Bottom line is there is low confidence on both
exact location and timing of precipitation for Tuesday night through
Wednesday night beyond the fact that there will be an increased
threat for storms sometime between Tuesday night and Wednesday
night. Temperatures will gradually warm through the period from mid
to upper 70s on Monday to mid to upper 80s by Thursday.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1240 PM CDT Fri Aug 11 2017

VFR conditions are expected through the TAF period. Diurnally
driven cumulus may briefly knock conditions to MVFR ceilings but
do not expect this to become widespread. Isolated showers or
thundershowers will also become possible, but this activity will
remain skittish through sunset. Late tonight, some scattered
showers will become possible near the highway 14 corridor after
06z, but will weaken as they move east.




LONG TERM...Schumacher
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