Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 251721
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1221 PM CDT THU AUG 25 2016

.SHORT TERM...(Today and Tonight)
Issued at 407 AM CDT Thu Aug 25 2016

Little difference in model solutions today except for the typical
late summer warm and dry bias of the GFS. For the first 24 h
basically kept the forecast as a blend of the ECMWF and NAM.

Much of the day will see partly cloudy skies, especially near the
Nebraska border, as mid and high clouds continue to move northward.
This may limit sunshine a bit during the day. In additiona, a weak
cold front moving through the area this morning is also bringing
cooler air into the region. This will both enhance mixing this
afternoon and keep temperature a couple of degrees cooler than
yesterday in the lower to middle 70s. Wind this afternoon may gust
to around 20 mph north of I90.

High pressure builds in tonight setting up the potential for
radiatinal cooling. Wind will be light and skies will be generally
clear although some higher clouds will be moving east of the
Missouri River toward down. Low over night will be in the upper 40s
to lower 50s.

.LONG TERM...(Friday through Wednesday)
Issued at 407 AM CDT Thu Aug 25 2016

Surface ridge will start to relax eastward early Friday, but will
still provide quite a blockade to advancing moisture at lower
levels. Little question that mid- to high-level clouds will
aggressively invade skies from southwest to northeast through the
day, but through at least mid-afternoon, should be able to hold off
most precipitation with exception of perhaps a few spotty locations
west of the James River.  Late in the day, increasing theta-e
advection could also spread a few showers and isolated thunderstorms
toward the lower Missouri Valley.  With much of the day dry, should
be able to get a bit more warming, but tempered by southeast winds
and increase in clouds, with most still below normal in the mid to
upper 70s.

There continues to be some fairly significant differences in model
depiction of the more significant trough swinging across the area
Friday night and Saturday. However, there is a bit more consensus
among 00z soothsayer set toward a slightly quicker solution, pushing
trough axis through by around midday Saturday. In general, will end
with a shorter duration to precip threat for most, between slowing
up the start on Friday afternoon, and exiting threat sooner.

Leading weak wave appears to remain in solutions and streaks
northeast across the CWA during the evening. Increase in moisture
transport should lead to development of showers and thunderstorms
through Friday night, gradually pushing east/northeast as trough
clears and weak low-level jet with accompanying advections veers
toward Minnesota and Iowa. 700 hPa gradient with fairly cool mid-
level temps has gradient up toward I-90 and northward, however, the
surface to 850 hPa boundary likely to remain across a more southerly
location, making into perhaps toward northwest Iowa during the
night. The more activity which develops near the surface-925 hPa
boundary on later Friday afternoon and Friday evening, the less
likely the more widespread rainfall threat will spread as far
northward as low-level jet veers more quickly eastward, and have
trimmed QPF back across the area, especially between the James river
and I-29 which could be between the more widespread precipitation
southeast, and the more scattered development moving out of central
SD.  Overall, the instability is quite weak, perhaps 500-700 J/kg
toward south central SD and also the toward northwest Iowa.  Two
things would seem to fall out of this, thunder coverage will be
substantially less the precip coverage, and, the severe threat is
fairly minimal Friday night and Saturday.

Much of the precip should be on the verge of exiting eastern
portions of southwest Minnesota and northwest Iowa by early
Saturday.  Many clouds will remain, with a struggle for temps over
much of the area in the 70s, perhaps some readings near 80 west of
the James valley where should break into some partial sunshine.
Will retain some chance level pops back toward the James valley and
expanding across mainly the Highway 14 corridor to account for
weakening frontal convergence and any more isolated storms which
could develop through early afternoon closer a bit more potential
instability. Have removed all pops on Saturday night with strong
subsidence behind trough, but remains some fairly weak and unfocused
thermal advection through lower to mid levels.

The remainder of the forecast period will be dominated by flat
ridging, amplifying to some degree early to mid next week. The
result of this flow bringing warming temperatures aloft and removal
of any meaningful boundaries will be for no widespread precipitation
chance.  Better trajectory of lower level moisture into northwest
Iowa may bring a couple of rogue storms toward Tuesday or Wednesday
east of I-29, but small pops in the forecast are about as likely to
be in the wrong location as the right location given varying
strength of the upper-level ridging.  Another potential fly in the
ointment will be quite a bit of lower level moisture pooled up to
the south, which eventually will be peeled northward by days of
south and southeasterly low-level flow. There may be a couple
periods of more extensive cloudiness, especially toward MN and IA
which could impact the warming.  Temperatures are generally expected
to be in the 80s and lower 90s, along with an increase in humidity
early next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1214 PM CDT Thu Aug 25 2016

VFR conditions expected through the period. Surface high pressure
will shift east through Friday morning with increasing
southeasterly flow on Friday. Could see some showers/thunderstorms
approach the Missori river valley after 18z on Friday.

&&

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

$$

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



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