Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 292050
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
350 PM CDT MON AUG 29 2016

.SHORT TERM...(This evening through Tuesday)
Issued at 350 PM CDT Mon Aug 29 2016

A rather complicated forecast is upon us tonight and Tuesday. Just
looking at the upper air, the pattern would appear to be rather
benign, with a split flow dominating this area. A northwest flow
aloft is present over much of the forecast area, colliding with a
sub tropical southwest flow aloft over our southern zones.  This is
quite evident on the satellite and water vapor moisture plumes as
upper moisture streams northward toward Sioux City.

There will be heating based isolated storms into early evening
across northwest Iowa and possibly even southwest MN, with any
storms creating outflow boundaries in which another storm may
develop on. An isolated storm could put down a quick inch or two
of rain over a very small area. But obviously most locations will be
dry. To the north, a weak cold front will shift southward tonight
and there may be isolated to scattered storms developing in our
northern zones along that boundary. The wind shear remains very
weak, so not expecting much in the way of severe chances with this
boundary this evening, and that activity will also be skittish in
coverage.

Later tonight, it still appears slight chance to lower end chance
pops are justified mainly east of the James River valley as the
aforementioned cold front moves southward, coupled with the
subtropical flow aloft adding some upper moisture. Again, coverage
of showers and storms will be limited with a lack of forcing and
wind shear in place, but if you caught under a shower it could bring
quite a bit of rain in a small area. On Tuesday, it appears at this
time the best focusing mechanism in place is along an axis from near
Mitchell and Sioux Falls, southeastward toward Yankton, Sioux City
and Storm Lake. This axis is along a decent 925-850mb shear zone
where northerly winds bend to an easterly direction, coupled with
the cold front stalling out around the Sioux City area. So this is
something to watch for Tuesday.

It will continue to be humid tonight, so thus had a tendency to go
with the warmer guidance values for lows which were the raw model
values. Tuesday highs are very tricky, in that it depends on the
degree of showers and cloudiness. The warmest readings in the mid
80s should prevail in our south central SD zones where more sunshine
may prevail.  But east of the James, temperatures may have trouble
warming more than the upper 70s and lower 80s.

.LONG TERM...(Tuesday night through Monday)
Issued at 350 PM CDT Mon Aug 29 2016

Showers and a few thunderstorms could linger in the confluence zone
as it settles southward from near the Missouri River early in the
evening hours, but any precipitation should be fairly short-lived as
axis of q-vector convergence sags southward along with loss of any
diurnal support. Flow remains fairly weak at night, but gradual
increase in north to northeast component should begin to flush
higher moisture levels out of the area.  Some hint that persistent
easterly flow and weak convergence could keep lower clouds or
perhaps a little fog across lower Brule areas through early
Wednesday morning.

The rest of the week looks to reside in seasonally cool conditions
with high pressure ridge sharpening over the northern plains,
keeping high pressure at the surface building through the
Mississippi valley. Thursday will average the coolest of the set,
with highs mainly mid 70s in southwest Minnesota to lower to mid 80s
west of the James river, with a bit of moderation by the time Friday
rolls around.

Conditions will begin to shift back toward more active through the
extended range of the forecast, which unfortunately covers a good
portion of the Labor Day weekend. Consensus between models of a
shift to southwest flow aloft and deep moisture transport setting up
by the middle of the weekend. A leading wave shoots out ahead of the
main western trough as early as Friday night, and could be
accompanied by a few elevated storms. Prospect for precipitation
will slowly increase by the time Saturday night/Sunday/Sunday night
come around with flow strengthening aloft downstream of trough axis
as deeper moisture increases. Main low-level boundary should remain
west of the CWA through early Sunday, with perhaps some elevated
showers and storms at terminus of low level jet near southeast South
Dakota.  Will have to watch later Sunday and Sunday night (perhaps
even parts of Monday across northwest Iowa if boundary suggest
enough sluggishness) for a severe weather threat. Instability will
be increasing by this time toward 1000-2000 J/kg, and much stronger
mid/upper-level winds will increase the deep and largely
unidirectional shear. However, any surface wave development along
the boundary will locally enhance the shear in lower levels to a
point where most modes of severe weather could occur.  Those with
outdoor plans this weekend should be sure to keep up on the latest
forecast.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1245 PM CDT Mon Aug 29 2016

Confidence is not real for the 18Z TAF set. Isolated thunderstorms
will be found across southwest MN and the southern sections of our
forecast area this afternoon. But only included a mention of TSRA
at the KHON TAF site this evening as a cold front shifts slowly
southward through the night. At KFSD and KSUX, the chances for
TSRA is more skittish so opted to leave the mention of
thunderstorms out of these two TAFs for now. However, would not be
surprised if MVFR fog prevailed at KFSD and KSUX around sunrise
Tuesday.

&&

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

$$

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


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