Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 150208
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
908 PM CDT Thu Sep 14 2017

.UPDATE...
Issued at 906 PM CDT Thu Sep 14 2017

Am in the process of updating the precip portion of the forecast
to delay the arrival of TSRA chances for the CWA until late
tonight. Right now, there is just not a lot out there, not even
Accas. So wanted to eliminate any chances for precipitation before
midnight. The chances for rain will remain very skittish tonight.

&&

.SHORT TERM...(This evening through Friday)
Issued at 449 PM CDT Thu Sep 14 2017

A large scale trough over the western U.S. and building upper level
ridge over the eastern U.S will put the northern Plains under active
southwesterly flow aloft. This will translate to an unseasonably
warm and moist air mass at the surface through Friday afternoon,
as well as southerly winds. Despite available instability aloft
this afternoon (Thursday), a strong mid-level capping inversion
will limit storm formation.

By late tonight into the early morning hours on Friday, high
resolution forecast models hint at scattered showers west of I-29.
An upper level shortwave trough moving northeast through the
Northern Plains will provide adequate lift for a few showers and
perhaps weak thunderstorms in south central South Dakota.
Atmospheric profiles suggest these showers would be based around
700 hpa, with relatively dry air underneath this layer. Given this
situation, precipitation totals are expected to be light at best.
These showers dissipate throughout the early afternoon on Friday,
and it`s not until later in the evening when thunderstorm
potential rises.

.LONG TERM...(Friday night through Thursday)
Issued at 449 PM CDT Thu Sep 14 2017

Friday night the area will be under broad low and mid level warm air
advection as a wrinkle in the southwest flow aloft moves through.
This should allow showers and a few thunderstorms to develop with
the better chances east of the James River. Instability and shear
are strong enough to support a few severe storms but organized
severe weather is not expected. Temperatures should stay very warm
ahead of the incoming cool front with lows across northwest IA in
the mid 60s.

Saturday will see a continued chance for showers and thunderstorms
but again, not expecting any organized activity. Suspect that most
of the area will be dry until late afternoon when a chance for
thunderstorms, a few severe, will occur over parts of northwest IA
and southwest MN. Instability a little better but shear still very
marginal so only isolated severe storms expected. Highs on Saturday
will range from the lower 80s near Storm Lake to 60 to 65 from Huron
to Chamberlain.

While Sunday should prove to be the coolest day overall, light winds
will allow for a pretty nice day as highs climb into the upper 60s
to lower 70s.

Monday will have southerly return flow set up as surface high
pressure drifts east. A jet max across the southern Rockies
extending towards the central Plains will bring a small chance for
showers and thunderstorms to locations south of Interstate 90 as
warm advection in the mid levels focuses in that area.

Some agreements and disagreements for Tuesday through Thursday in
the models but overall confidence is decent. A strong trough will
dig into the west coast and then across the Rockies. All of the
models agree with this overall pattern. The big difference is the
speed at which the GFS swings out the initial jet max which is quite
a bit faster than the ECMWF and Canadian. This would bring a better
chance for showers and thunderstorms Tuesday night into Wednesday
with winds turning to the west. The Canadian and ECMWF are much
slower and have the area deep into the southerly flow. In fact if
the pattern evolves close to the Canadian/EC we may have some record
warm lows on Wednesday morning in the upper 60s to lower 70s. Not
quite confident enough yet to go that warm, but the potential is
there. Nothing easily timetable at this time partially due to the
fact that this is day 5 through 7 and mostly because the models are
still a little less agreeable, but for now siding with the
Canadian/EC.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 639 PM CDT Thu Sep 14 2017

There is a good chance of stratus sneaking down into the KHON TAF
site late tonight and Friday morning due to northeast winds
ushering in low clouds from North Dakota. The wind shift to a
northeast direction stalls however north of Sioux Falls and Sioux
City, so at this time, the stratus is not projected to move into
these sites, although it may be close at KFSD. At this time, kept
the stratus at lower end MVFR for KHON but will monitor. Otherwise
for visibility, fog could set up around Brookings SD and parts of
southwest MN along and behind the wind shift near the stratus
line where the wind is very light. But otherwise have no
visibility reductions at the TAF sites, although will monitor for
smoke. The chances for TSRA are too skittish to put in the TAFs at
this time, and low level wind shear is a threat at KSUX once
again tonight.


&&

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

$$

UPDATE...MJ
SHORT TERM...VandenBoogart
LONG TERM...08
AVIATION...MJ



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