Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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672
FXUS63 KFSD 212033
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
333 PM CDT WED SEP 21 2016

.SHORT TERM...(This evening through Thursday)
Issued at 333 PM CDT Wed Sep 21 2016

A very warm and humid air mass in place this last day of summer. The
high to increasing dew points at the surface are definitely a sign
of a strong inversion which is keeping the moisture trapped near the
surface. In fact a look at this morning sounding from LBF shows some
very dry air from 925mb to 600mb advecting northward. The likely
inability to get thunderstorms to develop on or near the surface
front will likely leave the better chance for thunderstorms and
severe weather farther to the north and east. At 2 pm the 925mb
front was likely just north of highway 14 and as of 2 pm a few
thunderstorms were developing in northeast SD and west central MN,
closer to the 850mb to 800mb front. Will leave the opportunity open
for isolated to scattered thunderstorms over southwest Minnesota late
this afternoon and early this evening with a severe storm possible.
Fairly high CAPE values and very marginal shear do support this
threat. Later tonight as a wave passes through the low level jet
will increase and focus towards northeast Nebraska and northwest
Iowa which will bring another better chance for thunderstorms. Once
again the instability is decent but the shear is a bit lacking.
Will need to continue to watch the threat for flash flooding as
corfidi vectors do point towards a threat of backbuilding and
training.

A scattered thunderstorm threat will continue into Thursday with the
better chances to the east of the James River. If storms can hold
off and develop in the afternoon there may be a little more
organized shear to work with so storms could again become severe,
but at this time that threat looks pretty low. Otherwise will see
stratus expand overnight into the area along and behind the surface
front. This will likely set up some significant differences in high
temperatures Thursday. Current thinking is for near 85 from Sioux
City to Storm Lake to around 65 from Brookings to Huron to
Chamberlain.


.LONG TERM...(Thursday night through Wednesday)
Issued at 333 PM CDT Wed Sep 21 2016

Thursday night warm front will try to lift north through the area.
As it does so, there is a threat for showers and thunderstorms.
Atmosphere is not all that unstable, and shear is not all that
strong so would not expect any severe weather.  While weak
isentropic lift remains across the area on Friday, atmosphere looks
to be capped.  Have raised winds, but not as much as momentum
transfer on BUFKIT would suggest.  With lack of trigger, kept mainly
dry conditions Friday into Friday night.

Cold front works east through the region.  Appears to be much
breezier ahead of the front than behind. Have continued the chance
pops in the vicinity of the frontal passage.

Models have begun to evolve in the handling of large upper trough
over the west coast through the weekend into early next week.
Instead of splitting the wave energy, they keep the energy together
and translate it east as a full latitudinal trough.  Energy in the
base of the upper trough intensifies late in the weekend, and lifts
into the central and Northern Plains early next week. With the
evolving pattern, have a long period of pops through the extended
with not a lot of focus on timing.  Instability appears to be very
limited with the second system, and have therefore kept precip
mention to showers for now. With upper level low wandering around,
temperatures appear to be much more fall like with highs in the 60s.


&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1155 AM CDT Wed Sep 21 2016

Challenging aviation forecast this period. A band of stratus
across southwest Minnesota into central SD is expected to surge
southwest overnight, staying along and to the north of the surface
boundary. expect fairly widespread low end MVFR to IFR ceilings
from late tonight into Thursday morning. Otherwise thunderstorms
are still a possibility. Expect them to remain scattered across
southwest Minnesota late this afternoon, then isolated storms will
be possible into northwest Iowa and eastern South Dakota later in
the evening. Later tonight a more organized area of showers and
thunderstorms will be likely over northwest Iowa.


&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...Flash Flood Watch from 7 PM CDT this evening through Thursday
     evening for MNZ081-089-090.

IA...Flash Flood Watch from 7 PM CDT this evening through Thursday
     evening for IAZ001>003-012>014-020>022-031-032.

NE...None.
&&

$$

SHORT TERM...08
LONG TERM...BT
AVIATION...08



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