Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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989
FXUS63 KFSD 271736
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1236 PM CDT Tue Jun 27 2017

.SHORT TERM...(Today and Tonight)
Issued at 327 AM CDT Tue Jun 27 2017

Current IR satellite and water vapor imagery shows the next short
wave which will affect our weather tonight extending across western
MT and western WY. There is not a lot happening in the near term, as
a large area of surface high pressure nudges slowly eastward.
Because of this unseasonably strong high, low level dew points will
be slow to rebound today, and in fact staying fairly low especially
along and east of I 29. As the wave moves eastward today, surface
troughing will follow suit and move into the western plains. Tight
pressure gradient will result in our area with 30 to 35 knots of
mixed layer winds. Therefore it continues to look windy, with 20 to
35 mph southerly winds west of I 29, and a slightly less 15 to 30
mph east of I 29. In terms of high temperatures, bias corrected
values look the best, giving another day of mild mid 70s to lower
80s across the east, but in the upper 80s toward south central SD.

A few of the CAMs including the latest HRRR show some weak ACCAS
development forming above the cap very late this afternoon in the
James River valley, then nudging eastward to along the I 29 corridor
early this evening. Mid level moisture with steep lapse rates are
evident along with strong low to mid level warm air advection.
Therefore cannot rule out a few showers and thunderstorms ahead of
the main event. Otherwise overall, the pre convective environment is
extremely capped with CINs well over 400 j/kg. We just do not have
the low level moisture to help overcome this capping until the wave
is on top of this area and helps to allow some moisture pooling
along the surface trough. The atmosphere is only moderately
unstable, with ML CAPES averaging 1000 to 2500 J/kg once the
convection starts with the CIN finally breaking. Mid level lapse
rates are steep, but low level lapse rates are more stable so
much of the 0-1km helicity will not be tapped. We do have tremendous
wind shear however above 1 km. Directional sheering is highly
veering, with 1-6km bulk shear averaging 40 to 50 knots. The NAM
shows effective inflow of over 400 m2/s2 during the evening hours
for the convective areas before lessening overnight and late at
night. Therefore believe some severe weather is a threat through 06Z
generally along and west of I 29. If it persists after 06Z in our
east, it will be in the form of wind along a QLCS, but DCAPES
suggest this may not happen. Cells should be more discrete along and
west of the James River early this evening with deep moisture
lacking a bit, before morphing into a multicellular environment in
the late evening and overnight hours east of the James. Therefore
the threat for the largest hail will likely exist along and west of
the James.

.LONG TERM...(Wednesday through Monday)
Issued at 327 AM CDT Tue Jun 27 2017

Cold front will continue its eastward push Wednesday morning taking
the rain chances with it.  Instability will be to the east of the
area, so really don`t see much a chance for convection Wednesday
afternoon.

Next chance of rain comes into the area during the overnight hours
of Wednesday into Thursday as upper level short wave trof begins
to exert its influence on the area. However, the big questions
will be moisture availability especially thru Thursday morning.
As trof digs further into the area, storms should become more
abundant especially in areas of northwest Iowa. Most of the
moisture remains south of area due to the front and it shows up
well according models sounds and cross sections.

Once again, another ripple moves over the area in the upper levels
of the atmosphere which will trigger at least a chance of storms
forecast area wide.  Again, however the question will still be the
amount of moisture that will be returning into area.  Low level wind
flow remains less than favorable for much return moisture, but
can`t rule out at least a chance pop across the area.

High temperatures will remain below the season norms throughout the
seven day forecast period. Maybe a little hint of warmer air
into the area after the seven day forecast period is being seen,
but I won`t get my hopes up yet.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1236 PM CDT Tue Jun 27 2017

Primary aviation concerns through the afternoon/early evening will
focus on gusty southerly winds, gusting to 25-35kt, especially
along and west of I-29 corridor. After 01Z, attention turns to
increasing convective threat in central South Dakota, with the
potential for thunderstorms spreading east across the forecast
area through the late evening and overnight. Greatest chances for
thunder look to be at KFSD/KSUX after 03Z-04z, with more spotty
activity during the mid-late evening for KHON. Isolated stronger
storms could produce wind gusts in excess of 45-50kt, with KSUX
currently having the greatest risk of these damaging winds. Areas
of MVFR-IFR conditions will accompany the thunderstorms.

Any lingering convection in northwest IA/southwest MN at daybreak
Wednesday should exit to the east by the end of this TAF period.
Areas MVFR ceilings could lag behind the exiting precipitation,
but confidence not high enough to include this in the TAFs at this
time.

&&

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

$$

SHORT TERM...MJ
LONG TERM...Heitkamp
AVIATION...JH



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