Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 251958

Area Forecast Discussion
National Weather Service Sioux Falls SD
258 PM CDT Sun Jun 25 2017

.SHORT TERM...(This evening through Monday)
Issued at 255 PM CDT Sun Jun 25 2017

Latest water vapor and visible satellite loops showing a relatively
weak wave moving through  with some mid level moisture associated.
Surface and low level moisture pretty non existent so will need to
find the mid level moisture. At this time it looks like from about
850mb to 800mb has the best chance for a little moisture and minor
instability when lifted so will aim towards that as the timing and
locations for best chance for showers. Not out of the question to get
a little thunder late afternoon and early evening but not much more
than that. The lift and moisture associated with this system settles
south through the night with only a very small chance for showers
around Ida Grove and Sioux City from about 12z to 15z Monday
morning. Best chances will be east of the James Valley.

Cool air remains locked in place so lows tonight in the 40s and
highs on Monday in the lower to mid 70s. The big difference tonight
and tomorrow will be a much decreased wind leading towards a
beautiful day.

.LONG TERM...(Monday night through Sunday)
Issued at 255 PM CDT Sun Jun 25 2017

A warming trend with a more active weather pattern will take shape
by the middle of the week as the upper ridge axis begins to shift
east into the Upper Mississippi Valley.

After a mostly quiet and chilly night on Monday, warm and windy
conditions are expected on Tuesday as return flow on the backside of
the surface high settles in. This southerly flow will bring more low-
level moisture and dewpoints climbing into the 60s in the afternoon
into the overnight hours. Highs on Tuesday will range from the upper
70s east of I-29, and in the 80s west of I-29. Tightening pressure
gradients, and good mixing will allow windy conditions as the ridge
moves away of the area; thus increased winds from Monday night into
Tuesday evening, blending CONMOS/Superblend. Strongest wind gusts
expected west of the James River with gusts from 25 to 30 knots.

The scenario changes Tuesday night into Wednesday where models are
in good agreement showing a surface low developing over central
South Dakota. While southerly flow brings more moisture flow into
the region, strong convergence zone will build on the nose of the
LLJ to 35 knots (NAM)/ 35-40 knots (GFS). This will aid convective
development west of the James River. Despite a moderate 0-6 km bulk
shear vector at upper levels from 35 to 40 knots,  parameters do
favor strong to severe weather across the region. Steepening mid-
level lapse rates up to 8.5 C/km, and MUCAPE values to 2,500+ J/kg
along the convergence zone should be sufficient to support
convective development. With moderate winds/shear becoming
unidirectional with time, and strong DCAPE, organized MCS will be
possible along the boundary with large hail and damaging winds as
the main threats. This is definitely a period to keep watching.
At this point, the risk for strong/severe thunderstorms will be
late Tuesday night into early Wednesday morning along and west of
the James River.

The surface trough and associated cold front will push further east
into southwest Minnesota, and northwest Iowa through the day on
Wednesday, with isolated/scattered thunderstorm chances continuing
in the afternoon.

After this main shortwave exits the area, the long term brings
periodic chances of showers, and thunderstorms, mainly in the
afternoon and evening; but confidence is moderate due to model
inconsistencies. The Canadian and ECMWF advertise a wave
approaching the region Thursday afternoon, with southerly wind
flow bringing warmer and more humid conditions. The GFS remains
quiet during this time frame, with dry northwesterly flow already
over the area. The Canadian/ECMWF models seem like the best
scenario at this time; therefore went with Superblend with
scattered showers and thunderstorms in the forecast for Thursday
to Friday.

In terms of temperatures this week, a slight warming trend is
expected with temperatures near normal, at least though Friday.
Zonal wind flow aloft, and warm 850 mb temperatures to 19+C will
support highs from the upper 70s, and 80s. Lows will generally be in
the upper 50s and 60s.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1210 PM CDT Sun Jun 25 2017

There is a slight chance of a thunderstorms at KHON and KFSD
prior to 02Z Monday. However, the risk of both storms and showers
is 30 percent or less so did not include in TAF at this time. The
risk of a shower is a little higher in SUX after midnight but
timing remains uncertain and there is not expected to be thunder.
So also did not include in SUX TAF at this time. Ceilings and
vsbys are expected to be VFR through the period. Winds will be
around 10 kts at KHON and 10-20 kts at KFSD and KSUX during the
day becoming light and variable overnight. A slightly better
chance for showers and isolated storms over southwest MN.




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