Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 131945

Area Forecast Discussion
National Weather Service Sioux Falls SD
245 PM CDT Wed Sep 13 2017

.SHORT TERM...(This evening through Thursday)
Issued at 244 PM CDT Wed Sep 13 2017

A couple high based isolated showers remain mid afternoon around the
Sioux City area but should continue to shift out of our area.
Otherwise we are left from some residual clouds and temperatures
ranging from the low to mid 80s with low 90s across our western
counties where clouds have cleared out.

Models continue to show another weak shortwave overnight and into
Thursday, which combined with a LLJ nosing into the area, could be
enough to result in some convection. High res guidance doesn`t seem
too excited about this potential however and thus, in collaboration
with neighboring offices, have cut back and limited chances to the
slight category. If anything does sneak into our area, the favored
area would along and west of the James River.

Thursday will feature another warm day across the region with 850 mb
temps in the low to mid 20s C. This will again lead to high
temperatures ranging from the mid 80s across the east to low to mid
90s across our western counties, which are a good 10 to 15 F above
normal for this time of year. Look for some increased winds across
NW Iowa and adjacent areas as the gradient tightens during the
afternoon hours.

.LONG TERM...(Thursday night through Wednesday)
Issued at 244 PM CDT Wed Sep 13 2017

Thursday night will see a very marginal threat for showers and
thunderstorms. Weak mid level troughing and broad warm air advection
may be enough to support some isolated activity but weakly rising
heights aloft may suppress the activity or at least keep it to a
minimum. Otherwise warm overnight lows in the upper 50s to mid 60s.

Friday will see a stronger piece of energy rotate northward into
western and central SD. This should interact with the fairly strong
warm front from central into northeast SD. Shear is marginal at
about 30 knots with about 1500 J/kg CAPE possible so an isolated
severe storm will be possible. Clouds and any daytime isolated
activity could play a role in development.

As the main wave rotates north another piece of energy will rotate
northward ahead of it, keeping the threat for showers and
thunderstorms in place, especially east of the James River Valley.
Right now any activity Saturday and Saturday night does not look
severe. The cooler air at the surface will filter in Saturday into
Sunday bringing temperatures back below normal for a couple of days
with highs from about 65 to 75 Saturday and Sunday.

A strong wave expected to begin digging across the western U.S. by
Monday with a lead southerly jet max spreading across the Rockies.
This should allow for a warm and possibly breezy day on Monday as
southerly flow increases ahead of this energy. There should be a
decent chance for showers and thunderstorms on Monday as warm
advection aloft, but confidence in timing a little as each model
handling this advection differently, but for now would think the
better chances would be late Sunday night through early Monday
afternoon. Hints that a weak upper level ridge will build Monday
afternoon behind a wave expected to move through.

The models are not very agreeable Tuesday and Wednesday as this very
strong energy spreads from the west coast through the northern
Rockies and onto the Plains. These differences result in variability
mostly for precipitation as all of the models through Wednesday are
on the warm side ahead of the wave. More than likely there would be
at least a chance of showers and thunderstorms both days with the
strong upper level jet still to the west of the area. If the models
are correct this will likely be one of the stronger upper level jet
maximums that we have had in quite awhile with 250mb wind speeds
averaging about 125 to 130 knots Tuesday into Wednesday. Sometimes
these early Spring/late Summer strong west coast jet maxes dive
farther south across the west coast than expected, then low pressure
cuts off from the main flow across the southern Rockies so will
watch for any trends in that direction. Nothing real interesting
statistically as the ensemble MOS output trends back towards
climatology compared to the deterministic MOS output during all
periods for both the GFS and ECMWF, which is pretty common and not a
signal to go in any particular direction.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1212 PM CDT Wed Sep 13 2017

A few isolated high based sprinkles or light rain showers continue
to move through the area and may linger around KSUX over the next
few hours, but shouldn`t have much of an impact. Ceilings are
expected to remain VFR at all TAF sites. Another small chance of
showers or thunderstorms exists later tonight, largely west of the
James River.




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