Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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808
FXUS63 KFSD 212328
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
628 PM CDT THU JUL 21 2016

.SHORT TERM...(This evening through Friday)
Issued at 252 PM CDT Thu Jul 21 2016

Parameters appear to do a poor job of coming together for any storms
late this afternoon and tonight, despite decent instability and CAPE
of 2000-4000 J/jg mainly east of the James River. While mid level
temperatures eventually cool a little tonight north of Interstate
90, a developing boundary will allow moisture pooling south of the
interstate. This boundary however does not appear to be developing
nearly as sharply as some models especially the GFS indicated a day
or two ago, and we have no real upper impulse detectable to help
initiate convection. SPC idea of some conditional severe storm risk
seems quite reasonable, but getting any storms going and sustaining
looks like a difficult task. Any storms are likely to be isolated
and fairly short lived, given the lack of any real low level jet and
the modest to weak mid level flow. The experimental HRRR, unlike the
operational model, is going for a near dearth of storms over the
area tonight. Have decided to keep isolated storms over the area
closest to the developing boundary, meaning in the south. Would not
be shocked to see a stray storm or two in the north, again short
lived, but the coverage there looks to be below mention levels, that
is 10 percent or less.

Am keeping no mention of storms for friday daytime, as the upper
ridge line passes over the area, and capping holds sway. Again, a
remote threat in the early morning south appears to small to
mention, and any storms later are not expected to develop until
Friday night.

This all leaves the heat as still the big story through Friday, and
in fact there may not even be the slight drop off in the heat index
that we envisioned earlier, so, adding the lows in the 70s tonight,
and with highs in the 90s to 100 Friday, keeping the heat warning
through the day is an easy decision. Most notably, an earlier
anticipated drop off in dew points and humidity north of the
boundary is not there anymore, with a simple continued resurgence of
extremely high low level moisture over the area. Temperatures in
some areas might be slightly less hot than today, but very slightly.
Winds will be generally easterly tonight and Friday.

.LONG TERM...(Friday night through Thursday)
Issued at 252 PM CDT Thu Jul 21 2016

Friday night should be one of our better threats for showers and
thunderstorms. A wave will move from the western high plains early
Friday evening to near Interstate 29 by Saturday at 12z. The low
level jet should increase across Nebraska into southern SD bringing
the boundary that is slowly eaking southward today back to the
north. The better chances should be north and east of a Huron to
Yankton line and from about 6z to 15z. Suspect that these
thunderstorms will lift northeast into central Minnesota late
morning then another threat for thunderstorms will come late
afternoon and evening with an approaching cold front from the
west. There is a decent chance that little if any thunderstorm
activity will be nearby from about 10 am to 4 pm. As for the
severe potential with plenty of instability there will be a threat
on Friday night but the shear is pretty weak so only expect an
isolated threat. Saturday afternoon and evening have the potential
for a more organized threat for severe thunderstorms and even an
isolated tornado will be possible. The surface to 1 km shear has
favorable turning while the mid level winds steadily increase from
about 25 knots to 50 knots from 800mb to 200mb.

Late Saturday night into Sunday should prove to be quiet as much
less humid air spreads across the region. No threat for
thunderstorms, lows Sunday morning in the 60s with highs Sunday in
the 80s.

In the outer periods (Monday through Thursday), weak ridging on
Monday should keep thunderstorm chances to a minimum. But,
increasing west northwest flow at the jet level will being decent
thunderstorm chances the remainder of the period. No one day at this
time looks a whole lot better than another, but the wave Tuesday
night looks a bit stronger so will try to aim for highest chances
during this time. No big changes to temperatures with highs mainly
in the 80s and lows in the 60s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 627 PM CDT Thu Jul 21 2016

Spotty MVFR ceilings near/east of I-29 early in the TAF period are
expected to gradually diminish through the evening, leaving VFR
conditions through this TAF period. Very isolated storm threat
during the overnight hours, but potential too low to mention at
any of our TAF sites.

&&

.FSD Watches/Warnings/Advisories...
SD...Excessive Heat Warning until 7 PM CDT Friday for SDZ038>040-050-
     052>071.

MN...Excessive Heat Warning until 7 PM CDT Friday for MNZ071-072-080-
     081-089-090-097-098.

IA...Excessive Heat Warning until 7 PM CDT Friday for IAZ001>003-
     012>014-020>022-031-032.

NE...Excessive Heat Warning until 7 PM CDT Friday for NEZ013-014.

&&

$$

SHORT TERM...
LONG TERM...08
AVIATION...JH



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