Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 220902

Area Forecast Discussion
National Weather Service Sioux Falls SD
402 AM CDT Mon May 22 2017

.SHORT TERM...(Today and Tonight)
Issued at 402 AM CDT Mon May 22 2017

Large upper low centered just northeast of the CWA in northern MN
and western Ontario has a strong short wave on the back side of it,
currently moving into the western Dakotas. This short wave moves
into eastern SD by afternoon, then begins to strengthen further and
closes off in south central MN by late tonight. At the surface, a
cold front with accompanying wind shift will move eastward, helping
to focus some showers and thunderstorms in the area, with high pops
warranted generally along and south of I 90 as moisture depth
substantially increases in the mid levels. There is only a modest
risk for marginally severe thunderstorms this afternoon mainly
across northwest IA along and just ahead of the cold front. 0-1km ML
cape averages close to 500 J/kg in that area with mid level winds of
30 to 40 knots. However the best directional wind shear in the
forecast area is right now, with it becoming increasingly
unidirectional this afternoon limiting updraft potential. There
are steep lapse rates both in the low to mid layers with abundant
mid level moisture, which give the soundings a rather inverted V
look to them. So if a cold pool can develop in the vicinity of
showers, a rogue strong wind gust is possible in northwest IA this
afternoon, with maybe some small hail as the freezing level is below
700mb. The rainfall today will be more showery, as opposed to the
steady like rainfall we had last week and Saturday. With decent
mixing potential in the lowest kilometer, highs today will be in the
60s, perhaps lower 70s from Sioux City to Storm Lake despite
abundant cloud cover.

Tonight as the low moves further east, wrap around deep moisture
will move down from the north. There is enough ascent to rebuild
showers again after midnight after slackening off in the evening
hours due to a loss of heating. Lows look rather non eventful, with
a readings in the 40s.

.LONG TERM...(Tuesday through Sunday)
Issued at 402 AM CDT Mon May 22 2017

Tuesday appears to be characterized by self destructive sunshine and
instability showers as steep lapse rates from 850-700 mb leads to
precipitation.  The steepest lapse rates will be across the eastern
half of the forecast area, and have raised pops east of I-29 into
the likely range.  Expect showers to have a strong diurnal
characteristic, dying away quickly with sunset.

Wednesday into Thursday appears to be a nice warming trend with
strong southerly flow developing Wednesday night into Thursday.
Precip chances appear to be dwindling as a compact upper level low
moves across southern Canada.  Could see some trailing dynamics into
North Dakota, but further to the south it is all dependent on
shortwaves moving from southwest to northeast.  Quite a spread in
the trajectory of these shortwaves, with some models suggesting a
focus more on the I-80 corridor.  Have left limited slight chance
pops in, but am not overly confident.

That uncertainty continues into the weekend as it appears mid level
boundary appears to be focused further south in the I-70 to I-80
region. With low level moisture being focused further to the south,
precip chances through the first half of the holiday weekend appear
to be uncertain at best.  A second shot of cooler air appears to
work into the plains early next week, but the greatest effects
appear to be after Memorial Day.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1100 PM CDT Sun May 21 2017

VFR conditions are expected through the TAF period. Scattered
showers and isolated thunderstorms will develop after 18z, but
ceilings are expected to remain VFR. Could see some locally lower
visibility under stronger thunderstorms.




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