Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 220910

Area Forecast Discussion
National Weather Service Sioux Falls SD
410 AM CDT SUN MAY 22 2016

.SHORT TERM...(Today and Tonight)
Issued at 410 AM CDT Sun May 22 2016

Certainly not a clear-cut forecast today, especially during the
daytime hours when some increasing elevated instability will be
fighting limited moisture which is only slowly advancing toward the
forecast area. As was the case the past couple of nights, surface
dew points have been overforecast even by most 22/00Z models, with
early morning readings again down near 40 degrees along/east of
Interstate 29. We should be able to change this through the day
today, as low level flow through the eastern Plains begins to get a
good fetch from the Gulf instead of out of the departing dry high
pressure ridge. This should allow dew points in the 50s to make a
good push eastward toward the I-29 corridor by early this afternoon,
with readings climbing into the lower 60s west of the James River
Valley. This may be enough to allow for isolated development from
the James Valley east toward I-29 this morning through mid afternoon
as a subtle mid level wave shears north-northeastward across the
Dakotas. Feel any activity which does develop should wane as it
approaches I-29, as it will be moving into drier and more stable
air. Do not expect widespread severe threat from this activity,
however with axis of low level jet shifting into eastern SD as the
wave shears north, may have to watch for isolated strong winds.

Primary focus for strong to severe thunderstorms expected to develop
west of the Missouri River toward evening, as a stronger wave lifts
north into the High Plains and interacts with cold front/dry line,
and right entrance region of an upper level jet in the area. Storms
gradually expand east of the Missouri River through the evening, but
in doing so they work into area with less favorable shear profiles,
which should begin to limit ability to maintain severe threat as
storms progress east of the James River Valley overnight. As for
severe mode, think primary threat as storms work into our forecast
area would be strong winds given forecast DCAPE values near 1000
J/kg, though even this decreases sharply after 06z. Marginal severe
hail also possible with initial activity, but expect this threat to
lessen as the storms move into more weakly sheared environment as
mentioned above. Given the late evening/overnight timing and the
likelihood that storms will be outrunning the surface boundary,
tornado threat for our forecast area should be minimal.

Other concern is potential for locally heavy rainfall as storms
begin to expand into western portions of the forecast area tonight.
Storms should be fairly fast-moving with mean wind speeds 30-40kts,
and it has been fairly dry across the western CWA in recent weeks.
However strong 40-50kt low level jet oriented nearly parallel to the
mean wind could allow for backbuilding and/or training storms. In
addition, expected increase in moisture through today results in
precipitable water values nearing 175 percent of normal, which also
contributes to a locally heavy rain potential. As with the severe
threat, this should also diminish as the storms progress toward the
I-29 corridor late tonight.

.LONG TERM...(Monday through Saturday)
Issued at 410 AM CDT Sun May 22 2016

Monday is an interesting set up with deep southwesterly flow across
the region.  This should help in trying to erode cloud cover.
Relatively steep lapse rates exist across the forecast area, so
severe weather threat will be focused in areas that have adequate
low level moisture to create instability and areas that are able to
warm.  Not an overly capped environment, but in the same respect,
hard to identify any specific trigger other than diurnal heating.
With lack of strong capping inversion, may only see 1000-1500 J/kg
of CAPE develop in any clearing prior to redeveloping convection.
Lack of shear in the low levels but good speed shear in the mid
levels favors hail and wind as the main severe weather threats. Best
location for this to occur will be along and east of the James
River, though eastern portions of the area may have a hard time
clearing and thus destabilizing.

Expect convection threat to gradually shift further to the east
Monday night as low level jet sets up across the Upper Midwest.
Lingering boundary across the area will stall through the overnight
hours, and become more east/west oriented prior to lifting back to
the north as a warm front on Tuesday.  hard to follow this feature
in time models suggest a series of short waves moving
through the area leading to rounds of convection throughout the
week. Specifically, Tuesday during the day looks to be mainly dry
ahead of the waves, but thereafter it is difficult to pick a period
where there wouldn`t be the potential of convection. With adequate
instability, could see the potential for severe hail and wind
through Saturday. Thereafter, upper ridge starts to build it but it
may not last too long as large trough remains anchored along the
west coast.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1031 PM CDT Sat May 21 2016

For the remainder of tonight, the primary concern will be low
level wind shear. With a low level jet around 40 kts at 1500 ft
AGL, we are expecting low level wind shear to develop at Huron
around 06z and continue through 12z with winds aloft increasing
around Sioux Falls and Sioux City around 10Z leading to low level
wind shear around sunrise. The winds will pick up near the surface
shortly after sunrise ending the risk of low level wind shear by
13z. The low level jet will bring stratus northward as well and
with ceilings at or below 3000 ft in parts of central Nebraska and
South Dakota, expect MVFR ceilings to slowly spread northeast
through the night. The MVFR ceilings may reach Huron by mid
morning before ceilings lift back above 3000 ft over most of the
area. Thunderstorms look to reach Huron around midnight on Sunday
night which may result in MVFR or IFR conditions. Did add
thunderstorms to TAF at HON but did not lower ceilings below 3000
ft. While current forecast has VFR conditions at KSUX and KFSD
through the period, stratus moving north and lowering Sunday
evening could produce MVFR ceilings after 03z. While not in TAF at
this time, the possibility will be monitored.


.FSD Watches/Warnings/Advisories...


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