Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 211743
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1243 PM CDT Wed Jun 21 2017

.SHORT TERM...(Today and Tonight)
Issued at 501 AM CDT Wed Jun 21 2017

Abbreviated forecast discussion this morning because of ongoing
convection moving into the forecast area.  Elevated storms on the
leading edge of warm air expected to move through the forecast area
this morning. Could see gusty winds with wealth of dry air in the
sub cloud layer.  Have seen a few reports approach 60-65 mph across
central SD.

Biggest concern today is the potential for severe weather late this
afternoon/evening.  Have backed off on pops until after 22Z with
stout cap. Thereafter, CAMs seem to be in fairly good agreement that
storms will try to develop across north central Nebraska lifting
northeast into the forecast area along and ahead of the cold front.
Timing of the cold front is really uncertain, with the ECMWF close
to the James Valley at 00Z and the GFS near highway 60 across
northwest Iowa.  CAMs supporting more of the middle of the road,
closer to the NAM timing.  While the atmosphere is unstable with
around 3K surface based CAPE, Bulk shear in both the 0-1 km and 0-6
km layers appear to be the weakest around the southern half of the
forecast area.  Still think there is a potential for all three
severe weather modes: hail, damaging winds and tornadoes, especially
in the vicinity of boundaries left over from the morning convection.

.LONG TERM...(Thursday through Tuesday)
Issued at 501 AM CDT Wed Jun 21 2017

Seems fairly likely that overnight convective activity would
favor a more progressed surface frontal position at the start of
the day, likely draped through far southeast SD into northwest IA.
Larger scale forcing will be minimized early in the day, starting
to increase toward northwestern CWA as western trough begins to
encroach into the central Dakotas. Warm temps aloft will
effectively cap the lower level frontal locations with greater
potential instability, while the northwest is much more weakly
capped to elevated lift forcing over post-frontal cooler air push.
Prospects for thunderstorms should increase over the northern CWA
during the morning with advance of upper-level support, which
areas to the south remain largely capped with a plume of steeper
mid-level lapse rates. Most solutions indicate potential for
accumulation of perhaps up to 2000-2500 J/kg of modestly capped
MLCAPE by mid afternoon near location of boundary which will be
nearly stationary or perhaps creep northward a bit through the day
in advance of upper wave, with 1000-1500 J/kg elevated CAPE in
zone north of boundary where somewhat more substantial lift
forcing will occur through the afternoon. Overall, there will be a
threat for severe storms given deep-layer shear values by
afternoon as wave knocks back temps aloft and weakens capping,
with new convection growing or transitioning to closer to lower
level frontal boundary. Main threats hail and wind, but will be
somewhat conditional on where boundary is as better forcing
reaches by mid to late afternoon, most apt to be across northwest
Iowa and far northeast Nebraska.

After convection wanes on Thursday evening, likely will be in for a
cooler and generally dry period through Friday night as cooler high
pressure filters into the Plains.  Of note is the NAM which actually
produces some subzero 850 hPa temps in North Dakota Friday morning,
a curiosity in itself for late June.  Other, more modest solutions
suggest somewhat breezy conditions and mixing into the lower to
mid 70s for most.

There are deviations within the model solutions which begin to
enhance later Friday night into Saturday, mainly dealing with
reinforcing troughing into the northern Plains.  GFS is much quicker
and deeper with trough, while ECMWF to end of availability at 96h
indicates a slower and not as deep trough swinging toward Minnesota
by late Saturday. Continuity would suggest ignoring or minimizing
impact of the quickest solutions, which allows for a small
precipitation chance to continue Saturday with some frontogenetic
forcing in right entrance of jet across the lower Missouri River
valley. At this point, almost no instability worthy of a thunder
mention. Quite cool for June with mainly mid 60s to around 70,
threatening a few record cool highs.

Sunday brings some fairly cool mid level temperatures, and despite
the lack of consistency in whether there may be a weak focal point,
there looks to be enough surface-based heating beneath cold core
aloft to advertise a small chance for afternoon instability showers
and even a rumble of thunder or two generally near/east of I-29.
Temps should moderate back toward normal through early next week.
Initiation of some warm advection, although with fairly meek
moisture availability, could bring a return to low-end shower/storm
chances again on Tuesday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1233 PM CDT Wed Jun 21 2017

VFR conditions are expected through the TAF period. A few
thunderstorms could develop along the Missouri River Valley, and
move eastward around KSUX after 00Z. A second round of showers
and thunderstorms will also be possible Thursday morning at all
sites. Models are not in good agreement on its timing and coverage;
therefore, have opted to not mention in the TAFs.



&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...BT
LONG TERM...Chapman
AVIATION...05



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