Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 230555
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1255 AM CDT MON MAY 23 2016

.SHORT TERM...(This evening through Monday)
Issued at 416 PM CDT Sun May 22 2016

Active weather pattern setting up this evening and tonight, with a
frontal boundary and two shortwave tracking into the region. Upper
level southwest flow will aim a pair of shortwaves across the
northern plains. The first emerges from northern Colorado later this
afternoon and tracks quickly northeast into the upper Midwest around
daybreak Monday. Meanwhile a stalled frontal boundary in central
South Dakota nudges eastward into southeast SD overnight before
stalling/becoming washed out late tonight. Southerly winds tapping
into low level moisture from the gulf have allowed dewpoints to
skyrocket into the upper 50s to mid 60s along and west of I29 this
afternoon, with temperatures near or above 80 degrees, allowing for
strong instability to develop in a weakly capped environment.

With a strong low level jet developing this evening, the question
will be whether convection can develop out ahead of the front. CAMS
suggest that this is certainly possible, however the trend remains
that this activity would develop west of Winner and remain in south
central SD just to the west of our forecast boundary as it moves to
the north northeast. While CAPE is strong enough for severe weather,
effective shear is weak or very borderline west of the James River
this afternoon through mid evening, and poor to the east of the
James. It does improve in the Chamberlain and Huron areas late this
evening into the early morning hours. If an isolated storm or two
does wander into our western border, severe storms will be possible,
with the main concerns of wind gusts to 65 mph and up to ping pong
size hail.

Later tonight, expect the discrete thunderstorm cells to our west
merge into a line segment mid to late evening as low level jet
increases and the front/boundary starts advancing late evening.
Models continue to slow this band moving into our region until
nearly midnight. This activity looks to move into the
Huron/Chamberlain area around midnight, and into the I29 corridor
and points east, in the early morning hours. The severe threat wanes
pretty quickly, however there may be a brief hail and wind threat as
it initially moves into our area. Rainfall will be quite heavy, but
think that there will be enough movement with the band of
thunderstorms to keep the flash flooding threat low given our recent
dry spell.

Monday will start out wet, with the overnight precipitation
gradually decreasing as the as initial wave exits to the northeast
and the low level jet decreases some. There will likely be a
decrease in precipitation chances in the morning/midday. The
question will be whether we can clear out some of the lingering
cloud cover, which will allow us to heat out and break the cap. With
southwest winds, it is certainly possible, especially in the I29
corridor. The shortwave tracks northeast through the I29 corridor
and our Iowa/Minnesota counties in the afternoon, with a moderate
amount of potential instability developing. Again, shear is
marginal, but it does look like severe weather will a possibility if
enough heating and instability develops.

.LONG TERM...(Monday night through Sunday)
Issued at 416 PM CDT Sun May 22 2016

Will continue to have threat for thunderstorms mainly near and ahead
of the slowly moving frontal boundary. the fact that area is
basically nil on capping through the afternoon is concerning that
scattered storms will continue to fire even at periods of lesser
dynamic forcing. Broad lingering q vector convergence across the
area, and suggestion of at least a secondary coherent low level
convergence zone back across southeast South Dakota suggests
dragging pops a bit westward. With a decent amount of uncertainty on
how well conditions will recover through the day on Monday, have
generally kept precipitation at scattered to chance levels, but
somewhat higher across areas closer to highway 71. MLCAPE still
potentially in the realm of 500-1000 J/kg and with shear somewhat
better in the shallow and deep layers with increased mid level flow
and more turning in the 0-1km zone, would expect at least a nominal
severe threat during the evening near and ahead of the front.
Gradually, focus will shift south and east for storms, and
decreasing winds with boundary along with more stable air intrusion
should gradually knock out precip chances by late night.  In the
meantime, plenty of moisture around for storms to work with, so
could see some pockets of locally heavy rainfall as well.

Tuesday at this time likely to be the most quiet day of the upcoming
week as boundary nudges southward into Nebraska and Iowa, and more
stable push at lower levels nudges into the northern/eastern CWA.
As temps warm, expect will get a minor blow up of flat capped
cumulus development especially east of the James River and
near/south of I-90 on Tuesday late morning and afternoon...a more
ECMWF vision somewhere between the NAM`s unlikely instability and
the GFS more extreme drying.  Another warm day in the lower 80s
likely with a very modest east to southeast wind.

By very late afternoon, may begin to feel impact of lift from
stronger upper wave moving into the western plains. Should really
not be much of an impact until the overnight period, but have kept a
very small late day chance in the Missouri valley.  Low level jet
will increase overnight with frontal boundary starting to lift back
northward as a warm front. Very strong isentropic lift and
potentially development of decent elevated CAPE of 600-800 J/kg, but
effective shear remains fairly weak. Perhaps a few hail producers
hugging the southern CWA.  Not out of the question that storms
developing closer to the surface front toward I-80 could keep
boundary well south and any more significant activity as well.
Would have to watch for some better rains if inflow can push storms
in a more organized way toward northwest Iowa late night.
Gradually, the wave lifting north should shift the focus for
precipitation north and east through the morning and early
afternoon. Again, timing features almost impossible by this point,
so did not get too cute with timing, but more of a general southwest
to northeast dropoff with time, morning to afternoon.

Details toward the latter half of the week into the weekend really
become muddled in terms of signal. At best, can glean out that there
will likely be another fairly significant wave progressing northeast
across the area on Friday and Friday night which would look to
enhance precip threat once again, and even potentially bring a risk
for a few severe storms and heavy rainfall.  Overall, the pattern
does look to remain an active west to southwest flow, worthy of
watching for details as each day comes into view.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1255 AM CDT Mon May 23 2016

Line of weakening thunderstorms moving across the forecast area
through the early to mid morning hours. Brief MVFR-IFR conditions
are possible with the heaviest rain areas, but generally expect
ceilings and visibility to remain VFR. After 12Z, deeper moisture
does begin to push northward into northwest Iowa, which may bring
a period of MVFR ceilings to KSUX and other areas of northwest
Iowa mid morning through mid afternoon. Chance for additional
thunderstorms will also linger in areas mainly east of the I-29
corridor into this evening.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...
LONG TERM...Chapman
AVIATION...JH



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