Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 152014

Area Forecast Discussion
National Weather Service Sioux Falls SD
314 PM CDT Mon May 15 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 313 PM CDT Mon May 15 2017

Active forecast period in the short term. Surface boundary sliding
southward across northwest Iowa with much cooler stable air to the
north of the boundary.  Have significantly lowered forecast highs
for the late afternoon and evening hours.  40+ knot low level jet is
expected to develop this evening pushing the boundary back to the
north.  As the boundary works to the north, minimal capping is
expected to develop near 800 mb with 1500-2000 J/kg of elevated
CAPE. With relatively strong speed shear, models suggest 35-40 knots
of 1-6km bulk shear providing ample shear to organize storms to
create wind and hail threat.  Have expanded area of concern back as
far as the James River overnight into the HWO.  Most critical time
for severe weather potential appears to begin around 03z, lingering
through around 09z or so.  May see some lingering convection through
the early morning hours, but expect it to lift off to the northeast
as band of warm air advection lifts to the north.

Expect much of the day to be relatively quiet across the area on
Tuesday as the area is void of any strong forcing moving through and
strong capping inversion in place.  Concentrated pops across south
central SD along the Missouri River Valley late tomorrow after 21Z
as series of short waves approach from the southwest.  Capping
inversion appears to be fairly strong, so pops even at 21Z may be on
the fast side. Cold front moving through the area late in the day
may help to focus storm potential.  If storms are able to develop,
decent potential for strong damaging winds with dry soundings and
surface downdraft CAPE values of 1500-2000 J/Kg.

.LONG TERM...(Tuesday night through Monday)
Issued at 313 PM CDT Mon May 15 2017

Primary concerns in the medium and extended portions of the forecast
are focused on convective trends Tuesday night through Wednesday,
along with cooling temperatures towards the second half of the week.

Convection risks begin to increase Tuesday evening, as synoptic lift
overspreads a stalled frontal boundary across the region. An
initially well capped environment may begin to break down well
southwest of the CWA ahead of the dryline bulge in south central
NEbraska late in the afternoon. Further northeast there is some
uncertainty as to the impacts of downstream instability levels or
contamination from this initial convection early in the evening
to the southwest keeping convection at bay until well into the

Nevertheless, model dew points likely also boosted too high
for this time of the year, but will still result in MLCAPE values
approaching 2000-3000 J/KG in the evening. Initial shear looks
somewhat marginal, but should be enhanced by a 500 mb jetlet pushing
effective shear towards 40 knots.   While initial cells may be
somewhat discrete, unidirectional and somewhat marginal shear
profiles may support the rapid development of clusters as 02z
approaches. Steep mid-level lapse rates would support a large hail
risk (greater than golf ball), with a minor pocket of dry air around
750 mb supporting pockets of damaging winds within localized bowing
segments despite boundary layer stabilization. Greatest severe
weather risks will find themselves focused along and southeast of a
Yankton to Sioux Falls and Marshall line.

Once the LLJ increases after 03z and towards 06z, post-frontal
elevated convection rooted on the 850 mb boundary will begin to
increase substantially further north and west.  Much lower
instability further northwest of the surface boundary, but it may be
supportive of at least a marginal hail risk. However, as the
overnight moves onward and the surface boundary begins to flatten,
we`ll likely transition towards an overrunning heavy rain risk.

Rain will continue through Wednesday as the upper low begins to
pivot over the CWA.  As the upper low begins to stack on top or just
northwest of the surface low have some concerns that, if surface
based instability can grow, we could quickly jump towards a very
conditional low end severe weather threat over portions of the Tri-
State area, but especially across NW Iowa and adjacent areas of
MN/SD.  The primary risks would be hail with very low freezing
levels, but any mesoscale surface boundary could even force a minor
tornado risk given the very low LCLs.

High pressure sinks southward Wednesday night into Thursday with
scattered showers lingering overnight.  Seasonally cool conditions
with highs in the 50s and lower 60s return for Thursday and Friday.

While some uncertainty remains, increasing showers with isolated
thunder risks moves northeast into the CWA for Friday and Friday


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1246 PM CDT Mon May 15 2017

VFR conditions expected through the afternoon. Overnight, strong
low level jet is expected to develop and be the driving force for
thunderstorms. Best chances for widespread storms are along and
north of I-90. Storms are expected to be based high enough for VFR
ceilings, but could see MVFR visibilities in heavier downpours.
Convective cells may produce gusty winds and large hail.




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