Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 212051

Area Forecast Discussion
National Weather Service Sioux Falls SD
334 PM CDT Mon May 21 2018

.SHORT TERM...(This evening through Tuesday)
Issued at 334 PM CDT Mon May 21 2018

Active forecast across the area tonight as there are several
focuses.  Isolated- scattered showers associated with warm air
advection across southwest Minnesota into northwest Iowa are
expected to decrease in intensity and areal coverage through the
evening hours.  With the moist atmosphere and light winds, could see
fog/stratus develop in the near surface layer.  Best chances for fog
appear to be mainly east of I-29.

Other concern late tonight is convective chance across northeast
Nebraska along a subtle elevate warm front.  Forecast soundings
support the potential for elevated storms rooted in the 800-850 mb
layer.  While this could result in upwards of 1000 J/kg of CAPE, the
shear through the layer is quite weak. The 2-7 km bulk shear values
are in the 10-15 knot range, suggesting severe potential would be
quite pulpy and limited. Expect convection to lift to the northeast,
but dwindle a few hours after sunrise

Expect much of the day on Tuesday to be relatively quiet with a
growing cumulus field across the area.  Increasing southeasterly
flow will lead to increasing moisture and instability across the
area.  With lack of a trigger in the atmosphere, think the bulk of
Tuesday afternoon will be dry.

.LONG TERM...(Tuesday night through Monday)
Issued at 334 PM CDT Mon May 21 2018

As the low level jet develops across the Plains Tuesday evening, may
get storms to fire on the nose of the jet with mass convergence.
Increasing support comes later in the evening hours as leading
shortwave lifts from southwest to northeast across the plains.  With
minimal capping inversion and CAPE of 1500-2500, expect storms to
develop across the area.  A few storms could be on the strong side,
but with winds through the depth of the troposphere being fairly
weak, expect any severe potential to be limited. 0-6 KM bulk shear
values are generally in the 20-25 knot range.  With weak corfidi
vectors there is the potential to see some slow moving storms.
Precipitable water spikes as high as nearly 1.5", but appears to be
convectively inflated.

Wednesday into Thursday remains unsettled as a series of weak
shortwaves move through the area in southwest flow leading to
additional rounds of convection. On Wednesday, the forcing appears
to be more uncertain, though increasing southerly flow may draw
additional moisture into the region.  Low level jet develops across
the Northern Plains Wednesday night ahead of a stronger shortwave
moving from western Wyoming into north central North Dakota.

Much quieter conditions develop into the weekend as a relatively
slow evolving pattern develops across the US and an upper level
ridge develops across the region. This leads to mainly dry
conditions with above normal temperatures through the holiday


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1217 PM CDT Mon May 21 2018

A clearing line, currently extending from just west of KHON to
near KYKN South Dakota is pushing slowly eastward and northward at
this time. However, cumulus will likely develop on the western
edge of the cloud bank through the afternoon. Confident that KHON,
KFSD and KSUX will move solidly into the MVFR category, but its
possible that by mid and late afternoon, that the cumulus could
rise into the lower end VFR category and this scenario is
currently stated in the 18Z TAFs. Otherwise tonight should be VFR,
with a cloud deck near 800mb, or between 4,000 and 5,000 feet AGL
forming late tonight and affecting KSUX and possibly KFSD
associated with a warm front above the surface. Its possible that
KSUX may see an isolated TSRA threat from about 10Z to 15Z
Tuesday, but chances of this are too skittish to put in the KSUX
TAF site at this time.




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