Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 191153

Area Forecast Discussion
National Weather Service Sioux Falls SD
653 AM CDT Sat Aug 19 2017

.SHORT TERM...(Today and Tonight)
Issued at 422 AM CDT Sat Aug 19 2017

A sprawling surface ridge across the area this morning with a
residual moist low-level air mass is starting to yield the expected
fog early this morning, with genesis a bit earlier generally near
the track of the evening supercell through southwest MN.  For the
most part, observations are more pessimistic than any webcams so
far, but along with guidance from the hi-res solutions, expect this
could change fairly rapidly over the next couple of hours as the
general fog area expands southward through northwest Iowa, and
begins to spread back westward toward I-29.  Have discussed issuing
a small dense fog advisory in parts of SW MN and far northern IA,
but consensus was to monitor with the eventual area perhaps a bit
uncertain as to how widespread the dense fog may get. Fog should
become most significant toward 12-13z before slowly dissipating
through 15z.

Heading through the remainder of the day, will be watching a wave
across Wyoming slowly approach from the west, sliding into central
NE by early evening. Once the fog dissipates, will be open for temps
to climb to around seasonal normal east, and well above normal
heading west of I-29 as the southerly breeze becomes fairly strong.
Diurnal conditions will gradually destabilize with perhaps 900-1400
J/kg MLCAPE, but a cap will remain into early evening. Approach of
the wave will also bring an increase in mid-level lapse rates by
late day. At this time, initial development of convection is likely
to be tied to convergence boundary/main instability axis back
through central NE by very late afternoon into early evening, but
gradual strengthening of the low-level jet this evening should allow
this to grow eastward and expand in a more elevated sense into
southeast SD and northeast NE through late evening.  Coverage will
likely be scattered at best, and should be mainly near/south of I-90
until later into the night when veering jet focuses more toward SW
MN/NW IA.  Possible that more organized convection closer to the
synoptic warm front down toward I-80 could interfere with late night
low-level jet development if it gets too boisterous.

.LONG TERM...(Sunday through Friday)
Issued at 422 AM CDT Sat Aug 19 2017

Early week thunderstorm/severe chances later Sunday through Monday
night becoming a bit more muddled than previously, partially due to
uncertain effects of earlier morning activity on boundary location
and destabilization, and partially to lack of significant forcing
Sunday to break what looks to be a decent capping inversion.

For Sunday...models remain consistent in showing upper level jet
near/north of the Canadian border Sunday, with a wave passing
through the southern Canadian Prairies dragging a cool front across
the Dakotas/Minnesota. This front making a much stronger southward
push into the forecast area than was seen at this time yesterday,
though consensus does keep it within our CWA, from roughly KYKN to
KMJQ by late afternoon. With the stronger mid-upper level winds well
to our north, deep layer shear near/south of the boundary remains
rather modest, though low level shear should be locally enhanced
near the boundary. Varying degree of instability among the models,
with GFS most aggressive with warming and thus most extreme on its
instability. NAM on the other end of the spectrum, likely slower to
recover from the morning activity, and thus cooler and more stable.
In either case, soundings show a moderate to strong cap for much of
the area during the afternoon, and thus think development will be
isolated at best prior to 21/00Z, with late afternoon pops confined
to areas near/southeast of the boundary.

Through Sunday evening, increasing low level jet through eastern
Nebraska will push the boundary northward, while a subtle mid-level
wave moves east across South Dakota. This should result in at least
scattered development of storms through the evening/overnight. Still
uncertainty in timing of the wave, and in the northward extent of
instability though, so will keep pops in the chance range for most
areas through the night. Greatest threat of severe should be in the
evening to maybe early overnight hours, if storms are able to break
through the cap.

For Monday, thinking that overnight activity should be exiting the
area through the early-mid morning, and expect most areas to be free
of precipitation by late morning/early afternoon total eclipse time.
Unfortunately, starting to look like we could see lingering stratus
behind the exiting precipitation, so may be a fair amount of cloud
cover around this area during the eclipse. Stronger mid-upper trough
dropping southeast across the northern Plains by late afternoon will
trigger the next round of showers and storms, primarily affecting
the region Monday night. Cap appears weaker across the region, so
would anticipate fairly quick increase in coverage as the wave
approaches. Again boundary location is uncertain, though greatest
consensus places it across northeast Nebraska into northwest Iowa,
with increasingly stable air as one travels farther north toward
Highway 14. Isolated severe threat should be confined to southern
portions of the area if this holds true.

Remainder of the work week looks dry and seasonably cool, especially
in eastern areas which will see greater influence from a strong area
of high pressure slides through the Upper Midwest midweek, and
settles across the Great Lakes by Thursday.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 653 AM CDT Sat Aug 19 2017

The most significant concern for aviation interests is the areas
of VLIFR fog present across much of southwest MN and northwest IA,
along with adjacent areas toward Interstate 29. The fog should
reach its most widespread coverage by 13-14z, then begin to
gradually dissipate through 15-16z. KHON likely to escape any
non-VFR conditions, with KSUX next to most fortunate likely only
dipping into a period of MVFR visibility for a couple hours prior
to 15z. KFSD a very tricky forecast with webcams showing less
1SM visibility around 5 miles north and east from the TAF site,
but generally most significant in the shallow areas near the
river. Have gone ahead and indicated the uncertainty by taking
visibility into the IFR range in the immediate term, breaking an
overall forecast of MVFR visibility. After fog burns off, should
remain VFR through the remainder of the period. Potential for a
bit of LLWS around KFSD during the upcoming night as low-level jet
increases late evening.




SHORT TERM...Chapman
AVIATION...Chapman is the U.S. government's official web portal to all federal, state and local government web resources and services.