Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 150409

Area Forecast Discussion
National Weather Service Sioux Falls SD
1109 PM CDT Mon Aug 14 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 419 PM CDT Mon Aug 14 2017

High pressure will slide east across the CWA and through MN
overnight as low pressure deepens in NE Panhandle. With fairly light
easterly winds and dewpoints around 60, fog will again be probable
later tonight into Tuesday morning, mainly in southwestern MN where
the flow is slightly upslope on the Buffalo Ridge. Don`t expect
widespread dense fog as winds will be a little stronger than what we
saw this morning. For Tuesday, the low pressure will lift
northeast toward the area bringing an increasing chance of showers
and thunderstorms from west to east through the afternoon.

Overnight lows will be at or above normal with high dewpoints and
increasing cloud cover. Highs Tuesday will continue to be a few
degrees below normal and very similar to today.

.LONG TERM...(Tuesday night through Monday)
Issued at 419 PM CDT Mon Aug 14 2017

Convection is on the lead of the forecast for the start of the
intermediate period.  Very little will be present to prevent the
development of convection heading into the evening, with temps aloft
remaining cool enough while increasing warm-advective profile to
expect convection to become more widespread through the evening
along two main areas, more elevated activity toward east central SD
and southwest MN on nose of low-level jet, and a secondary
development/enhancement to convection near the Missouri River where
there is greater instability to the south. Afternoon clouds/precip
perhaps limiting the instability to a greater degree, which is
expected from 1000-1500 J/kg early evening, especially south of I-
90. Shear is not strong, but deep-layer balanced and more than
sufficient to get organized structures. Again, problem to getting
more than an isolated severe threat will be competing updrafts with
a lack of capping leading to more numerous storms, especially as the
deep layer forcing approaches during the evening.  Most of any
severe threat should be contained to the evening hours before storms
get too widespread, although potential is for perhaps an organized
wind event a bit longer into the night in the Missouri River valley.

ECMWF seems much too slow with progression of the boundary
considering the fairly consistent PV signature with other solutions,
and have sided with a blend of other model timing, which takes the
main synoptic boundary to near I-29 early Wednesday, with rainfall
continuing in earnest in parts of NW IA into the morning hours.

Concerns likely a bit greater for some areas of heavy rainfall, with
the presence of "double wave" structure working to slow effective
boundary progression and a very deep moist adiabatic profile with
deep warm cloud layer. Slight risk of excessive rainfall for much of
the area does seem appropriate even with variations in precipitation
location, with several solutions showing that potential rainfall
amounts could exceed 2-3 inches.

Secondary wave lifts across the post-frontal area starting midday
Wednesday. Dynamically strong, but there will be an influx of dry
layer coming in behind the leading wave. Likely strong enough to
result in scattered precipitation, but some of the hi-res solutions
of spotty heavy rainfall is not likely.  Wednesday does not appear
as if it will be terribly pleasant for most, with a lot of clouds,
northerly winds picking up behind the front, and areas of
precipitation. Temps should remain generally in the 70s, even after
a start in the mid to upper 60s.

Weak ridge settling in at the surface through Thursday should keep
active weather at bay for a short time, but temps remaining near to
slightly below normal.  Next focus for convective threat appears to
come in on Friday with stronger northwest flow wave.  GFS much
stronger and slower with feature. With timing of the system still up
in the air somewhat, there is potential that diurnal cycle could tie
up fairly well with frontal passage at some point across the area.
Shear certainly enhanced with strong directional flow change and
increasing wind speeds aloft, so would have to think at least a back-
burner thought to severe storms.  Ridging again should quiet down
the pattern for most of the weekend, with the next chance for storms
coming with warm advection later Sunday into Sunday night as next
wave flattens ridge along the International Border.  Temps generally
will be near normal to perhaps a bit above from time to time.

All eyes on the forecast for Monday, with the eclipse working across
the area midday. For now, it appears we may be dealing with some
exiting precipitation and lingering clouds during the start of the
day, but much much time for the timing/location of features to


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1108 PM CDT Mon Aug 14 2017

The main concern will be some low stratus or patchy fog development
late tonight into Tuesday morning. Models now suggest that this
will mainly be limited to the Buffalo Ridge area into the Iowa
Great Lakes, however will continue to monitor if this layer will
spread further south and west. Some scattered showers and
thunderstorms in southwest SD may drift into the southern
forecast area late tonight into daybreak Tuesday, with additional
development developing in south central SD late morning or midday
Tuesday and tracking east through the end of the forecast period.




LONG TERM...Chapman
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