Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 232308

Area Forecast Discussion
National Weather Service Sioux Falls SD
608 PM CDT Thu Mar 23 2017

.SHORT TERM...(This evening through Friday)
Issued at 354 PM CDT Thu Mar 23 2017

A couple of elevated waves have generated small complexes of
thunderstorms across far southeast SD and northwest IA today, the
first of which produced some isolated severe hail as it tracked from
near Yankton into areas south of the Iowa Great Lakes. The second,
weaker complex is lifting northeast into northwest Iowa as of 20Z.
Movement on both waves has been quite fast, on the order of 50-60kt.

For tonight, watching a line of enhancement developing on IR and
water vapor satellite imagery from west of KONL into far northwest
Kansas late this afternoon. As upper trough over the Rockies begins
to kick eastward and interact with this feature, expect thunderstorm
development in western Nebraska, expanding northeast into southeast
South Dakota through the early-mid evening as a 40-50kt low level
jet develops in eastern Nebraska and enhances convergence along an
inverted trough, progged to lie from central Nebraska, toward KYKN
and KFSD, and into southwest MN by 03Z. Unlike activity earlier
today, for which the most high-res models were playing catch up, the
HRRR/ARW/NAMNest, and even lower-res GFS/CMCreg are similar in their
depiction of development through the evening. Given the increasing
elevated instability ahead of the wave, and increased support from
the low level jet, cannot rule out another isolated strong-severe
storm. However, with deeper moisture becoming available, locally
heavy rain may be a greater concern.

Thunder threat expected to wane after 06Z as column becomes more
moist-adiabatic, but widespread showers should continue with deep
moisture feeding into what becomes a nearly stationary low-mid level
front across the forecast area. Surface front does eventually push
southeast of the area by 12Z-15Z, with 850mb front lingering near
KSUX-KSLB line into the afternoon. This will keep potential for a
few thunderstorms over the far southeast part of the forecast area
on Friday, with widespread rain across at least the southeast half
of the area as deep moisture continues to rotate northward around
the slowing system shifting east through the Plains. With rain band
likely to persist over the same area, certainly potential for some
rainfall amounts in excess of an inch southeast of Lake Andes-Sioux
Falls-Windom line just tonight and Friday. That combined with the
more localized rainfall from today`s activity could result in some
river rises for the Lower Big Sioux and our rivers across northwest

.LONG TERM...(Friday night through Thursday)
Issued at 354 PM CDT Thu Mar 23 2017

Primarily continued to follow a blend of the GFS and ECMWF for the
track of the surface and upper low, and their corresponding
precipitation trends for Friday night and Saturday. High pops are
still warranted Friday night generally along and southeast of a Lake
Andes SD to Windom MN line as a deep trowal centered in the 290-310K
layer becomes the dominant player to the north of the wrapped up
upper low. That said, the QPF trend on the NAM looks too heavy near
the I 90 corridor given the degree of dry air which is beginning to
nudge into the mid levels in that area. Therefore trended closer to
GFS/ECMWF/WPC values for Friday night rain amounts. The
aforementioned trowal still hangs around in northwest IA on
Saturday. However with ever increasing dry air advecting in, pops
will will slowly wind down in that area. All told for the rain
event, looking at a one inch to inch and a half band throughout the
southeast half of our forecast area, lets call it along and south of
Lake Andes SD to Windom MN. Amounts then rapidly taper off north of
there to just very minimal amounts in our northwest around Huron.
This is because the track of the low across the southern and central
plains is not friendly to that area, too much dry air in the mid
levels cutoff from the deep moisture supply.

By Sunday and through the rest of the forecast, the upper flow is
very wavy. One wave moves through Sunday and Sunday night but will
only give minimal chances for measurable rainfall. Then another
strong upper low digs into the desert southwest by Tuesday. After
that some moderate discrepancies still exist in the track of it
similar to yesterdays model runs. In the large scale, the
deterministic ECMWF and GFS are not horribly dissimilar at 500mb. But
by mid week the ECMWF is more cutoff with the low placing the
northern plains in a dominant northern jet. The GFS is also wrapped
up but does bring up a greater influence of moisture into our area
from the Gulf. Interestingly, both models bring in another strong
upper trough on its heels along the Pacific coast by Day 7 Thursday.
Chance pops for mainly rain brought on by warm air advection ahead
of the aforementioned upper low are still warranted Tuesday night
and Wednesday. Chance pops are continued to be warranted into
Thursday as the low gets closer. But taken literally the ECMWF would
be dry by then. At any rate, a typical spring like active pattern.
Because of this, temperatures will be above normal but not greatly
above normal, as the periodic waves continually knock back daytime
highs. Highs in the mid 40s and 50s will dominate Saturday through
next Thursday.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 602 PM CDT Thu Mar 23 2017

Showers and thunderstorms will develop across the area
tonight, with MVFR ceilings and visibilities. There could be brief
reductions of visibility into the IFR range in heavier storms.
Winds will transition to northeasterly overnight, picking up and
gusting 20 to 25 kts later tonight through Friday.




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