Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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161
FXUS63 KFSD 211131
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
631 AM CDT WED SEP 21 2016

.SHORT TERM...(Today and Tonight)
Issued at 424 AM CDT Wed Sep 21 2016

Potent system with former tropical connection will lift into the
region today and tonight, bringing unsettled weather for the next
few days. Initial shortwave trough nudges northeast into western
Nebraska by midday today. Meanwhile, surface low pressure deepens
over the central high Plains, with an attendant warm front draping
across our forecast area, roughly from around Tyndall to Sioux Falls
to Windom. This morning, stratus will expand across the area, and
with ample moisture advection, a few sprinkles or patchy drizzle may
develop north of the boundary in the highway 14 corridor.

Despite the cloud cover, temperatures will warm nicely today south
of the front, with highs reaching the 80s. With dewpoints in the
60s, and rising in the 70s this afternoon south of the front,
instability will increase markedly, however capping will hold off
thunderstorm initiation for much of the day. Models suggest that
some scattered elevated convection may develop midday into the
afternoon north of the boundary. As these storms strengthen,
outflows may become the catalyst for storm initiation to the south
across northwest Iowa and far southeast SD. With strong instability
and marginal shear, a few storms may become severe this evening into
the early morning hours, with hail up to ping pong size and wind
gusts up to 70 mph. Cannot completely rule out the possibility of an
isolated tornado, mainly near the warm front, if storms become more
surface based. As the storms continue mid evening into the early
morning hours, the severe threat will transition to primarily a wind
threat.

As mentioned, ample moisture and very high precipitable water values
will make heavy rainfall the main forecast concern. With the low
level jet aimed at northwest Iowa and parts of southwest Minnesota
and the slow moving nature of this system, thunderstorms will become
widespread by late evening and persist overnight, especially in far
southeast SD, northwest IA, and southwest MN. Expect training of
showers and thunderstorms to further heavy rainfall concerns and
localized flash flooding. While the heavy rainfall looks to shift
slightly south and east of the previous forecast, still think there
is enough concern of flash flooding and localized heavy rainfall to
keep the watch going in northwest Iowa and southwest MN.

.LONG TERM...(Thursday through Tuesday)
Issued at 424 AM CDT Wed Sep 21 2016

Unclear just how much convection will be lingering at the start of
Thursday period, as veered low-level jet will be focused across Iowa
and into south central MN, just ahead of residual wave from Paine
which looks to be through much of the CWA, but becoming diffuse
within mean ridge axis.  Cold pool from overnight convection will
likely mean that a more southern solution to front/outflow hybrid
will favor the southern end of solution set, and thus expect any
continued lift with last of wave forcing/theta-e advection over cold
pool will primarily be focused across northwest Iowa.  The entire
area remains in a weak flow regime with slowly building heights to
accompany a persistent lower- to mid-level theta-e advection on
Thursday as boundary reaches a southward extent or perhaps starts a
late day slow crawl northward as a warm front near the far southern
CWA.  Bountiful moisture will linger across the area, and especially
near the boundary, resulting in potential instability from 1500-2000
J/kg, but perhaps struggling with capping for much of the day. Best
chance for additional storms to redevelop will be near the boundary
on the southern/southwest flank of what should be fairly extensive
cloudiness.  CAPE profiles are not terribly thick with a more
tropical feel, but enough along with deep layer shear near 30 kts
that should be a bit concerned with a potential late afternoon or
evening severe storm near the highway 20 corridor, which will spread
northeast with strengthening low level jet and lifting warm front
during the evening/early overnight. Farther northward, would expect
a great deal of postfrontal cloudiness which will be hard to break
with east to northeast low-level flow through the day. Would not be
surprised to find a little drizzle or a few light showers north of I-
90 with the bulk of moisture locked below inversion, and temps
struggling to reach around 70. Temps will show a strong variation at
some point, perhaps sharply reaching the lower 80s heading toward
areas near/south of KSUX, but boundary location will be everything
to temps on Thursday.

Thursday night will see warm front working northward of the
Missouri River by late night. Scattered thunderstorms potentially
will develop near 925-850 hPa frontal zone, and of concern other
than some spotty severe chance would be the additional threat for
heavy rainfall with deep warm cloud profiles, and Corfidi vectors
suggestive of regenerative development. Gave some consideration to
extending the Flash Flood Watch into Thursday night, but may be
somewhat other areas and really would like to see where boundary
will set up, but northwest Iowa would probably be a preliminary
area of concern.

On Friday, have continued a mention for thunder working north of the
northward advancing warm front, but by afternoon should not be much
of a threat remaining with degree of warming aloft pushing across
the frontal location.  Modestly cooler and damp ahead of the front
through the day, which could linger in the far northeast corner near
KMML where highs are expected in the mid 70s, while warm and humid
to the south with 80s south of I-90.

Quiet conditions will continue into Friday night under capping, but
main upper trough should begin to push into the western plains
during the night. Meridional frontal boundary will develop in the
western plains, and strong low level jet will keep mild conditions
overnight.  With lobe of lift wrapping into the central plains, will
likely see some convection ignite in the convergence and spread
north and northeast in meridional flow, perhaps reaching south
central SD later in the night.  As energy with trough rotates out
across the plains and lifts to the northeast, will break down
capping along the advancing frontal boundary and allow a greater
degree of showers and storms to break out Saturday afternoon and
evening as the front moves through the I-29 corridor in the
later afternoon or evening.

Been fairly entertaining watching the models struggle with the main
trough over the last several days, and today is no exception in
diverse solutions by later in the weekend.  GFS has settled on
putting all eggs into one basket, namely the initial trough lifting
into the northern plains becomes the one and amplified moving into
MN on Monday and WI on Tuesday.  ECMWF on the other hand breaks off
a significant piece of energy into the southern Rockies, which then
slows up the progression of entire trough, and melds into a large
negatively tilted wave lifting into the region midweek.  Each day
seems to bring a new solution, so doubt whether we have seen the
right answer yet.  As a result, have not made any appreciable
changes to longer range grids off initialization.  In general,
expectations for cooler temperatures by early next week along with
periodic chances for rainfall after a brief break around Sunday
/Monday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 627 AM CDT Wed Sep 21 2016

Patchy MVFR and IFR stratus continues to expand along the highway
14 corridor. A few sprinkles may be possible in this stratus.
Otherwise, main concern will be thunderstorm chances this evening
through the end of the TAF period. Best thunderstorm chances will
be east of I-29 where very heavy rainfall will likely produce
reduced ceilings and visibility at times. Additional MVFR and IFR
stratus may also develop after 03z behind the slowly advancing
frontal system and surface low.

&&

.FSD Watches/Warnings/Advisories...
SD...None.
MN...Flash Flood Watch from this evening through Thursday evening for
     MNZ081-089-090.

IA...Flash Flood Watch from this evening through Thursday evening for
     IAZ001>003-012>014-020>022-031-032.

NE...None.
&&

$$

SHORT TERM...
LONG TERM...Chapman
AVIATION...



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