Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 180917
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
417 AM CDT Tue Apr 18 2017

.SHORT TERM...(Today and Tonight)
Issued at 417 AM CDT Tue Apr 18 2017

Currently, well advertised short wave is scooting eastward. The
track of the wave is best defined by 700-500mb QG forcing, which
exits the wave into our extreme southeast zones by 18Z today. A line
of convection has entered our western zones at this time and is
filling in as it encounters a stout low level jet of 35 to 45 knots
and moderately strong thetae advection. In front of this line, are
isolated bursts of convection associated with warm air advection in
extreme eastern SD and southwest MN. The chances for severe weather
early this morning are pretty remote. There is elevated CAPE in the
850-750mb layer averaging 500 J/kg right on the line of convection.
But all of the wind shear in the lowest 1 to 2km is unusable due to
stability. The mid levels are conditionally unstable but with
unidirectional wind shear, both in speed and direction, suggesting a
multi-cellular environment which is what we are getting. Furthermore
the stability in the mid levels increases heading eastward toward I
29 as the saturated profile becomes deeper. Despite the saturation,
rain amounts will be held down due to the speed of the individual
cells moving pretty rapidly. Overall the forecast area will receive
a tenth to a third of an inch on average this morning, perhaps a
rogue quick half inch in isolated locations if you get under a
stronger cell.

As the system exits, boosted wind speeds due to subsidence behind
the wave. In addition, low level moisture could curl down into east
central SD causing widespread strato cumulus in that area later this
morning and afternoon. And in far southeast corner, stratus may hang
on into early afternoon around the Storm Lake vicinity. But with a
strong mixing west to northwest wind, we are still looking at much
above normal highs for one more day. Liked the north to south
thermal gradient given by the bias corrected ECMWF raw model value
and the weighted model. This scenario warmed Sioux City up into the
mid 70s. Conversely, east central SD should be held into the lower
60s with the aforementioned strato cumulus possibly affecting their
highs a bit. But it is still above normal.

A stronger wave with more moisture associated with it will move
east overnight. The models are in general agreement in bringing
the left exit region of the jet and associated PV advection into
south central South Dakota after midnight with PV advection
approaching I-29 by 12Z Wednesday. Instability is very limited so
not expecting any thunderstorms overnight. However, with a strong
700 mb front established near I-90, expecting that precipitation
will quickly develop along the front after midnight. Rain may get
as far east as I-29 by sunrise. With increasing clouds across the
area and strong easterly flow, lows are expected to stay in the
40s overnight.

.LONG TERM...(Wednesday through Monday)
Issued at 417 AM CDT Tue Apr 18 2017

The primary concern in the long term is the strong system
moving across the eastern Plains Wednesday and Wednesday night.
The models are generally in good agreement with timing of the
upper level wave and associated jet streak. The primary
differences are with the amount of instability that will exist and
how far north the surface warm front will get to Wednesday
afternoon. The NAM had the most instability while the GFS and
ECMWF bring the surface front fairly close to Highway 20 during
the afternoon. In general favored the GFS and ECMWF evolution for
the system.

In terms of lift and associated with the precipitation, the upper
wave will continue to slowly move east-northeast through the
morning approaching the SD/IA/NE border during the afternoon. The
700 mb front is expected to slowly lift to the north from near
I-90 to near Hwy 14 (KHON to KMML) through the day. Expect that
for much of the day a band of rain will continue along this front.
This band of rain will spread from central South Dakota into
southwest Minnesota during the morning and continue into the
afternoon.

The big question is when and where convection will develop in the
afternoon. 700-500 mb lapse rates rise above 7 C/km in
northwestern Iowa and far southeast South Dakota by early
afternoon. Examination of model soundings show that instability
exists for parcels lifted from 850 mb or higher in the NAM and
GFS. However, the NAM shows MUCAPES over 1000 J/kg while the GFS
is closer to 500 J/kg as its lapse rates are not as high. In both
models the convective inhibition gradually decreases and by late
morning or early afternoon, thunderstorms should begin developing
near the Sioux City area. Farther south toward I-80, there is
surface-based instability but convective inhibition remains closer
to 100 J/kg until later in the afternoon. So the expectation is
that elevated storms will develop in northeastern Nebraska and
then expand and spread northeast through the afternoon. CAPE is
likely to be 500-1000 J/kg from Sioux City to Spencer, IA during
the afternoon with effective shear around 30 kts. If CAPE is
closer to 1000 J/kg, a few storms during the afternoon could
produce 1 to 1.25 inch hail in parts of northwest Iowa. Otherwise,
much of northwest Iowa should see precipitation from these
storms. The storms may get as far north as Sioux Falls and
Windom. As storms evolve through the afternoon, the frontal band
should slowly decrease in strength along Highway 14 with the focus
for heavier precipitation with the convection near the Iowa and
Minnesota border. During the late afternoon, additional storms
could develop near the surface boundary by I-80 in western Iowa.
If those storms develop quickly and grow upscale, that may limit
the duration of any rainfall in northwestern Iowa. Regardless, as
the evening progresses, any thunderstorms should be east and
rainfall will gradually end after midnight. With the clouds and
rain, temperatures will stay in the 40s and 50s for highs except
near Highway 20 east of Sioux City where the surface front may get
close enough for temperatures to rise into the lower 60s.

The exact location of the heaviest rainfall remains uncertain as
it will depend upon how quickly convection develops during the
late morning in northeast Nebraska and again in the afternoon
closer to I80. Currently we are forecasting between 0.5-0.75" with
the frontal band north of I-90 and 0.5-1" with the convection
moving across northwestern Iowa. If the convection is slower to
develop then rainfall with the frontal band will be heavier. If
convection develops rapidly and quickly grows upscale then less
precipitation will fall with the frontal band - especially in
southwestern Minnesota - but heavier rainfall would be expected in
parts of northwestern Iowa.

Much quieter weather is expected from Thursday through Sunday.
Temperatures will generally be at or above normal with the warmest
day expected to be on Sunday. The next chance of precipitation
will be late Sunday night and Monday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1029 PM CDT Mon Apr 17 2017

Primary aviation concerns through the TAF period will be low level
jet driven low level wind shear and showers and thunderstorms that
will swing through the region 06z - 14z. Scattered nature of the
impending precipitation precludes including anything more than a
couple hours of MVFR ceilings at FSD and SUX. Brief MVFR
reductions are possible at HON from 06z - 10z as well. After the
precipitation moves through, winds will turn to west and
eventually the northeast near by end of the TAF period.


&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...MJ
LONG TERM...Schumacher
AVIATION...Ferguson/JH



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