Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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000
FXUS63 KFSD 191653
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1153 AM CDT Wed Apr 19 2017

.SHORT TERM...(Today and Tonight)
Issued at 410 AM CDT Wed Apr 19 2017

The primary concern in the short term is rainfall and possible
severe weather though tonight.

Latest water vapor imagery shows and shortwave and associated jet
streak across North Dakota with a second jet streak moving into
western Kansas. As a strong low level jet brings moisture
northward, convection has developed across southeast Nebraska.
Both NAM and GFS show lapse rates between 6.5 and 7 C/km moving
toward northwest Iowa this morning resulting in MUCAPES of 100-300
J/kg by 15Z. With strong lift associated with an approaching 850
mb front, expect that convection over southeastern Nebraska will
continue to develop as it moves north this morning...reaching
northwestern Iowa and far northeast Nebraska by 15Z. The forcing
for ascent will only increase through the morning so that even
with weak convective instability, the thunderstorms should expand
in coverage later this morning and early this afternoon such that
most areas south and east of a Yankton to Sioux Falls to Windom
line should see some rain with embedded thunderstorms by early
this afternoon. This area of rain and embedded thunderstorms will
continue to move northeast through southwestern Minnesota and
adjacent areas of South Dakota this afternoon where many places
will see over a half of rain this afternoon. With instability
rather weak, none of these storms this morning are expected to be
severe. Farther to the west, large scale ascent associated with
the upper wave will help enhance lift along a front between 800
and 700 mb. This should allow an area of rain to develop across
south central South Dakota this morning and then lift north toward
Chamberlain and Mitchell by late this morning eventually to Huron
and Brookings this afternoon. Even though thunderstorms are not
expected many areas could see a quarter to half inch of rain from
the steady rainfall. This area of rain will only slowly dissipate
so that light rain is expected through at least 06Z in the
vicinity of Highway 14 - especially in southwestern Minnesota.
The rain should gradually end across southwestern Minnesota late
tonight or early Thursday morning.

After the storms move north toward I-90, there may be a break or
much less coverage in rainfall across northeastern Nebraska and
northwestern Iowa later this morning and early this afternoon. All
models then show 700-500 mb lapse rates of 7-7.5 C/km moving over
northwestern Iowa as the 850 mb front approaches the Minnesota and
Iowa border. While clouds will not allow any sun, the strong warm
advection will bring in additional warmth and moisture above the
surface resulting in MUCAPES of 500-1000 J/kg by mid afternoon.
The NAM hints that MUCAPES could be even higher but am concerned
the NAM is overforecasting moisture return which results in MUCAPE
that is too high. As the upper level jet, currently moving into
western Kansas moves into eastern Nebraska after 21Z, expect
thunderstorms to develop across northwestern Iowa. These storms
will quickly move to the east and should be east of Highway 71 by
02Z Thursday. If there are scattered to numerous showers through
the afternoon such that MUCAPEs are closer to 500 J/kg, then
rotating updrafts are as the cloud depth will be more shallow
resulting in effective shear of 20-30 kts. This would not be be
supportive of supercells or severe weather. This is the more
likely scenario. However, if showers basically end for a period or
are very isolated such that MUCAPES approach 1000 J/kg, deeper
convection is more likely resulting in effective shear of 30-40
kts and that would allow a few rotating updrafts to develop. It
will be too stable in the boundary layer for tornadoes but hail up
to golf ball size would be possible in that case. There could
also be isolated wind gusts of 40-60 mph for storms closer to
Storm Lake or Ida Grove where the stable layer will be more
shallow. Again, this is a less likely scenario which is why SPC
only has a marginal risk for most of northwest Iowa.

With all the clouds today, highs will generally be in the upper
40s to lower 50s. Only around Storm Lake and Sioux City will
temperatures approach 60 degrees. With colder air coming in behind
the low tonight, lows will be in the upper 30s to lower 40s.

.LONG TERM...(Thursday through Tuesday)
Issued at 410 AM CDT Wed Apr 19 2017

Quiet period of weather expected through the middle range of the
forecast, Thursday through the weekend. Upper low giving us the
widespread rain through tonight will be pulling away from the area
Thursday, with gradually decreasing clouds and breezy northwest
winds. The lingering clouds could hold temperatures a few degrees
below seasonal normals along/east of I-29, but should be quite a bit
warmer than today across the area.

Gradual warming trend expected through the weekend as upper ridge
builds eastward over the northern Plains behind the departing wave
Friday. Upper ridge is undercut by a southern stream wave sliding
through the central/southern Plains Friday night/Saturday, but with
low level boundaries and upper wave track south of I-70, any threat
of precipitation will remain well south of our area. Northern stream
wave slides along the Canadian border toward Lake Superior Saturday
night/Sunday, but this system also too far removed from our region
to bring any potential for rain to our forecast area. Temperatures
Friday through Sunday will warm a few degrees each day, topping out
Sunday with highs from the mid 60s in southwest Minnesota to lower
70s in south central South Dakota. May have to monitor Sunday for
potential elevated fire danger, with some models hinting at lower
humidity levels and slightly stronger winds in areas west of the
James River Sunday afternoon, but for now conditions look very
marginal.

Pattern returns to a bit more active setup early next week, starting
with a mid-upper trough moving across the region Monday. Timing of
the wave pretty similar between the various models, though strength
varies. Moisture also somewhat limited initially, so would expect
only spotty light showers. Should see stronger winds develop with
this system, which could boost the fire danger a little higher
Monday. However, with modest increase in low level moisture and
light precipitation chances ahead of the surface boundary, and
current timing not bringing drier westerly flow into the forecast
area until late afternoon/evening, do not expect significant fire
issues at this time. Cooler Tuesday with rain chances returning to
mainly our western areas later in the day as the next system begins
to swing out into the Plains. Greater impact from this system looks
to hold off until after this forecast period, though, as a deeper
trough develops and moves slowly through the Plains through the
middle of next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1151 AM CDT Wed Apr 19 2017

Widespread IFR conditions in stratus, rain and occasional
thunderstorms. Some patchy small hail also possible south of a
Vermillion to Jackson MN line. The rain and thunderstorms will
come to an end this evening with IFR to MVFR stratus in place
tonight into Thursday morning.

&&

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

$$

SHORT TERM...Schumacher
LONG TERM...JH
AVIATION...08



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