Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 242320

Area Forecast Discussion
National Weather Service Sioux Falls SD
620 PM CDT Mon Oct 24 2016

.SHORT TERM...(This evening through Tuesday)
Issued at 612 PM CDT Mon Oct 24 2016

Will see gradually increasing clouds overnight, largely mid to high
cloud cover streaming in from the west ahead of approaching wave
through the first half of the night. However, lower clouds expected
to spread into areas west of I-29 from the south during the latter
half of the night as well, making for largely cloudy sky by daybreak
Tuesday. As mentioned in previous discussion, initially dealing with
some saturation issues in the mid levels (800-600mb layer), and this
could result in some patchy drizzle as low level moisture deepens in
locations from south central South Dakota toward the I-29 corridor
through the morning. This layer eventually moistens as well as the
lift deepens with the incoming wave, so think potential for bonafide
drizzle will lessen in the afternoon.

Late morning through the afternoon will also have potential for some
thunderstorm development, as steepening mid-level lapse rates lead
to elevated instability above 850mb. Greatest instability indicated
by high-res models with CAPE values at or above 1000J/kg south of
Yankton SD-Spencer IA line, and will go with higher thunder chance
in that area with just an isolated mention farther north. Strongest
storms in our southern areas may produce some small hail with the
steep mid-lapse rates, but severe threat appears minimal.

Temperatures tonight will be mild with the increasing cloud cover,
with lows largely in the 40s. Small diurnal range under the thicker
cloud cover, with highs ranging from lower 50s in southwest MN to
lower 60s through the Missouri River Valley.

.LONG TERM...(Tuesday night through Monday)
Issued at 612 PM CDT Mon Oct 24 2016

The most significant weather of this forecast period is right at the
start, as the upper system continues to move over the area Tuesday
night. Models have been very consistent on the general strength and
movement of this system, and the prospects of rain and thunderstorms
to mainly the area of Interstate 29 and east. During the last two
days there has been a shift of where the parameters are indicating
the most rain, from southwest Minnesota to northwest Iowa, including
a small part of southwest Minnesota around Jackson. While thermal
and theta e banding and advection initially spreading toward
southwest Minnesota has weakened a bit, feel that the change is due
mostly to the great depth of the dynamic lifting the models are
indicating for northwest Iowa close to the upper system.

There appears easily enough instability and modest shear, not to
mention copious moisture flowing into the system to produce plenty
of thunderstorms in this Interstate 29 and east area Tuesday night,
especially in the evening in northwest Iowa. The elevated nature of
the storms and modest wind fields will preclude a damaging wind
threat, and the copious moisture fields would seem to greatly limit
hail potential. However, over northwest Iowa, where the instability
is forecast to be modestly decent, the deep nature of the upper
forcing and lift may work to produce at least small hail with some
of the elevated cells. The copious nature of the H8 moisture values,
with dew points getting to the mid 50s, would also seem to present a
modest heavy rain threat, which models in their QPF mostly place
just east of the area, but close enough to keep an eye on. At least
the system looks progressive enough to prevent a flooding threat,
since we should not be dealing with midsummer thunderstorm rainfall

Showers and storms should taper off steadily from west to east late
Tuesday night. Am going with guidance pops generally ending the
chance of measurable rain by daybreak Wednesday. However, clouds and
low level moisture behind the system could conceivably produce a few
sprinkles or areas of drizzle Wednesday morning. After coordination,
have elected to leave out for now, given the system will be moving
steadily east away from the area. For the same reason will keep the
idea of steadily decreasing low clouds Wednesday.

After Tuesday night lows in the 40s, Wednesday highs should vary
from near 50 in southwest Minnesota, to the mid 60s southwest from
Chamberlain south where there will probably be some decent sunshine.

Thursday through Monday looks like a dry period, with warming late
in the week, cooling behind a front over the weekend, and warming
again by Monday. The EC is slower than the GFS on the weekend
cooling, not really beginning to bring it in until Saturday, with
the GFS starting it Friday afternoon. In any event, extremes are not
expected. Highs should mostly going back and forth in the 50s and
60s, with some lower 70s in the Chamberlain to Pickstown area. Above
normal overall; still holding off on any wintry temperatures.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 612 PM CDT Mon Oct 24 2016

Quiet tonight with VFR conditions. MVFR ceilings will begin to
develop tomorrow late morning into the afternoon. Not real
confident on drizzle threat as the low levels look a bit mixed to
reduce visibility so will not have any reduced visibilities. The
threat for rain and thunderstorms will increase through the day
with the better chance late in the period and the best chance
from about 0z to 6z.




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