Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 241716

Area Forecast Discussion
National Weather Service Sioux Falls SD
1216 PM CDT Mon Oct 24 2016

.SHORT TERM...(Today and Tonight)
Issued at 320 AM CDT Mon Oct 24 2016

Models have backed off of stratus potential for today with no
development overnight. Otherwise, will just have a few high clouds
streaming across the area with a surface ridge axis over the
Northern Plains shifting into the Mississippi Valley by afternoon.
It will be seasonably cool today with highs in the mid and upper 50s.

By tonight, moisture profiles begin to deepen with increasing
midlevel theta e advection across the area. With that, will have
increasing cloud cover, though a stubborn dry layer remains between
800 mb to 600 mb, so thinking that any rain chances will remain
minimal in spite of some models spitting out QPF late. Did allow for
a slight pop through portions of the northern Highway 14 corridor
after 09Z where moisture profiles are slightly deeper. With cloud
cover and warm air advection, it will be a much milder night with
lows in the 40s, nearing 50 along the Missouri River corridor.

.LONG TERM...(Tuesday through Sunday)
Issued at 320 AM CDT Mon Oct 24 2016

Still looking at our well advertised short wave moving across the
forecast area late Tuesday afternoon and night, preceded by a bout
of warm air advection during the day on Tuesday. Initially Tuesday
morning, mid levels from 800-500mb remain dry, with slightly deeper
moisture in our northern zones. Therefore included a possibility of
drizzle where the mid level moisture is the driest up the I 29
corridor zones. In addition to this, have a mention of isolated TSRA
not including our zones in southwest MN. Elevated instability is
abundant except in southwest MN. But for any embedded TSRA to occur,
the convection will have to be very elevated tapping into positive
CAPE and negative lifted index values generally in the 800-750mb
layer. By Tuesday afternoon, the moisture increases in the mid
levels as the short wave gets closer. Therefore eliminated the
drizzle threat for the afternoon hours. Strong thetae advection
exists in the 850-700mb layer, first in our western zones early
Tuesday, then migrating eastward into central MN by late Tuesday.
Along this boundary aloft is fairly strong isentropic lift in the
295-310K layer where the moisture is abundant. All of these factors
come together primarily along and east of I 29 by mid afternoon
Tuesday where likely pops are warranted. Right around Sioux City by
late afternoon, the instability deepens coupled with deep layer
shear. Therefore in the Sioux City vicinity, just went predominant
TSRA, with isolated embedded TSRA elsewhere where the instability is
more aloft.

Tuesday evening is still the period in which the bulk of the short
wave affects the forecast area. Moisture and deep layer veering
shear still exist generally along and east of Interstate 29, with
the moisture depth less deep heading west of I 29. Categorical to
likely pops are therefore still warranted throughout the eastern
half of our forecast area, with chance pops in the James River
valley for only light rain. The instability and shear factor is a
concern Tuesday evening in southeast SD, extreme northeast Neb, and
much of northwest Iowa, maximized in the Sioux City area. As stated
above, a strongly veering profile is noted with the low level jet
showing 40 knots just south of our forecast area impeding into only
20 knots just north of Sioux City. Therefore low level jet speed
convergence is rather strong in our far south central zones, and the
925mb-800mb CAPE values are very close to 1000 J/Kg not too far
south of Sioux City, about midway between Sioux City and Omaha.
Therefore agree with the current Day 2 severe weather outlook from
SPC in bringing the marginal risk to just south of Sioux City. This
is something that will need watched Tuesday evening in our far south
central areas because any further northward movement of this
instability will place it at Sioux City. The wave rapidly exits
however after midnight Tuesday night, leaving what looks like now a
dry day for Wednesday.

This system really does not have any cold air with it. So we are
looking at highs in the mid 50s to lower 60s on Tuesday despite the
cloud cover, as warm moisture advection should allow for some
warming. With lingering stratus, warmed up the lows Tuesday night
from consensus values by loading in some raw model guidance values.
Wednesday will be seasonable with lower 50s east, to lower 60s in
our MO River valley zones.

The next interesting feature to watch for will occur late this week
on Friday. Both the deterministic ECMWF and GFS show a short wave
diving down the northwest flow aloft. However the GFS is quite a bit
stronger with it and about 6 hours quicker. With the stronger
solution, the GFS drags quite a strong cold front through on Friday
which would give highs markedly cooler then what is currently in the
blended guidance. In fact taken literally, we would probably have
steady to falling temperatures in this forecast area Friday
afternoon using the GFS. The ECMWF remains rather mild. Therefore
hedged toward the blended consensus solution for now with mild highs
of 60 to 70, but with cooler temperatures on Saturday. A chillier
Friday is certainly something to watch for as the week progresses.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1216 PM CDT Mon Oct 24 2016

VFR conditions will dominate across the forecast area through
tonight, followed by gradually deteriorating conditions after
12Z-14Z Tuesday. Increasing low-level moisture will bring a
lowering of ceilings into MVFR range through Tuesday morning.
There is a potential for spotty drizzle and/or isolated thunder
accompanying the increasing moisture, with the chance of drizzle
somewhat higher than any thunder threat through 18Z. However,
confidence in either event is too low to include in TAFs at this




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