Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 040452
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
1052 PM CST Sat Dec 3 2016

.SHORT TERM...(This evening through Sunday)
Issued at 415 PM CST Sat Dec 3 2016

Vigorous short wave moving eastward across the forecast area at this
time associated with a moderately strong PV surge. Primary
precipitation type is snowfall as cold air aloft exhibited by
soundings displays a dendritic layer in the mid levels of the
atmosphere. Therefore despite the relatively mild surface
temperatures, ice crystals are abundant through a saturated cloud
depth and able to produce snowfall. Snowfall amounts will be minor
with the mild surface temperatures, with generally just a dusting.
However this evening after sunset, there could be around a half inch
to just under one inch amounts along and south of Highway 60 in
northwest Iowa and extreme southwest MN with the surface
temperatures cooling a bit. One big question tonight is cloud cover.
Both the NAM and GFS linger a lot of stratus mainly east of the
James River valley. This solution was followed but there is a
clearing line encroaching into our western zones late this
afternoon. One interesting item however is that low stratus has
developed in Beadle county just behind the main shield of clouds. So
beginning to wonder if this is what the models are trying to pick up
on for locations elsewhere east of the James as the moist southerly
flow continues. At any rate noting this southerly flow, this will
help to keep lows mild with or without clouds, and the precipitation
threat will end east of I 29 after evening.

Not a lot happening on Sunday except for a probable decrease in
clouds. A trailing short wave will move through late tonight and
Sunday morning which will help to scour remaining cloud cover out of
the area as winds turn to a westerly flow. So the forecast area will
likely see some Sunshine on Sunday. Manually warmed up temperatures
in our far southwest corner well into the mid 40s after seeing what
Winner SD did today. In fact our entire MO River valley zones should
get into the 40s with a lot of upper 30s elsewhere. Bias corrected
values were too cold so went with a blend of the more mild consensus
raw model values, although the NAM is too cold also as it lingers
stratus for too long.

.LONG TERM...(Sunday night through Saturday)
Issued at 415 PM CST Sat Dec 3 2016

Weak low-level ridging and at least some partial clearing is likely
to let temps fall fairly quickly in Sunday evening, but it will not
take long for southerly gradient to begin to strengthen along with
an increase in mid- and high-level clouds for a slight non-diurnal
trend to overnight temps.  Gradient strengthens enough by late night
with favorable inversion height and flow direction to support a
threat for enhanced winds along and downstream of the Buffalo Ridge
through early Monday morning before winds even out with mixing.

Strong western trough will begin to slide out into the Northern
Plains on Monday and continue to shear northeast on Monday night.
Warm layer aloft will push across the area on Monday, and dry enough
that any elevated lift will likely produce little more than a
enhancement to mid clouds as area of moisture convergence aloft
lifts northeast early morning.  A frontal boundary will drag through
the area and push to around a KMWM to KSUX line by Monday evening.
Ahead of this front, despite cloudiness, there will be warm enough
conditions aloft to favor highs well into the 40s, if not perhaps a
rogue 50 degree reading around northwest Iowa if can sneak in a
little decrease in clouds during the midday hours.  Progression of
the strong frontal zone aloft during the evening should be
accompanied by a few higher based rain-to-snow showers near and
behind the surface frontal boundary, but dry intervening layer
likely to diminish QPF potential.

The main story for the forecast will be the much colder weather
which will settle into the region for Tuesday through Friday, the
coldest so far this season. Winds from 25 to 35 knots at times
through the mixed layer will mean blustery conditions, with wind
chills as cold as 5 below to 15 below during the day, and 10 below
to 20 below at night.  Cannot agree with a forecast of no
precipitation during the Wednesday-Thursday time frame, given a
secondary wave sliding through the area around Thursday morning, a
good slug of low-level humidity with temperatures well within the
dendritic growth layer. Have coordinated to add back in scattered
snow showers, which in the end will likely be mixed with occasional
flurries, perhaps even into portions of later Wednesday or Thursday
evening. Eventually, the larger scale subsidence Thursday night will
end any meager precipitation threat, and leave a quiet period with a
weak ridge axis passing across the area on Friday. Depending on
whether could time breaks in cloudiness with light winds, could
sneak a few temps to near zero by Friday morning.

Another trough will punch into the plains on Saturday. Issue to
consider here is the fact that there is continued indication of a
split flow, and models have little consistency in which branch to
have greater amplitude. Confidence in location for snow is fairly
low with Friday night into Saturday system. A good share of GFS
ensemble solutions along with the ECMWF indicate more
splitting/digging and a little more southern track for better
snowfall, compared with GFS which is far north. Therefore, have not
changed from initial grids.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1028 PM CST Sat Dec 3 2016

IFR fog expected to continue from FSD/SUX into mid-morning. HON
may end up just on the edge of this stratus, but will tempo a
couple hours after midnight. Visibilities shouldn`t fall too far
as winds begin to increase, generally MVFR at most locations.

Stratus will move east by mid-day with lingering upper level
clouds and a southwesterly wind remaining into the evening.

&&

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

$$

SHORT TERM...MJ
LONG TERM...Chapman
AVIATION...Dux



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