Area Forecast Discussion
Issued by NWS Sioux Falls, SD

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

135
FXUS63 KFSD 022212
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
412 PM CST Fri Dec 2 2016

.SHORT TERM...(This evening through Saturday)
Issued at 411 PM CST Fri Dec 2 2016

Plenty of stratus still exists throughout the Sioux Falls forecast
area at this time with a west to northwest flow having locked it
into place. The formation of patchy light freezing drizzle or snow
flurries remains a question mark tonight as the cloud tops cool.
Cannot totally rule out the possibility of this type of
precipitation falling, however the lower portions of the soundings
are not overly unstable as time progresses plus the stratus depth
gets more shallow through the evening hours leading into overnight.
So therefore left things dry for now. Otherwise, as high pressure
moves off to the east tonight, the surface flow and winds off the
surface will become southerly. This should eventually usher our
stratus northward, as central to southeast Nebraska are free of
stratus. But this scenario will take a while. Followed the NAM12
soundings and humidity time sections for guidance on the low level
cloud fields, and they do not clear Huron out of stratus until about
09Z tonight, with Sioux Falls and Sioux City not following suit
until very late tonight and Saturday morning. It will obviously take
even longer for locations such as southwest MN to break out of this
stratus. All this said, sky conditions will not become overly clear
when the stratus breaks, as we have a lot of mid and high level
clouds moving in from the southwest ahead of the next short wave.
These higher level clouds will become quite dense and should keep
fog formation away late tonight in our western zones. Concerning
temperatures, they will be modified by the cloud cover this evening
and not drop a whole lot despite the winds going extremely light or
calm. Lows will stay in the 20s either due to stratus, or high
clouds moving in.

On Saturday, our weather will be influenced by another short wave
moving into our area. It is currently in WY, and will deepen a bit
as it moves into the plains. PV at the 1.5 pressure surface is
fairly strong with this wave by Saturday afternoon, with a fairly
deep fetch of moisture. Therefore chance pops are warranted by
afternoon generally up the spine of the I 29 corridor. Most of the
precip should be liquid rain, with perhaps a light snow mix north of
I 90 toward east central SD but with little to no accumulation due
to mild surface temperatures. Highs across the CWA will be 35 to
40.

.LONG TERM...(Saturday night through Friday)
Issued at 411 PM CST Fri Dec 2 2016

Broad northern branch through will push the Northern Plains on
Saturday night and early Sunday. Trajectory of lower level flow will
continue to bring moisture northward into areas east of I-29.  Zone
of isentropic ascent will precede passage of the mid-level trough
axis, and lift also supported by strong PV advection.  As a result,
have boosted pops above initialization especially toward areas near
KSPW and KSLB, but generally for most locations east of I-29.

Precip type could find a bit of activity in the form of rain during
the early evening where near surface layer will remain warmer toward
the lower Missouri valley, but most indications are that wet bulb
and diabatic impact would bring snow to the forefront. As band of
precipitation gets more organized heading eastward, QPF amounts
should also enhance eastward toward highway 71, but overall values
should remain fairly low such that amounts should only reach a half
inch to inch at most. Gradual influx of milder air should keep low
temps in the mid 20s to around 30.

Quiet weather reigns into the start of the work week. Weak ridging
Sunday and Sunday night as flow near surface works from most
west/northwest toward southerly and clouds with the trough continue
to pull toward the east and leave southwest MN and northwest IA by
early afternoon. Temps should return to above normal levels and work
to melt any remaining snowcover, from upper 30s to mid 40s. The
increasing southerly gradient Sunday night will help keep any fog at
bay, as will increasing higher level cloudiness. Lows will remain
mild from 25 to 30, and could perhaps remain a bit higher at
elevation across southwest MN.  The increasing winds are indicative
of a strong upper level wave moving into the northern Rockies by
Monday morning, and will see progression of a frontal boundary to
near the I-29 corridor by evening as trough swings out into the
Northern plains.  Trends in models are quite a bit quicker with the
boundary, averaging 6-9 hours faster with 12z solution set. Areas
remaining ahead of the front in the warm push, especially east of I-
29, can expect very mild temperatures despite an abundance of mid- to
high-level cloudiness. A somewhat conservative estimate applied
would bring temps into the mid 40s, but would not be surprised by
some near 50 readings into northwest Iowa, nor would some readings
remaining in the 30s toward central SD.

Main energy lifting northeast toward eastern North Dakota and
northern MN on Monday night will pretty much preclude the area to a
cold frontal passage. Could be a little light precipitation making a
transition from rain to snow as colder air rushes in, but the big
story will be the drastic change of temps to be ushered in.
Temperatures Tuesday are not likely to see much of a climb at all,
and mostly a steadying to slow fall.

As secondary trough drops into the larger scale troughing across the
northern plains from later Wednesday into Thursday, even colder air
likely to spill into the region, but tempered somewhat by the
wind/clouds. Dendritic growth potential in the near surface layer is
much stronger with this secondary wave and push of low-level RH from
later Wednesday night through Thursday evening. Have coordinated
addition of at least some chance level PoPs to the forecast for the
Thursday and Thursday evening time frame as wave wraps through the
flow.  Both GFS and ECMWF show potential area of weak off-surface
warm advection wrapping into areas east of I-29, which could enhance
the precipitation chance. For sure, a blustery and cold period
starting Tuesday, and especially with the secondary push by later
Wednesday through Thursday night. Quite the change from our
conditions for most of the last few weeks.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1158 AM CST Fri Dec 2 2016

The 18Z TAFs reflect primarily lower end VFR to MVFR conditions
through the evening hours. However later, as high pressure slides
off to the east, surface and low level winds will become southerly
late tonight. There is currently clear or mostly clear skies in
central and southern Nebraska. Therefore when the winds become
southerly, it is likely only a matter of time before the stratus
advects to the north. Reflected this in the 18Z TAF sites by
scouring out the lower deck from west to east late tonight and
Saturday morning. That said, a higher deck of clouds firmly in the
VFR category will then move in when the stratus clears out.


&&

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

$$

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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.