Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 121802

Area Forecast Discussion
National Weather Service Sioux Falls SD
102 PM CDT Mon Mar 12 2018

.SHORT TERM...(Today and Tonight)
Issued at 243 AM CDT Mon Mar 12 2018

A cool, but dry start to our work week as stratus has exited the
area to the southeast. Starting to see a bit of fog developing
this morning and this will continue through daybreak, with
localized visibilities under 1 mile.

Today: We`ll start the day with plenty of sunshine and that
lingering fog, but may see a broad increase in upper level clouds
through the day as a shortwave drops southeast across western and
central Nebraska this afternoon. Continued cold advection aloft
will keep temperatures from climbing to or above normal in most
areas. In fact, some areas of SW Minnesota may not reach freezing.

Tonight: High pressure moving through the Dakotas will allow winds
to weaken overnight.  A bit of low level stratus may be possible
overnight as a thin area of enhanced RH over southern Canada moves
into the area. Temperatures will again fall below normal, with
single digit lows in north central South Dakota.

.LONG TERM...(Tuesday through Sunday)
Issued at 243 AM CDT Mon Mar 12 2018

The medium and extended range forecast is one of split certainty.
Decent model agreement through Thursday remains, however beyond
Thursday there is little agreement, standard deviations are
high, overall confidence in specifics will remain minimal at best.

Tuesday: As western troughing begins to approach the EPAC (Eastern
Pacific), we should see an amplification of the downstream ridge,
along with the return of low level warm advection. That said, cool
Canadian high pressure remains in control for Tuesday, which will
actually keep temperatures from rising at all, and highs will again
top out very similar to those of Monday.

Wednesday: A series of shortwave troughs dropping south across the
Great Lakes will bring a backdoor cold front into the area on
Wednesday. This will be problematic to the temperature forecast as
it will certainly limit the warmup over portions of the CWA, and yet
allow others to jump quickly.  For now the coldest areas will be
focused in the northern and northeastern portions of the CWA where
mixing should be fairly minimal. Temperatures will warm through
the 40s as you move towards the MO river valley, with 50s and even
60s expected further south of the boundary in Nebraska.

Thursday: This is probably the final period of the forecast where
there is any reasonable forecast confidence, and even that is in
question. It`s certainly beginning to look like eastern troughing
will have a larger influence on Thursday, with the EC showing
strong low level cold advection and cool easterly sfc flow. The
GFS meanwhile has even stronger easterly flow, but neutral
temperature advection. For now, prefer a blend of both, which
would keep the warmest readings in the MO river valley, and cool
temperatures further northeast. Have lowered temperatures slightly
from blends.

Friday-Tuesday:  There`s really no better way to say this, than
the overall confidence in the forecast is minimal at best. Why is
this? Well, most of the uncertainty in the forecast revolves
around the previously mentioned upper trough that approaches the
EPAC by Wednesday. Mostly likely due to poor oceanic sampling of
the system, ensemble solutions are highly deviant with the GEFS
showing significant height deviations by Thursday. At this point,
there seem to be fewer early week departures in the eastern CONUS
trough and thus more confidence in it`s placement at the end of
the week.

How does this impact the forecast? All three models (CMC, EC, GFS)
all try to bring energy out of the Pacific trough by Friday.
However, they are all extremely different in their solutions. GFS is
weaker and further north, CMC is stronger and centered over the
Plains, and EC is deeper but also exhibits a much stronger blocking
high to the east that pushes the system through the Central and
Southern Plains.  Thus, the very low forecast confidence for Friday
and Saturday. It`s certainly possible the warmer GFS could verify,
but it`s just as possible that the much colder and drier EC could
verify.  With EC/GFS ensemble temperature deviations approaching 6
and 7 (and operational models both towards the extremes), decided to
play more of the mean with PoPs and temperatures.

An already low confidence forecast won`t improve as we look to the
start of next week. This is evident in the GEFS forecast uncertainty
plots of surface temperatures with show temperature deviations
across central and eastern Canada approaching 8-12 SD, indicative of
the highly uncertain evolution of the eastern CONUS trough in the
GFS ensembles. GFS/GEM support another system moving through the
region Monday, but the ECMWF keeps this system again to the south,
thanks to eventual development of a Greenland block.

So what does all that mean?  I`m avoiding putting all my eggs in a
basket when it comes to one solution or another. I do foresee
warmer temperatures (or at least finally climbing above normal),
but wouldn`t go extreme temperatures. At this point, any solution
is possible and enjoy the ride in getting there.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1259 PM CDT Mon Mar 12 2018

Light winds and mostly clear skies are expected over the next 24
hours. During the early morning hours Tuesday, patchy fog
development is likely, and in the James River Valley including
KHON, fog could become more widespread. Where fog does become
established, light winds tomorrow morning will cause it to linger
through mid-morning, before enhanced daytime mixing is able to
dry out the lower atmosphere.




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