Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
532 PM CST Tuesday Jan 27 2015
Short term...(this evening through wednesday)
issued at 229 PM CST Tuesday Jan 27 2015
Tricky forecast tonight in terms of cloud cover. Stratus along I-29
north of I-90 is gradually eroding...though there is some question
as to whether it will be able to erode through the afternoon hours.
With southerly flow drawing more moisture into the region...there is
the potential for stratus to expand tonight...but may have a harder
time tonight as stronger winds develop ahead of an approaching
trough. NAM soundings suggest widespread stratus developing across
the area...but sided closer to the GFS and European model (ecmwf) which keeps the
stratus more contained to the north. There is an outside chance
that stratus could become thick enough to produce drizzle...but am
banking against that at this point so have kept forecast
precipitation free. With modest warm air advection through the
overnight into Wednesday and strengthening pressure gradient...sided
with warmer guidance lows.
On Wednesday...looks like one final warm day as 925 hpa temperatures surge
above zero across the region. Cold front is expected to work
through during the afternoon hours. With the cold air advection and
925 hpa winds increasing into the 25-30 knot range...expect winds to
increase through the afternoon hours with steady to slowly falling
temperatures. Should be a fair amount of clouds through the
afternoon hours behind the front.
Long term...(wednesday night through tuesday)
issued at 229 PM CST Tuesday Jan 27 2015
The medium and extended portions of the forecast continue to be
dominated by rather progressive flow aloft...with several shortwaves
capable of impacting the region into next week.
Wednesday night and Thursday...two distinct shortwaves will drop
through the region late Wednesday night into Thursday. The GFS
remains the strongest with these features...especially a northern
stream wave. Moisture is not overly impressive...but soundings do
show a fairly shallow moisture within the 900:800 mb layer...so
would not discount the potential for a few convective show showers
The other concern Wednesday night into Thursday will be with
winds. Guidance suggests a corridor of 40-50 knot flow at the top
of the mixed layer late Wednesday night into Thursday. Have bumped
up wind speeds through the time frame and it is not out of the
possibility that a Wind Advisory could be needed.
Another warmup will be in store for Friday and Saturday as upper
troughing develops in the southwestern Continental U.S. And progressive
northerly flow remains steadily located north of the area.
Temperatures both Friday and Saturday will climb above normal.
Saturday night and Sunday...model solutions begin to diverge with
energy lifting northeast into the central and Southern Plains and
with a secondary trough dipping into the northern plains from
British Columbia. The general consensus is that the northern
stream will win out...pushing an energetic baroclinic zone south
of the forecast area and keeping the local area cold with heavier
precipitation focused over the lower Missouri River valley. Given
this is a timing issue...southern counties will need to continue
to monitor forecast trends for any shift northward.
Early next week...model differences prevent any substantial shifts
in the previous forecasts. The GFS and Gem remain much stronger
with troughing over Hudson Bay...with sharper northwesterly flow
over the area. The European model (ecmwf) on the other hand has a more broad
solution...keeping warmer temperatures in the area. Generally
preference currently is with the GFS/Gem.
Aviation...(for the 00z tafs through 00z Wednesday evening)
issued at 532 PM CST Tuesday Jan 27 2015
MVFR ceilings persist along and east of the Buffalo Ridge at the
start of the taf period...and main question is whether those MVFR
conditions will once again expand westward later tonight and into
Wednesday morning. Given stronger boundary layer wind profile
which shifts from southeast at the surface...to south-southwest
in the cloud layer and above...think MVFR-locally IFR stratus may
see slight redevelopment into east-central South Dakota. However
at this time expect it to remain North/East of kfsd taf location.
Thus will carry VFR conditions at all taf sites through midday
Cold front will push across the forecast area in the late morning
and afternoon. Models pointing to some MVFR stratus in the colder
Post-frontal air...but at this time uncertain whether this stratus
will develop a broken-overcast deck...and if so...how quickly.
Primary threat of MVFR conditions within this taf period is west
of I-29 corridor...mainly affecting khon. Will carry sct015-025
for now...but this could transition to MVFR ceilings as we get
closer to the frontal passage.