Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Sioux Falls South Dakota
647 am CDT Thursday Oct 2 2014

Short term...(today and tonight)
issued at 403 am CDT Thursday Oct 2 2014

Several features of interest in the first 24 hours of the forecast
today. First is northeastward shearing wave which will lift across
the area this morning and early afternoon. Strong middle level
frontogenetic forcing will accompany passage of wave...and will
result in a fairly widespread coverage of rainfall over the
southeastern two thirds of the County Warning Area...generally east of the James
Valley. Development should not be a problem even with more
convective nature occurring southward toward I 80. For the most
part...not enough instability to be concerned with in terms of a
widespread thunder threat...but not out of the question to see an
early morning rumble or two wander toward the lower Missouri Valley
and into northwest Iowa. With wave axis passing by during the
afternoon...and fairly quick decrease in upper level pv advection
and divergence q signatures...have put in a bit quicker west to east demise
to rainfall...leaving only a scant slight chance in the extreme
southeast by 00z. Abundance of clouds and the early day precipitation will
have impact on temperatures...more so heading east through the County Warning Area...with
mainly lower 60s for highs east...and some readings pushing toward
70 degrees in the James Valley.

Winds should decouple quite effectively around sunset...but this
will be a far cry from conditions developing during later into the
evening and overnight. Very strong jet focused across the northern
rockies early this morning will be digging sharply toward the
Central Plains by early evening... amplifying the upper trough
across the northern plains. In response...a very strong cold front
will surge southward overnight...followed by a pressure rise of 4 to
9 hpa/3h. This front should be poised just northwest of the County Warning Area
around sunset...near a Jackson to Vermillion line by midnight...and
through the entire County Warning Area by 3 am. Given the strength of the 925-850
hpa winds...would not be surprised to get a brief period of advisory
level winds for an hour or two behind the front as it passes...with
a decrease later in the night...but still quite windy at a gusty 20
to 30 miles per hour.

Perhaps a couple of light showers will brush the western Missouri
Valley locations as wave digs past late evening and early in the
overnight...but not Worth much more than a lower chance pop at this
time.

Long term...(friday through wednesday)
issued at 403 am CDT Thursday Oct 2 2014

Main story on Friday is obviously the wind concern. By daytime...the
cold air advection and strong pressure surge has ended...generally
leaving just a strong synoptic wind to linger through the day. The
GFS and NAM soundings mix to 800mb or even slightly higher...which
will have no trouble in likely mixing down 35 knot or slightly
greater gusts through the day. For sustained winds...mainly stuck
with values about 5 to 10 knots lower than the 925mb wind speeds.
Therefore it looks like a Wind Advisory may be needed...and will hit
it hard in the forthcoming severe weather potential statement. Decided to eliminate the probability of precipitation for
light showers that we had going. Subsidence appears to be shredding
apart the organized cumulus deck...and both the NAM and GFS show
only a thin deck of what will likely be stratocumulus clouds based
near 5500 feet. Concerning temperatures...highs still look to mix to
a couple of degrees either side of 50...with possibly slightly
cooler readings in southwest Minnesota where the cumulus may be a bit more
intense.

Still looking at lows around freezing Friday night. The MOS guidance
values are astoundingly cold...with many 25 to 30 degree readings.
However as mentioned below...believe the surface winds will stay up
enough to keep readings from plummeting that cold. Conversely...the
bias corrected values are too warm so thoughts are that the truth
lies somewhere in between the two. Doubt there will be a frost
Friday night because of the winds...and also because of the Bone dry
air.

Otherwise through the weekend...both the GFS and European model (ecmwf) continue to
show the sharp...cold upper trough actually retrograding a bit
westward by Sunday night. With the cyclonic flow...the surface winds
will stay stirred up east of the James River Valley Friday night and
Saturday...but not near as strong as they will be Friday. On
Sunday...a weak disturbance will move down the sharp northwest flow
aloft and will give our entire forecast area breezy to windy
conditions. There may even be some light rainfall following on
Sunday night. However left it dry for now because there is a large
amount of discrepancy on exactly where to place the rain chances.
The European model (ecmwf) moves it across our western and southern zones...while the
opposite is true of the GFS. As is so often the case...their
respective dynamics are out of phase across our forecast area that
far out...and since any rainfall will be very light...and at
night...will wait to pin it down later.

The upper flow begins to flatten somewhat by the middle of next week
which will help to usher in milder temperatures. Breezy conditions
will linger with a well mixed atmosphere on Monday...then the winds
should begin to notch down Tuesday and Wednesday.

For temperatures...a very slow warming trend will take place
beginning on Saturday...and as expected...the consensus model values
are continuing to trend our highs down for the weekend however. But
now that the GFS and European model (ecmwf) have settled down on the jet stream
placement...with the low level features following suit...most of the
cooler correction has probably taken place. By Tuesday and
Wednesday...our highs might be back to slightly above normal
again.

&&

Aviation...(for the 12z tafs through 12z Friday morning)
issued at 642 am CDT Thursday Oct 2 2014

Portions of northwestern Iowa will continue to find vlifr
conditions around kckp and kslb and southeastward through
at least middle morning. Showers working northward east of
the James Valley will be primarily VFR...but a brief
ceiling or visibility into MVFR range will be possible
around ksux. Otherwise...the main aviation concern will
center around very strong northwest winds pushing in
behind a cold frontal boundary tonight. Winds will gust
to 30 to 40 knots for a couple of hours behind the
boundary...then settle back a bit later in the night.

&&

Fsd watches/warnings/advisories...
South Dakota...none.
Minnesota...none.
Iowa...none.
NE...none.
&&

$$

Short term...Chapman
long term...mj
aviation...Chapman

National Weather Service Glossary of Abbreviations