Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
635 am CDT Thursday Oct 30 2014
Short term...(today and tonight)
issued at 339 am CDT Thursday Oct 30 2014
Relatively cool pattern setting up in the short term with a strong
cold front moving through the area with breezy conditions to follow
behind the front. Middle level short wave across the eastern portions
of the forecast area will slide southeast yielding clearing skies
temporarily this morning. However...cold front is expected to move
through the area with an expansive area of stratus behind the front.
With 925 hpa temperatures falling 3-5 c through the afternoon
hours...have been conservative with forecast highs...except for
northwest Iowa where front may take longer to get to. With strong
low level cold air advection...have raise winds over guidance but
still below advisory criteria.
With large area of stratus to the north...went ahead and kept clouds
around through the evening hours and gradually shift stratus
southward. With 925 hpa temperatures falling through the overnight
hours...this looks to be the first cool widespread hard freeze
tonight. Have sided with cooler lows closer to the center of the
high building in...but wasn/T as confident further to the south with
lingering cloud and wind concerns.
Long term...(friday through wednesday)
issued at 339 am CDT Thursday Oct 30 2014
Not a lot of changes needed for the longer term. Synoptically the
upper air flow exhibits a high amplitude pattern late in the
week...with upper ridging moving slowly eastward Friday through
Sunday across the middle part of the country. With strong and
chilly surface high pressure squarely planted over our area on
Friday...temperatures will be well below normal...generally mixing
only into the upper 30s to middle 40s. But there will not be a lot of
wind to contend with.
The wind issues begin in our western zones Friday night...then
move eastward across much of the forecast area on Saturday.
Because of the return flow Friday night...lows are a challenge.
With light winds much of the night...dropped the lows below
consensus guidance for locations such as Windom and Worthington
Minnesota...as well as Spencer Iowa. Conversely...the western zones should
have lows in the upper 20s. But many areas will have steady or
slowly rising temperatures Friday night after evening...especially
along and west of I 29.
On Saturday...the relatively lighter winds will be confined to
along and east of Highway 60. But elsewhere...the southerly flow
will average 20 to 35 miles per hour...with gusts approaching 40 miles per hour in south
central South Dakota. Highs will still be below normal with the strong
influence of the cold high just off to our east...as middle 40s east
to lower 50s west are expected. With dry air in place however...
minimum humidity values will be between 30 and 40 percent...which
will give a very high grassland fire danger index for all areas
except along and east of Highway 60. That location should stay in
the high category due to a bit less wind.
Breezy conditions will persist Saturday night keeping lows fairly
mild. Then expect windy conditions again on Sunday east of the
James River Valley...but the humidity values will be somewhat
higher than they are on Saturday helping to mute the fire danger.
The western zones will have less wind as low pressure moves into
the central Dakotas ahead of a large upper trough over the
intermountain west. Sundays consensus highs still look a little
cool given the 900mb forecast temperatures. Therefore continued to
blend the warmer European model (ecmwf) values into the mix to give middle 50s
east...to middle 60s west.
Both the deterministic GFS and European model (ecmwf) show a marked split flow in
the upper pattern as the aforementioned upper wave moves into the
plains on Monday. The northern stream takes over our area and thus
measurable rainfall chances do not look overly great at this
time. Breezy northwest winds will follow the wave on Monday...with
temperatures not too far from normal on Tuesday and Wednesday.
Aviation...(for the 12z tafs through 12z Friday morning)
issued at 631 am CDT Thursday Oct 30 2014
Cold front is expected to move through the area this morning.
Winds will become gusty behind the front...with MVFR ceilings
developing behing the front. The low lying stratus will raise to
low end VFR this afternoon...and spread south of the area after
00z/31st leading to clearing from north to south. With the
clearing...gusts will decrease.