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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
607 am CDT Monday Apr 21 2014

Short term...(today and tonight)
issued at 427 am CDT Monday Apr 21 2014

Area of low pressure centered over southwestern Minnesota very early
this morning will continue to push off to the east through the
morning hours...pulling the associated frontal boundary eastward and
bringing an end to scattered showers and isolated thunderstorms
shortly after 12z. Otherwise...surface high pressure will begin
to build southward into the region through the day...bringing
drier air to the area and kicking up the winds in response to a
tightening gradient and cold air advection behind the exiting
system. With good mixing...again went with model bias corrected
output with regard to dew points this afternoon...which will yield
afternoon relative humidity percentage values in the lower to middle
20s across much of the area...leading to fire weather concerns as
outlined in the discussion below. While it will be much cooler
today than yesterday...good mixing will allow highs to climb into
the middle 60s to lower 70s across the area.

The surface ridge axis slides across the area tonight...and as it
does so winds will become very light. This in conjunction with
mostly clear skies and cold air advection will result in a cool
night...and went on the low side of guidance with lows in the middle
and upper 30s.

Long term...(tuesday through sunday)
issued at 427 am CDT Monday Apr 21 2014

Fairly active pattern developing for the longer range period...
beginning later Tuesday night with warm advection beneath the upper
ridge...then late Wednesday through early Thursday as strong upper
trough swings through the region. Minimal instability Tuesday night
through Wednesday morning...but strong enough lift and increasing
moisture transport with the warm advection to warrant mention of
isolated thunder through this period. Greater focus for strong to
possibly isolated severe storms would be late Wednesday afternoon
and upper trough and associated cold front move into
our western County Warning Area. Stronger instability looks to remain south of our
forecast area...from central Nebraska into the Southern Plains.
However...models show strong deep layer shear with the approaching
trough in our west late afternoon/early evening Wednesday...which
may be enough to overcome weaker instability and produce a few
stronger storms. More widespread rain/embedded thunderstorms are
expected to spread across our eastern County Warning Area ahead of the trough later
Wednesday evening through the overnight...exiting the County Warning Area Thursday
morning as the trough shifts east of the area. Highs Tuesday and
Wednesday will remain on the mild to warm side...generally in the
60s east and 70s west...with lows in the middle 40s to lower 50s.

Tuesday will be another day to watch for fire weather concerns...
especially in our central and west with southeast-south low level
flow increasing through the day as surface ridge shifts east...and
strong mixing leading to humidity levels of 20 to 25 percent. Winds
at this time look to remain shy of red flag criteria...but will
likely be another day of very high danger for parts of our area.

While models appear to be fairly close in timing of first upper
trough through Thursday/Thursday night...they become more and more
divergent as we head through the extended portion of the forecast
Friday through Sunday. GFS digs the eastern trough farther south
into New England through the weekend...which forces the incoming
western trough to progress more slowly eastward. This solution
leaves our area drier but somewhat cooler. On the other hand...
European model (ecmwf) is more progressive with eastern trough early in the weekend
which allows the western trough to swing much farther northeast into
the plains next weekend...producing much wetter period for our area.
Needless to say...confidence in one direction or the other is quite
low...and have strayed little from model consensus blend which keeps
chance probability of precipitation across the area Saturday through Sunday. In either case
below normal temperatures look to return. The question will be just
how far below normal they will be. Again...did not move too far from
consensus at this point...but hints that next weekend could be even
cooler than current highs in the 40s to lower 50s.


Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 604 am CDT Monday Apr 21 2014

VFR conditions through the taf period. Northwesterly winds will
increase through the morning into the afternoon...gusting at or
just above 25 kts at times. Winds will then die off this evening.


Fire weather...
issued at 427 am CDT Monday Apr 21 2014

Northwesterly winds are expected to increase to 20 to 25 miles per hour by this
afternoon with gusts to 35 miles per hour across most of the area. In
addition...with good mixing in a drying atmosphere...afternoon
relative humidity will range from around 20 to 25 percent...though
slightly higher over southwestern Minnesota. In light of
this...elected to upgrade the Fire Weather Watch to a red flag
warning...which covers all but the southwestern portions of the County Warning Area.
The reasoning behind leaving out that portion of the County Warning Area is that
lower relative humidity values are required for that area...and winds are expected
to be less there in a weaker gradient as high pressure builds in
from the northwest. The most critical fire danger period will be
from early afternoon into early this evening.


Fsd watches/warnings/advisories...
South flag warning from 1 PM this afternoon to 7 PM CDT this
evening for sdz256>258. flag warning from 1 PM this afternoon to 7 PM CDT this
evening for mnz900. flag warning from 1 PM this afternoon to 7 PM CDT this
evening for iaz300-301. flag warning from 1 PM this afternoon to 7 PM CDT this
evening for nez249.



long term...jh