Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
334 am CDT Thursday Apr 2 2015
Short term...(today and tonight)
issued at 330 am CDT Thursday Apr 2 2015
A cooler day looks to be on tap behind the exiting frontal boundary
to the east. Will have decreasing clouds in the morning...before
more middle and high clouds increase from the west during the afternoon
out ahead of another shortwave which will push across Nebraska
tonight. It will be another breezy day...especially through our
north where the gradient will be a little tighter and mixed layer
winds are stronger. With very deep mixing today...stayed below
guidance with respect to dew points...and this in combination with
the winds will again result in fire weather concerns...which will be
addressed in a separate discussion. With the good mixing it will
still be a fairly mild day with highs generally running from near 60
in the north to the middle 60s through the lower MO River Valley.
The aforementioned shortwave scoots across Nebraska tonight...and
models are still split on whether any of the precipitation with this
system will make it into our area. Still appears that the better
chances will remain well to the south of our County Warning Area...but did include a
slight chance in our far southern northwestern Iowa and Nebraska
zones late tonight. A secondary more significant shot of cold air
begins to filter into the region overnight...with lows dropping down
into the middle 20s to middle 30s.
Long term...(friday through wednesday)
issued at 330 am CDT Thursday Apr 2 2015
Friday will exhibit our ever present northwest flow aloft with high
pressure dominating at the surface. It will be another well mixed...
dry day. However winds aloft are not projected to be all that strong
at 850mb compared to some other days we have had recently. For
instance 25 knots is forecast off of the various models at 18z in
our eastern zones at 850mb...much lighter along and west of I 29.
But with the high settling across our forecast area during the
afternoon...the winds should not increase after 18z and may in fact
notch slowly down a bit. For highs...mixed temperatures to just
above 850mb but at any rate it will be substantially cooler...more
seasonal with middle 40s to middle 50s. Lows Friday night are tricky and
will be elevated dependent with a light...dry surface flow. May not
have gone quite chilly enough at some locations such as Sioux
City... Spencer...Huron and Chamberlain. But there is no risk for
precipitation and it is so dry it may not even frost.
On Saturday...low pressure develops over the Western Plains with a
warm front extending eastward...the question is exactly where. The
GFS is an outlier in showing cool high pressure clipping southwest
Minnesota during the afternoon draining down from western Ontario. Went
with a NAM and European model (ecmwf) compromise which places the warm front across
our northern zones. 850mb temperatures really warm up big time along
and south of the boundary...so continued the trend of raising highs
for Saturday. Any guidance values which are not bias corrected were
immediately tossed...and preferred the warmest bias corrected values
such as MOS and the European model (ecmwf). Therefore have 70 to 75 degrees for the
MO River Valley...and 60s elsewhere.
Saturday night is interesting in that all of the various model
solutions show a weak shortwave drifting southeastward. Would not be
surprised to see some altocumulus castellanus thunderstorms and rain develop in parts of our area east
of I 29 where a Stout 45 to 55 knot low level jet is feeding in from
the south into the warm front providing localized lift. Middle level
moisture looks fairly decent in those areas but not sure if that is
a feedback from the model produced convection or not. Middle lapse
rates are pretty unstable however.
Sunday is another problematic day for highs in that there is quite a
difference in how aggressive the models are in plunging the
aforementioned warm front southward. The European model (ecmwf) is most aggressive
with this and positions the frontal boundary close to Sioux City by
00z Monday. The Gem global barely moves it and the GFS is in
between. Noting this...our far northern zones along Highway 14 may
have trouble getting out of the 50s. But conversely...the warm bias
corrected values once again look King for our south...and have Sioux
City close to 70 degrees.
Then on Monday and Tuesday a wavy pattern finally ensues...and this
has been advertised by the deterministic models for a while now as
an upper trough moves into the western states. Still looks like a
chance for warm air advection precipitation Sunday night with enough
instability to produce elevated thunderstorms and rain in the southeast quadrant of our
forecast area. Monday still could be quite a damp and chilly day for
a change...much needed if it pans out. The European model (ecmwf) still shows a
vigorous short wave overriding the surface boundary and placing a
decent amount of rain in this area. However the Gem global is
further north with it. 850mb temperatures are a bit chilly and the
superblend looked too warm...especially if rainfall pans out.
Therefore blended the much cooler European model (ecmwf) into the superblend on
Monday. Tuesday and Wednesday now look dry behind the wave passage
but with temperatures at or just below seasonal normals.
Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1103 PM CDT Wednesday Apr 1 2015
Showers will impact Sioux City for the upcoming 1 to 2 hours
otherwise little to no aviation impacts expected at the local
airports. Winds will continue to howl from the west on
Thursday...gusting as high as 30 knots at times. Increasing middle-
upper level clouds will arrive Thursday afternoon.
issued at 330 am CDT Wednesday Apr 1 2015
A red flag warning will remain in effect for this afternoon for
southeastern South Dakota and southwestern Minnesota. Low dewpoints
are expected across the area with deep mixing...yielding afternoon
relative humidity values in the range of 15 to 20 percent through
most areas. This in combination with northwesterly winds of 20 to 30
miles per hour will result in an extreme fire danger for those areas mentioned
above. Elected to keep the northwestern Iowa and northeastern
Nebraska zones out of a red flag warning as winds will be a little
lighter in those areas...as well as the fact that some of the areas
in those fire weather zones received rainfall overnight.
The low humidity continues Friday into Saturday...15 to 25
percent...but winds should be lighter. Will have to closely
monitor mixing potential on Friday noting a northwest flow. But at
this time...it appears the wind speeds should maximize during the
middle of the day...then notch down gradually in the afternoon as
high pressure becomes more centered over our area. Saturday will
see a south to southwest flow for many locations...but again wind
speeds do not look terribly strong. This should keep US below red
flag criteria both days...but still expect high to very high fire
danger given dry fuels and low humidity. Relief...at least
temporary...will be found across this area by Monday next week.
South Dakota...red flag warning from 11 am this morning to 7 PM CDT this
evening for sdz255>258.
Minnesota...red flag warning from 11 am this morning to 7 PM CDT this
evening for mnz900.