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Area forecast discussion 
National Weather Service Sioux Falls South Dakota
1246 PM CDT Wednesday Apr 1 2015

Short term...(today and tonight)
issued at 415 am CDT Wednesday Apr 1 2015

Highly challenging short term weather for today...with multi faceted
concern on weather and severe weather fronts.

Quiet weather generally starting the day with strong ridging across
the northern plains...but decent height falls crashing across the
northern rockies with strong wave pushing along the international
border. Low level jet really responding with latest kfsd vwp product
showing 50-60 knots right off the deck. Elevated locations to the
west have been warm and breezier thus far into the night...and in
the last half hour are seeing marked increase in winds and temperatures for
Buffalo Ridge locations. Will continue to see increasing winds
across elevated and leeward of Buffalo Ridge locations with flow
direction and inversion height almost classic for our version of a
mountain wave. Will likely see some winds gusts approaching 45 miles per hour
by early morning...and strong winds will expand across

As diurnal heating becomes a bit more established across the eastern
County Warning Area with maximum wind corridor shifting eastward...will likely see
advisory winds of 25 to 35 miles per hour...and a few gusts around 45 miles per hour...
expand through eastern portions of northwest Iowa by later morning and
continuing through middle afternoon. Have expanded the Wind Advisory to
cover this across the eastern two tiers of northwest Iowa.

Not an April fools joke that there is a risk for severe storms
developing middle to late afternoon and continuing through around
sunset. Atmosphere is quite dry at the moment...but have been
working to establish a strong elevated mixed layer with 8.5-9.5c/km
lapse rates. Low level moisture will be somewhat slow to respond
today...and have slowed up the increase quite a bit...but eventually
will see narrow band of moisture stream up in south-southwest flow ahead of
advancing frontal boundary. Likely that NAM is well overdone with
low level moisture...and has somewhat polluted its convective
potential toward the quick the optimistic side as cap is eroded far
too easily along front earlier in afternoon. More realistic to have
a 50-100 j/kg cin in place holding back any surface based convection
until late afternoon...when will start to get some better height
falls and cooling work in tandem with strong frontal
forcing. By around 20-21z...the boundary will be closing on a
Slayton to Vermillion line...and it is from this point and eastward
which will have an increasing risk for thunderstorms by later
afternoon and early evening. As it looks...greater Sioux Falls area
certainly is on the edge of any potential early development. Deep
layer mainly unidirectional shear sufficient for blend of multicell
to supercell development early on...transitioning toward more linear
structure with shear vectors shifting more cross frontal with time
and likely development of stronger cold pool. Cape looks to be
somewhat limited to 1000 j/kg or a bit higher in the southeast
County Warning Area...with a fairly skinny profile. Damaging wind gusts to
65 miles per hour and isolated hail to half dollar size are most likely
threats. Expect coverage of storms to increase toward 6 to 7 PM
along the boundary...and amalgamation of storms should mostly
progress out of the County Warning Area by 11 PM or midnight.

Tempwise...dry airmass suggest that full mixed forecast will
probably come up a couple degrees short as it has most recent days.
Likely that highs will be reached early in the afternoon across the
western third before a steady to slow fall commences. 70s to lower
80s expected today...before drifting off tonight into the upper 30s
and lower 40s with winds temporarily diminishing.

Long term...(thursday through tuesday)
issued at 415 am CDT Wednesday Apr 1 2015

Behind the short wave on Thursday...the northwest surface flow will
become quite brisk with plenty of drying. Fire weather concerns are
detailed below. But otherwise it will be another day of strong
mixing potential with conditions likely mixing above 850mb. Highs
should be in the upper 50s across our northern and far western middle and upper 60s in northwest Iowa.

Thursday night and early Friday are getting a bit interesting for
our southern zones. The next short wave heads eastward and the European model (ecmwf)
and Gem global are beginning to generate some light rain in our
southern zones including Sioux City and Storm Lake. These two models
have more of a hookup between the northern and southern portions of
the wave...allowing a deeper fetch of moisture to form in our
southern zones. The NAM and GFS close off the moisture potential by
allowing the northern stream to takeover...and thus pushing the
rainfall to the south of our forecast area. Until the American
models come around...decided to leave those locations dry but they
will be considerably cloudy.

Friday will have seasonal temperatures with less wind but still a
tad brisk. Model discrepancy begins on Saturday between the various
deterministic models. The European model (ecmwf) is cooler than the GFS on Saturday
and a lot cooler by Sunday. Then early next week on Monday...the GFS
tracks a surface low just to the north of our forecast area which
would continue to provide a substantially warmer solution on Monday
than what the European model (ecmwf) is showing. The European model (ecmwf) keeps a deep east to
southeast flow of air going Sunday and Monday...and if this pans
out...the day on Monday would probably be pretty chilly. The Gem
global is in between the two solutions. For now...primarily blended
the European model (ecmwf) and Gem global...liking the wpc wind solution for these
two days. Concerning precipitation...there could be some warm air
advection rainfall Sunday night but European model (ecmwf) elevated lifted index
values at 850mb are extremely stable at this point. This stability
continues into Monday where more light rain could occur with
isentropic lift present. Interestingly...the European model (ecmwf) continues to
hold onto the idea of a vigorous short wave passage on Tuesday as
it has been doing...which would finally give a decent widespread
rainfall for our entire forecast area which we desperately need.
But with such a variance between it and the GFS...only chance probability of precipitation
are still warranted until we get closer to the event. Temperatures
are tricky Sunday and early next week due to such a wide variety
in solutions. For now blending is the way to go...however due to
anticipated cloud cover...I did narrow the diurnal temperature
trends early next week.


Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1231 PM CDT Wednesday Apr 1 2015

Several aviation concerns today...with strong winds...a sharp
wind shift and convection expected through the evening. Cold front
is now moving into our western border and will dive southeast
through the region through around 03z. Ahead of the front...very
strong south southwest wind are averaging 20 to 35 miles per hour with higher
gusts. Expect these winds to decrease slightly as the front
arrives then increase again to 15 to 25 knots immediately behind the
front. As the evening progresses...overall wind speeds
decrease...then drop to less than 10 knots after 06z. Thunderstorms
are expected to develop along the cold front late afternoon -
after 22z...through around 04z...primarily east of a Sioux City
to Orange City to Windom line. It still looks like any storms
would develop southeast of will not mention in tafs. Any
storm that develops will be capable of becoming severe with small
hail and wind gusts to 60kt.


Fire weather...
issued at 415 am CDT Wednesday Apr 1 2015

Red flag warnings continue today in fire weather zones 255 and 256
in south central South Dakota to the middle James conditions behind
advancing cold front will degrade humidity to 18 to 25 percent... as
northwest winds become a gusty 25 to 35 miles per hour. Did add in additional
red flag warning in the far southeast South Dakota fire weather zone
258. Concerns are for similar conditions to those zones to the west
developing across the western half of the zone...while those areas
east of Yankton toward Sioux City and Beresford are unlikely to have
conditions reach either wind or humidity criteria. Another area of
concern will be parts of SW Minnesota and northwest Iowa...where strong low level
jet will increase winds during the morning and continue strong
southerly winds through at least middle afternoon. At this time... even
slowing up the moisture increase have forecast relative humidity
remaining lowest midday and early afternoon of 30 to 37 percent. If
these area fail to see increasing dewpoint by later morning...a
short lead time red flag warning may be needed with wind gusts of 35
to 45 miles per hour likely.

Have decided to issue a Fire Weather Watch for Thursday for the
southeast quarter of South Dakota and southwest Minnesota. Behind
tonights short wave...the northwest winds will once again become
quite strong on Thursday...averaging 20 to 35 miles per hour...with minimum
humidity values falling into the upper teens to middle 20s percent. For
extreme northeast Nebraska and northwest Iowa...there was too much
of a question to include them in the Fire Weather Watch. The reason
is two fold. depends on how much wetting rain those areas
will get later today from thunderstorms. The wetting rain could be
significant and if this scenario happens...northwest Iowa and
extreme northeast Nebraska will get one day of damp fuel conditions
which will be Thursday before drying out on Friday again. Second...
the gradient is a little baggier in those zones on Thursday with
less wind in the mixed layer. Therefore there is some question as to
how strong the wind will be...although it should be quite breezy in
those areas by the end of the afternoon.


Fsd watches/warnings/advisories...
South flag warning until 8 PM CDT this evening for sdz255>258.

Fire Weather Watch from Thursday morning through Thursday
evening for sdz255>258.

Wind Advisory until 4 PM CDT this afternoon for sdz040-055-056-
062. flag warning until 8 PM CDT this evening for mnz900.

Fire Weather Watch from Thursday morning through Thursday
evening for mnz900.

Wind Advisory until 4 PM CDT this afternoon for mnz071-072-080-
081-089-090-097-098. flag warning until 8 PM CDT this evening for iaz300-301.

Wind Advisory until 4 PM CDT this afternoon for iaz001>003-
012>014-021-022-032. flag warning until 8 PM CDT this evening for nez249.



Short term...Chapman
long term...mj
fire weather...Chapman/mj

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