Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1219 PM CDT Friday Sep 19 2014
Short term...(today and tonight)
issued at 329 am CDT Friday Sep 19 2014
Low level stratus continues to stream northward this
morning...blanketing the bulk of the forecast area. High resolution
models suggest that this stratus will linger through around middle
morning and finally dissipate from southwest to northeast through
around noon as the low level jet Cranks up to around 50 knots and some
middle level drying manages to mix down late morning and early
afternoon. With a very strong middle level inversion in place on the
order of around 20 to 25 c at 850 mb...temperatures will likely be
above normal...particularly in south central South Dakota and the Missouri
corridor...however full mixing will be unlikely with this pattern.
Still...should manage to reach near 90 close to the Missouri
River...especially if the stratus can manage to exit this morning.
With low clouds lingering longest across our east...did lower highs
a few degrees...but should still be a pleasant late Summer day with
highs in the low to middle 80s.
Dry conditions will prevail today with strong capping holding off
convective activity through the afternoon. Cold front dives into the
western forecast area middle to late afternoon and arrives in the i29
corridor early to middle evening before stalling out and slowly sagging
across northwest Iowa and south central Minnesota late evening through much
of the night. Upper level support increases late this evening into
the overnight hours as increased vorticity and a jet maximum starts
nosing into the area and shifts east through the northern plains.
With decent low level moisture...but a persistent cap...any
significant thunderstorm activity will likely be near or Post
frontal passage. Steep middle level lapse rates and decent instability
suggests that storms will be mainly elevated with marginal severe
threats - perhaps an isolated storm with half dollar hail and 60 miles per hour
gusts at best late evening into the early morning hours along and
south of a Vermillion to Windom line. Activity will taper off to the
southeast through daybreak Saturday.
Long term...(saturday through thursday)
issued at 329 am CDT Friday Sep 19 2014
Shaping up to be a nice weekend across the region as high pressure
builds in. Given the mostly sunny skies and deep mixing...makes
sense to stay on the warm side of guidance for Saturday...with highs
in the upper 70s to low 80s...about 5 to 10 degrees above average.
Will be a breezy one though...with northwesterly winds gusting up to
around 30 miles per hour. As the upper trough axis swings across...could see a
sprinkle or isolated shower near Marshall during the afternoon...but
the rest of US will stay dry and mostly sunny. Cooler air aloft will
move in by Sunday. Still should be a pleasant day though...with
lighter winds...sunny skies and highs in the low to middle 70s. Sunday
night will be the coolest night of the forecast period...with high
pressure nearby allowing for good radiational cooling and lows in
Next week will feature large scale ridging building into the plains.
However will likely have to deal with a cutoff upper low that should
reenter the flow and move across the northern plains. GFS...European model (ecmwf)
and Gem actually in really good agreement with the progression and
track of this low. They keep the cutoff low over California through
Saturday...then eject it eastward as it gets caught back up in the
westerly flow aloft. As forcing and moisture increases...shower
chances should increase Monday night across our west...spreading
across the County Warning Area Tuesday into Tuesday night. Still some uncertainty
with timing of when the low exits...so will keep some low probability of precipitation in
Wednesday ND Thursday as well. Instability seems pretty minimal so
should mainly be a shower threat...but could be enough elevated
instability to get some thunder. Based on these model trends...did
go ahead and speed up and bump up probability of precipitation some Monday night through
Tuesday night. However did not want to go too high yet...as models
usually have a tough time with cutoff lows like this...so even
though models in good agreement for now...not completely sold on
their forecast evolution just yet. If model consistency increases
will have to continue to increase probability of precipitation on future shifts.
Given shower and cloud cover uncertainty...stayed
with consensus values for highs and lows Monday through
Thursday...giving highs in the low to middle 70s and lows in the 50s.
However model trends are suggesting we get quite warm again behind
this cutoff low. Uncertainty on when exactly it gets out of the
area...but highs by Thursday could end up warmer than forecast...and
above normal highs are looking probable by the end of the week into
Aviation...(for the 18z tafs through 18z Saturday afternoon)
issued at 1217 PM CDT Friday Sep 19 2014
Scattered to broken middle level clouds will persist into the evening
hours as a front drops southeast through the region. Considerable
uncertainty on convective chances along the front...with highest
probabilities at sux. Activity should remain rather scattered...so
will only include a thunderstorms in the vicinity at this time.
Hon/fsd should remain dry with middle level clouds through the night.
Winds will gradually turn to the west and northwest by daybreak.