Scientific Forecaster Discussion

Return to Local Conditions & Forecast
Area forecast discussion 
National Weather Service Sioux Falls South Dakota
647 am CDT Wednesday Apr 16 2014

Short term...(today and tonight)
issued at 413 am CDT Wednesday Apr 16 2014

Fairly intense upper wave pushing across Wyoming early this morning
with leading pv surge into southwestern South Dakota. Lift increasing with
broad divergence q over eastern South Dakota...however...airmass is very
dry down low...and increasing returns on radar have yet to reach the
surface at reporting sites. Likely that will start to get some
sprinkles or very light showers developing through the next several
hours with increasing middle level moisture and lift...with 700-500
lapse rates from 7.5 to 8 c/km. The far northwestern County Warning Area could see a
more dynamically significant band start to wander in around
daybreak. With large temperature/dewpoint spreads heading to the
northeast...not out of the question that a little mix to any light
leading showers will Grace the leading edge of any showers to start
to morning.

Challenging little evolution to system today as it moves across the
County Warning Area in fairly quick fashion. Majority of the precipitation will
focus through the northern third of the County Warning Area with support from the
middle level frontal boundary as a sharpening wavelength system
translates through the area. Should be a sharp lift/descent couplet
with such a system...with precipitation really starting to increase
by middle to late morning across the north. Some warmer air wrapping
northward ahead of the feature will battle the wet bulb for a while
likely shifting all eastern areas to rain...while the initial
cooling behind the trough axis by middle to late morning starts a
change over to snow for the northwest first...working mainly east
and a little south during the day. Early to middle afternoon looks to
be the window where cooling thermal profiles along with lift and
pivoting of the thermal boundary will focus a period of more
significant snowfall mainly closer to the the Highway 14 corridor.
Timing of the change is difficult given how warm the warmer airmass
is ahead of the wave...along with monitoring of a dry surge
northward at low levels moving north through eastern Nebraska...
which will cut back on potential snow amounts especially heading
south and east from the far northern County Warning Area. All things considered...
have increased snow accumulations somewhat along the far northern
areas...especially from around Delaware smet toward areas just west of
kmml...where up to 2 inches potential. With wet nature of
snowfall...unlikely that much of any of the visibility reduction
will ee from wind...but rather from snowfall intensity with a little
instability above the frontal zone indicative of more intense

By 00z...practically all of the deeper forcing will have ended with
the exception of perhaps the far east/NE areas...with a more shallow
lingering frontal forcing wrapping through the northeast third
during early evening. Will maintain some higher chance to lower
likely probability of precipitation at the very onset of the evening...but rapidly ramp down
expectations. Still will have to keep an eye on the eastern County Warning Area
later in the night after deeper saturation pulls away. Still appears
that stratus will be a formidable foe through the night and into
Thursday morning as flow really never gets a good chance to flush
out low level moisture before starting to hint toward an easterly
component. As lose temperatures in the moisture profile that could support
ice formation...could have a period of drizzle/freezing
drizzle...perhaps led into by a few flurries.

Long term...(thursday through tuesday)
issued at 413 am CDT Wednesday Apr 16 2014

Models still not showing much of any consistency on Thursday and
Thursday night. Broad troughing settles over the area...however the
big question is whether a shortwave will track along the northern
stream and across the forecast area. The GFS...NAM and sref are most
aggressive this wave...with the GFS moving it into the area
earliest...while the NAM is a bit stronger. European model (ecmwf) and Gem meanwhile
are more aggressive with building ridging into the forecast area -
thereby also warmer and much drier. It does look like even with with
wetter NAM and GFS...there will be some dry low levels to overcome am thinking that Thursday should remain dry
regardless. The NAM especially brings in some pretty significant
quantitative precipitation forecast...which would mainly fall as snowfall. Will continue to favor
the more consistent Gem and European model (ecmwf) solutions...but did not want to
entirely discount the NAM and GFS precipitation potential. Added
mention of isolated precipitation mainly for collaboration
purposes...but with low inter model confidence will cap probability of precipitation below
25 percent for light rain or snow Thursday night into Friday
morning. Temperatures are also problematic with large pattern
differences. Did nudge slightly cooler for with greater cloudiness
on Friday...falling between the two extreme solutions.

Ridging shifts over the area Friday night and Saturday...with mixy
southwest to west flow ushering in much warmer readings. Despite
clouds building into the region ahead of the next system on
Saturday...highs should still manage to reach the upper 50s to low
60s. Plenty of moisture advects into the region ahead of an
advancing cold front Saturday evening and Saturday night. Some weak
elevated cape values developing throughout the County Warning Area suggest that a
few isolated thunderstorms will be possible. Weak shear will prevent
any real severe threat for our forecast area...however with
precipitable water values as high as 1... fairly heavy
rainfall to accompany any showers or storms.

Ridging quickly rebounds over the Western Plains Sunday and
gradually shifts eastward across the plains into Tuesday. Look for a
warming trend during this period...with highs in the 60s on Sunday
and mainly in the 70s by Tuesday. Continued the trend of bumping
highs slightly above guidance during this period with all models
indicating very warm low and middle level temperatures.


Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 647 am CDT Wednesday Apr 16 2014

In the immediate term...non convective low level wind shear will
continue in low level jet area ahead of advancing system...
impacting kfsd and ksux terminals. Low level jet will veer east
as the area of low pressure moves along Interstate 90. Conditions
ahead of the boundary will continue VFR...while periods of MVFR
visibilities and some MVFR to upper end IFR ceilings develop
back in the colder air. Rain will change over to snow for several
hours around khon and a somewhat shorter time for kfsd. With
surface temperatures expected to remain near to just above freezing
during snowfall...accumulations will be minimal on paved surfaces.
While have mentioned some widespread MVFR ceilings into the
night... there is potential that such ceilings could linger a bit
longer into the night and morning on well as sink
toward higher end IFR levels.


Fire weather...
issued at 413 am CDT Wednesday Apr 16 2014

One area to watch from a fire behavior standpoint today will be
extreme northeast Nebraska...extreme southeast South Dakota...and
much of northwest Iowa. Surge of drier low level air will work up
toward northwest Iowa through early afternoon ahead of approaching
system. Majority of model guidance does not appear to be handling
this feature well...and have worked in much lower dewpoints to the
current grids. As temperatures mix in the south southwest gradient ahead of
the advancing frontal boundary...likely to get some winds 20 to 30
miles per hour winds to collocate with humidity from 25 to 30 percent. There is
certainly some indication dewpoints could even be a touch lower...
and in this case...would potentially be looking at a brief period
from late morning to middle afternoon of extreme fire danger and
critical fire behavior. As it stands...will end up with very high
fire danger.


Fsd watches/warnings/advisories...
South Dakota...none.


Short term...Chapman
long term...
fire weather...Chapman