Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1029 PM CDT Tuesday Sep 16 2014
Short term...(this evening through wednesday)
issued at 357 PM CDT Tuesday Sep 16 2014
Current synoptic situation shows surface low pressure near northeast
Colorado and a large area of high pressure over the lower Great Lakes.
This is ushering in a southerly flow in the low levels. Tonight...a
subtle warm front will nudge northeastward primarily through the
southwest portion of our forecast area. In response...the winds
shift to the northeast north of this boundary around Huron late
tonight...then become east to northeasterly throughout our entire
forecast area as Wednesday progresses with the warm front stalling
and shifting a bit southward again.
The main concern tonight and Wednesday is higher based stratus
nosing into our forecast area from the Central Plains. Moisture
maximized around 850-800mb is currently nudging into our forecast
area near the I 29 corridor...with more of it extending
southwestward through central Nebraska. This stripe of moisture is
prognosticated to move slowly eastward through the night and into the
eastern half of our area by 12z Wednesday. During the day on Wednesday...
the clouds should slowly mix out...at least by afternoon in our
eastern zones leaving a sunny to partly cloudy afternoon for the
forecast area. Right now it appears the moisture depth is not deep
enough to warrant a mention of rainfall. But it could keep lows a
bit above guidance consensus readings throughout much of our eastern
half tonight. On Wednesday...highs look mild with temperatures in
the 70s with not much wind to contend with. Should be a nice day by
afternoon in our east...and mainly sunny skies west.
Long term...(wednesday night through tuesday)
issued at 357 PM CDT Tuesday Sep 16 2014
Drier air wedging in through the east will attempt to be displaced
by an increase in moisture Wednesday night as flow turns toward
southerly beneath more westerly and warming flow aloft. Best
moisture return will occur at lowest levels through the Missouri
River corridor westward...and while not an optimal set up to get
drizzle or fog...at least a mentionable percentage of sref
soundings lean strongly toward that probability of occurrence...
along with operational NAM/GFS soundings. Kept mention at patchy
fog/drizzle...but very well may be a bit more significant than
that. Otherwise...will have clouds increasing later in the night
from the south.
Clouds could have a significant impact on temperatures on
Thursday...as most solutions are not receptive to more than a
partial breaking of clouds during the afternoon. Best mixing
depth are likely in the east with drier low level trajectory for
much of the day...resulting in a higher ceiling...while lower
ceilings...and perhaps some early day fog/drizzle hover across the
South Dakota County Warning Area. Highs kept pretty much where they were in earlier
forecast...from around 70 in The Heart of the County Warning Area...to lower 70s
east and middle 70s in the far west. If clouds do not break much
around the I 29 corridor and westward toward the James Valley...may
hold temperatures in the 60s.
A little elevated Theta-E advection above the pooling lower level
moisture could yield a spotty shower or thunderstorm on Thursday
night on nose of steeper lapse rates spreading across the County Warning Area...
as wave moving through North Dakota flattens upper ridge a bit
later night. Southerly winds certain to increase Thursday night
into Friday over what was some breezy Friday conditions...so low
temperatures nudged up a bit...and trend will likely either be a little
non diurnal at times at night...or at worst a definite slowing of
the fall under abundant clouds east.
Warmest day of the forecast period will be Friday...as strong
southwesterly gradient and good pre frontal warm push spreads west
to east across the County Warning Area. Should be able to shake clouds fairly
early in the day east...and will be at least middle afternoon before
any thicker clouds would threaten the western/northern areas with
advance of next wave and frontal boundary. Highs may threaten 90
in a few Lower Brule to middle Missouri Valley locations...with 80s
otherwise the flavor of the day. Temperatures aloft warm to 11 to 13c at
700 hpa...with indications of as strong as a 200-300 j/kg cap in
the prefrontal environment. This should preclude any daytime
development near the boundary...with better upper support also
lingering into the nighttime hours.
Continued to carry higher chance to likely level probability of precipitation mainly east
through south on Friday night with fairly strong Post frontal
forcing. Good news is that tropical tap of moisture from remnants
of odele will likely not be involved in any precipitation across
our area. Rapid west/ northwest flow pattern would seem to favor a
somewhat quicker solution...so pushed the ending of precipitation up a
bit on Saturday with a slightly quicker cold frontal push as well.
Very well mixed days Saturday and Sunday...so did boost winds
several knots over initialization...and most guidance suggests
daytime mixing temperatures would be several degrees warmer...at least
on Saturday ahead of secondary cool push Saturday night.
The remainder of the extended range Sunday through Tuesday appear
quiet with amplifying upper flow keeping strong north to northwest
component across the plains. Subtle fly in the ointment could be a
Monday speed maximum digging past to the east with back door boundary...
with temperatures cool enough aloft to perhaps scare up a nuisance
shower or storm. Will let such a subtle signal sit around for
another couple of forecast cycles before cluttering up the
forecast. Temperatures should be fairly seasonable through the period.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1025 PM CDT Tuesday Sep 16 2014
Stratus has redeveloped in areas from the Interstate 29
corridor late this evening. Ceilings are expected to remain in the
upper MVFR range overnight. Otherwise...winds will be light and
variable...with patchy fog possible around ksux after 10z.