Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
1054 PM CDT Friday Apr 17 2015
Short term...(this evening through saturday)
issued at 413 PM CDT Friday Apr 17 2015
Very pleasant afternoon across the tri-state area today as
temperatures have risen into the lower 70s and light northerly winds
prevailed. Main story heading into the first half of the weekend
will be the influence of a rather large upper trough centered over
The Rockies today.
This afternoon/evening...weak shortwave continues to pivot
northwards through Nebraska. Scattered showers and a few
thunderstorms have developed along a deeper moisture axis just south
of the County Warning Area. Given warming this afternoon...soundings show a rather
uncapped environment south of I-90 through sunset...however very
little instability. There may be enough lift to produce a sprinkle
through sunset. The best forcing with this weak wave will shift
westward this evening...with most areas dry overnight.
Saturday morning-day...as a strong jet maximum rotates into the Southern
Plains and another wave eject around the upper low this
evening...convection will Blossom over the Central Plains. This
convection will gradually lift northward through the night...and
will begin to impact the Missouri River area towards daybreak. In
general...models have continued to narrow and slow down the arrival
of precipitation in the area. This leads to warmer temperatures in
areas north of I-90. A narrow band of rain with embedded
thunderstorms will likely lift through the County Warning Area through The Heart of
Depending on the progression of rain through the area...it is
possible that areas south of the Missouri River could see partial
scattering as a dry slot moves into the region. Depending on the
amount of clearing we could potentially see MLCAPE upwards of 500-
800 j/kg form by middle-afternoon along and south of the MO river. While
effective shear is not all that impressive...freezing levels are
rather low at or above 7k feet above ground level...which would support the possibility of
marginal hail should storms develop.
Long term...(saturday night through friday)
issued at 413 PM CDT Friday Apr 17 2015
Precipitation distribution continues to be larger challenge
heading into the intermediate term. Some question on progression
and orientation of dry slot by 00z sun...which may push a majority
of the more steady precipitation across the far north/eastern County Warning Area.
Hard to picture coverage within the middle level dry slot...but
should not be more than a Middle Range scattered coverage with a
fairly distinct weakness between dynamic focus of the filling
upper low to south and middle level forcing of northern stream.
Plenty of moisture with little overall focus...as even isentropic
forcing will have faded significantly. Have worked a diminishing
trend in probability of precipitation over much of the area...but not before perhaps
another quarter to third of an inch rainfall over the eastern County Warning Area.
Likely to also find some lighter precipitation in middle level
frontal zone wrapping in with northern stream energy which could
wander into the western County Warning Area by late night. With appreciable low
level moisture...areas ahead of frontal boundary mainly in lower
50s overnight...with lower to middle 40s filtering into the far west.
Precipitation Sunday will be mainly forced in a couple areas...along and
ahead of advancing cold front in southwest Minnesota/northwest
Iowa...and a fragmenting shield with northern stream upper front
wrapping gradually eastward through the County Warning Area. Amounts should be
fairly inconsequential...but given the timing and amount of
earlier precipitation...not thinking the afternoon increase in northwest
winds and relative humidity drop to around 30 percent in west will
result in as much degradation of fire weather conditions as
otherwise would expect. Winds staying up Sunday night should keep
lows from dropping too far.
Monday will not be a pleasant day on many fronts. On the southwest
flank of the upper low...strong gradient aloft will quickly
surface and would not be surprised to see marginal advisory level
winds /30 to 45 mph/. Fire weather conditions may also become a
great concern as afternoon relative humidity drops to 20 to 25
percent in the west. Further east...will be dealing with colder
temperatures...and impact of some very cold air aloft. Likely to
see development of instability showers...but coverage may be
limited a bit by larger scale tendency for subsidence. With small
bit of low topped cape present and all but near surface layer well
into ice phase range...suggestive of perhaps a few graupel
For Tuesday to Friday...pretty much will be a period of below
normal temperatures...moderating late week. With some inconsistency
in handling transition gradually out of influence of large upper
low toward the Great Lakes...could not justify keeping any threat
of precipitation in...which was introduced in blended guidance
through the southern half of the area from Thursday through
Friday. Otherwise...GFS and more recently the latest European model (ecmwf) are now
trying to sneak in a little bit of moderation of temperatures aloft on
Tuesday with more westerly push to low level flow...ahead of next
frontal push later in the day. Nudged temperatures up just a bit...
especially through the Missouri Valley. On the other hand...
Wednesday blended temperatures came out unreasonably warm and
unattainable by full mixing of any operational solution...so
collaborated a general lowering of readings. Many lows over the
Tuesday to Friday morning will reach upper 20s to middle 30s...and
will bring into question the eventual need for frost or freeze
mentions...currently mentioned in the severe weather potential statement and social media.
Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1048 PM CDT Friday Apr 17 2015
The 06z tafs will reflect VFR conditions for the remainder of the
night...with the weather deteriorating on Saturday. Closed upper
low pressure currently centered over Colorado will inch eastward
into eastern Kansas by 06z Sunday. Ahead of this low...a short wave
will lift northeastward through the day on Saturday affecting the
weather in this area. VFR will give way to MVFR to possibly even
upper end IFR conditions from south to north...reflected in the
timing of the tafs by affecting ksux first around 16z Saturday...
to very late in the day at khon. Still kept the possibility of a
few thunderstorms in the early and middle afternoon for a couple of
hours at ksux just to the south of a subtle warm front.