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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
757 PM MDT sun may 3 2015

Update...
issued at 753 PM MDT sun may 3 2015

00z kunr sounding had well mixed and fairly dry boundary layer
below 700mb...fed by north/northeast flow off surface anticyclone
centered over the Dakotas. Overnight...high shifts east and low
level flow will turn upslope with 20kt southeast boundary layer
flow Monday morning. This will feed low level moisture into the
County Warning Area and slowly erase drier air. Aloft...right entrance region of
90kt jet streak will bring synoptic lift per upper level
divergence. Modest 850-700mb Theta-E advection develops over
northern portions of County Warning Area. Activity over Wyoming associated with weak
shortwave should push slowly northeast overnight and thus have
upped probability of precipitation across far northeast Wyoming/far northwest South Dakota.

Rest of forecast tweaked for latest trends.

&&

Short term...this evening through Monday night
issued at 220 PM MDT sun may 3 2015

Ll frontal boundary backed up against The Rockies this
afternoon with disturbed fast westerly flow aloft. SW Continental U.S. Upper
low coming on shore and is providing backing ll flow over the
southern High Plains into the Central Plains...and will support a
slow increase in deep layer moisture in the period. Weak upper
level impulse now spreading into the northern plains as evidenced
by water vapor. This will support an increase in shower activity
to the west of the region. Uptick in lsa with strong western bh
eddy convergence should support isolated to scattered rain showers there this
afternoon...with other showers possibly spreading into the far SW
forecast area /NE Wyoming/ this evening. Have retained low chance probability of precipitation for this.
Aforementioned impulse will shift east overnight with increasing
deep layer ascent coupled with lingering convective driven mesoscale
forcing. This will support increasing shower chances over the
western forecast area overnight with activity slowing shifting east. Upper
impulse will shift east with rain showers waining over the western
forecast area...with little eastward progression expected across the forecast area. Did
shift/retain low chance probability of precipitation over the central South Dakota plains tomorrow
afternoon. However...the majority of activity should be over the
western 2/3 of the forecast area Monday morning. Did remove thunder mention
give lack of sufficient instability...with prognosticated mu convective available potential energy under
200 j/kg. Lingering chances for additional showers were retained
over scentral South Dakota where the northern extent of the deepest moisture
will graze with a fgen response. Otherwise...lingering rain showers may be
possible over scentral South Dakota into Monday night...with the remainder
of the forecast area expected to be dry under weak subsidence. Temperatures look
tricky Monday...struggling with expected clouds and moisture in
the area. Did cut highs...especially over the west...with many places
struggling out of the 50s where insolation is hampered.

Long term...Tuesday through Sunday
issued at 220 PM MDT sun may 3 2015

Upper flow becomes southwesterly Tuesday as a deep trough
develops over the western Continental U.S.. a strong disturbance will eject
into the central/northern plains...bringing increasing chances for
precipitation. GFS and ecm are still showing much of the precipitation
beginning Tuesday night...with around a half to one inch of quantitative precipitation forecast
for many locations across western South Dakota by Thursday night. Cyclonic flow
will keep chances of showers around through next weekend...and
both models are showing the main upper low becoming negatively
tilted next weekend just west/SW of the County Warning Area. Latest guidance is
delaying the storm until Sunday. The GFS has now brought in colder
air into the northern plains wrapping around the low. This solution
could mean the precipitation falls as snow...but still much uncertainty
exists. Either way the area could see another one or more inches
of quantitative precipitation forecast. Temperatures through the extended period will be near to
slightly above average...in the 60s to lower 70s. However cooler
temperatures are possible with the storm next Sunday.

&&

Aviation...(for the 00z tafs through 00z Monday evening)
issued at 503 PM MDT sun may 3 2015

Scattered -shra will expand across northeast Wyoming/Black Hills this
evening and slowly spread into western South Dakota overnight into Monday.
VFR conditions expected initially...but persistent upslope flow
combined with expanding precipitation area will create areas
MVFR/IFR ceilings in/near precipitation as well as local MVFR
visibilities...especially over the higher terrain.



&&

Unr watches/warnings/advisories...
South Dakota...none.
Wyoming...none.
&&

$$

Update...helgeson
short term...jc
long term...13
aviation...helgeson

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