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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
548 PM MDT Wednesday may 27 2015

Update...
issued at 547 PM MDT Wednesday may 27 2015

Latest water vapour/radar loops showed primary wave -tsra heading
east/northeast and will exit the County Warning Area later this evening. Afternoon
rap picks up on next shortwave over southwest Wyoming...which will push
into western reaches of the County Warning Area overnight...so have upped probability of precipitation in
the west late to account.

&&

Short term...this evening through Thursday night
issued at 225 PM MDT Wednesday may 27 2015

Closed upper low over the Pacific northwest will continue to slowly push
east in the period. A eastern periphery weak impulse/jet streak is
in the process of lifting NE out of Wyoming...supporting scattered-numerous
rain showers and ts across the forecast area. Jet streak will lift NE this
evening...with forcing and associated convection lifting NE. Hires
analysis fields indicate at or above 1000-1500 j/kg of cape mainly in a
north/south axis on the South Dakota plains. As convection lifts NE into
this area of higher instability...a few stronger storms can be
expected. Best chances/coverage of rain showers/ts will be across northwest South Dakota
where the best ll convergence and lsa will overlap. Ll surface trough
will shift southeast into the forecast area this evening...eventually stalling over
scentral South Dakota. Main upper low over the Pacific northwest is expected to drift
east overnight...eventually supporting an increase in lsa and
associated deep cyclonic flow...potentially supporting lingering
rain showers into the far western forecast area by late in the night. Dried most
places out after midnight...with a slight uptick to chance probability of precipitation
over the far west toward dawn. Deep cyclonic flow will spread over
the area and support increasing lsa over most of the region
Thursday. More than sufficient ll moisture in place/diurnal
heating/and enhanced convergence along a stalled surface trough will
support widespread connective showers over the forecast area...especially in the bh
where cin will be negligible. A few stronger storms will be
possible over scentral South Dakota ahead of the surface trough where ongoing
moisture transport in the warmer sector will support ml convective available potential energy of
around 1500 j/kg by afternoon. However...shear remains quite
limited with 20-25 knots prognosticated where the highest cape will be.
The main concern is locally heavy rainfall over the bh. Steep ll
lapse rates/high precipitable water/S over an inch/enhanced convergence per eddy
effects/and slow storm motion vectors will be in place over the bh
for much of Thursday afternoon. Would not be surprise to see a
widespread half inch of rain across much of the Black Hills which
could aggravate ongoing flooding concerns. A secondary trough will
shift south Thursday night and support an enhanced area of convergence
with rain showers chances continuing overnight. Kept probability of precipitation fairly high
through Thursday night...with a southward shift as the main trough
axis shifts south along with lsa.

Long term...Friday through Wednesday
issued at 225 PM MDT Wednesday may 27 2015

Upper level trough will shift east of the northern plains on Friday
as surface high pressure drops south out of Canada. Lingering
showers are expected on Friday...shifting south throughout the day
as drier air moves in. Cool high pressure will keep temperatures
slightly below average on Saturday...with a few showers and
thunderstorms possible in the afternoon as a weak wave passes
through. Thermal ridge will build into the region Sunday and
Monday...allowing for a warmup...with small chances of showers and
thunderstorms. Greater precipitation chances return for the middle
of next week as upper flow becomes southwesterly.

&&

Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 535 PM MDT Wednesday may 27 2015

-shra/-tsra will diminish this evening with VFR conditions
expected outside of precipitation. Additional -shra will develop
over northeast Wyoming later tonight and then expand over the area
Thursday...including increasing pockets of thunderstorms and rain. MVFR ceilings/visibilities
will be associated with the heavier precipitation.

&&

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

$$

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

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