Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
946 am MDT Sat Aug 23 2014

Update...
issued at 945 am MDT Sat Aug 23 2014

Main shortwave ahead of the Idaho/Wyoming upper low is now crossing western
South Dakota. Subsidence behind the wave has ended precipitation across SW South Dakota...the
Black Hills...much of NE Wyoming...and scntrl South Dakota. No flooding issues have
been seen with area streams...thus have cancelled the Flood Watch.
The watch does remain for northwest South Dakota and portions of NE Wyoming where
heavier rainfall fell last night. Short term models show
additional storm development this afternoon and migrating eastward
tonight. Large scale flooding is not expected from this additional
precipitation later today. Instability will be relatively weak across
most of the County Warning Area...except 1000 to 2000 j/kg over south central South Dakota
later in the afternoon into the evening. Hrrr/NAM suggest a line of storms
pushing into scntrl South Dakota through this time period. Ll shear is
marginal...but with instability present...some of those storms
could end up being strong or even severe.

&&

Short term...today through Sunday night
issued at 318 am MDT Sat Aug 23 2014

09z surface analysis had frontal boundary extending across the County Warning Area
from southeastern Wyoming into north central South Dakota. Latest
water vapor imagery shows a digging trough across the Pacific
northwest as a potent shortwave moves northeastward from Colorado.
Radar imagery is showing widespread precipitation across the
forecast area. Latest blended total precipitable water products
indicate 1.0 - 1.5 inches. These precipitable water amounts combined with
east-southeasterly flow will continue to allow rain to fill
in through the morning hours. Current forecast precipitation amounts
of 1-2 inches still seem to be on track with some locally higher
amounts possible.

Overall model precipitable waters have trended back from previous runs with average
precipitable waters of 120 to 150 percent of normal. This seems reasonable...thus
expect the heaviest rainfall to continue through this afternoon.

Temperatures ahead of the frontal boundary this afternoon will reach
into the 80s. Forecast shear profiles and instability in south
central South Dakota could support supercells. With the moist airmass
in place will have to monitor conditions for the possibility of hp
supercells producing large hail and strong winds.

The system will become stacked late tonight over Montana/ND with dry air
intruding across the County Warning Area. Most of the rain will end across the
area tonight into Sunday morning as dry air works in from the
west-southwest. Colder air will advect in tonight bringing below
average temperatures on Sunday.

&&

Long term...Monday through Friday
issued at 318 am MDT Sat Aug 23 2014

On Monday morning the western Continental U.S. Trough will begin to move eastward
toward the forecast area. Given the residual moisture and
instability there will be a chance of showers and thunderstorms from
Monday into Wednesday morning. Temperatures will be 5-10 degrees
fahrenheit below average Sunday through Tuesday.

On Wednesday the trough will move east and a large-scale ridge will
move over the central/eastern rockies. This will persist into Thursday and
move over the County Warning Area...but with a general flattening. There is some
signal that one or two short-wave troughs will top the ridge across
the northern Continental U.S.. if this comes to pass there would be isolated rain showers/thunderstorms and rain
across at least the northern County Warning Area. Zonal flow with additional shortwave
troughs will be possible through Friday...thus do not expect the
weather to be completely dry. Nevertheless...Wednesday through
Friday should see a warming and drying trend.

&&

Aviation...for the 12z tafs through 12z Sunday morning
issued at 318 am MDT Sat Aug 23 2014

A strong low pressure system will bring widespread rain showers/thunderstorms and rain to the
forecast area this morning with the area of precipitation moving north in
the afternoon. Widespread MVFR/IFR ceilings/visibilities with stratus and some
fog will occur with the precipitation across much of northestern Wyoming and western
and central South Dakota...persisting through most of this morning. Conditions
will begin to improve from south to north late this
morning...although MVFR ceilings are expected to persist over parts of
northestern Wyoming and northwestern South Dakota through the afternoon and
overnight.



&&

Hydrology...
issued at 945 am MDT Sat Aug 23 2014

Flood Watch continues across northwest South Dakota as a potent
shortwave continues across the western South Dakota plains. Given the vigorous
upper level dynamics and the ample moisture and easterly low level
flow...moderate to heavy rain will continue through the early
afternoon...then decrease in the late afternoon hours.

&&

Unr watches/warnings/advisories...
South Dakota...Flood Watch until 6 PM MDT this evening for sdz001-002-012>014-
032-073.

Wyoming...Flood Watch until 6 PM MDT this evening for wyz054-056-071.

&&

$$

Update...13
short term...mls
long term...10
aviation...10
hydrology...13

National Weather Service Glossary of Abbreviations