Area Forecast Discussion
Issued by NWS Rapid City, SD
FXUS63 KUNR 210315
Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
915 PM MDT FRI MAY 20 2016
Issued at 912 PM MDT Fri May 20 2016
Thunderstorms are finally decreasing in intensity and coverage.
Made some adjustment to pops for the rest of the night.
.SHORT TERM...(This Evening Through Saturday Night)
Issued at 219 PM MDT Fri May 20 2016
20z surface analysis had southerly flow over the CWA ahead of lee
trough from central MT into central CO. 0.5-1KJ/kg MLCAPE has
developed along and ahead of boundary. MLCIN eroding over
northeast WY, but in better shape over SD plains. TSRA have
developed over the higher terrain as southeast boundary layer flow
converges over the northern and central Black Hills. Activity
having trouble coming off the Hills intact, but some returns
floating east- northeast on rather weak 850-300mb steering flow.
Water vapor loop showed main weather feature of interest,
spinning over northern CA this afternoon.
Tonight, convection should be largely diurnal in nature with
activity waning after sunset. Low level jet does return on the SD
plains overnight, but main theta-e advection shifts north of CWA.
With diurnal boundary layer cooling, suspect a stratus deck will
develop over western SD, eventually reaching the Black Hills and
creating patchy fog. Temperatures will be near guidance for minimums.
Saturday, shortwave ejects from upper low complex over the
northwest US into MT. Surface low develops along lee trough with
tightening pressure gradient and resultant moist flow. 1-2KJ/kg
MLCAPE develops ahead of low across southeast MT into northeast
WY. 0.5-1KJ/kg over western SD. MLCIN looks pretty staunch over
western SD until peak heating and terrain interaction. Convection
should develop along lee trough in the afternoon over northeast WY
with 15-20m/s 0-6km bulk shear. Slight risk of severe
thunderstorms with large hail and damaging winds will be main
effect. Wind advisory may be needed in northwest SD given gradient.
Temperatures will be near guidance, but perhaps be held down on the
SD plains if stratus doesn`t erode fast enough in the morning.
Saturday night, convection pushes east and trends downward as it
encounters less favorable atmosphere over western SD. Temperatures
will be slightly above guidance given cloud cover and dew points in
the mid 50s -- tropical compared to conditions so far this spring.
.LONG TERM...(Sunday Through Friday)
Issued at 219 PM MDT Fri May 20 2016
On Sunday, the upper low strengthens as it shifts northeastward
into the Canadian Prairie provinces, while a cold front slowly
slides through the forecast area. Chances for showers and
thunderstorms continue on Sunday as weak upper energy associated
with the low moves over the region. With dew points in the 50s
across central SD, MLCAPE is progged near 2000 J/kg. May be a few
severe thunderstorms there, as 0-6 km shear will be around 35 kts.
Some weak instability and lingering moisture may allow for a few
showers/storms around the Black Hills area on Monday; however, most
areas should remain dry. Persistent southwest flow with embedded
shortwaves, combined with marginal instability, will result in daily
chances of thunderstorms next week. Temperatures will be near to
slightly above average, with a slight cool down towards the end
of the week.
.AVIATION...(For the 00Z TAFS Through 00Z Saturday Evening)
Issued At 514 PM MDT Fri May 20 2016
Scattered thunderstorms are expected across northeastern Wyoming
and over far western South Dakota into this evening. Local MVFR
conditions are possible with the thunderstorms. An area of low
stratus is expected to develop over parts of western South Dakota tonight,
with some patchy fog possible along the eastern foothills. MVFR to
IFR conditions are possible in the fog.