Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 181732
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1132 AM MDT THU AUG 18 2016

.SHORT TERM...(Today Through Friday Night)
Issued at 326 AM MDT Thu Aug 18 2016

Upper level analysis has central CONUS ridge shifting south with
strong cyclonic flow developing across the northern CONUS. Strong
shortwave in this flow is moving southeast through central Montana
at the moment. Skies are partly to mostly cloudy across the
Dakotas with isolated to scattered convection moving west to east
across the area. At the surface, low pressure resides across north
central SD with cold front from western ND into north central WY.
Temperatures at 3 am are in the 60s and 70s, with light west to
southwest winds.

Today...Strong cold front will move into Northwestern South
Dakota and Northeastern Wyoming by mid morning. This front will
move quickly across the region and should be nearly out of the CWA
by late in the afternoon. Wide range in high temperatures expected
with low 70s in the west and upper 90s expected across south
central SD.

Models prog strong forcing along the cold front and should see
showers and storms re-develop along and behind the cold front as
it pushes south through the CWA. Best parameters for severe
weather and storms occur behind the cold front, where models
project 1-2kJ/KG of CAPE and around 40kts of 0-6km shear. Ahead
of the cold front, shear and instability parameters are rather
meager. Storm motion near the front is to the northeast, nearly
parallel to the cold front. Due to the expected storm motions, not
expecting much in the way of isolated convection, but rather a
potentially strong to severe line of storms behind the cold front
where damaging winds and hail would be the main threats. Might
have to monitor the threat for flash flooding as training storms
will be possible due to storm motions along cold front.

Tonight...Strong cyclonic flow remains across the area, with
showers and thunderstorms continuing across the area. Best chances
for precipitation will be across northeast WY and southern SD.
Much cooler air will move into the area behind the cold front,
with low temperatures falling into the upper 40s and 50s across
the CWA.

Friday...Strong upper level shortwave will dive south through
Montana and into western SD by midday. Widespread rain and
embedded thunderstorms are expected for much of the day along with
very cool temperatures. With widespread cloud cover and rain, high
temperatures will struggle to make it into the 60s, with 50s in
the Black Hills. Upper shortwave will move through quickly, with
precipitation diminishing Friday evening from west to east. Low
temperatures Friday night will fall into the 40s to low 50s across
the area. Could be some upper 30s in a few locations across the
higher elevations of the Black Hills.

.LONG TERM...(Saturday Through Wednesday)
Issued at 326 AM MDT Thu Aug 18 2016

Progressive but amplified flow expected in the period. Staunch
upper trough will depart the region Saturday with very dry air
advecting into the region behind this system. Strong cp push will
shunt deep moisture well south of the region, supporting dry wx
for most of the extended. Did remove pops for all areas outside of
the BH on Sat given more aggressive dry push under a subsidence
inversion. BL moisture does begin to increase by Tues of next
week, possibly supporting a shower or storm over the hills then.
However, the gfs and ecmwf are indicating a stronger northern
stream trough tue night into wed, which would shift a frontal
boundary south of the region in a stalling fashion. Perturbed
westerly flow may then support clusters of shra/ts invof the
boundary by late wed/thur next week...mainly across the southern
third to possibly half. As for temps, after a cool Sat, a warming
trend can be expected through tues, with cooler temps mid next
week on given the active amplified northern stream.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Friday)
Issued At 1129 AM MDT Thu Aug 18 2016

Showers and thunderstorms will increase in coverage through the
period with widespread precipitation expected over the area by
late tonight into Friday. Areas MVFR cigs over northeast WY will
improve some this afternoon, but then spread across the western
South Dakota plains tonight into Friday with local IFR CIGS/VSBYS
at times Friday. Local IFR conditions will occur with the
strongest thunderstorms, especially southeast of the Black Hills
late this afternoon and early this evening.

&&

.UNR Watches/Warnings/Advisories...
SD...None.
WY...None.
&&

$$

SHORT TERM...15
LONG TERM...JC
AVIATION...Helgeson



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