Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 172007

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
207 PM MDT WED AUG 17 2016

.SHORT TERM...(This Evening Through Thursday Night)
Issued at 205 PM MDT Wed Aug 17 2016

Current surface analysis reveals a 1010mb surface low across Eastern
Montana. Position of low has CWA in warm sector with mainly
southeasterly surface flow being observed across the region. Weak
surface trough starting to push through Campbell County where some
isolated convection has already started to develop. Convection is
also firing off the Black Hills at this hour.

For rest of this afternoon and tonight...fairly narrow tongue of
instability, generally 1-2kJ/KG, develops along eastern fringes of
CWA through Northwestern South Dakota. High-res CAMs show convection
currently in higher terrain of Black Hills and Northeastern Wyoming
gradually moving toward the east where the higher
moisture/instability axis resides. Shear will be supportive of
organized convection, so cannot rule out a couple of strong to
severe storms late this afternoon and evening. Models keep isolated
to scattered storms across a good portion of the CWA this evening
ahead of a shortwave and strong cold front, so will keep precip in
the forecast for those areas this evening.

By Thursday morning, cold front will already be in portions of
Northwestern South Dakota and Northeastern Wyoming. This front will
move quickly across the region and should be nearly out of the CWA
by 00z Friday. Models prog strong forcing along the cold front and
should see showers and storms develop along/behind the cold front
Thursday. Interestingly, the best parameters for severe weather and
storms will be behind the cold front, where models project 1-2kJ/KG
of CAPE and 40kts of 0-6km shear, though mid-level flow is rather
weak. 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.

.LONG TERM...(Friday Through Wednesday)
Issued at 205 PM MDT Wed Aug 17 2016

An upper trough extending from the Hudson Bay low will affect the
forecast area on Friday. The combination of strong QG forcing and
steep lapse rates will result in widespread showers and
thunderstorms. Clouds and precipitation will keep highs in the
60s, 15-20F below average. The trough axis will shift eastward
into Minnesota on Saturday. Models continue to indicate a
secondary shortwave dropping southward into the ern MT/WY and the
Dakotas on Saturday afternoon creating scattered showers and
isolated thunderstorms. The upper trough will move into the Great
Lakes region Sunday as an upper ridge builds across the Rockies
into the northern plains. This will lead to dry weather and warmer
temperatures for the first half of next week.


.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 205 PM MDT Wed Aug 17 2016

Isolated to scattered showers/thunderstorms are expected across
parts of northeastern Wyoming into western South Dakota this
afternoon and evening. Local MVFR conditions are possible with the
precipitation, otherwise VFR conditions through tonight.


.UNR Watches/Warnings/Advisories...


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