Area Forecast Discussion
Issued by NWS Rapid City, SD
Issued by NWS Rapid City, SD
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819 FXUS63 KUNR 180515 AFDUNR Area Forecast Discussion For Western SD and Northeastern WY National Weather Service Rapid City SD 1115 PM MDT Tue Sep 17 2024 .KEY MESSAGES... - Dynamic negatively-tilted upper trough brings slight risk of severe weather later this afternoon/evening (mostly wind) - Elevated fire weather conditions through Wednesday - Low confidence long term forecast with cool weather potentially delayed until the weekend && .UPDATE... Issued at 725 PM MDT Tue Sep 17 2024 At this time, the threat for severe storms has diminished considerably across northeast WY and the Black Hills area, as temps have cooled and buoyancy has decreased behind an outflow boundary from earlier convection that is now located just to the east of the Black Hills. The main severe threat over the next few hours now appears to be from southeast of the Black Hills into portions of the west central SD plains, where warmer temps, along with better buoyancy and shear, linger ahead of the outflow. Gusts of 60 to over 70 mph are possible with the stronger storms, along with some small hail. Further west into the Black Hills area, far northeast WY, and northwest SD, cannot rule out some strong gusts to around 50 mph with some of the storms moving north across the area. Lingering severe potential should diminish toward late evening as atmosphere stabilizes further and main shortwave energy starts to move north and east of the area. Not much change needed to the forecast at this time, but will make needed adjustments as the evening progresses. && .DISCUSSION...(This Evening Through Tuesday) Issued at 228 PM MDT Tue Sep 17 2024 The latest analysis places the upper low over western Wyoming, producing southwesterly flow aloft over the forecast area. A surface low is centered over SE WY/WRN NEB/NE CO with breezy southerly winds over western SD. As the surface low moves northeastward into South Dakota early this evening the surface pressure gradient will tighten, increasing the winds over south central South Dakota. A wind advisory remains in effect there into this evening. The upper low will lift northeastward as it becomes negatively tilted this evening. A jet streak approaching the region from the southwest will deepen the surface low. A band of thunderstorms is expected to develop over northeastern Wyoming late this afternoon and accelerate eastward across western and central South Dakota through this evening. Initially, buoyancy will be limited with high-based convection anticipated over WY. CAMS continue to show a developing squall line pushing into western SD with 0.5-1KJ/kg MUCAPE available this evening, along with moderate deep-layer shear, which will enhance thunderstorm strength over western SD. The main severe threat will be damaging wind gusts, although marginally severe hail could occur. Activity will quickly exit to the east of the forecast around 06Z. Thee tightening surface pressure gradient to the east of the surface low will promote gusty southerly winds ahead of the front, particularly over south-central SD where chance of 45+ mph wind gusts 50%+, supported by Bufkit momentum transfer profiles. A wind advisory remains in effect through this evening over south central SD. On Wednesday, the stacked low will be located over southeastern MT resulting in a with tight surface pressure gradient over the forecast area. May eventually need a wind advisory for parts of western SD on Wednesday, but will hold off for now to reduce confusion with the expected strong to severe thunderstorm winds tonight. Backwash moisture lends credence to low pops along the WY/MT/SD/ND border areas. Temperatures will be near guidance. Thursday through next weekend, west/northwest flow aloft expected with periodic shortwave troughs bringing PoPs. Confidence is low with details as probabilistic guidance varying greatly. For now, the major cooldown that was thought possible has been pushed back to the weekend. The chance of >0.25" QPF during any 24-hour period in the long term is less than 30%. && .AVIATION...(For the 06Z TAFS Through 06Z Wednesday Night) Issued At 1115 PM MDT Tue Sep 17 2024 Showers and thunderstorms will continue to move north-northeast across northwest to south central SD, exiting the area by around 09z. Gusty winds will continue to accompany stronger storms. VFR conditions can be expected tonight and Wednesday, along with breezy/windy south to southwest winds in many areas. Some low level wind shear can be expected across at least the Black Hills area late tonight into early Wednesday, including KRAP, with stronger southwesterly winds just off the surface. Some wrap around showers are likely on Wednesday from far northeast WY to northwest SD as upper level system passes north of the region. && .FIRE WEATHER... Issued At 228 PM MDT Tue Sep 17 2024 Elevated fire weather conditions will continue through Wednesday. Strong/severe thunderstorms are possible beginning over northeastern Wyoming this afternoon, reaching the Black Hills during the late afternoon/early evening and then spreading eastward across the rest of western and south central South Dakota during the evening. Gusty southerly winds are expected ahead of the front, especially over south central South Dakota where wind gusts up to 50 mph are possible during the late afternoon/evening. The line of thunderstorms may be accompanied by strong to severe wind gusts as they move through. There is a good chance of wetting rain with any thunderstorms. Elevated fire weather conditions are expected through Wednesday due to gusty southwest winds. On Wednesday, the MinRH will average 20-30%, but areas of 15-20% are likely over southern SD. The cooldown for the end of the week looks to show up for the weekend. Thus, Thursday and Friday will be warmer and drier than previously thought. && .UNR WATCHES/WARNINGS/ADVISORIES... SD...Wind Advisory until midnight MDT /1 AM CDT/ tonight for SDZ044- 046-047-049-077. WY...None. && $$ UPDATE...26 DISCUSSION/KEY MESSAGES...10 AVIATION...26 FIRE WEATHER...10