Area Forecast Discussion
Issued by NWS Bismarck, ND
Issued by NWS Bismarck, ND
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521 FXUS63 KBIS 170237 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 937 PM CDT Mon Sep 16 2024 .KEY MESSAGES... - Severe threat is becoming confined to far north central North Dakota into northeast North Dakota. Key threats are damaging wind gusts, large hail, and very heavy rain. - An active pattern continues through the week, with strong to severe storms again possible Tuesday night. - Temperatures will trend cooler through the week, after Tuesday, with forecast highs in the 60s to lower 70s by next weekend. && .UPDATE... Issued at 929 PM CDT Mon Sep 16 2024 For late evening update have trimmed McLean and Bottineau Counties out of the watch. Have also trimmed down thunderstorm chances over areas either not affected or in the path of the ongoing convection. Only exception is a few showers/storms lifting through eastern Montana which may enter western North Dakota later tonight per latest CAMs, but chances for this appear low. For additional info on ongoing storms in north central North Dakota, please refer to last mesoscale discussion. UPDATE Issued at 757 PM CDT Mon Sep 16 2024 ** Mesoscale Discussion ** The severe-thunderstorm risk will be greatest over north central and northeastern ND the remainder of the evening, with a lower risk of severe storms as far south as the Highway 200 corridor. As of mid-evening a cluster of strong-severe storms has evolved east-southeast of Minot, north of a surface frontal zone/outflow boundary. These storms are elevated per trends in low-level CIN, but low-level moisture has been gradually increasing (surface dewpoints rising into the mid 60s F) downstream of the storms in McHenry, Pierce, and Rolette Counties. This is resulting in a deeper moisture profile that is sufficient for increasing MUCAPE on the order of 2000 j/kg for the elevated storms, and objective analysis suggests 30-35 kt of effective-layer shear ahead of the ongoing storms, favoring a continued large hail and transient damaging wind risk with storms as they evolve generally eastward the next few hours. An increasing low-level jet combined with the eastward advance of an upper-level vorticity max will both likely result in development of additional storms across north central and northeastern ND the next few hours. However, simple extrapolation of ongoing storms would have them exiting the Rolette and Pierce County areas by around 11 pm CDT. Upscale growth and storm interactions with the elevated storms will likely limit peak hailstone sizes, though the deep-layer shear and CAPE combination still supports quarter to golf ball size hail potential with the most-intense storms. Further south, congested cumulus in the Bismarck/Mandan area reflected what was only a weakly-capped boundary layer, but it appears background ascent has been insufficient for sustained convective initiation even as this agitated cumulus moved north closer to the surface front in eastern McLean, Sheridan, and Wells Counties. The boundary layer will gain appreciable CIN on the side of the frontal zone receptive to boundary-layer based storms with sunset, so the conditional odds of severe storms is low and diminishing quickly south of the Highway 200 corridor. This will also result in an end to the earlier-mentioned tornado risk, which was predicated on a surface-based storm developing in close proximity to the aforementioned surface boundary. CJS UPDATE Issued at 713 PM CDT Mon Sep 16 2024 Have issued a severe thunderstorm watch for most of north central North Dakota until midnight tonight. Otherwise, remainder of forecast remains in tact. Please see mesoscale discussion in previous update for details. UPDATE Issued at 516 PM CDT Mon Sep 16 2024 ** Mesoscale Discussion ** As of late afternoon, a quasi-stationary/remnant outflow boundary extends from southern Mercer county northeast toward Turtle Lake-Mercer in McLean County, to just north of Harvey, in southern Pierce County. Surface winds have veered slightly south of that boundary (e.g., at Bismarck and Linton), suggesting that low-level confluence is increasing in its vicinity. One-minute satellite imagery reveals shallow cumulus along that boundary, and earlier some orphan anvils focused in Mercer and Oliver Counties, but those have become less-focused in the last 30 minutes. Overlap of mid 60s F surface dewpoints and mid 80s F temperatures is greatest from northern Burleigh into Sheridan and Wells Counties, along and just south of that boundary, and reflects the relatively-greatest (albeit still only medium) probability for surface-based thunderstorms to develop before 00 UTC. RAP-based objective analysis has ~2000 J/kg of MLCAPE in that corridor and 30-40 kt of effective layer shear, favoring a mix of multicellular to transient supercellular storms if sustained initiation occurs. However, as mentioned, there are only medium probabilities of that sustained initiation occurring, which will be driven by whether or not sufficient ascent related to the main vorticity maxima aloft further northwest in the Minot vicinity overlaps with the buoyant sector on the warm side of the frontal zone. Hodographs are not very lengthy aloft, and deep-layer shear vectors oriented with a large parallel component to the surface boundary suggest that if surface-based storms develop they will likely grow upscale into messy modes after 1 to 2 hours. As a result, potential hazards remain large hail up to golf ball size (potentially largest in the first 1 to 2 hours of any surface-based development, before smaller sizes become favored with upscale growth), damaging wind gusts up to 60 mph, and a potential tornado owing to sufficient low-level SRH and ambient vorticity along with large low-level CAPE along the boundary. However, this is all conditional on sustained surface-based initiation occurring. Across north central ND to the north of the boundary, where surface temperatures are in the 60s to lower 70s F, storms are expected to be elevated in nature. Sufficient bouyancy (MUCAPE on the order of 1000 J/kg) exists for strong-severe storms, but effective-layer shear is weaker, on the order of 25 kt, which may marginalize the hazard magnitude in north central ND north of the boundary (including in places like Minot and Rolla). The forcing for ascent will be greater in those area, closer to the vorticity maxima aloft, so storm coverage will be greater to the north of the boundary, in north central ND, though. CJS && .DISCUSSION... Issued at 230 PM CDT Mon Sep 16 2024 Currently, large cutoff upper level low pressure centered over central California, with a broad southwest flow aloft pattern over the Dakotas. Well defined embedded wave continues to lift northeast across the region (now over mainly northeastern ND), along with all associated showers and thunderstorms north of I-94 and mainly across northern areas of the state. We remain fairly unstable this afternoon, with Mixed Layer CAPE near 2500 and MUCAPE up to 3000, especially for those areas that cleared this afternoon across much of the south. 0-6km wind shear is modest, around 30-40 knots. CAMs have been consistent (as the midnight shift last night pointed out) of most if not all convective activity across north central into the Turtle Mountains later today into the evening, along with some members showing signs of activity in the James River Valley. This is where the Slight Risk is maintained by SPC. Still, with the amount of unstable air, the amount of moisture, and the potential forcing with any subtle mid level wave lifting through, will keep at least a slight chance of thunderstorms all areas through this evening. S/WV ridge momentarily builds over the Northern Plains later tonight through most of Tuesday, as the aforementioned upper low starts to meander east-northeast into the Intermountain West. This will result in a warm and dry day for tomorrow, along with ample sunshine. Later on Tuesday and into Tuesday night, a warm frontal boundary will be lifting north across the region, as the upper low swings northeast into the Northern High Plains of eastern Wyoming and Montana. This will result in another round of showers and thunderstorms (some strong to severe) across the region during this timeframe. We will remain in an active upper level pattern through the work week, with multiple periods of showers and potential thunderstorms. There is also a very favorable probability for temperatures trending cooler, with highs in the 60s to lower 70s by next weekend. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 713 PM CDT Mon Sep 16 2024 Scattered to numerous thunderstorms this evening over north central North Dakota, including the KMOT terminal. Isolated thunderstorms possible over remaining areas, but have refrained from mentioning in individual TAFs given low confidence in timing or development. Later tonight low status is expected to move into north central and parts of northwest North Dakota, leading to areas of IFR ceilings through late Wednesday morning. By Wednesday afternoon, VFR conditions are expected across the area. && .BIS WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...CJS/JJS DISCUSSION...NH AVIATION...NH