Area Forecast Discussion
Issued by NWS Bismarck, ND
Issued by NWS Bismarck, ND
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751 FXUS63 KBIS 251402 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 902 AM CDT Tue Jun 25 2024 .KEY MESSAGES... - A few isolated strong to severe thunderstorms are possible late this afternoon and evening from far northwestern North Dakota through central North Dakota, including the James River Valley. Potential hazards include hail up to the size of quarters and damaging winds as high as 60 mph. - High chances for showers with medium chances for thunderstorms Thursday afternoon through Thursday night. Isolated to scattered severe storms are possible over this time period. - Below normal temperatures Friday through Sunday, with highs mostly in the mid 60s to mid 70s. && .UPDATE... Issued at 902 AM CDT Tue Jun 25 2024 Sunny skies are present over all but far southern parts of the state, and any rogue showers from earlier this morning have dissipated. SPC has expanded the marginal risk to north central and far northwestern ND, which makes sense as the environment isn`t much different from the south central as mentioned below in the full discussion. However, with low instability expected, the threat remains very conditional with a few isolated strong to severe storms possible. UPDATE Issued at 630 AM CDT Tue Jun 25 2024 Since 530 AM CDT, a rogue shower or two developed over southern Logan County and passed over Fredonia and Kulm into northwest Dickey County. Reflectivity has approached 50 dbZ at times, but no lightning has been observed. Additional stray showers cannot be ruled out across the far south this morning, but most areas will remain dry. && .DISCUSSION... Issued at 415 AM CDT Tue Jun 25 2024 Quasi-zonal flow aloft lies over the region early this morning, with surface ridging building from southern Alberta into eastern Montana and the western Dakotas. A weak mid level shortwave crossing the Northern High Plains with downstream theta-e advection over a zonal thermal gradient has generated widely scattered showers along the ND/SD border early this morning, with a couple thunderstorms just across the border on the South Dakota side. With SPC mesoanalysis still showing the northern gradient of MUCAPE extending into far southern North Dakota, we will keep the low chance of a thunderstorm in the forecast through early morning, but it is more likely that there will be no lightning on this side of the border. Any lingering showers should exit our forecast area to the south and east no later than an hour or two after sunrise. Quiet weather with a mostly sunny sky is expected this morning, with flow aloft beginning to transition to northwest as a Hudson Bay trough begins to dig into the Great Lakes. A trailing shortwave ejecting off the southern Canadian Rockies early this morning is forecast to reach northern parts of the state this afternoon. The northwest and north central will be one of the two focal areas for potential shower and thunderstorm development this afternoon and evening. The other focal area is from south central North Dakota into the James River Valley where forcing for ascent is much weaker, but greater boundary layer instability is forecast underneath a cyclonic northwest flow pattern. Multiple CAMs have simulated convective initiation in this part of the state later this afternoon, and even a few have produced UH tracks. But with only around 500 J/kg CAPE to work with and model soundings suggesting low EL heights, think it will be difficult for any updraft to tap into the 50-60 kts of deep layer shear that is forecast. Effective shear could still be as high as 30 kts though, which could support a few stronger to borderline severe storms, should convection initiate. The SPC has introduced at Marginal Risk (level 1 out of 5) for severe storms this afternoon and evening from Lake Oahe to the southern James River Valley. It should be noted that even though there is no forecast risk of severe weather across the north, boundary layer thermodynamics are only slightly less favorable there compared to the south, with similar shear and stronger forcing. That is to say, a few stronger storms cannot be ruled out with the arrival of the shortwave in northern North Dakota later this afternoon into the evening, and this could translate southeast through the James River Valley into the early overnight hours. As alluded to above, it will be around 5-10 degrees warmer across the south than the north today, with forecast highs in the mid 70s to mid 80s. It will also be breezy across the north, with northwest winds increasing to around 20 mph this afternoon and gusting as high as 30 mph. Later tonight into Wednesday, high pressure sliding down from Alberta/Saskatchewan will become the dominant synoptic feature for the Northern Plains, with the northwest flow aloft transitioning from cyclonic to anticyclonic. This will bring a period of much quieter weather to the region tonight through the day Wednesday, which may allow overnight temperatures to fall into the upper or perhaps mid 40s in the favored low-lying areas from Beulah/Hazen to Hebron/Glen Ullin to Mott and Hettinger. Highs on Wednesday are forecast to range from around 70 northeast to 80 southwest. A well-advertised Pacific Northwest trough is forecast to move ashore Wednesday night, amplifying an upper ridge over the Northern High Plains. This should allow the nocturnal low level jet to strengthen, which could result in scattered shower development across western and south central North Dakota Wednesday night into Thursday morning. MUCAPE is forecast to be very low to nonexistent (only a few hundred J/kg at most) over this time period, but steepening mid level lapse rates could promote a weaker thunderstorm or two. During the day Thursday, the Pacific Northwest trough will begin crossing the Northern Rockies, forcing the downstream upper ridge axis through the Dakotas. This setup should result in low level southerly flow advecting 60s surface dewpoints into the region while strengthening CIN under an EML. A lee trough is also forecast to develop near the MT/ND border Thursday afternoon. Lowering heights in response to the approaching mid/upper trough should help erode CIN along the lee trough, which seems like the most likely area for surface-based (or mixed layer) convective initiation late Thursday afternoon. There is still ensemble spread in the placement of the lee trough and tangential instability axis, which can be seen in ensemble cluster analyses that show two lower-membership clusters that contain nearly all GEFS members but very few members from other ensembles that have everything shifted farther east into western North Dakota. However, all clusters show an environment favorable for severe convection, with mean CAPE around 1000-2000 J/kg and deep layer shear around 30-40 kts, and deterministic models suggest values of both could end up higher. Deep layer shear and mean wind vectors orthogonal to the lee trough imply a discrete, supercellular mode could be favored initially, with upscale growth likely following outflow interactions and the arrival of stronger forcing. Even if surface-based convection fails to initiate or sustain, widespread showers and storms are still likely later Thursday evening through Thursday night with the arrival of strong DCVA attendant to the mid/upper trough. All hazards could be in play for discrete storms, including a tornado or two, though any tornado risk would likely be much smaller in space and time compared to the overall severe threat, and it could also be limited by both residual MLCIN and a shortage of streamwise vorticity per forecast hodographs. A transition to a linear mode would greatly favor a damaging wind threat over other hazards. The risk for localized flash flooding appears lower given the synoptic setup, but if any training convection develops, anomalously high precipitable water with warm cloud depths exceeding 3000 m could promote a heavier rain concern. The passage of the trough on Friday will drag much cooler air into the region, with below normal temperatures expected through the weekend. The NBM is focused on Saturday being the coolest day with highs only in the mid 60s to lower 70s, and Friday could be just as cool in the northwest. Lower chances for showers and thunderstorms remain in the forecast on Friday, but the risk for severe weather should be very low to nonexistent. The synoptic pattern for Wednesday through Friday could repeat itself almost verbatim on Saturday through Monday, but confidence in any potential for severe storms on Sunday is lower given the forecast time range, and ensembles do not favor below normal temperatures following the passage of this trough. && .AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 630 AM CDT Tue Jun 25 2024 VFR conditions are likely through the forecast period. However, there is some potential for MVFR ceilings to move into northern and eastern North Dakota late tonight. Introduced a scattered 2,500 ft mention to KXWA, KMOT, KJMS for this TAF issuance. A few showers and thunderstorms could develop from northwest to central North Dakota later this afternoon and evening, but the probability of impacts to aviation at any given terminal is low. Light winds early this morning will become northwest around 12-17 kts this afternoon, strongest across the north where gusts to around 25 kts are possible. && .BIS WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...Telken DISCUSSION...Hollan AVIATION...Hollan