Area Forecast Discussion
Issued by NWS Grand Forks, ND
Issued by NWS Grand Forks, ND
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636 FXUS63 KFGF 161818 AFDFGF Area Forecast Discussion National Weather Service Grand Forks ND 118 PM CDT Mon Sep 16 2024 .KEY MESSAGES... - There is a level 2 out of 5 risk for severe storms this evening. Damaging winds and hail will be the primary threats, with an isolated tornado possible. - There are multiple chances for rain and thunderstorms through this weekend. && .DISCUSSION... Issued at 1245 PM CDT Mon Sep 16 2024 ...Synopsis... 18z surface analysis indicates two pressure troughs across the region, with the main density gradient along outflow from this mornings thunderstorms. Thunderstorms are attempting to fire along a weak surface theta-e gradient in the James River Valley. These parcels are likely surface based, but as time goes on and storms track northeastward, they will likely quickly become elevated should they survive long enough. Severe chances are likely to develop later this afternoon into the early evening hours. The environment is favorable for primarily hail, damaging winds, and heavy rainfall. There is a low chance for an isolated tornado for surface-based supercells south of the front, but that probability heavily hinges on supercells arising. The pattern through the week will feature southwest flow, with multiple impulses of shortwaves. This will bring the potential for continued rainfall and thunderstorms through this weekend. Severe storms are possible but mesoscale features would need to be resolved to feel confident enough to up messaging. ...SEVERE STORMS TODAY... The main surface features to watch through the day today will be two theta-e boundaries, one that appears closer to the international border and the other from a remnant outflow boundary/reinvigorated by differential diabatic heating along the central Red River Valley. Guidance continues to be very strong in its assertion that this outflow will begin to act as a warm front and surge northward through the day today. While this is appearing to begin to occur now, there is high uncertainty in how far north this mesoscale feature will get. Synoptic forcing will be more than enough in northeast North Dakota given a solid CVA plume pushing into eastern ND and ageostrophic divergence at the apex of the ridge. For this reason, there is high confidence in thunderstorms arising north of US Highway 2. The probability decreases as you get further and further south away from synoptic forcing. It does appear based on initiating convection upstream that mesoscale forcing alone may be enough, so it does appear surface based convection will arise this afternoon. As storms generate north of the boundary, these are likely to be very elevated. Shear is more than enough for organization (generally effective bulk shear of 20-30 knots). The bulk of the shear is within the 1-3km layer, so 0-3km bulk is 30 knots. The main issue right now is how the convective mode evolves this evening. The two main scenarios that are possible are multicellular/transient supercells. This scenario would feature more potential for larger hail to golf ball size hail and heavy rainfall. The other scenario would be a more organized linear structure that is aligned south to north, but this is a much lower probability scenario. If this arises, 0-3km shear will be more than enough for damaging winds to make it to the surface despite the elevated nature. Further south, more surface-based thunderstorms are possible, but this will be further away from synoptic forcing. As such, there is high confidence that convection down here will be much more sparse as reflected by CAMs. The downside is that curvature within the lowest 3kms appears sufficient for potential surface-based supercells. Within the lowest 1km, there is solid streamwise vorticity to facilitate a tornado threat this afternoon if supercells can remain discrete. Thunderstorms will eventually transition to a heavy rain threat if linear structures perpendicular to 0-3km shear do not arise. Training thunderstorms would become fairly likely, especially if the frontal boundary can align much better with shear vectors (more southwest to northeast oriented front). Flash flooding is not likely to be a widespread threat but may arise in isolated locations. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 1201 PM CDT Mon Sep 16 2024 TSRA is the biggest impact for the TAF period, although location is very uncertain. The greatest chances are in the vicinity of GFK and DVL. Storms will be capable of 50 knot wind gusts and hail. Thunderstorms will exit the area by 10z for most, with the potential lingering at BJI through 16z. && .FGF WATCHES/WARNINGS/ADVISORIES... ND...None. MN...None. && $$ DISCUSSION...Perroux AVIATION...Perroux