Area Forecast Discussion
Issued by NWS Grand Forks, ND
Issued by NWS Grand Forks, ND
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256 FXUS63 KFGF 270906 AFDFGF Area Forecast Discussion National Weather Service Grand Forks ND 406 AM CDT Thu Jun 27 2024 .KEY MESSAGES... - There is a level 1 out of 5 risk for severe thunderstorms across a portion of eastern North Dakota this evening. - There is a level 1 out of 5 risk for severe thunderstorms across the region Friday afternoon and evening. - There is a low chance for severe thunderstorms again Monday across the region. && .DISCUSSION... Issued at 406 AM CDT Thu Jun 27 2024 ...Synopsis... Ridging is currently in place, with a mid/upper low deepening in the Pacific NW. Pattern across the Northern Plains remains progressive however, and ensemble trends continue to show strong consensus on this general zonal/quasi-zonal flow to continue into next week. The variances will be on the timing, track, strength of each passing mid level trough and associated surface low/front evolution that causes variances in timing, location, and magnitude of any severe impacts or heavy rain. Temperatures vary with passing shortwave ridges/troughs with a tendency for below average temperatures at time (notably this weekend behind the trough on Friday. Each passing trough carries at least some potential for severe thunderstorms, with details tied to the mentioned uncertainties. ...Severe threat this evening... SW flow ahead of the wave in the Pacific NW is building into the Northern Rockies and several surface troughs/warm fronts to the west eventually will shift east towards eastern ND as a LLJ develops this evening. Current consensus shows a modest elevated instability axis (1000-2000 J/KG) developing into our west around 00Z but quickly diminishes this through 03Z due to the shifting surface pattern. The window for organized/severe convection seems to be tied to upstream activity arriving as the LLJ strengthens in the warm sector and instability remains high enough. The WAA/LLJ should support increasing coverage of showers/elevated convection, but if profiles are as narrow/weak as shown by consensus/mean HREF when upstream convection arrives then severe convection is much less likely. Slightly higher elevated CAPE lingering longer in our west or south (as a subset of guidance shows) would still support a lingering severe threat into parts of eastern ND, but activity should be very elevated. Primary threats remain hail, and wind where momentum transfer of heavy rain/elevated downdrafts can mix down higher gusts in the higher low level shear environment. ...Severe threat Friday afternoon/evening... Deformation zone is favored by latest trends to shift north through Friday morning towards the US/Canada border, lingering into the afternoon. A mid level dry slot is shown to allowing for some clearing across the southern Red River Valley into west central MN. This could aid in destabilization and there is a signal for SB CAPE to increase above 1000 J/KG in some of these areas. This will tend to be in a post frontal shear environment (backing low levels) but deep layer shear remains high (top heavy shear profiles). Still, due to the strong effective shear we could see supercell development carrying and larger hail threats. The two main scenarios based on current trends include elevated clustering or isolated supercells with larger hail to ping pong ball size (maybe larger), but less of a tornado threat. A less likely scenario (shown by one cluster of ensembles) brings better instability farther north where low level veering near the surface front and slower moving frontal zone could allow for a conditional tornado threat. ...Severe threat Monday.... Another mid/upper wave follows a very similar track and evolution as the system Friday, with a period of widespread showers or embedded thunderstorms. There is a signal for a deeper warm sector building into our region within most clusters, and higher probs for SB CAPE to exceed 1000 J/KG during the peak heating/diurnally favored period of the afternoon/evening. CSU GEFS based machine learning (15-30% severe probs) are supported by 100% chance for greater than 55 NBM CWASP and 50-60% probability for NBM CWASP values exceeding 65. That aligns with a threat for isolated to scattered severe convection. This is still farther out, so details/impacts are still inherently uncertain. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 1135 PM CDT Wed Jun 26 2024 Mid level stratus persists in the vicinity of KDVL, but is expected to gradually clear by around 10Z. VFR conditions then prevail through the remainder of the morning hours, with increasing cloud cover during the afternoon. Showers and scattered thunderstorms are expected to work into the area through the evening hours, then persist overnight. Went with the mention of VCSH for now until confidence increases regarding onset timing of better rain and thunderstorm coverage. Winds shift to the southeast by mid morning, then increase heading into the afternoon with gusts approaching 25 knots. && .FGF WATCHES/WARNINGS/ADVISORIES... ND...None. MN...None. && $$ DISCUSSION...DJR AVIATION...Lynch