Area Forecast Discussion
Issued by NWS Grand Forks, ND
Issued by NWS Grand Forks, ND
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897 FXUS63 KFGF 271457 AFDFGF Area Forecast Discussion National Weather Service Grand Forks ND 957 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. && .UPDATE... Issued at 957 AM CDT Thu Jun 27 2024 Patchy dense ground fog has dissipated. Radar/satellite imagery indicates showers/embedded thunder continues to slowly propagate east/northeast, now located across south central North Dakota. This activity will be the main concern for today, and is being forced by elevated (700mb) warm air advection - with guidance also indicating weak MUCAPE near this level. Guidance indicates this forcing/instability will continue to slowly propagate northeast, across the valley by 6pm. The main uncertainty at this point is coverage of showers through today. This evening and overnight, attention turns to more widespread showers/thunder and the marginal severe risk (will continue to assess through the day). UPDATE Issued at 650 AM CDT Thu Jun 27 2024 Patchy dense ground fog has developed over parts of west central MN, with highly variable impacts. So far webcams and obs have been trending towards this improving quickly this morning. Showers and isolated/embedded thunderstorms are southwest of our CWA and spreading northeast (handled well by current HREF) and this is accounted for in the current forecast. This initial area of showers should spread into our area later this morning/towards midday and then transition north. && .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 /12Z TAFS THROUGH 12Z FRIDAY/... Issued at 650 AM CDT Thu Jun 27 2024 VFR is currently prevailing in eastern ND and most of northwest MN. Patchy ground fog in west central MN is resulting in variable flight conditions near KBJI, which could lift/clear out with daytime mixing quickly this next hour or two. A period of showers will arrive from the southwest midday in eastern ND and this progress northeast with lower chances for thunderstorms near the TAF sites this afternoon. Widespread showers and thunderstorms develop this evening and overspread the region along with MVFR and IFR ceilings. A few stronger or severe storms will be possible in ND (lower coverage and harder to anticipate impacts at this range). & .FGF WATCHES/WARNINGS/ADVISORIES... ND...None. MN...None. && .FGF WATCHES/WARNINGS/ADVISORIES... ND...None. MN...None. && $$ UPDATE...TG/DJR DISCUSSION...DJR AVIATION...DJR