Area Forecast Discussion
Issued by NWS Bismarck, ND
Issued by NWS Bismarck, ND
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222 FXUS63 KBIS 231407 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 907 AM CDT Thu May 23 2024 .KEY MESSAGES... - Scattered severe thunderstorms (risk level 2 out of 5) across southern North Dakota this afternoon and evening. Expected hazards are large hail up to 2 inches in diameter and damaging winds up to 70 mph, with a tornado or two possible. - Periods of scattered to widespread showers are expected today through Friday, especially tonight through Friday morning when some snow could briefly mix with rain. - Well below normal temperatures for most areas today through Friday night, with widespread overnight lows in the 30s. - Low to medium chances (20 to 40 percent) for showers and thunderstorms over the holiday weekend, with temperatures warming back closer to normal. && .UPDATE... Issued at 900 AM CDT Thu May 23 2024 Showers continue to lift northeast through central ND. A strong shortwave currently moving through southern MT/Nrn WY will be the focus for strong to severe storms this afternoon/evening across southern ND. At this time there are no significant changes from the previous forecast. SPC update this morning chances little from overnight. For this mornings update only some minor tweak to sky cover and pops based on latest radar and satellite imagery. UPDATE Issued at 645 AM CDT Thu May 23 2024 The forecast for today remains on track. Scattered showers will continue to move northeast across northern, central, and eastern North Dakota this morning. Much of western and central North Dakota, except for the far north, is likely to see a period of dry weather in the late morning before showers, and thunderstorms in the south, become widespread in the afternoon. && .DISCUSSION... Issued at 421 AM CDT Thu May 23 2024 A closed upper low at the base of a positively tilted trough sits over southern Idaho early this morning. Shortwave energy eminating from the trough combined with low level warm air advection has generated and maintained scattered showers across northern and central North Dakota. This activity is expected to move northeast through the morning, with the northern extent stalling near the Canadian border. Later this morning and into the afternoon, the upper low/trough will kick into Wyoming with rapid lee cyclogenesis forecast near the Black Hills. This should lead to an increase in shower activity (and eventually thunderstorms), especially over western North Dakota. The warm front attendant to the surface low is forecast to advance northward into far south central and southeast North Dakota this afternoon, bringing afternoon temperatures there into the 60s and 70s. Highs will otherwise be limited to the 50s today, and perhaps only upper 40s in parts of the northwest. The warm frontal boundary and adjacent warm sector will be the focal point for scattered severe thunderstorms this afternoon and evening. Latest HREF guidance brings mean SBCAPE around 1000-2000 J/kg from the Standing Rock Reservation to the southern James River Valley by mid afternoon. Some capping is noted in model soundings, and only a slight cooling of the mid levels is forecast. Due to the strong forcing at all levels of the atmosphere, the capping would not prevent convective initiation, but it could act to slightly dampen the threat ceiling. That threat ceiling could reach into significant severe territory, with effective bulk shear of at least 50 kts and very long hodographs with low level curvature. Confidence in the development of strong to severe convection is high, with all CAMs showing at least a few UH tracks of 100 m^2/s^2 or higher. Forecast confidence in storm mode is lower though, with shear and anvil level wind vectors pointing ENE and 850-300 mb mean wind vectors pointing NNE over a W-E warm front and a N-S dryline/surface trough. Thinking conceptually and pairing that with CAM reflectivity output, there would seem to be a slight favoring of an initial discrete mode followed by a possible evolution into linear/bowing segments. With higher deep layer RH acting to reduce dry-air entrainment of updrafts, a shallow depth of the LFC-FZL layer, and ELs extending to the tropopause, discrete storms are likely to produce up to 2-inch diameter hail given the CAPE/shear parameter space. Forecast DCAPE less than 1000 J/kg and the lack of a more focused linear mode point toward damaging winds being a lesser concern, but the 0-3 km shear forecast of around 35-50 kts has prompted us to increase our forecast wind threat to 70 mph. Finally, there is a small window both spatially and temporally for a tornado risk. The area of concern is most likely to be confined to along and just south of the warm front, with the highest HREF probabilities of STP exceeding 1 focused over Logan and McIntosh Counties. 0-500 m SRH increasing to 100 m^2/s^2, 0-1 km shear increasing to 10-20 kts, low LCLs, and a large streamwise vorticity component in modeled hodograph analysis are all contributing factors to the tornado risk. Aside from what has been discussed, there is also a lower severe storm risk north of the warm front from around the southwest corner of the state to the northern James River Valley. This would mainly be elevated convection with large hail as a primary hazard. The expected timing window for the overall severe threat is from around 3 PM to 11 PM CDT. The highest severe probabilities (risk level 2 out of 5) are located along and south of a line from near Mott to Bismarck to near Carrington. The surface low is forecast to deepen and lift east-northeast into the southern Red River Valley through the night. After evening convection pushes east of the forecast area, the highest precipitation chances will focus on the wrap-around deformation band, which is forecast to be located from north central to southwest North Dakota around midnight, shifting to the Turtle Mountains/ Devils Lake Basin to near Bismarck around 7 AM CDT Friday morning. Forecast temperatures under the axis of highest precipitation rates have risen by a degree or two, which has lowered the probability of snow mixing in. We are still carrying a brief period of a rain/snow mix during the late night and Friday morning hours, per NBM precipitation type probabilities. The highest chance of snow mixing in is now located in the Turtle Mountains. Temperatures will be unseasonably cold for most areas tonight through Friday night. Overnight lows will mainly be in the 30s both of these nights, and frost could be a concern behind the departing low Friday night. Highs on Friday will be highly dependent on lingering showers and residual cloud cover. The Turtle Mountains will likely be the coldest part of our forecast area, where highs could actually struggle to climb out of the lower 40s. In contrast, the southwest corner of the state could see highs in the lower 60s, which is still around 10 degrees below normal. A lower amplitude shortwave trailing the departing low could bring some showers and perhaps a few thunderstorms into western North Dakota Friday afternoon and evening. While forecast CAPE is only a few hundred J/kg, effective bulk shear around 35-45 kts could support a stronger storm or two. Warmer temperatures are expected over the holiday weekend, but it will remain cooler than normal with highs mainly in the 60s and lows mostly in the 40s. North Dakota is now forecast to lie in more of a split flow regime, with the strongest forcing to our north and south. There could still be enough forcing and instability for some showers and thunderstorms at times, but aside from Sunday afternoon and evening when the left exit region of an upper jet is forecast over the area, NBM PoPs have lowered to around 10 to 20 percent at most. A more appreciable warm up is finally favored by the middle of next week, with ensemble mean height fields showing ridging over the Northern Plains. This would coincide with a couple days break from shower and thunderstorm chances until later in the week when ensembles hint at a transition to southwest flow aloft. && .AVIATION /12Z TAFS THROUGH 12Z FRIDAY/... Issued at 645 AM CDT Thu May 23 2024 There are multiple aviation hazards this forecast period as a strong low pressure develops southwest of North Dakota. Scattered showers will remain possible across northern, central, and eastern North Dakota this morning, including KXWA, KMOT, and KJMS. Showers will then become more widespread across all of western and central North Dakota this afternoon and evening, along with thunderstorms across the south. Some storms could be severe with large hail and damaging winds as high as 55-60 kts. Significant visibility restrictions can be expected with any thunderstorm, and prevailing MVFR/IFR visibility is otherwise possible under showers. Ceilings are forecast to lower to MVFR/IFR levels from north to south through the day,with improvement at KXWA later tonight. Northeast winds will increase to 15-20 kts today, with gusts to 25-30 kts. && .BIS WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...TWH DISCUSSION...Hollan AVIATION...Hollan