Area Forecast Discussion
Issued by NWS Bismarck, ND
Issued by NWS Bismarck, ND
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190 FXUS63 KBIS 161958 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 258 PM CDT Sun Jun 16 2024 .KEY MESSAGES... - Windy this afternoon, especially in the northwest. - Widespread showers with isolated to scattered thunderstorms Monday through Tuesday. - Isolated to scattered severe storms possible late Monday afternoon through Monday night, with expected hazards of hail up to golf ball size and winds up to 60 mph. Very heavy rainfall is also possible with any storm. - Below normal temperatures expected this week, especially Tuesday and Tuesday night. && .DISCUSSION... Issued at 256 PM CDT Sun Jun 16 2024 A closed upper low at the base of a negatively-tilted trough is spinning northeast across the Canadian prairie provinces this afternoon, with a deeper trough further upstream over the Pacific Northwest. This places the Northern Plains under southwest flow aloft. The attendant surface low is located just slightly northeast of the upper low, placing the Northern Plains in the dry and relatively cool sector. The associated strong pressure gradient has maintained sustained winds around 30 to 35 mph and gusts to around 50 mph in northwest and far north central parts of the state. A Wind Advisory remains in effect for much of western and north central North Dakota until 7 PM CDT, but southern portions of the advisory are not expected to be as windy as it is to the north. The southwest flow aloft will become more energized tonight as the Pacific Northwest low begins to kick into the Northern Rockies. Most guidance projects widespread convection developing along and north of a thermal/buoyancy gradient draped across South Dakota and southern Minnesota. The northern fringes of this activity may push into the James River Valley late tonight into Monday morning, but even the most aggressive guidance with the northward extent of the MUCAPE gradient now keeps it just south of the ND/SD stateline. We have therefore ceased messaging an isolated severe threat for this forecast time period. There could still be a few rumbles of thunder in the southern James River Valley late tonight into Monday morning, and small hail is a reasonable worst case scenario. Increasing cloud cover, and to a lesser extent chances for rain, are in the forecast for Monday morning and afternoon. Confidence in the timing of PoPs in this part of the forecast is not particularly high given the broad but weak synoptic scale ascent. There are low chances for thunderstorms during the day Monday, but SBCAPE is nonexistent and MUCAPE will be slower to increase. From late Monday afternoon through Monday night and into Tuesday morning, showers should become much more prevalent across the entire state as a 700 mb trough approaches from the west while strong low to mid level moisture transport arrives from the south. Thunderstorm probabilities over this time period increase from northwest to southeast, with projected MUCAPE ranging from near zero in the northwest corner of the state to possibly as high as 3000 J/kg in the James River Valley. A quick examination of deterministic model soundings from Ashley to Jamestown shows there could be little to no convective inhibition atop the surface inversion layer (which could be nearly completely saturated with mist/drizzle/fog at the surface), so there is at least some potential for more robust convection. While shear at all layers originating from the surface is quite high, the values significantly drop off when taking into account that the storm inflow layer is forecast to be 1-2 km AGL, which could render effective bulk shear lower than 30 kts. Because the difference between a 1.5 km AGL and 2 km AGL storm inflow level could result in a substantial difference in effective bulk shear, which is within the realm of possible outcomes, we will continue to message golf ball size hail as the main expected hazard from the strongest convection. We did however lower our wind gust forecast to 60 mph, owing to the stability of a deeper, nearly saturated boundary layer with negligible DCAPE forecast at this time. Another hazard that bears monitoring with this system is the potential for very heavy rainfall considering a very deep warm cloud layer (LCL-FZL) and anomalously high PWAT. Showers remain in the forecast on Tuesday as the trough shifts into the Northern Plains, which should push the instability required for deep moist convection off to the east. An unseasonably cold air mass will also filter into the region, which combined with cloud cover could keep afternoon highs lower than 60 in western parts of the state. There seems to be a favored solution for surface high pressure developing near or a little north of the Black Hills Tuesday night. Should this occur under a mostly clear sky, southwest parts of the state could see overnight temperatures colder than 40, which is already being advertised by deterministic NBM output. A somewhat active southwest flow pattern is favored to persist from mid to late week, with a modest warm up back to mostly 70s for highs. The highest chances for showers and thunderstorms per NBM guidance are from late Thursday through late Friday, higher south than north. && .AVIATION /18Z TAFS THROUGH 18Z MONDAY/... Issued at 1227 PM CDT Sun Jun 16 2024 VFR conditions are expected through the forecast period. There is a chance of showers and perhaps a few thunderstorms across far southern North Dakota this evening through Monday morning. At this time, the rain is expected to remain south of KDIK, KBIS, and KJMS through the remainder of forecast period. Strong westerly winds will continue this afternoon, gusting as high as 40 kts in the northwest. && .BIS WATCHES/WARNINGS/ADVISORIES... Wind Advisory until 7 PM CDT /6 PM MDT/ this evening for NDZ001>005-009>013-017>019-021-031>033. && $$ DISCUSSION...Hollan AVIATION...Hollan