Area Forecast Discussion
Issued by NWS Bismarck, ND
Issued by NWS Bismarck, ND
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649 FXUS63 KBIS 300902 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 402 AM CDT Sun Jun 30 2024 .KEY MESSAGES... - Isolated to scattered strong to severe thunderstorms are possible tonight, with up to a Level 2 of 5 risk. The main hazards are hail up to quarter size, and wind gusts up to 60 mph. - Breezy to windy conditions are expected today, especially southwest. - There is a Level 2 of 5 risk for severe thunderstorms Monday in southern North Dakota, and a Level 1 of 5 risk for much of the rest of the area. The main hazards are hail up to half dollar size and wind gusts up to 60 mph. && .DISCUSSION... Issued at 402 AM CDT Sun Jun 30 2024 Early this morning surface high pressure is centered across the eastern Dakotas, and is beginning to shift eastward while broad surface low pressure deepens over the Northern Rockies region. The result is increasing southeasterly low-level flow over the High Plains downstream of an upper-level trough moving across the Pacific northwest. Overnight IR satellite imagery reveals a band of cooler cloud tops aloft associated with a shortwave trough in eastern WA/OR, which will be a feature of interest for our convective potential tonight. For today, midlevel warm air advection will become pronounced as southeast flow increases. This warm advection pattern has already been manifest as a midlevel cloud deck and evidence of high-based sprinkles or showers in central MT overnight, and 00 UTC guidance suggests midlevel saturation in the developing warm air advection regime aloft will lead to midlevel clouds crossing southwestern and south central ND today. There is a very low chance of a shower with this setting, but confidence in that was not high enough to include a mention in the forecast. Otherwise, increasing winds will be the main story of the day as the surface pressure gradient tightens, with southeast winds increasing to 20 to 30 mph by afternoon over most of western and central ND. Winds will be strongest in southwestern ND where the surface pressure gradient will be strongest, but forecast soundings suggest peak speeds at the top of the mixed layer will only be around 30 kt, and thus forecast gusts will likely fall short of Wind Advisory criteria. Moreover, ensemble guidance including the ECMWF EFI all suggests the probability of advisory criteria winds is less than 50 percent, so we held off on a headline. Highs will be warmer today, ranging from near 70 in east central ND to the upper 70s in southwestern ND. Tonight the main concern will be potential thunderstorms, which are expected to develop over the higher terrain of central MT today as the aforementioned shortwave trough reaches that area. The low-level jet will increase significantly downstream of that shortwave trough tonight, with 40-50 kt speeds centered on the 850 mb level over much of western and central ND. Steep midlevel lapse rates will exist, all of which favors convection. However, low-level moisture return will be immature owing to the primary low- and midlevel frontal zone that marks the boundary between a recycled continental polar moisture profile and the rich maritime moisture regime initially being displaced well south, along the Kansas/Oklahoma border. Upstream observations at this writing accordingly have surface dewpoints only in the 50s F as far south as Kansas. This seasonably-tempered moisture return, combined with a comparatively-warm elevated mixed layer noted on 00 UTC Salt Lake City and Riverton soundings, is apt to yield significant inhibition/capping across western and central ND tonight. Forecast soundings call for 1000-1500 J/kg of MUCAPE for parcels originating in the 850 to 700 mb layer tonight, but with MUCIN generally larger than -75 J/kg. Thus, while we expect intense convection in eastern MT by this evening in a hotter, more deeply-mixed and less capped environment, there are medium to high odds that storms will weaken as they move into western ND after sunset. Nonetheless, a risk of damaging winds (up to 60 mph) and large hail (up to quarters) exists, conditional on the Montana convection developing sufficiently-deep cold pools and/or shear-related organization to sustain itself into the increasingly-capped air mass for some length of time into North Dakota tonight. Even if the intensity of storms wanes, the background environment will favor elevated showers and storms progressing eastward tonight, as most CAMs and global ensembles suggest, supporting 50 to 80 percent chances of precipitation tonight, highest in northwestern ND. Interestingly, by late tonight and Monday morning, the return of richer low-level moisture could become sufficiently-deep enough for parcels originating around the 850 mb layer to have reduced CIN. If the ascent associated with the passing shortwave trough and/or the weakening convection itself has not moved too far to the east, outpacing the richer moisture return, then there will be a window of opportunity for intensification of storms late tonight into Monday morning, from approximately 09 to 15 UTC. This risk is highly conditional, as it will be very sensitive to the low- and midlevel moisture profiles that will determine elevated parcel trajectories, and highly dependent on the timing of ascent aloft and residual convection that could have upwind/ upshear re-intensification on its back side if moisture profiles improve in time. This could result in a somewhat bi-modal timing of tonight`s risk of severe storms, and is sufficiently high for maintenance of the SPC Level 1 of 5 severe risk well into central ND. On Monday, another severe storm Risk (up to Level 2 of 5 from SPC) will exist as a weak cool frontal boundary moves eastward across western and into central ND during the day. Richer low- level moisture marked by dewpoints in the 60s F is forecast to reach the area, resulting in 1500-2000 J/kg of MLCAPE. However, the boundary layer may still have residual capping, dependent in part on how much stratus the low-level moisture advection generates, and whether or not warming is offset by the richer moisture. Early-day convection could also result in residual cloud cover. This all results in a wide envelope of possible outcomes, from little in the way of late-day storm development, to upscale-growing clusters of storms. The 00 UTC NAM has an especially cool and moist boundary layer with significant capping, likely owing to a known PBL parameterization scheme tendency, which suggests that its more lackluster convective environment is likely a low-probability outcome. The 00 UTC GEFS-based machine learning severe weather guidance increased its signal for Monday, especially in central ND, and effective layer wind shear of 30 to 40 kt supports multicellular to supercellular storms if initiation occurs. However, parallel alignment of shear profiles to the north-south-oriented surface front, and hodographs that have largely straight-line geometry, favor cluster-or upscale growing storms. Given the expected mode, and background uncertainty in convective initiation, we have chosen to message potential hazards of wind gusts up to 60 mph and hail up to half-dollar size, even though the CAPE-shear setting itself favors potential hail sizes of 1 to 1.5 inches or slightly greater. Any strong-severe storm risk will continue into Monday evening, but gradually shift eastward. Thereafter, persistent cyclonic flow aloft is still expected to prevail through the week with low to medium precipitation chances most days. Guidance continues to suggest a notable shortwave trough could cross the region on Independence Day, which is supporting 60 to 80 percent chances of showers and storms that day. Machine-learning severe weather probabilities are low through this period, though. Highs Tuesday through Friday will generally be in the 70s to lower 80s F, with small spread among NBM membership. By next weekend, roughly 60 percent of global ensemble members favor some magnitude of upper-level ridging developing, which could result in a step upward in temperatures, and results in signs of a drier period setting up. However, the other subset of ensemble members suggest lower heights aloft with somewhat cooler (but still seasonable) temperatures persisting longer. && .AVIATION /06Z TAFS THROUGH 06Z MONDAY/... Issued at 402 AM CDT Sun Jun 30 2024 VFR conditions are forecast to prevail through the 06 UTC TAF cycle, but a band of midlevel clouds is forecast to develop in southwestern and central ND Sunday as southeast winds increase. There is a low, 5 percent chance this pattern leads to a brief period of MVFR ceilings in southwestern ND Sunday morning, too. Otherwise, southeast winds will increase Sunday in western and much of central ND, with gusts of 25 to 35 kt (strongest in the southwest including at KDIK). Those gusty winds will continue into Sunday night. Storms will develop in eastern Montana by Sunday evening, but will likely just be reaching the North Dakota/Montana state line near the end of the 06 UTC TAF cycle. && .BIS WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...CJS AVIATION...CJS