FXUS63 KBIS 230923 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 423 AM CDT Sun Aug 23 2026 .KEY MESSAGES... - Isolated to scattered severe thunderstorms are possible across western and central North Dakota this afternoon and evening. Potential hazards include hail up to the size of ping pong balls, damaging winds up to 70 mph, and brief torrential downpours. - Conditional risk for isolated severe thunderstorms in central and eastern North Dakota Monday afternoon and evening. Potential hazards include large hail up to 2 inches in diameter and damaging winds up to 70 mph. - Temperatures generally favored to remain near average through next week. - Near-critical fire weather conditions could develop in parts of western North Dakota on Monday, and the far southwest on Tuesday. && .DISCUSSION... Issued at 422 AM CDT Sun Aug 23 2026 An upper level ridge axis extends from the Texas Panhandle through western North Dakota early this morning, with a shortwave ejecting off the Northern Rockies preceding a deeper closed low over western Washington. The shortwave will be the main focus for showers and thunderstorms developing in western and central North Dakota later this morning through this evening. A handful of models bring some weaker elevated convection into southwest North Dakota by mid morning, but there is better consensus for an increase in both coverage and intensity beginning in northwest North Dakota early this afternoon and spreading south and eastward through the late afternoon and evening hours. Showers and storms will develop over a boundary layer with a southerly, moisture-rich return flow, although there is some question whether an MCS over southwest South Dakota and northern Nebraska early this morning could impede moisture transport to some extent. That uncertainty aside, diurnal heating should contribute to CAPE increasing as high as 2000 J/kg with deep layer shear forecast to increase from 30 kts in the afternoon to 40 kts in the evening. There are some factors that could dampen the intensity of convection, including mid level lapse rates less than 6 C/km and a capping inversion that boundary layer mixing and cooling aloft from height falls may not be able to fully erode. Nevertheless, recent CAMs have certainly honed in on an outcome that includes a few severe thunderstorms over parts of western and central North Dakota from mid afternoon through this evening. A lack of a well-defined surface boundary and a mixture of shear and deep layer wind vector orientation to the dewpoint/CAPE gradient would seemingly point toward a multicell/cluster storm mode, which fits with CAMs' reflectivity simulations. Confidence in sustained supercellular structure is low, so have limited our most likely maximum hail size forecast to ping pong balls for now. If upscale growth can lead to more organized convection, damaging winds up to 70 mph could emerge as the dominant severe hazard, especially in the evening when 0-3 km shear could increase to 35 kts. Despite low LCLs and 0-1 km shear greater than 10 kts, model hodographs do not appear to be supportive of tornadogenesis. While instantaneous rainfall rates could be quite high with these storms, their forward propagation should limit the risk for localized flooding, with urban areas being a possible exception. Back to the west, the deeper closed low is forecast to eject off the Canadian Rockies this evening. Lee cyclogenesis is already underway from Alberta through central Montana, but the lee trough/cold front is not forecast to reach western North Dakota until later tonight. Models have shown mixed signals on whether strong to severe convection associated with the trough in central/eastern Montana this afternoon and evening will survive into western North Dakota later tonight. More recent guidance has trended to a lower probability of convective maintenance, which may be due in part to the increased probabilities of thunderstorms in North Dakota stabilizing the air mass in between the two convective systems. MUCAPE is actually projected to remain as high as 2000 J/kg through the night in western North Dakota, but model soundings reveal the most-unstable layer to be highly-capped. Depending on the timing of decaying convection, there could be some higher wind gusts sneaking across the Montana border around midnight. A conditional threat for severe storms continues in central and eastern North Dakota on Monday, but confidence in convective initiation is quite low. Even though several forcing mechanisms are forecast to be present, including mid level height falls/ DCVA, low level warm air convergence, and surface convergence along the frontal boundary attendant to the Canadian low, there may not be enough of an alignment of these features to ensure deep moist convection initiates, and there are a number of possible scenarios and combinations thereof being displayed by the suite of CAMs. First, there is potential for elevated morning convection moving across northern North Dakota ahead of the surface front in an area where stronger warm air advection on the mesoscale could overlap with stronger height falls as the Canadian upper low spins over southern Saskatchewan. This may or may not play a role in afternoon convection if it occurs. A capping inversion will almost certainly delay surface-based convection until late afternoon and early evening. Then it becomes a question of whether strong enough forcing will come into alignment for sustained deep moist convection. Despite the low probabilities for convective initiation and coverage, the threat ceiling may be higher than today, with CAPE around 2000-2500 J/kg and deep layer shear closer to 40-50 kts. Unlike today, there is a clear favoring of a discrete storm mode with shear and deep layer wind vectors perpendicular to the surface boundary. Model hodographs are long and mostly straight, but with enough curvature to support splitting supercells. The most likely maximum hail size forecast for Monday has therefore been increased to 2 inches in diameter, with the maximum wind gust forecast remaining at 70 mph on account of forecast DCAPE around 1000-1500 J/kg. Similar to today, the tornado risk on Monday appears to be low on account of modeled hodograph structure, despite low LCLs and modest low level shear. In non-convective forecast news, today will be seasonably warm and humid with highs mostly in the 80s and dewpoints rising into the mid 50s to mid 60s. It will also be a bit breezy with southeast winds around 15-20 mph. Monday is favored to be the warmest day of the upcoming week for most areas, with widespread highs around 85 to 90. The post-frontal air mass in western North Dakota will become very dry and could feature gusty northwest winds Monday afternoon, which may lead to near- critical fire weather conditions. The southern Canadian low, which will become nearly-stacked Monday night, is forecast to dive southeastward into the Red River Valley on Tuesday. This brings slightly cooler daytime temperatures and stronger northwest winds into the forecast as the surface pressure gradient gets squeezed between the low and high pressure building over Alberta and Saskatchewan. RH could be low enough in the far southwest for another afternoon of near-critical fire weather conditions on Tuesday. The NBM has also increased PoPs for diurnally-driven showers in the northwest cyclonic flow regime Tuesday afternoon, just in the Turtle Mountains area for now but could see that expanding with future forecast updates. The downstream low continues a southeastward trajectory toward the Great Lakes on Wednesday as surface ridging slides into the Northern Plains and the upper level ridge begins to reamplify over the Rockies. A deeper trough digging into the Pacific Northwest on Thursday could push the upper ridge back into the Northern Plains by the end of the week, with ensemble spread on the synoptic pattern increasing thereafter. Ensemble clusters now tend to favor more of a quasi-zonal flow pattern heading into next weekend, which is evident in deterministic guidance. But there is still some support for yet another reemergence of the upper level ridge somewhere between the Rockies and Great Plains. The potential quasi-zonal flow is reflected in the NBM returning daily low to medium shower and thunderstorm chances to the forecast from Friday onward. The temperature outlook for mid to late next week remains close to average, with highs mostly in the upper 70s to mid 80s and lows mostly in the 50s. && .AVIATION /06Z TAFS THROUGH 06Z MONDAY/... Issued at 422 AM CDT Sun Aug 23 2026 Chances for showers and thunderstorms will increase from west to east this morning through this evening. A few storms could be severe, particularly in southwest and south central North Dakota from mid afternoon through early evening. If a storm moves over a terminal, IFR visibility and strong wind gusts can be expected, with large hail and MVFR ceilings possible. A few showers and storms could linger into the overnight hours or re- enter western North Dakota late tonight. Outside of showers and storms, VFR conditions are expected. Southeast winds around 10-20 kts from this morning onward. && .BIS WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Hollan AVIATION...Hollan