Area Forecast Discussion
Issued by NWS Bismarck, ND

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000 FXUS63 KBIS 122054 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 354 PM CDT Wed Aug 12 2020 .SHORT TERM...(This evening through Thursday) Issued at 353 PM CDT Wed Aug 12 2020 The main concern for the short term period is chances for thunderstorms, some possibly severe, mainly across the northern half of the state this evening and tonight, shifting to the eastern half on Thursday. A transient upper level ridge is drifting across the Northern Plains this afternoon ahead of deep troughing that extends from the Canadian Rockies into the Pacific Northwest. A N-S oriented surface trough has sharpened over eastern Montana, but is not quite as far east as originally expected at this time. Water vapor imagery does suggest weak impulses embedded in the southwest flow over eastern Montana, and SPC Mesoanalysis shows no CIN in the vicinity of the surface trough. Current visible satellite and radar suggest convective initiation is underway along and ahead of the surface trough. The highest probability of development, sustainment, and greatest coverage of convection is over northwest North Dakota where better forcing is forecast. But isolated to scattered storms cannot be ruled out as far south as the South Dakota border. Storm intensity appears a little more uncertain at this hour compared to earlier in the day. A local minimum of 1500 J/kg SBCAPE is currently analyzed where convection is growing, and effective bulk shear is only around 20-30 kts. However, there are other parameters that are still favorable for severe weather, including mid level lapse rates near 9 C/km and DCAPE over 1500 J/kg. It is beginning to look like severe wind gusts might be a greater threat than large hail, with discrete supercells now appearing more unlikely. The most likely outcome is for storms to weaken or altogether dissipate as they move into north central North Dakota later this evening into tonight, but MUCAPE is actually forecast to increase near 3000 J/kg. Deep layer shear will likely remain meager though. Additional thunderstorms may develop over central North Dakota early Thursday morning as the surface trough drifts into the area in tangent with another piece of mid level energy. Most guidance clears western and central North Dakota of convection by early afternoon, at which time the surface trough is forecast to be in the vicinity of the James River Valley. Convective initiation may occur here during the mid to late afternoon, but will be discussed in more detail in the long term discussion. It will be slightly cooler and less breezy on Thursday, but perhaps a bit more humid over the eastern half of the state. .LONG TERM...(Thursday night through Wednesday) Issued at 353 PM CDT Wed Aug 12 2020 The main concern for the long term period is chances for severe thunderstorms Thursday afternoon through Friday morning. By Thursday evening, a surface trough is forecast to be located from the northern Red River Valley to southern James River Valley. Most models initialize convection along this boundary, but there are timing, placement, and coverage differences. An earlier (later) initialization time would favor a greater (lesser) threat in the southern James River Valley due to the eastward propagation of the surface trough. 12Z guidance came in with a stronger mid level capping inversion, which would favor a later and farther east start time. Extreme instability on the order of 4000-5000 J/kg CAPE is more than sufficient for severe storms despite perhaps only marginal deep layer shear of around 30 kts. Shear and mean wind vector orientation favors a messy multicellular/linear mode, but both large hail and damaging winds will be possible with any storm. There could be a tornado threat with any storm that remains more discrete given the presence of the surface wind shift, relatively lower LCL heights, and extreme instability. There is a possibility, albeit low, that training convection could develop over the southern James River Valley if low level moisture transport is strong enough to influence the orientation of the low level convergence zone. But this is likely contingent on a slower progression of these features, which recent models have trended away from. Back to the west, a synoptic scale trough axis with a strong vorticity max at its base is forecast to eject off the Northern Rockies by Thursday evening, resulting in convective initiation over the southeast quadrant of Montana. These storms will initially form in an environment with less than 1000 J/kg CAPE. But if convection is able to sustain itself, which it should given the amount of forcing and over 50 kts of effective bulk shear, it will encounter greater instability on the order of 1000-2000 J/kg MUCAPE as it enters southwest North Dakota late Thursday evening. Additional support for convective maintenance may be supplied by a low level jet/moisture transport response to the approaching shortwave. All things considered, including elongated hodographs, this setup has the potential to produce a strong damaging wind threat in the form of a bow echo/QLCS type system moving from southwest to east central North Dakota Thursday night into Friday morning. Confidence in this scenario is still medium at best, with the greatest source of uncertainty being the degree of instability. But it needs to be acknowledged that this potential exists, and 06-12Z guidance has trended more in this direction, particularly the last three consecutive runs of the NAMNest. Even if the instability is much lower than current consensus indicates, sub-severe showers and storms would still be expected over the same area and time. Behind the departing complex of storms, high pressure will begin to build over the region Friday afternoon. Cyclonic flow aloft may produce some isolated afternoon thunderstorms, and there may be enough instability for a few storms to become strong. Beyond Friday, the weekend into early next week looks relatively quiet, with surface high pressure and northwest flow aloft in between a Hudson Bay low and Rocky Mountains ridge. This pattern will produce a strong temperature contrast across the area, ranging from below normal (70s) northeast to near normal (mid 80s) southwest. && .AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon) Issued at 118 PM CDT Wed Aug 12 2020 VFR conditions are likely through the forecast period. A frontal boundary will bring a chance of showers and thunderstorms from western into central ND late this afternoon into tonight. There are better chances for storms across the north, so have kept a VCTS mention at KXWA and KMOT. Any storm could produce a brief period of MVFR to IFR visibility and sporadic wind gusts. Additionally, there is a low probability that MVFR to IFR ceilings may develop at KMOT late tonight into Thursday morning. Expect south-southeast winds around 15-20 kts through the afternoon, with some gusts around 30 kts in western ND. Winds will diminish to around 10 kts later this evening and turn northwesterly behind the frontal passage. && .BIS WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...Hollan LONG TERM...Hollan AVIATION...Hollan

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