Area Forecast Discussion
Issued by NWS Bismarck, ND

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000 FXUS63 KBIS 192018 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 318 PM CDT Sat Jun 19 2021 .SHORT TERM...
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(This evening through Sunday) Issued at 317 PM CDT Sat Jun 19 2021 The main concern for the short term period is a chance of strong to severe thunderstorms mainly over western North Dakota late this afternoon into tonight. Flow aloft over the Northern Plains is quasi-zonal early this afternoon as a mid level shortwave ejects off the Northern Rockies in advance of an upper level jet streak. Surface high pressure has shifted into the Red River Valley while a low sits over eastern Wyoming, with an inverted trough sharpening into eastern Montana. The approaching shortwave will force the strengthening of a W-E oriented baroclinic zone near the southern Saskatchewan border later this afternoon and drive it southward this evening. The resultant areas of low level frontogenesis, as well the surface convergence along the inverted trough, could provide enough forcing for convective initiation along the ND/MT border by the early evening. Showers and thunderstorms should then become more widespread with the arrival of the shortwave later this evening and track eastward across the state through the night. There is some uncertainty regarding the evolution and intensity of convection through the evening due to mixed signals in several forecast parameters. Guidance has shown varying degrees of CAPE and CIN, but there is consensus that the greatest amount of CAPE (as high as 1500 J/kg) will be located over southwest North Dakota. The likely presence of surface-based CIN per model sounding analysis introduces uncertainty on whether storms will be able to initiate prior to the arrival of the deep layer forcing, and also whether they would be rooted in the boundary layer or elevated. Think that elevated storms would be more probable farther north due to poleward decreasing MLCAPE and frontogenetical processes located closer to the top of the boundary layer. Surface-based convection appears slightly favored farther south, at least initially, where the inverted surface trough could be the main forcing mechanism in the presence of slowly falling heights aloft. However, with possible initiation not expected until around 23Z, and possibly occurring over far eastern Montana, the window of opportunity for surface- based storms appears to only be 2 to 3 hours at most before decoupling of the boundary layer commences. Regardless of whether convection is elevated or rooted in the boundary layer, favorable hodographs/shear profiles with 0-6 km bulk shear around 50 kts will support a severe threat for any storm that can ingest at least a few hundred J/kg CAPE. This threat should mainly be focused over the southern two thirds of western North Dakota, where shear vector orientation to the inverted trough and weaker deep layer forcing are more favorable for discrete supercells, and instability is greater. The main expected hazard is large hail. Damaging winds are possible given inverted-v soundings and around 30 kts of 0-3 km bulk shear, but DCAPE is rather unimpressive. If convection develops and remains surface-based longer than expected, increasing low level shear/SRH and lowering LCLs could result in a brief, localized tornado threat near the I-94 corridor around 01-04Z. But this threat carries a very low probability overall, and is highly conditional based on several parameters needing to align. The overall severe threat should diminish with time as eastward propagating convection encounters lower instability, but downstream shear will remain on the higher end of the spectrum. The larger complex of showers and storms should exit most of western and central North Dakota by late Sunday morning as an initial surge of cooler air advects in from the north. Breezy, if not windy conditions are expected Sunday with highs only reaching the mid 60s to lower 70s. Cyclonic flow aloft and diurnal heating may result in isolated to scattered afternoon showers ahead of a secondary cold front moving down from Canada. .LONG TERM...(Sunday night through Saturday) Issued at 317 PM CDT Sat Jun 19 2021 Surface high pressure is forecast to build over the Northern High Plains Sunday night as flow aloft remains cyclonic. This could result in chilly temperatures, especially if clouds are not present. The NBM advertises lows around 40 to 45 for most areas, but the typical cold spots from Beulah-Hazen to Hettinger could see temperatures fall into the upper or even mid 30s if radiational cooling can be maximized. This area will need to be monitored for a patchy frost potential Sunday night into Monday morning. Flow over the Northern Plains is forecast to remain cyclonic through Tuesday as the surface high shifts south and eastward. Deterministic models suggest streaks of cyclonic shear vorticity crossing the region Monday night, but guidance is failing to generate any precipitation in response, probably due to a lack of low level moisture and weak mid level lapse rates. Slowly rising heights over time will allow high temperatures to climb back to near average by Tuesday. But Monday will be seasonably cool, with highs once again limited to the mid 60s to lower 70s. Model consensus favors a mid-week transition to zonal flow aloft with a transient thermal ridge on Wednesday before swinging a deeper shortwave into the Upper Midwest by next weekend. Widespread highs in the 90s are possible on Wednesday, which could be followed by a slow cooling trend. There is large ensemble spread on whether the mid to late week pattern will bring shower and thunderstorm chances to the region.
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&& .AVIATION...
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(For the 18Z TAFS through 18Z Sunday afternoon) Issued at 317 PM CDT Sat Jun 19 2021 VFR conditions are expected through the afternoon. Isolated thunderstorms could develop over western North Dakota early this evening. By sunset, showers and thunderstorms are expected to become more widespread across western North Dakota and track eastward across the state through the night. The strongest storms may produce erratic wind gusts and hail. MVFR visibility and ceilings are possible under any shower or storm. A period of prevailing MFVR ceilings is likely Sunday morning behind the departing showers and storms, except at KXWA where a quicker return to VFR conditions is expected. North-northwest winds will increase to 15-20 kts late Sunday morning.
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&& .BIS WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...Hollan LONG TERM...Hollan AVIATION...Hollan

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