SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0753 AM CDT Wed Jun 09 2021 Valid 091300Z - 101200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF NORTH-CENTRAL AND WESTERN MONTANA... ...SUMMARY... The best-organized severe-thunderstorm threat appears to be over portions of north-central and western Montana, with damaging gusts and large hail expected. ...Synopsis... In mid/upper levels, a highly amplified pattern will cover the western and central CONUS, featuring a major northeastern Pacific trough that now extends southward from a cyclone off Vancouver Island. A strong, basal shortwave trough is apparent in moisture- channel imagery west of the central CA coastline, near 130W, which will pivot east-northeastward and inland during the later half of the period. By 12Z tomorrow, this shortwave trough should be located near the Sierra Nevada. Downstream, a weakening trough and accompanying broad area of modest cyclonic flow will contribute to an extensive area of general thunderstorm potential from the Mississippi Valley across the eastern CONUS. Any severe threat would be very localized and conditional, with potential too unfocused and marginal for a specific categorical area at this time. At the surface, 11Z analysis showed a cold front across ME to southeastern ON, becoming a quasistationary to warm front over Lower MI, through a low near MSN, to near MSP, then a warm front northwestward to an occlusion triple point over southeastern MB. A cold front was drawn from there over central SD, becoming quasistationary from there to a weak low near 2WX, southwestward over eastern/southern WY. The northern Plains frontal segment was diffuse amidst a broad area of convective outflow from last night's MCS activity. Additional surface cyclogenesis is likely across parts of western to southern MT from this evening onward, as synoptic-scale height falls spread over most of the central/northern Rockies after 00Z this evening. This should relocate/redevelop the synoptic frontal zone across northeastern/central MT this evening. A dryline -- initially from southeastern CO to far west TX - will mix eastward this afternoon to the eastern TX Panhandle and Permian Basin regions. ...MT... Widely scattered thunderstorms should develop late this afternoon and evening, initially over higher terrain of southwestern MT, then in areas northeastward across northwestern/north-central MT through the evening. Some of this activity should evolve into supercells, offering damaging gusts and large hail, and a tornado cannot be ruled out. Bowing clusters also are possible, locally augmenting the wind threat. A substantial easterly component of low-level flow, north and east of the cyclogenesis area, will advect higher theta-e into the region in support of convective potential. Though the source region over the western Dakotas and eastern MT includes a convectively processed boundary layer, surface dew points in the 50s should become common over northern MT, with advective and vertical-mixing processes roughly counterbalancing each other in maintaining a fetch of favorable moisture into the region. Still, diurnal heating that drives that mixing will also steepen low-level lapse rates and deepen the subcloud layer, beneath steep midlevel lapse rates of around 8 deg C/km. MLCAPE around 1000-1500 J/kg should develop. Meanwhile, gradually strengthening mid/upper flow and relatively backed near-surface winds will support effective-shear magnitudes of 45-55 kt near the international border -- less farther south into smaller buoyancy related to more vertical mixing. ...West TX/western OK... Isolated to widely scattered, slow-moving, high-based thunderstorms are possible for a few hours late this afternoon into this evening, in a narrow corridor near the dryline from the Big Bend/Davis Mountains north-northeastward to the eastern Panhandle/far western OK area. Severe gusts and large hail are possible, though coverage likely will be very limited. Strong directional shear is probable across the entire corridor, though wind speeds will be modest through most of the troposphere. The most confident area of convective initiation is in the southernmost part of the outlook, where diurnal heating of higher terrain will help to remove MLCINH relatively efficiently. On lower elevations farther north, intense heating is likely on both sides of the dryline, with surface temperatures in the upper 90s to low 100s F. Very deep/well-mixed boundary layers are expected, supporting potential for strong-severe downdrafts, as well as maintenance to the surface of any large hail. MLCAPE will be stronger in northern areas where low-level moisture is less mixed-out, but also, under stronger capping that may preclude development altogether. However, the conditional severe potential with any cell that can develop in the northern 1/2-2/3 of the corridor is strong. ...Northern MN/northeastern ND region... Widely scattered to scattered thunderstorms may persist through much of the day, and into the evening, across this area and northward bast the international border. Isolated damaging gusts and severe hail will be the principal concerns. A convectively reinforced shortwave perturbation aloft -- now over portions of southern SK, western ND and western SD -- is expected to shift northeastward, with the part over the CONUS deamplifying through the evening as it penetrates the larger-scale ridge over this area. The most-favorable large-scale and frontal lift will be north of the border; however, convection should develop near the front and in the weakly capped, prefrontal sector's warm-advection regime. Rich low-level moisture -- with surface dew points generally in the mid 60s to low 70s F -- will underlie 6.5-7 deg C/km midlevel lapse rates, in support of diurnally peaked MLCAPE around 1500-2500 J/kg. Lack of more-robust deep shear will limit organization -- with effective-shear magnitudes forecast only in the 25-35 kt range over most of the area. ..Edwards/Kerr.. 06/09/2021 Read more LIVE: spc.noaa.gov
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