Convective Outlook
SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1223 PM CDT Sat Aug 15 2020 Valid 161200Z - 171200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHWEST MN INTO PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS VICINITY...AS WELL AS ACROSS SOUTHERN AZ... ...SUMMARY... Clusters of strong to isolated severe thunderstorms are possible across portions of the central Plains into the southern High Plains and northern Texas. Additional strong storms are possible across southern Arizona. Locally damaging gusts and hail will be possible with the strongest storms. ...Synopsis... An amplified upper level pattern will exist over the CONUS on Sunday. An upper anticyclone centered over southern portions of the Great Basin and the attendant ridge extending well north into the Canadian Rockies. Meanwhile, an upper trough will be in place from the Hudson Bay vicinity south/southeastward through the upper Great Lakes to the Southeast. The northern branch of the eastern upper trough will pivot eastward across the upper Great Lakes vicinity toward NY and the Mid-Atlantic by the end of the period with the upper ridge persists across the West. At the surface, a cold front will extend from far eastern Lower MI south/southwest into northern AR and arcing northwest toward the OK/KS border. A secondary cold front will be shifting southeast across the northern Plains during the morning, reaching southwest MN and central NE by mid-afternoon. The boundaries will be the main focus for strong to isolated severe thunderstorm development across portions of the central/southern Plains Sunday afternoon and evening. Further west, a seasonally moist environment will exist beneath the upper ridge across parts of southern AZ. Strong heating will lead to afternoon thunderstorm clusters which could produce strong winds. ...Southwest MN into parts of the Central Plains vicinity... Thunderstorms are expected to develop along the surface boundary draped from southwest MN into southwest NE Sunday afternoon. A narrow corridor of low-to-mid 60s F dewpoints will be in place beneath modestly steep midlevel lapse rates. This will result in a corridor of moderate to strong instability ahead of the boundary. Forecast soundings indicated steep low level lapse rates, with LCLs near, to just above, 850 mb. As such, isolated convection will be rather high-based. Low to midlevel flow will not be particularly strong, but directional shear will provide effective shear values from 30-40 kt, supporting high-based supercells. Forcing for ascent will not be very strong, and only a narrow corridor of favorable conditions will exist from southwest MN into southwest NE/northwest KS. A few strong gusts and/or hail will be possible, but isolated coverage and somewhat short-duration of the threat will preclude higher probabilities. ...Southern High Plains into North Texas... A surface boundary will sag southward across OK/AR through the morning hours and extend from near the Red River, arcing northwestward through the TX Panhandle. Resulting east/southeasterly low level flow will maintain upper 50s to mid 60s dewpoints across the region, and moderate to strong destabilization is expected by afternoon. Thunderstorms are expected to first develop over higher terrain across NM and CO and shift eastward over the High Plains in a weak-shear environment. High-based cells will generally be capable of strong downburst winds given very steep, sub-cloud layer lapse rates. Given steep midlevel lapse rates and effective shear values around 25-35 kt, an occasional hail producing storm is also possible. Additional thunderstorm development is expected along the instability gradient and surface boundary east/southeastward from the TX Panhandle into northern TX. Thunderstorm clusters will mainly be capable of wet microbursts. Some guidance suggest a westward-propagating cluster could develop via storm mergers/outflow interactions. If this occurs, it could increase the threat for strong/locally damaging gusts. ...Southern AZ... High-based thunderstorms are expected to develop during the afternoon in a weak to moderately unstable airmass. Steep low level lapse rates, with inverted-v low-level thermodynamic profiles evident in forecast soundings, suggest strong outflow winds may occur with this activity. Hi-res CAMs suggest a westward-propagating line of convection could move across the lower desert vicinity during the late afternoon. Strong to isolated severe gusts could occur with these storms. ..Leitman.. 08/15/2020Read more
Updated 15 August 2020 | 12:30 pm
SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1129 AM CDT Sat Aug 15 2020 Valid 151630Z - 161200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS NORTHERN MN AND THE SOUTH-CENTRAL HIGH PLAINS... ...SUMMARY... Scattered severe storms will be possible across northern Minnesota and a portion of the south-central High Plains during the late afternoon and early evening. ...Northern MN... The left-exit region of an embedded mid-upper speed max over the Prairie Provinces will translate southeast across the Red River Valley by early evening. While boundary-layer dew points should largely hover around the mid 50s in the wake of convection and a cold frontal passage yesterday, cooling mid-level temperatures and robust surface heating should support MLCAPE of 500-1000 J/kg. Deep-layer vertical shear will be sufficient for a few supercells and multicell clusters during the late afternoon to mid evening, with a primary risk of severe hail. ...South-central High Plains... Weak low-level easterlies have yielded upper 50s to mid 60s surface dew points across the region. Surface heating beneath steep mid-level lapse rates will result in a plume of moderate-strong buoyancy at peak heating (MLCAPE of 2000-3000 J/kg), within a modest northwest flow regime aloft characterized by effective bulk shear increasing to near 30 kts. This will conditionally support a few transient supercells and multicell clusters. Storm initiation north of the Palmer Divide is highly questionable given upstream mid-level drying overspreading the region per water vapor imagery and minimal simulated convection in morning guidance. Greater confidence in convective development is off the Sangre de Cristos and Raton Mesa region. This activity should develop along the periphery of the buoyancy plume and its uncertain whether it can spread northeast towards the richer low-level moisture centered on southwest KS prior to nocturnal MLCIN increase. Convection may tend to spread southeast in the weaker shear regime of eastern NM and the western TX Panhandle. Otherwise, low-level warm advection may yield a cluster of overnight/early morning convection across southwest/south-central KS and northwest OK with perhaps a marginal severe risk. ...IL to the Ozarks... A weakly convergent cold front slowly settling southeastward should be the focus for isolated late afternoon and early evening thunderstorms. A confluent belt of modest 20-25 kt mid-level northwesterlies should overlie this portion of the front, with the greatest buoyancy expected from MO southwestward. Setup should support isolated damaging winds region wide, with some hail also possible over the Ozarks. ..Grams/Wendt.. 08/15/2020Read more
Updated 15 August 2020 | 11:38 am