Severe Storm Outlook Narrative (AC)
Issued by NWS
Issued by NWS
468 ACUS02 KWNS 201732 SWODY2 SPC AC 201731 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1231 PM CDT Thu Jun 20 2024 Valid 211200Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS BLACK HILLS VICINITY EAST INTO THE UPPER MISSISSIPPI VALLEY... ...SUMMARY... Scattered strong to severe storms will be possible from the northern High Plains into the upper Mississippi Valley on Friday, with a few strong storms over parts of the Northeast. A few severe gusts are also possible over parts of the eastern Great Basin. ...North-Central High Plains to the upper MS Valley... An organized cluster of showers/thunderstorms will likely be ongoing early Friday morning in the vicinity of southern MN along a west-east oriented frontal zone. It is possible a rejuvenation of this activity or additional storms on the leading edge of the outflow will continue to develop east along the warm front during the late morning into the early afternoon east of the MS River. Some model data suggest appreciable destabilization will occur ahead of this thunderstorm cluster, supporting storm intensification and severe risk. Farther west, diurnal storms likely develop over the high terrain of eastern WY into north-central CO, and likely produce hail and wind as they move across western and into central NE. Ample instability will remain overnight ahead of High Plains activity to support a hail and wind risk as it persists all night across SD and into MN. ...MT... A weak mid-level trough over the southern Canadian Rockies will pivot east and glance the MT/Canadian border. Weak easterly low-level flow will advect increasing moisture with upper 40s to lower 50s dewpoints beneath steep 700-500 mb lapse rates (7.5 to 8 deg C/km). Model forecast soundings show 1500-2000 J/kg MLCAPE developing by mid afternoon as isolated storms begin to develop over terrain-favored locales. Ample mid to high-level westerly flow will favor storm organization, including the possibility for a couple of supercells. Large hail and severe gusts will be the primary hazards with the stronger storms. A thunderstorm cluster or two will probably evolve by evening and eventually move downstream into eastern MT by the late evening. ...Lower Great Lakes into the Northeast... Models agree in showing a moist air mass and heating contributing to destabilization and renewed thunderstorm activity developing from eastern OH into southern New England. Forecast soundings indicate perhaps poorer midlevel lapse rates overall compared to the previous day, with more of a subsidence inversion around 700 mb. However, heating will easily support at least isolated development by early afternoon along the weak front. Isolated to scattered strong to damaging gusts (50-65 mph) will be the primary concern. ...UT...northern AZ...western CO into WY... Early Thursday morning observed upper-air raobs and surface data showed a very moist airmass has advected westward into the Four Corners. A mid-level trough initially over CA/NV will weaken as it moves eastward into the central Rockies by early Saturday morning. However, ample moisture and diurnally steepening lapse rates coupled with weak ascent associated with the disturbance, will support scattered to numerous thunderstorms during the period. Severe gusts (60-75 mph) are possible with the stronger downdrafts. ...Southeast AZ... Isolated to widely scattered thunderstorms are forecast to develop during the afternoon as temperatures warm into the 90s to the lower 100s. An appreciably moist airmass (1.25 inch PW; observed on the 12 UTC El Paso, TX observed upper-air sounding) and the development of 9.5 deg C/km 0-3 km lapse rates will favor isolated microbursts capable of severe gusts (60-70 mph) with the stronger evaporatively cooled cores. ..Smith.. 06/20/2024 $$