SPC Sep 13, 2017 1300 UTC Day 1 Convective Outlook

By Newsroom America Feeds at 13 Sep 2017

SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0750 AM CDT Wed Sep 13 2017 Valid 131300Z - 141200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Severe thunderstorms are not expected today or tonight. ...Discussion... The overall large-scale pattern will remain zonal on Wednesday, with the westerlies confined to along/near the United States-Canadian border. This mid-level pattern will be augmented, however, by two large, cut-off cyclones south of the main westerlies: one along/just off the coast of California and the other associated with the remnants of Tropical Cyclone Irma over the Ohio Valley. The western United States low will begin to move east today, ending up near southern Nevada by early Thursday. The remnants of Irma will move northeast during the day, nearing southwest Pennsylvania and western West Virginia by Thursday morning. At the surface, the remnants of Irma, located in the vicinity of western Kentucky this morning, will continue to slowly weaken/fill during the day as it moves off to the east-northeast. Cooling/moistening thermodynamic profiles will support thunderstorm potential ahead of Irma across the Northeast, but a continued decrease of the overall kinematic fields and weakening lapse rates will limit any severe potential. Another area of lower surface pressure will be located across the northern Plains in association with a weak front to the south-southwest of a low moving across southern Canada. A secondary low is expected to develop along the front over western South Dakota during the afternoon, with an attendant lee trough strengthening to the south during the afternoon. Modest low-level convergence within the trough may lead to a few thunderstorms by late day. Additional thunderstorms will be possible within the weak upslope regime to the north of the developing surface low across northeast Wyoming. In both regimes, surface dewpoint temperatures near or below 50F should limit CAPE and the overall severe potential. Farther northeast, along the weak front, a highly conditional hail threat will exist across northern Minnesota. Although most 00Z convection-allowing model guidance fails to initiate convection, at least some guidance does develop a thunderstorm or two. Additionally, more recent convection-allowing guidance, such as the last several runs of the HRRR, have trended toward the idea of isolated late-afternoon convection. If thunderstorms can develop, the overall environment will support a conditional severe hail risk. Here, steep mid-level lapse rates (approaching 9C/km per upstream 12Z upper-air sites) atop surface temperatures around 80F and dewpoint temperatures up to 65F will yield surface-based CAPE locally in excess of 2000 J/kg. However, confidence in initiation at this time, along with the small spatio-temporal nature of the afternoon threat, is too low to warrant 5 percent unconditional probabilities. Overnight, increasing warm-air advection to the north of the front will likely initiate additional, albeit elevated, thunderstorms. Most of these thunderstorms should remain along/north of the United States-Canadian border. Should it become apparent that more of these thunderstorms will occur on the United States side, marginal hail probabilities may be necessary for overnight. Elsewhere, thunderstorms will be possible across the far southeast United States, as well as from the Pacific Coast of central California east into the Rockies. Although a severe hail or thunderstorm wind report may occur within these broader areas, the overall large-scale environment in either area is not conducive to organized severe thunderstorms. ..Marsh.. 09/13/2017 Read more



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