SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0635 AM CST Mon Nov 16 2020 Valid 161300Z - 171200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... Severe thunderstorms are not expected across the contiguous United States through tonight. ...Synopsis/Discussion... The mid/upper-level pattern will amplify somewhat over the CONUS, while remaining progressive. Ridging now over the West Coast States will shift eastward to the Rockies and build through the period, while a large cyclone over the Gulf of AK shifts/redevelops southeastward toward the high-seas areas west of the BC coastline. As this occurs, a strong, basal shortwave trough -- now apparent in moisture-channel imagery near 145W between 30N-40N -- will pivot east-northeastward toward the northern CA and OR coasts while gradually losing amplitude. This perturbation should move ashore early day-2, but its low-level frontal zone and associated band of convection may reach parts of the coastline by 12Z. Conditions favorable for isolated thunderstorms are expected to remain offshore, behind this band. However, some thunder may occur within a precursory warm-advection plume today over the Olympic Peninsula and adjoining waters, where forecast soundings show sufficiently steep midlevel lapse rates above a moist layer near 850 mb to yield around 100-300 J/kg elevated MUCAPE. The eastern trough's broadly cyclonic flow regime will be maintained by a series of shortwave perturbations, following the departure of the strong shortwave trough now over central/southern QC and New England. The associated surface cold front was analyzed at 11Z from the western parts of the Canadian Maritimes southwestward over Atlantic waters offshore from the Carolinas, to north-central FL, and the north-central/northwestern Gulf. This front will move southward down the FL Peninsula through the period, decelerating overnight in southern portions. Associated convergence generally will weaken with time. However, diurnal boundary-layer moisture/ destabilization along and ahead of the front will weaken MLCINH enough for lift near the front to support isolated thunderstorms, mainly this afternoon into early evening. ..Edwards/Leitman.. 11/16/2020
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