SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1203 AM CDT Sat Oct 16 2021 Valid 161200Z - 171200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON ACROSS EASTERN PENNSYLVANIA...SOUTHEASTERN NEW YORK STATE...MUCH OF NORTHERN AND WESTERN NEW JERSEY AND ADJACENT PORTIONS OF MASSACHUSETTS...CONNECTICUT...LONG ISLAND...DELAWARE AND MARYLAND... ...SUMMARY... An organized cluster of thunderstorms may evolve this afternoon across parts of eastern Pennsylvania into southeastern New York state and New Jersey, accompanied by the risk for damaging wind gusts and perhaps a tornado or two. ...Synopsis... Models indicate that the mid-latitude westerlies will remain amplified, with flow remaining a bit more progressive across the Pacific into western North America than across downstream eastern North America into the western Atlantic, as blocking mid-level ridging centered near/southeast of Baffin Island maintains influence. Within this regime, a significant mid-level trough is forecast to approach the northern U.S. Pacific coast today through tonight, while broader downstream large-scale troughing (encompassing most areas east of the Mississippi Valley through the Atlantic Seaboard) only very slowly shifts eastward. In between, broad mid-level ridging will prevail, with an expansive area of cold surface ridging east of the Rockies, and generally dry/stable conditions being maintained in southerly return flow across much of the West. Surface cyclogenesis is forecast to proceed across western Quebec, as one or two significant short wave perturbations within the Eastern mid-level troughing pivot across and north/east of the lower Great Lakes and Ohio Valley. In response, a sharp trailing surface cold front is expected to advance east of the Appalachians through much of the Atlantic Seaboard by 12Z Sunday. It appears that a narrow plume of moisture along and ahead of this front will become supportive of sufficient instability to support a risk for thunderstorms across much of the Northeast. Some of these could become strong to severe, particularly in an area centered across the northern Mid Atlantic region. ...Eastern Pennsylvania/southeastern New York/New Jersey... Despite some early day mid/high cloudiness, latest model output suggests that a pre-frontal corridor of stronger daytime heating across eastern Pennsylvania into southeastern New York may become a focus for the development of modest boundary-layer CAPE by this afternoon. It appears that this will coincide with the northeastward progression of an intensifying southwesterly mid-level jet streak (including 50-70 kt in the 700-500 mb layer), beneath difluent upper flow, where strongest lift may become focused by late afternoon. This forcing for ascent, coupled with the strong deep-layer shear, may provide support for the evolution of an organized cluster or line of storms, which should tend to progress northeastward and eastward through early evening. Although weak lower/mid tropospheric lapse rates will not be optimal, eventually heavy precipitation loading should aid the downward transfer of higher momentum to the surface and contribute to the potential for damaging surface gusts. Given the expected convective mode, the risk for tornadoes remains more unclear, but forecast soundings do indicate that sizable, clockwise-curved low-level hodographs may develop. This may become conducive to the evolution of mesovortices with the potential to produce tornadoes, before outflow becomes more prominent. ..Kerr/Wendt.. 10/16/2021 Read more LIVE: spc.noaa.gov
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