Severe weather on Monday

As an intense upper level trough provides upper level divergence, a mid-level vorticity max provides cyclonic spin, and locally backed surface winds ahead of a cold front provide shear, an extremely moist surface layer will advect across much of New Jersey and New York. With dewpoints expected to range from the lower 70s to the mid 70s on Monday June 19th, ample fuel for destructive storms will be present and ready to be tapped into. NAMNSTMA_sfc_dewp_045.png

Around 21Z, favorable shear moves in from the west as the upper level trough moves overhead. In addition to the establishment of a LLJ off the coast, there will be a shear sweetspot across the region. A NAM 3km sounding shows the tornadic potential within the forecasted environment on Monday night at 21Z.2017061800_NAMNST_045_40.38,-74.51_severe_ml.png

With 1km SRH at only 69 m2/s2, however, more local backing of surface winds would be required to create some rotating thunderstorms across central New Jersey. This is the wildcard to be watched for. If winds become more easterly on Monday, the risk for rotating supercells increases significantly. Instability will be more than sufficient with MLCAPE values above 2000 J/kg and dewpoints above 70F. The LCL is very low, which is favorable for the formation of funnel clouds and then eventually a tornado.

The NAM 3km shows the possibility of a squall line developing late Monday afternoon over Pennsylvania, which would mean more straight line winds than anything. However, the risk for discrete supercells ahead of the squall line is certainly possible and can not be ruled out with the parameter space in play for Monday.