Updated Upton discussion
Strengthening low pressure moves from the Ohio Valley this evening
to Upstate New York by late. LLJ helps push a warm front farther
north, with the entire forecast area in the warm sector. A trailing
cold front then reaches west of the Hudson towards daybreak. There`s
enough moisture and lift for chances of showers at any given point
tonight, but high res models continue to key on the main focus for
showers and thunderstorms after midnight. The main line of showers
and thunderstorms will enter western zones 06z to 09z, and then
quickly move across the NYC/NJ metro 09z to 12z, and then across
eastern zones shortly thereafter. The line will be forced by
convergence of a +3 to +4 standard deviation PWAT subtropical
moisture plume ahead of the front as well as deep layered lift
enhanced by upper jet dynamics.
Forecast rainfall amounts are generally 1/2 to 1 inch across the
region with locally higher amounts possible. Much of this rain may
come within the line of showers and thunderstorms. Minor urban and
poor drainage flooding is the main threat. The relatively fast
movement of the showers and thunderstorms should limit any flash
flooding threat, but WPC does have marginal to slight risk for
excessive rainfall from around NYC and points west.
SPC has placed portions of NE NJ, NYC, and the Lower Hudson Valley
in a slight risk for severe thunderstorms. This is where there is
some surface based CAPE progged by some models. The main threat
appears to be damaging wind gusts. The environment is highly sheared
with low CAPE, so there is potential for rotation, but the potential
is very low. A stronger low-level inversion should limit the severe
threat east of the Hudson River where instability is elevated.