WPC update—mod risk SW CT into NYC metro as expected.
12z suite of guidance has narrowed the corridor of expected heavy
rainfall that will transpire thanks to a prominent easterly fetch
event expected on the northern periphery of an inverted trof in-of
the northern Mid-Atlantic coast. The signal for a prolonged heavy
rain event has increased markedly over the past 24 hrs with the
current deterministic and ensemble mean guidance now pinpointing a
swath of 2-4+" rainfall footprint across the NNJ and the adjacent
NYC metro and southern NY state towards Poughkeepsie. 12z HREF
mean is approaching 3-4" for just the NYC metro which would cause
considerable problems due to urban sprawl and inability for
rainfall to penetrate the impervious surface. 12z HREF
probabilities are also depicting an upper threshold event within
some of the probabilistic signatures. 3"/3 hr rainfall probability
is exceeding 70% for the NYC area up through the exo-burbs outside
the metroplex. 1-2"/hr rainfall rate probabilities are also into
the 70-80% territory which is typically found in the most robust
setups, usually within a tropical moisture field which is what is
expected given the enhanced IVT signatures on global deterministic
correlating well with the axis of heavy rainfall. GFS/ECMWF PWAT
anomalies are both approaching the 1.5-2 deviations above normal
range focused in the corridor of higher forecast QPF. Instability
will be lackluster for the most part, but some theta-E advection
into coastal NJ, NYC, and Long Island will pose a lower-end
convective threat across the region which would only exacerbate
flash flooding concerns.
The biggest change outside the magnitude of rainfall is the
propagation of the main QPF field further inland thanks to the
advection of low-mid level moisture being forecast due to very
anomalous U-vector wind fields generated from our inverted trof.
Enhancement on the NW side of the precip field will be possible
Friday morning and early afternoon as the shortwave trough over
the Great Lakes pivots into the northern Mid-Atlantic and closes
off across northern PA. This would allow for a more dynamic
signature within the northern and eastern fringes of the
upper-level circulation creating localized banding structures over
the terrain as depicted by a few hi-res deterministic. This is one
of the reasons we see secondary and tertiary maxes within the
general QPF forecast from deterministic as banding structures will
be found during the event evolution. Local maxes between 6-8" will
be plausible across the outlined Moderate area and even within the
confines of the SLGT risk. This has evolved into a higher impact
potential and will be monitored closely for future updates to the
orientation of the MOD risk with coordination from the local FO's
involved.
Little sucker holed here with 1.62” (big surprise) and at 12.20” since September 8. Incredible. July and September over a foot of rain.
