KOKX AFD from this morning:
NEAR TERM /THROUGH TONIGHT/...
For this morning, weak shortwave energy may continue to produce
some showers mainly across Long Island and southeast Connecticut.
The support for the showers should push south and east shortly
after 12z as middle level energy passes. There may then be a
relative min in showers for the rest of the morning. Some clouds
will likely linger and there may be some patchy fog along the
coast.
Attention then turns to the potential for severe thunderstorms
this afternoon and evening as a cold front moves across the
region. A pre-frontal trough will move in ahead of the front
early this afternoon and will likely be the convergence
mechanism to initiate convection around 1-3pm well north and
west of the NYC metro. The convection should continue to develop
and organize into a line of storms that will move south east
across the area through the rest of the afternoon and early
evening. The actual cold front will follow quickly behind the
pre-frontal trough and could initiate a few more showers and
storms early in the evening before any lingering convection
pushes offshore.
The main threat from the thunderstorms will be from damaging
wind gusts in excess of 58 mph. There is also a risk of hail
and an isolated tornado cannot be completely ruled out. SPC has
continued to highlight a slight risk across the entire area. The
timing of the highest risk for severe thunderstorms appears to
be between about 3pm and 9pm based on the SPC experimental
timing graphics. The main uncertainty with the timing is whether
or not the pre-frontal trough convection will be predominate
and have the greatest chance at being severe or will it be with
the actual cold front. In either case, the earlier timing in the
aforementioned range favors the interior with the later
afternoon and early evening for locations closer to the coast
including the NYC metro and Long Island.
The ingredients for potential severe thunderstorms include an
unstable environment with MLCAPE values 1500-2500 J/kg and
SBCAPE values 2000-3500 J/kg. Some CAMS even indicate SBCAPEs
pushing 4000 J/kg, especially away from any maritime influence.
Bulk shear values increase through the day with an approaching
middle and upper level shortwave. There looks to be about an
average of 35-45 kt of 0-6 km shear, especially in the afternoon
and evening. Winds are mostly unidirectional, but enhanced
surface convergence from storm outflow could support a few
rotating updrafts and an isolated or brief tornado. The region
will also lie in the right entrance of a jet streak over
northern New England and southeast Canada, which should help
support the convection into the early evening.
Cloud cover to start the day should diminish in coverage late
morning and early afternoon, but there may some lingering low
clouds close to the coast. Even just a few breaks in the clouds
will bring quick surface heating and quick destabilization with
surface dew points in the lower 70s.
PWATs look to range from around 1.75 to around 2 inches with
subtropical moisture pooling along the approaching boundary.
While locally heavy downpours are likely, the flash flood risk
is low and isolated due to the relatively fast steering flow
and progressive nature of the convection. Minor urban and poor
drainage flooding is most likely.
Another concern with the convection will be from frequent
cloud-to-ground lightning. Forecast soundings show a decent
amount of CAPE in the -10C to -30C region of the profile which
typically is a sign that once the convection organizes there
will likely be frequent lightning. The CAPE in this zone also
supports the risk of hail.
Highs today will be in the middle to upper 80s for most with
potential of hitting 90 in urban NE NJ. Heat indices should
reach around 90 for much of the area with close to 95 in the
NYC and urban NE NJ corridor. The cold front will push south and
east of the area this evening and should be offshore after
midnight. Some showers/storms may linger near the coast after
9pm, but most of the activity should be ending thereafter. Much
drier and cooler air will begin working in behind the front with
dew points falling into the 50s and low 60s by early Monday
morning.
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