Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service New York New York
726 am EDT sun Apr 30 2017
high pressure building into New England slowly moves east of the
area through tonight. A warm front moves northward through the
area tomorrow morning, followed by a cold front moving in from
the west Monday night and passing through the area by Tuesday
morning. High pressure builds in briefly for the middle of the
week. An area of low pressure will bring unsettled weather for
the end of the week into the weekend.
Near term /until 6 PM this evening/...
only minor updates needed to reflect current temperature and dew
point trends, as cooler air has been slower to advect southward
than previously forecast. Previous discussion follows.
A surface high will continue to build into New England beneath
confluence aloft between a departing trough and building high
pressure centered off the southeast coast. Onshore flow
gradually develops by afternoon, with a strengthening low-level
inversion and increasing cloud cover through the day. Despite
elevated instability, the strengthening subsidence inversion and
overall lack of forcing mechanism will limit any thunderstorm
chances. Highs will be much cooler than yesterday, even a few
degrees below climatological normals, follow this morning's cold
frontal passage and with the afternoon/evening onshore flow and
Short term /6 PM this evening through 6 PM Monday/...
continued onshore flow will lead to low-level saturation, with
increasing chances for light drizzle through the evening and
into Monday night, particularly as warm advection strengthens
ahead of the returning/modified cold front. Low temperatures
will be closer to climatological normals. The warm front is
expected to move north of the area by morning/early afternoon
with brief clearing possible behind the front. Meanwhile, the
upper high off the southeast coast will gradually shift eastward
ahead of a closed upper low. At this time, there is no clear
signal for a pre- frontal trough, though there is a possibility
of a convectively induced vorticity maximum moving through the
area ahead of the main upper trough that may be sufficient
enough to produce isolated afternoon thunderstorms, primarily
north and west of the city. Afternoon highs largely depend on
how quickly the warm front moves through and how much clearing
can occur following its departure, though highs will likely be
much warmer than Sunday.
Long term /Monday night through Saturday/...
the cold front moves through from west to east Monday night,
with rain and isolated thunderstorms clearing the area by
Tuesday morning. Low temperatures will be mild amidst cloud
cover and south flow, generally 5 to 10 degrees above
climatological normals. Flow becomes more westerly following the
frontal passage for Tuesday, and despite cold advection the
westerly component will be favorable for downslope warming which
may keep high temperatures a few degrees above guidance. Expect
above normal highs for Tuesday. By Wednesday, a subtle upper
trough and attendant cold front pass through the area by
afternoon/evening, with a reinforcement of cold advection and
temperatures closer to or slightly below normal. By late week,
upper difluence ahead of a developing system across the central
US will support moisture advection along the East Coast and the
intensification of a surface low across the southeast that will
gradually traverse northeastward into the region, leading to a
period of unsettled weather into the weekend.
Aviation /12z Sunday through Thursday/...
high pressure builds down from southeastern Canada today. This
high retreats to the northeast tonight, as a warm front
approaches from the southwest.
VFR through at least 20z, but will see 4000-5000 feet ceilings by
around midday. MVFR conditions develop by late afternoon/early
evening, IFR conditions are likely by late evening/around
midnight at all but kswf and kgon, where they should develop
after midnight. Airport minimums for ceiling and/or visibility could
possibly be reached late tonight/early Monday morning.
Winds continue to veer around to the east this morning, with
speeds generally around to just under 10 kt. Winds veer to the
east-east-southeast this afternoon, with speeds mainly around 10kt, with some
13-14kt sustained speeds possible at city terminals. Occasional
gusts to 15-20kt are possible this afternoon. Winds become east
throughout this this evening, with speeds 10 kt or less, then
become light and variable overnight.
Outlook for 12z Monday through Thursday...
Monday...IFR or lower likely through morning, with a chance
for improvement to MVFR or VFR in the afternoon.
Monday night...IFR or lower conditions, low level wind shear and scattered
thunderstorms are possible.
Tuesday-Wednesday night...VFR. SW-west-southwest winds g15-20kt possible
Tuesday. West-northwest winds g15-25kt possible Wednesday/Wednesday
Thursday...MVFR or lower conditions developing is possible.
a light to moderate pressure gradient over the waters through Monday
morning will limit winds to 15 knots or less. The pressure gradient
increases Monday afternoon, with winds up to 20 knots. Coastal ocean
waters could see some gusts up to 25 knots by Monday afternoon, with a
persistent onshore flow building seas to 5-6 ft over southern
portions of the coastal ocean zones on Monday as well.
The winds increase on Monday night, with solid small craft
conditions on the coastal ocean waters. As a result, have issued an
Small Craft Advisory for the coastal ocean waters for Monday and Monday night. On the
non-ocean waters Monday night, gusts to small craft levels possible,
but given limited mixing, confidence is not high enough to warrant
headlines at this time.
From Tuesday into Tuesday night, all waters likely will experience
Small Craft Advisory conditions with gusts of 25-30 knots.
The pressure gradient relaxes to light to moderate again by
Wednesday, and remains so through Thursday, limiting winds to 15 kt
or less. Small Craft Advisory level seas should linger on the coastal ocean waters
into Wednesday, then possibly return again by late Thursday ahead of
the next storm system.
no hydrologic impacts are expected through the forecast period.
Marine...Small Craft Advisory from 6 am Monday to 6 am EDT Tuesday for