Scientific Forecaster Discussion
fxus61 kokx 280214
Area forecast discussion
National Weather Service New York New York
1014 PM EDT Thu Jul 27 2017
a weak cold front moves across the region from central New York
overnight before stalling just south of Long Island on Friday.
Waves of low pressure develop along the front Friday night into
early next week. By mid next week, the low pressure area moves
farther offshore with weak high pressure building in from the
west. Weak high pressure remains over the area until Thursday
of next week when a cold front approaches from the northwest.
Near term /until 6 am Friday morning/...
main update this evening was to lower the chance of showers and
remove mention of thunder. Although the 00z sounding showed
instability aloft, the convective inhibition around 750 mb may
prove to strong for any saturation/lifting from the weak cold
front. The isolated showers across the area have been devoid of
any lightning so far.
Otherwise, cloudy conditions and and increase in humidity will
lead to warmer temperatures than last night...with lows
generally in the low 60s to near 70 - closer to climatological
normals for this time of year.
Short term /6 am Friday morning through Saturday/...
the front settles just south of Long Island on Friday and have
gone with a dry, but cloudy, forecast. Over running sets up
Friday evening as cyclogenesis begins as shortwave over the
Great Lakes closes off. This cause rapid deepening of the
surface low over the Delmarva. The upper dynamics cause the
surface low to sag south as wave "spin off" to the east.
Main potential hazard is heavy rain. Having looked at a lot of
guidance, we have sided with the idea that the heaviest rain
stays south of our region and will not issue a Flood Watch - at
least not yet. In fact just saw the 15z sref and amounts are
Severe weather potential statement (hazardous weather outlook) continue to highlight for the
potential for flooding - but the potential is still in the low
See Hydro section to see rainfall forecast amounts.
Long term /Saturday night through Thursday/...
the upper level trough remains over much of the eastern U.S. For the
remainder of the weekend. The trough axis will slowly move east
during this time but will still be west of the local region. The
local region will be near the inflection Point of this trough. The
upper level jet stream affiliated with the trough will be situated
generally from the western Atlantic waters not that far offshore
northeast through Nova Scotia. This will help sustain the low
pressure along the stalled front at the surface. The region will
remain in a northeast flow as a result. The pressure gradient will
still be relatively tight Saturday night, making for gusty winds
mostly between 30 and 40 mph. Winds decrease Sunday into Sunday
night as low pressure becomes weaker south of Long Island, hence
weakening the pressure gradient. The winds finally switch in
direction mid to late next week as the upper level flow flattens and
low pressure moves farther offshore. More typical southwest flow
will come into place during this timeframe with weak high pressure
building in from the west. A cold front approaches from the
northwest next Thursday.
In terms of weather, cooler than normal temperatures and rain will
still be in the forecast Saturday night and Sunday, albeit mostly
light rain. Higher chances for the light rain will be across Long
Island and offshore while lower chances will be farther north. Rain
could even linger especially across Long Island going into Monday
and Tuesday of next week as some models convey a retrograding
movement back to the west of the offshore low. There are even some
slight increases in elevated instability, making for a slight chance
of thunderstorms on the east end of Long Island Monday night into
early Tuesday. Drier conditions and temperatures closer to normal
return Tuesday and more so for mid to late next week.
Aviation /02z Friday through Tuesday/...
a weakening cold front slowly crosses the area into Friday
morning. Low pressure begins to develop over the central
Appalachians by early Friday afternoon, then tracks into the
mid-Atlantic states by Friday evening.
VFR for a least the remainder or this evening (becoming VFR
kjfk), with ceilings lifting above 3500 feet. Could see some
patchy MVFR fog early Friday morning at kteb/khpn (with a chance
of IFR at khpn), otherwise VFR at city terminals after midnight
Elsewhere, mainly MVFR conditions (with pockets of vfr) become
MVFR throughout by midnight. IFR conditions are probable at
kgon/Long Island terminals and kswf overnight. Conditions
improve to VFR everywhere by around 12z Friday (at tad later at
Winds become light and variable throughout by around midnight.
NE-east winds at under 10kt develop mid morning-early afternoon.
Exception is at kgon where a southerly seabreeze is expected by
Outlook for 00z Saturday through Tuesday...
Friday night-Saturday...IFR developing with heavy rain and
possibly a tstm. NE winds g25-35kt likely Saturday, with
isolated 40kt gusts possible at Long Island/southeast CT terminals.
Saturday night-Sunday...VFR. North-East winds g15-30kt (strongest
gale watch in effect for Saturday. Tranquil though Friday
afternoon, then low pressure to the south increase NE winds
with gales forces gusts likely on Saturday. As a result, seas
build rapidly on all waters.
Active time period to start Saturday night with gales probable
across the waters due to a relatively tight pressure gradient. The
gales could even extend on the ocean into Sunday with otherwise Small Craft Advisory
winds Sunday morning. The Small Craft Advisory winds are forecast for most waters
Sunday afternoon and evening. Then for non-ocean waters, there will
be sub Small Craft Advisory conditions. For the ocean, both winds and seas are
forecast to be in Small Craft Advisory range late Sunday night through Monday
evening. Thereafter though, ocean seas are forecast to still be
meeting low end Small Craft Advisory with 5-6 ft values.
there is the potential for a heavy rainfall event Friday night
through Saturday, especially across portions of northeastern New
Jersey, New York City and Long Island. From Friday night through
Saturday night rainfall is expected to range from around 1/2 to
3/4 of an inch across the lower Hudson Valley and interior
southern Connecticut to 1 to 2 inches across northeastern New
Jersey and across Long Island.
There is though the potential for significantly higher amounts
if the surface low passes farther north than forecast. The
forecast will need to be monitored closely.
despite the expected strong northeast winds on Saturday, only
minor coastal flooding appears possible on the South Shore bays
of western Long Island at this time.
Marine...gale watch from Saturday morning through late Saturday night