Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service New York New York
1056 PM EDT sun Jul 23 2017
a frontal boundary remains to our south tonight. Low pressure
then moves along the front, tracking off the mid-Atlantic coast
Monday. High pressure builds over the northeast late Tuesday
through Wednesday, then drifts off the coast Wednesday night.
Another wave of low pressure follows for Thursday into Saturday.
High pressure returns Sunday.
Near term /until 6 am Monday morning/...
precipitation probabilities look to be on track for timing later
this evening. Otherwise, updated temperatures and dew points,
for trends and current conditions. Also, with cloud cover
overcast or nearly overcast, and remaining into Monday night,
increased coverage to overcast into late Monday night.
A stationary front will remain to the south of the County Warning Area tonight.
A weak area of low pressure develops along this frontal
boundary and slowly tracks northeast, situated near the New York
City Metro area late tonight into early Monday morning. This
will be the focus for showers to start off with, but a more
widespread rain will develop as the low approaches. There is a
period of heavy rainfall possible, generally from 2 am to 8 am
for New York City and points west and from 4 am to 10 am for
points east. All the deterministic models have increased quantitative precipitation forecast
from their previous runs, and in particular, the 12z GFS had
twice as much quantitative precipitation forecast from 2 am to 8 am Monday. There is also the
possibility of 1 to 2 inch rainfall rates in the Metro area as
per the ncar ensemble, which shows a 50-70% chance of seeing
rainfall rates greater than 1" at some point overnight, from 4
am to 6 am, just before the morning rush. Thereafter, a period
of moderate rain is still possible. Also of note, the 12z European model (ecmwf)
had a bulls eye of over 2" of rainfall just north of the city.
Therefore, a Flash Flood Watch was issued for all of New York
City, western Long Island, northeast New Jersey, and portions of
the lower Hudson Valley. The rest of the County Warning Area has a chance of
seeing flash flooding, however, there is more uncertainty with
where the axis of heavy rain will be for these areas, so did not
issue a watch for these areas. This will need to be looked at
Storm Prediction Center has placed portions of New York City and northeast New
Jersey in a marginal risk for severe thunderstorm through 8 am
Monday. However, stability does not look impressive with this
low, and did not mention any severe, though an isolated severe
is not out of the question.
Short term /6 am Monday morning through Monday night/...
low pressure will pull away from the area Monday afternoon. This
will allow rainfall to taper off from west to east from Monday
morning into the afternoon. However, chances for rain continue as
the upper level trough remains over the area through Monday
night, with just a slight chance for the entire area from 3 am
Onward Monday night. Rainfall amounts will not be nearly as high
as for Monday morning.
Storm Prediction Center has placed much of the cwa, except southwest Connecticut in
a marginal risk for severe thunderstorm Monday into Monday night.
However, stability does not look impressive with this low, and
did not mention any severe, though an isolated severe is not out
of the question.
There is a high risk for the development of rip currents at
ocean beaches Monday.
Long term /Tuesday through Sunday/...
the extended forecast mainly remains consistent with the
previous forecast with the exception that precipitation is
expected remain into Saturday as was hinted at by the European model (ecmwf) late
Saturday, as an upper trough is slower to move east.
The northern stream flow along the US/Canadian border remains
nearly zonal and progressive with embedded shortwaves moving
through the flow.
One shortwave will be exiting the northeastern coast, along
with the accompanying surface low, Tuesday. Cape and instability
remain limited so will continue with showers.
A weak ridge axis builds into southern Canada from the southern
ridge over the southwest and moving into the plain states mid
Another shortwave was coming onshore of the Canadian Pacific
coast as seem on the upper level water vapor loop. And these
system has been picked up by the models. This shortwave open and
tracks quickly across southern Canada then digs and large trough
along the eastern United States late in the week into the weekend
as a western North Atlantic ridge builds and blocks the eastern
trough. This will keep waves of low pressure tracking along a
frontal boundary Thursday into Saturday before the last one exits
later Saturday. So now have chances of precipitation through
Friday into Saturday.
The upper trough weakens Sunday as surface high pressure builds
to the north resulting in a dry Sunday.
Temperatures through the extended period will be near seasonal
normals, except for being below normal Tuesday.
Aviation /03z Monday through Friday/...
a stationary front will remain to our south into Monday
morning. A wave of low pressure will move along the front,
passing south of the area early Monday.
Conditions become MVFR at all terminals by late evening/early
morning. Showers will overspread the area, mainly after
midnight. In addition, an isolated rumble of thunder cannot be
ruled out during the overnight hours as well, with the best
chance from the city terminals on N/W. IFR conditions are
probable by around sunrise, then should improve back to MVFR by
Winds become east throughout by early morning at around 10-15kt.
Gusts to around 15-25kt are probable overnight for CT/Long
Island terminals/klga and briefly at kjfk. Widespread 15-20kt
gusts are probable throughout by mid Monday morning, as the
winds back to the NE.
Amendments may be needed for fine tuning of MVFR to IFR cigs and
any heavier rain with embedded thunder.
Outlook for 00z Tuesday through Thursday...
Monday night-Tuesday...MVFR likely. IFR, NE-east winds g15-25kt
and isolated-scattered showers are possible.
Tuesday night...MVFR likely. IFR and NE winds g15-25kt are possible.
Wednesday-Thursday...VFR. There is a chance of showers and possibly
a thunderstorm mainly to the north/west of city terminals Thursday
Thursday night-Friday...MVFR or lower conditions and isolated to
scattered showers and thunderstorms are possible.
winds and seas forecast on track.
Small Craft Advisory waves of 5 ft are possible over the ocean waters tonight,
generally after 10 PM as a persistent easterly flow allows waves
to build. Winds gust of up to 30 kt are possible over the ocean
after 2 am, and the sound, Peconic Bay, and gardiner's bays
after 7 am Monday morning as a weak area of low pressure
develops over a stalled frontal boundary and heads northeast,
passing south and east of Long Island into Monday morning. Winds
should diminish below 25 kt over most waters by the afternoon,
except for eastern areas, which may remain above 25 kt through
the day Monday. Waves remain above 5 ft through Monday night.
A persistent easterly flow will be on going Tuesday, with winds
and gusts below Small Craft Advisory levels. However, ocean seas
will be at small craft levels. The easterly flow continues into
Wednesday and then gradually diminishes and becomes southeasterly
as high pressure moves well east of the forecast waters. Ocean
seas are expected to subside below 5 feet during Wednesday. Then
winds and seas remain below small craft levels Wednesday night
heavy rainfall is possible late tonight into Monday morning for
the entire area. Generally, 1 to 1.5 inches is forecast, with
locally higher amounts possible in any thunderstorms. A Flash
Flood Watch has been issued for portions of the area (see above)
for this threat. In particular, the New York City area could see
a period of heavy rainfall late tonight into early Monday
morning, leading to urban flooding.
Additional rainfall is possible Thursday through Friday night.
Hydrologic impacts are not expected at this time.
with the new moon that occurred early Sunday morning, tides are
running astronomically high. Tidal departures of 1/2 to 1 ft
are needed for minor flooding during the night time high tide
Sunday night into early Monday morning. With a southeast to east flow
expected, and looking at what occurred last night and with this
mornings high tide, have continued the coastal flood advisory
for western Li sound, the South Shore bays of Li, and lower New York
Harbor for the evening and night time high tides (will add East
Union with this update). A few locations could reach or slightly
exceed moderate benchmarks across the South Shore bays of
Nassau and Queens.
The threat for minor flooding could continue into Monday (possibly
the morning and likely the night time high tide), with east/NE flow
progged to continue.
CT...coastal flood advisory until 2 am EDT Monday for ctz009.
New York...Flash Flood Watch through Monday morning for nyz067-069-
Coastal flood advisory until 2 am EDT Monday for nyz071-073-
High rip current risk from 6 am EDT Monday through Monday
evening for nyz075-080-081-178-179.
Coastal flood advisory until 11 PM EDT this evening for nyz074-
New Jersey...Flash Flood Watch through Monday morning for njz002-004-006-
Coastal flood advisory until 11 PM EDT this evening for njz108.
Marine...Small Craft Advisory from 7 am to 2 PM EDT Monday for anz330-
Small Craft Advisory until 6 PM EDT Monday for anz350-353-355.