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Area forecast discussion
National Weather Service New York New York
1100 PM EDT Thu may 25 2017

Synopsis...
low pressure tracks through the tri-state region tonight and
off the New England coast Friday into Friday night. High
pressure builds down from southeastern Canada for the weekend.
This high then retreats to the northeast early next week as
weak low pressure tracks to the south. A cold front approaches
from the west on Tuesday, then crosses the area Tuesday night.
This front then stalls out nearby as high pressure builds in
from the southwest, producing potentially unsettled weather for
the middle of next week.

&&

Near term /until 6 am Friday morning/...
updated probabilities and weather across the far eastern zones
for showers and scattered thunderstorms with area moving north
northeast over the ocean waters. Hrrr indicating area will
remain next couple of hours, so have categorical and likely
probabilities.

Middle level dry air continue moving over the region into this
evening as impressive upper level low begins tracking from the
Ohio Valley into the middle Atlantic region. A vigorous
vorticity Max will round the base of the upper trough and
approach the northeast as the trough GOES negatively tilted
tonight.

Light rain remains across southern Connecticut with patchy
drizzle into northeastern New Jersey. Patchy to areas of fog
with visibilities around 1 mile, and up to 3 miles, were across
the region. No significant rain is expected however as lift is
weak and saturation is confined to the lowest 5 kft.

Moisture and lift increase after 03z from south to north as the
upper low and energy approach. Significant DPVA and steepening lapse
rates will support showers and scattered thunderstorms. The cape is
elevated as the low levels are inverted. The best instability
appears to lie along and east of the Hudson River. Due to the
anomalous nature of the upper low and strength of the energy,
feel thunder is possible anywhere across the area overnight.
High resolution mesoscale models, such as the hrrr, NAM-3km, and
ncar and Storm Prediction Center sseo ensemble all support this reasoning. The
normalized probability of greater than 40 dbz on the Storm Prediction Center sseo is
over 90 percent across Long Island and southeast Connecticut
06z-12z with slightly lower percentages further west. No severe
weather is forecast.

Surface low pressure will move along or near the Long Island coast
overnight so winds will not be as strong as earlier today.
The warm front may even briefly move across portions of eastern Long
Island before the low lifts to the north and east.

&&

Short term /6 am Friday morning through Friday night/...
upper low and surface low become vertically stacked as they move off
the New England coast on Friday. Wrap around showers and possible
thunder linger for a few hours around day break across eastern
Connecticut and The Forks of Long Island. Otherwise, clouds
will be slow to clear as moisture lingers below 10 kft. Northwest
downsloping flow should help to create some breaks in the
clouds, especially across the western half of the area. These
breaks will help boost temperatures into the 70s, with cooler
readings in the middle to upper 60s further east where clouds
linger longest.

Model soundings indicate some instability between about 5 and 10
kft, so clouds may fill back in with any breaks. There is also the
possibility of a few iso-sct showers. No thunder is forecast since
the instability does not look to get much above -5c with heights
building aloft behind the upper low.

Weak ridging passes Friday night as high pressure moves into the
region from the west. Lingering moisture will keep skies partly to
mostly cloudy with lows near normal values.

&&

Long term /Saturday through Thursday/...
the region is in between two shortwaves passing to the north and
south Saturday and Saturday night, so with most significant forcing
outside of the area, have at most slight chance pops in this time
frame, with temperatures near to slightly above normal.

Onshore flow will dominate from Sunday through Monday.

The result will be mainly gray sky conditions with spotty light rain
or drizzle mainly over higher elevations mainly to the north/west of NYC
from Sunday into Sunday night as a northern stream shortwave ridge
crosses the area. A 700-500 hpa shortwave approaches late Sunday
night and crosses the area on Monday, making for a more widespread
rain over the region then.

With the onshore flow/damming high, have undercut guidance by mixing
in NAM and European model (ecmwf) 2-meter temperatures. Highs will run 5-10 degrees
below normal Sunday and Monday and lows a few degrees above normal
Sunday night (due to reduced diurnal range).

A broad closed low slow tracks from Ontario into western Quebec from
Monday night through Thursday, with shortwaves rotating around the
base of the low from time to time. The timing of these shortwaves is
somewhat difficult this far out, but for now it appears that most
energy will be focused mainly to the north of the region. As a result,
there is a chance of isolated to scattered showers - mainly over
northern portions of the tri-state. Also, cannot rule out an
isolated rumble of thunder Tuesday/Tuesday night with the passage of
a surface cold front.

Temperatures Tuesday-Thursday should run near to slightly above
normal.

&&

Aviation /03z Friday through Tuesday/...
a warm front lies just south of the terminals this evening. A
weak low center then passes through during the late night
hours.

IFR/LIFR conds improving to MVFR at times with shra/tstms. Thunderstorm
chances drop off after around 06-08z with a return to prevailing
IFR/LIFR for the rest of the night.

East to NE winds around 10 kts for most terminals this evening,
turning variable in direction for a couple of hours within
roughly 04-08z for most terminals before a northwest flow on the
backside of the low gets established.

Improving back to VFR by Friday afternoon. West-northwest gusts 20-25 knots.
Winds generally south of 310 magnetic. Rain showers possible.

Outlook for 00z Saturday through Tuesday...
Friday night...VFR.
Saturday...VFR.
Saturday night-Monday...MVFR/IFR possible in showers. East/southeast
winds.
Tuesday...improving to VFR. S/southeast winds.

&&

Marine...
updated weather and probabilities for showers and scattered
thunderstorms early this evening across the far eastern ocean
waters and into the eastern Long Island Sound.

With nearshore wind gusts, and across the waters, below 25 knots
the Small Craft Advisory for the bays and New York Harbor and
Long Island Sound was allowed to expire. Easterly winds will
gradually weaken into this evening as the pressure gradient
relaxes. Seas on the ocean will remain at small craft levels
through Friday and the small craft was converted to a hazardous
seas small craft. These seas will gradually subside Friday night
with sub-Small Craft Advisory conditions elsewhere.

A light pressure gradient over the region Saturday-Tuesday should
limit winds to 10 kt or less. Given the absence of any significant
swell, in addition, conditions should be below Small Craft Advisory
conditions over the waters around Long Island Saturday-Tuesday.

&&

Hydrology...
an additional 0.50 to 1.0 inches of rain is forecast through
Friday morning. The highest amounts will likely occur across
eastern Connecticut and eastern Long Island. Locally higher
amounts are possible where thunderstorms occur overnight. Minor
urban and poor drainage flooding is possible.

It should then be mainly dry Friday night-Sunday.

Another round of rain is possible from Sunday night into late
Monday. At this time, it is to soon to specify, what, if any,
hydrologic impact will be experienced from this system.

&&

Tides/coastal flooding...
a coastal Flood Warning continues for moderate flooding
potential for the South Shore Back Bay locations of Nassau
County for tonight, and advisories are in place for minor
flooding potential for areas adjacent to western Long Island
Sound, New York Harbor, and the South Shore back bays of the western
Long Island. Minor flood thresholds could briefly be touched
along the eastern bays of Li and southeast CT.

How quickly easterly winds begin to weaken this
evening will be key to the potential for seeing widespread
versus localized moderate coastal flooding along the southern
bays of western Li. Less than 1/2 ft of surge is needed in many
places to reach minor flood thresholds.

The threat for minor coastal flooding is likely to continue during
the nighttime high tides for the southern bays of western Li/NYC
through the Memorial Day weekend. While along western Long Island
Sound, lower New York Harbor, and eastern Great South Bay...minor coastal
flooding is possible at those Times.

Beach erosion is expected with the prolonged easterly
sweep...but the potential for dune erosion looks to be low and
localized.

&&

Okx watches/warnings/advisories...
CT...coastal flood advisory until 1 am EDT Friday for ctz009-010.
New York...coastal flood advisory until 1 am EDT Friday for nyz071>073-
078-176-177.
Coastal flood advisory until midnight EDT tonight for nyz080.
New Jersey...none.
Marine...Small Craft Advisory for hazardous seas until 6 PM EDT Friday
for anz350-353-355.

&&

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