Area Forecast Discussion
Issued by NWS Upton, NY

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FXUS61 KOKX 270944

Area Forecast Discussion
National Weather Service New York NY
544 AM EDT Tue Jul 27 2021

A cold front approaches today and moves through tonight. High
pressure builds in for Wednesday. A warm front will approach
from the south on Thursday, followed quickly by a cold frontal
passage Thursday night. The front will linger to the southeast
on Friday, then high pressure will build in Friday night into
Saturday night. Another frontal system will approach early next


The forecast is on track with only minor changes made.

An upper level trough will amplify over the Northeast today, while
at the surface, high pressure weakens and a cold front will approach
from the north. However, any precipitation from the front looks to
hold off until the tonight period. 850 mb temperatures will
range from 16-18 C, translating to highs in the upper 80s to
lower 90s. Dew points will be in the 60s today, with some
slightly lower dew points expected this afternoon due to mixing
from aloft. This will keep heat index values below 95 for much
of the area, with an isolated reading possible in the New York
City metro area. Some haze may start to move in aloft from
wildfires out west, but this looks to happen late into the
afternoon or evening this evening.


The trough remains over the Northeast aloft tonight as the cold
front continues its approach and moves through. Showers and
thunderstorms will move through beginning early tonight. While
SBCAPE and MUCAPE values seem unimpressive, with a few hundred to
around 1000 J/kg, low to mid level lapse rates will steepen as the
front moves through the region. Dew points will rise somewhat, into
the 60s with 70 possible in some spots and PWATs in the 1.5 to 2.0
inch range, and with the cold front providing the lifting mechanism,
showers and thunderstorms are possible generally between 8 pm and 2
pm. 0-6 km bulk shear generally ranging in the 25-35 kt, with
isolated values of greater than 40 kts will lead to organized
multicells, with some isolated supercells possible. Any storm
that becomes severe will likely be associated with damaging
winds. However, this is occurring generally after sunset, and
the timing of the front with the decreasing diurnal instability
will be key to the development of any severe storms. SPC has
placed southern Connecticut and the Lower Hudson Valley in a
marginal risk. So, while widespread severe is not expected,
isolated severe storms are possible. Additionally, WPC has
placed the same areas in a marginal risk for flash flooding,
given the high PWAT values and generally wet antecedent

Showers and thunderstorms may continue into Wednesday under weak
high pressure, but generally speaking mainly dry conditions are
expected. Conditions will be cooler, with highs in the upper 70s to
middle 80s.


The big picture will continue to feature a longwave trough in the
eastern CONUS and a ridge in the west, with western Atlantic ridging
well to the southeast of the area and out of the picture. Transient
shortwave disturbances will bring chances for showers and tstms. The
first of these will take place on Thu as sfc high pressure over New
England moves east and allows a warm front to slowly approach from
the south, followed by a cold frontal approach/passage late day into
the evening. Showers/tstms will be likely Thu afternoon/evening
especially from NYC west in the afternoon, spreading east into Long
Island and S CT as well from late day into the evening.

The front should clear the area later Thu night and stall nearby on
Fri while most of the associated deep moisture continues moving E.
So daytime Fri should be mainly dry, with only some leftover early
AM showers out east, then an isolated afternoon shower/tstm.

As a closed upper low moves across Quebec and far northern New
England Fri night into Sat, the front should get a push farther
south/east, with a shot of CAA making for a cooler night Fri night
as high pressure builds in.

The high should be in control from Fri night into at least Sat
night/Sunday morning. Timing of the next frontal system to impact
the area is questionable, so both Sunday/Monday carry diurnal
chance PoP for showers/tstms.

High temps through the period should be a little below avg through
Sat, then should moderate to near avg Sunday/Monday. With the
exception of Fri night with cooler lows from the mid 50s to mid 60s,
nighttime temps will be mostly near avg.


Weak high pressure will remain over the region today. A cold front
will approach from the north late in the day, and move across

KGON up to its usual summertime late night vsby fluctuation as
radiation fog drifts in/out. Otherwise, some outlying terminals may
see some haze into the early daylight hours. Some haze aloft
possible this afternoon.

Light and variable winds should become W/NW less than 7 kt this
morning, and may back SW late morning. S/SE 8-12 kt afternoon sea
breeze development expected at most terminals. Potential for
scattered showers/tstms after 00Z.

  ...NY Metro (KEWR/KLGA/KJFK/KTEB) TAF Uncertainty...

Light W-NW winds (5 kt or less) for the AM push. S/SE seabreeze 8-12
kt for evening push. Potential for scattered showers/tstms from

.Late tonight...Chance of brief MVFR/local IFR cond mainly N of
the NYC metros and Long Island with any showers/isolated tstms.
.Wednesday and Wednesday night...VFR. Slight chance of a shower or
.Thursday and Thursday night...Showers/tstms likely especially in
the afternoon/evening, with MVFR/local IFR cond.
.Friday...Mainly VFR. NW winds 10-15G20kt late day/evening.

Detailed information, including hourly TAF wind component forecasts,
can be found at: https:/


A weak pressure gradient will be in place across the waters
with conditions expected to remain below SCA thresholds through
Wednesday night.

Sub SCA conds expected through the period. Wind gusts in S-SW flow
could approach 20 kt late Thu night ahead of a weak cold frontal
passage. Winds could also gust up to 20 kt with seas up to 4 ft on
the ocean Fri night in NW post-frontal flow.


PWATs between 1.5 and 2 inches Tuesday night may lead to
isolated flash flooding, mainly across southern Connecticut and
the Lower Hudson Valley, but minor flooding with thunderstorms
in low lying and poor drainage areas with locally higher amounts
of rainfall Tuesday night will be the more likely outcome.

Looking ahead, no hydrologic impacts are expected Wednesday
through early next week.


A moderate risk of rip current development is forecast for the
ocean beaches for Tuesday, with a mix of 2 ft S and SE swells.




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