Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 152339 AFDOKX Area Forecast Discussion National Weather Service New York NY 739 PM EDT Sun Jul 15 2018 .SYNOPSIS... Weak high pressure will remain in place through tonight and then move farther offshore and weaken Monday and Monday night. Meanwhile, a cold front will approach from the west. This cold front moves through Tuesday into Tuesday night, with high pressure following for Wednesday into Friday. An area of low pressure then impacts the region for the weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Isolated thunderstorms will continue to dissipate with the loss of diurnal heating/instability. Updated to reflect lower precipitation chances this evening, along with a reduction in cloud cover. Otherwise, forecast remains on track. For tonight, the region is left in broad western periphery of an Atlantic based high pressure area. There will not be much change of airmass, still warm and humid. More stable weather expected due to an upper level ridge axis that will be moving across. This will bring an associated negative vorticity advection. This lack of vertical forcing will keep weather conditions dry much of tonight. Lows were a blend of 2/3 MET and 1/3 MAV with a range from the mid 60s to mid 70s. A moderate risk for rip current development remains expected at the ocean beaches into early this evening. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... For Monday, the ridge axis over the region initially will keep subsidence strong and weather dry. However, this ridge axis will be moving eastward and behind it, there will be another shortwave moving in with positive vorticity advection during the afternoon. At the surface, high pressure weakens and a cold front approaches from the west. There will be increased warmth and moisture with higher temperatures and dewpoints. More of an unstable environment and daytime trough development will enable for some showers and thunderstorms to develop mainly N/W of NYC. The coverage will be isolated in the morning to scattered in the afternoon. Much of the day is expected to remain dry however and with that, took a blend of MAV/MET/GMOS for highs. Dewpoints are expected to increase as well with the continued southerly flow. 850MB temperatures trend a few degrees higher compared to the previous day. Highs are forecast in the upper 80s to lower 90s. The urban areas are especially warmer and here there is high enough confidence with the higher dewpoints that heat indices are expected to be mostly in the 95 to 99 degree range for NYC, Northeast NJ, and Lower Hudson Valley. As can be seen in the first period of the long term, heat indices are forecast to be 95 to 99 degrees for these areas for two consecutive days with higher confidence. These values have also been forecast over the last few forecasts, lending more confidence especially for the urban areas near New York City of reaching at least 95 degrees heat index early this week for both Monday and Tuesday. Hence, a heat advisory for these areas has been issued. For Monday night, the cold front moves closer to the region and a larger upper level trough will be approaching along with the front. More height falls aloft will portend more synoptic forcing, allowing for chances of showers and thunderstorms to increase and spread east across the entire forecast region. Compared to previous model runs, the model timing of the front seems to have slowed down along with the precipitation. Therefore, do not have much eastward advancement of chance POPs for showers and thunderstorms until late. Like Monday afternoon, Monday night has higher chances for showers and thunderstorms N/W of NYC. Lows were kept warm in the low to mid 70s. There will be a moderate risk for rip currents again at ocean beaches Monday. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Prolonged moisture advection from both the Gulf and Atlantic in south to southwest flow will set the stage for a heavy rainfall event as a cold front moves through Tuesday into Tuesday night. PWAT values are anticipated to rise to within the 90th to 97th percentile range for time of year, with maximum forecasts of close to 2.25". Convectively induced vorticity maximums ahead of the main trough may trigger more discrete activity ahead of the cold front at any point during the day. With an initially dry subcloud layer, these discrete initial cells may be capable of locally gusty winds, in addition to heavy downpours. Otherwise, flow aloft will primarily be parallel to the approaching frontal boundary, so expect storms to quickly spread/develop into a line. Heavy rainfall will be the primary threat, with local accumulation rates of 1"/hr possible. In addition to the threat of heavy rainfall, increasing humidity and the delayed timing of the front (not expected til afternoon to evening) will allow heat indices to approach 95 degrees across portions of the New York City metro and surrounding suburbs. The cold front and associated heavy rainfall then move offshore by Wednesday morning, with a return to more seasonable temperatures and humidity levels as high pressure builds in from the north and west. By the weekend, there is some consistency in a stormy pattern as several areas of low pressure approach, though details about timing and placement are too uncertain at this time to offer further detail. && .AVIATION /00Z MONDAY THROUGH FRIDAY/...
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High pressure will remain offshore tonight into early Monday with weak low pressure troughs nearby. The high moves farther offshore Monday afternoon with a weak trough developing inland. VFR. There is a chance of MVFR visibilities in fog 07Z-13Z at KSWF, KHPN, and KGON. Then in the afternoon with the chance of showers and thunderstorms, mainly inland, local MVFR conditions will be possible. Winds will be light SW in the NYC metro area, to light and variable to calm at the outlying terminals. A south flow develops Monday morning, increasing to 10-15 KT in the afternoon. .OUTLOOK FOR 00Z TUESDAY THROUGH FRIDAY... .Monday night...VFR. A chance of showers/tstms, mainly to the N and W of the NYC area terminals, with local MVFR conditions. .Tuesday...Afternoon and evening showers/tstms likely with MVFR or lower conditions. S winds G20-25KT into early evening. .Wednesday...VFR. A slight chance of showers in the morning east of the NYC terminals, with MVFR possible. .Thursday-Friday...VFR.
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&& .MARINE... Sub-SCA conditions expected tonight through Monday and ocean seas trend to marginal SCA criteria late Monday night. Due to uncertainty and SWAN being a little overdone currently, held off on any SCA issuances. So still expected mainly sub-SCA conditions much of Monday night. Winds and seas will gradually increase ahead of an approaching front into Tuesday, with SCA-level seas expected on the ocean waters by Tuesday. Although the cold front moves east of the waters Wednesday, continued swell may allow 5 ft seas to persist on at least the eastern ocean waters into Wednesday. Thereafter, high pressure builds into the waters, allowing winds and seas to subside into late week. && .HYDROLOGY... No significant widespread rainfall expected into tonight and through Monday night. However, with the humidity increasing, any shower or thunderstorm will be capable of producing locally heavy downpours, with potential for minor localized flooding. Another 1 to 1 1/2 inch basin avg rainfall likely with the slow moving frontal system from Tue into Wed morning. Locally heavy rainfall is possible with any thunderstorms, with some potential for minor urban and poor drainage flooding. && .TIDES/COASTAL FLOODING... Astronomical tides will continue to run high due to a combo of a recent new moon and onshore flow/incoming long period swells promoting tidal piling. Additional rounds of localized minor flooding are likely tonight and again Monday night along the back backs of southern Nassau. && .EQUIPMENT... NYC NOAA Weather Radio Station KWO-35 (162.55 MHz) remains off the air for an extended period of time. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...Heat Advisory from noon Monday to 6 PM EDT Tuesday for NYZ067>075-176>179. NJ...Heat Advisory from noon Monday to 6 PM EDT Tuesday for NJZ002- 004-006-103>108. MARINE...None. && $$ SYNOPSIS...MD/JM NEAR TERM...MD/JM SHORT TERM...JM LONG TERM...MD AVIATION...19 MARINE...MD/JM HYDROLOGY...MD/JM TIDES/COASTAL FLOODING... EQUIPMENT... is the U.S. government's official web portal to all federal, state and local government web resources and services.