Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 210254 AFDOKX Area Forecast Discussion National Weather Service New York NY 1054 PM EDT Fri Jul 20 2018 .SYNOPSIS... High pressure off the coast will retreat to the northeast through Saturday. An intensifying non-tropical low pressure system will move up the coast Saturday into Saturday night, and pass to the west late Saturday night into Sunday. Another low and associated frontal boundary will remain to the west Sunday night through the upcoming work week, while high pressure remains offshore. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Forecast remains on track, with just minor adjustments to reflect current conditions. High pressure should remain in control overnight. with mostly clear skies and light onshore winds. Had been concerned earlier about potential for patchy low clouds and fog to develop late, but a push of dry air coming in from the east should prevent this from happening except maybe along the immediate coast out east. Low temps should be in the 50s inland and across eastern CT/Long Island, and in the 60s elsewhere. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY/... As a deep closed low digs slowly S from the western Great Lakes region into the Ohio Valley, a non-tropical low will move up along the coast from eastern North Carolina, then likely hook NNW around the larger closed low, passing across NJ. Late afternoon guidance may be coming into closer agreement on timing of associated rain and wind, with the 18Z NAM now in agreement (at least for this cycle) with the 12Z GFS on leading vort max bringing showers especially to the NYC metro area and Long Island by late day Sat, while E winds slowly increase. Main impact from this low should arrive during Sat night mainly from late evening on, with heavy rain, and a windswept one at that along the coast, where E-SE winds just E of the low and N of a warm front to its east will likely be sustained 20-30 mph with gusts 40-45 mph. A few gusts to 50 mph may be possible along the ocean shores of NYC and western Long Island. Main question regarding heaviest rainfall will be whether low level moisture transport east of the low continues straight up the coast toward the NYC metro area and Lower Hudson Valley, or also hook NNW with the surface low more into NJ and eastern PA. Due to this uncertainty, and with the 18Z NAM wavering from its 12Z predecessor, also HREF/SREF/GEFS probabilities of 2+ of inches of rain all still on the low side, have not yet issued any flash flood watches, yet cannot dismiss the possibility, so mention continues in the HWO. As seems to be customary at this time of year, have to closely watch east of the low and along the warm front lifting northward for tstms that could produce isolated damaging winds or even a brief tornado, as low level SRH tends to maximize, and as air mass quickly destabilizes via an eroding low level temp inversion, either via heavy rain, or via sudden destabilization right along the warm front via descending air in a relatively dry slot right behind the front. Greatest potential for this appears to be across Long Island and SE CT. On Sunday, heaviest rain should shift mainly into E CT and E Long Island during the morning. After a short break, chances for showers and isolated tstms may increase into the afternoon as tropical moisture continues to stream northward and a weak mid level vort max rides northward as it pivots around the closed low well to the west. The rip current risk will increase through the day Saturday, with a moderate risk in the AM and high risk PM. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Guidance remains consistent with keeping a closed low from the Ohio Valley into the Gulf coast states Sunday night with the low opening into a trough Monday. This feature will remain within a nearly blocked flow as the northern Atlantic ridge remains offshore. There are indications that the upper trough will retrograde Monday into early Tuesday as the Atlantic region builds to the north. This push es the trough south into the Gulf coast states and cuts off from the northerly flow. Periods of showers and thunderstorms are expected to remain and will keep likely probabilities through Monday night, especially to the west. If the upper ridge pushes far enough inland, much of the area may be dry Tuesday and Tuesday night. With the uncertainty, and to keep some measure of consistency with the previous forecast, only lowered probabilities across the eastern zones to slight chance and kept chance probabilities to the west. The northern flow will then dominate Wednesday into Thursday as the ECMWF pushes the low to the Gulf coast where the low then mostly dissipates, while the GFS moves the low south, and maintains some circulation along the Texas Gulf coast. For the area, a weak cold front will develop and begin to slowly approach Wednesday and Thursday, and moves through Thursday night into Friday. A tropical like airmass will remain in place Sunday night through next week with highs each day in the lower to mid 80s and lows mostly lower to mid 70s. Humidity levels remain uncomfortable with dew points in the lower to mid 70s. && .AVIATION /03Z SATURDAY THROUGH WEDNESDAY/... High pressure gradually shifts towards the Canadian Maritimes tonight and early Saturday. An area of low pressure will approach from the south and pass west of the NYC terminals Saturday night. Heavy rain and gusty winds expected Saturday night. VFR through mid-afternoon Saturday then chance of MVFR arriving Saturday late afternoon in scattered showers. Winds diminish tonight, becoming more easterly by 12z Saturday. Gust E-SE winds 15-25kt developing at the coast around 20z Saturday. Gusts will increase through the evening and overnight, with gusts to 30kt after 00z Sunday for the NYC terminals. Will need to keep an eye on LLWS if the gusty surface winds don`t materialize. .OUTLOOK FOR 00Z SUNDAY THROUGH WEDNESDAY... .Sat night...Rain, heavy at times. MVFR/IFR develop in the evening. E-SE gusts 25-30 kt. Chance of gusts to 35 kt at coastal terminals. LLWS possible. Isolated TSTMs. .Sunday...MVFR/VFR with periods of SHRA/TSTMs. S gusts 20-25 kt. .Sunday night...Mostly MVFR with a chance of IFR in SHRA and isolated TSTMs. S gusts 20-25 kt. .Monday-Wednesday...MVFR/VFR with periods of SHRA and a chance of TSTMs. S gusts 20-25 kt. && .MARINE... Gale warning issued for all waters as sustained E-SE winds increase to 25-35 kt, with gusts 35-45 kt, with the non-tropical low moving up the coast. Can`t rule out a gust up to 50 kt on the ern ocean waters late Sat night, but since any winds of that magnitude should peak at this level and not last very long, opted not to issue storm warning. Gales should continue out east into Sunday morning as a strong southerly low level jet shifts east into those waters and into the waters off SE New England, with elevated seas continuing on the wrn ocean waters. Winds will remain below SCA levels on the forecast waters Sunday night through Wednesday. However, with a prolonged southerly flow and southerly swells, ocean seas will remain at SCA levels Sunday night through Tuesday. Seas may briefly fall below 5 ft Tuesday night into Wednesday as winds diminish slightly. && .HYDROLOGY... Rainfall of 1-2 inches is likely in most areas from late Saturday into Sunday morning. Locally higher amounts are possible, especially in any thunderstorms that develop. Minor urban and poor drainage flooding looks to be the main threat. A localized threat for flash flooding exists with any training convection, and also if heavy rain coincides with high tide levels approaching minor flood levels along the coast Sunday morning. Additionally, if high-end rainfall amounts are realized across the more quickly-responding NE NJ and Lower Hudson Valley river basins, minor stream flooding would be possible as well. A prolonged period of rainfall is possible from Sunday night through the end of the next workweek. Currently, an additional 1/2 inch of rainfall is likely Sunday night into Monday. Then, periods of rain and isolated thunderstorms will be possible into Friday. The focus of higher rainfall amounts are expected along a frontal boundary setting up inland. && .TIDES/COASTAL FLOODING... While astronomical tides remain low with the full moon still a week off, a slight (10-20%) chance continues for locations in/around the NY Bight, including the shores of Staten Island and the back bays of southern NYC and southern Nassau, to experience minor coastal flooding with the high tide cycle early Sunday morning, especially if strongest winds coincide closely enough with the high tide. Runoff from heavy rain bands could increase this potential, but this remains uncertain. && .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...High Rip Current Risk from Saturday afternoon through Saturday evening for NYZ075-080-081-178-179. NJ...None. MARINE...Gale Warning from 6 PM Saturday to 7 AM EDT Sunday for ANZ335- 338-345-353-355. Gale Warning from 6 PM Saturday to noon EDT Sunday for ANZ330- 340-350. && $$ SYNOPSIS...Goodman/MET NEAR TERM...Goodman/JP SHORT TERM...Goodman LONG TERM...MET AVIATION...BC MARINE...Goodman/MET HYDROLOGY...Goodman/MET TIDES/COASTAL FLOODING... EQUIPMENT... is the U.S. government's official web portal to all federal, state and local government web resources and services.