Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 170553 AFDOKX Area Forecast Discussion National Weather Service New York NY 153 AM EDT Tue Jul 17 2018 .SYNOPSIS... High pressure remains offshore tonight. A cold front will approach from the west on Tuesday and move through late in the day into Tuesday night. High pressure will build across the region Wednesday through Thursday, then remain over the region through Friday. The high will then move offshore on Saturday with low pressure and its associated warm front approaching for Saturday night into Sunday. The weakening warm front will then linger near the region into Monday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Forecast is mainly on track. Only changes were to decrease POPs for showers and thunderstorms and to push that timing back a few hours during the overnight timeframe. There are still not much of any reflectivity echoes on radar so no convection has redeveloped yet as surface instability has been waning this evening compared to earlier. However, on the other hand, overnight, heights aloft will lower a little more rapidly with trough approaching and this will lead to more vertical forcing along with elevated instability. Any showers and storms will be isolated to scattered in nature and occurring overnight, primarily inland such as the Lower Hudson Valley and interior Northeast NJ where boundaries from earlier convection that dissipated will serve as a focus for low level convergence. This is also where heights aloft will be lower. Otherwise, it will remain muggy tonight with lows in the 70s and some increase in clouds late tonight. For some locations that are expected to stay dry and have less clouds such as Eastern Long Island and Southeast Connecticut, have some patchy fog forecast as these locations cool to their dewpoints overnight into early Tuesday. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... A cold front approaches and is expected to pass through the tri-state area mostly during the late afternoon to evening hours. This will bring showers and thunderstorms to the area. SBCAPEs could range 1500-2500 J/KG, coinciding with unidirectional 0-2 KM shear of 20-30 kt. Strong wind gusts with TSTMs are possible along with a marginal potential for severe gusts. The main limiting factor for severe weather appears to be the high moisture content limiting the potential for steep lapse rates. Another threat with these showers and storms will be flash flooding. Very high moisture content will be in place with PWATs around 2.25, a warm cloud layer depth of 13-14 KFT, and a theta-e ridge shifting through. Best chances for heavy rainfall will be in the afternoon and evening hours when shortwaves and a llj pass through and tap into this deep moisture. Limiting the chances of flash flooding will be the forward speed of the cells as the upper flow will not be aligned with surface cold front, which will also limit the chances of storm cell training. Although models signal higher rain totals over the eastern half of the area, flash flood guidance and antecedent dry conditions don`t support flash flooding there. Any flash flooding would more likely be localized. After collaboration with the surrounding office, have decided to hold off on any flash flood watches. Given the dry pattern we`ve been in and the convective nature of the rainfall, have doubts with some of the guidance that show widespread 1.50 - 2.50 inch accumulations. Will go with the lower side of guidance for total rainfall. The remaining concern will be heat indices and the need for heat advisories. 850mb temps are expected to be similar to today`s however there should be more cloud cover tomorrow in addition to rain-cooled air. Expecting highs in the mid-upper 80s for the locations currently under a heat advisory. Dewpoints should however be higher than today`s. Forecast dewpoints at the top of the mixed layer are progged to be 15-16C and a S-SW wind flow will prevail, so surface dewpoints are not expected to drop much until after the cold front moves through. Expecting dewpoints in the mid 70s, leading to heat indices in the mid 90s for areas within the advisory. This is the only area that`s expected to have 2 consecutive days of 95+ heat indices, so the advisory will not be expanded in area at this time. There is a high risk of rip currents for the ocean beaches Tuesday into Tuesday evening from a 5 ft onshore swell component and stronger onshore flow. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... An Upper level trough will linger across eastern portions of the region through Wednesday. Have trimmed back any slight chance POPs just a touch from previous forecast, as low level cold and dry advection as depicted by most modeling should keep precip chances near a minimum, even for eastern sections. Upper level ridging will then build Wednesday night through early Friday. Dry conditions with comfortable humidity levels are expected during this timeframe. An upper level trough will then approach out of the Upper Midwest and Southern Canada into the weekend. With high pressure slipping off the east coast early in the weekend, a humid southerly flow will return to the region. With upper and mid level forcing precip chances will increase through the weekend, and peak by Sunday with a warm front just south and southwest of the region along with greatest height falls just NW of the region. The upper level system is then progged to weaken and fill to some degree on Monday. Forcing should still be sufficient to carry chance POPs across the region. Global ensemble mean 500 mb height forecasts still indicate some mean troughing over the eastern CONUS through early next week which would indicate at a minimum diurnal driven convective chances. Ensemble viewer indicates PWs running about 1.5 to 2 standard deviations above normal late in the weekend into early next week as actual PW values should be approaching 1.75 inches during this time. Too early to get into details, but there may be some localized pockets of heavier rainfall during this timeframe with broad deep southerly flow established. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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High Impact Day today with Numerous Showers and Thunderstorms in the Afternoon... A cold front approaches today, moving through the terminals late in the afternoon, into early evening. Mainly VFR this morning, except across coastal locations which could observe patchy fog and stratus (mainly LI and SE CT). Local MVFR or IFR is possible through 12Z. Then, scattered showers develop late this morning, followed by a line of thunderstorms during the afternoon, moving west to east. MVFR conditions and gusty winds of 30-35 KT are possible in the stronger thunderstorms. Heavy downpours could lower visibility to LIFR briefly if any individual storm passes over a terminal. A few showers and thunderstorms may linger into the evening hours as well, then clearing should take place after midnight. S to SW winds around 10 KT increase up to 15 KT after 12Z, with occasional gusts up to 20 KT. Winds lighten and become variable as the front slowly passes late this afternoon/early evening. Winds eventually turn to the NW late this evening. .OUTLOOK FOR 06Z WEDNESDAY THROUGH SATURDAY... .Late tonight-Saturday...VFR.
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&& .MARINE... Ocean seas tonight so far remain near 3 ft with gusts between 15-20 kt over the ocean and less over the non-ocean waters. The forecast remains mainly on track. Sub-SCA conditions continue to prevail tonight with light winds and seas below 5 ft. Winds and seas then increase Tuesday ahead of a cold front. SCA conds expected on the ocean waters by mid- morning and lasting into Tuesday night. Lingering S-SE swell component may result in SCA conditions due to seas for eastern ocean waters during Wednesday afternoon and evening. Otherwise, sub SCA conditions will persist through Friday as high pressure builds across the coastal waters with a light pressure gradient. High pressure will then move east and northeast of the coastal waters on Saturday resulting in an increasing SE swell component. This could result in small craft conditions due to high seas for the ocean waters by Sunday. && .HYDROLOGY... Showers an thunderstorms will be capable of producing locally heavy rain with the potential for primarily minor urban/ poor drainage and small stream flooding. The best overall chances for flooding will be Tuesday afternoon into evening. While not likely to be widespread, localized flash flooding cannot be ruled out. The convective nature of this event will produce a wide range of total accumulated rainfall. Most spots probably end up with around a half inch of rain with amounts ranging up to 1.50 inches less common. Isolated amounts of 2 to 2.50 inches are possible. No hydrologic impacts are expected Wednesday through Monday. && .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...
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CT...None. NY...Heat Advisory until 6 PM EDT this evening for NYZ072>075-176- 178. High Rip Current Risk from 6 AM EDT this morning through this evening for NYZ075-080-081-178-179. NJ...Heat Advisory until 6 PM EDT this evening for NJZ004-006- 103>108. MARINE...Small Craft Advisory from 8 AM this morning to midnight EDT tonight for ANZ353-355. Small Craft Advisory from 8 AM this morning to 6 AM EDT Wednesday for ANZ350.
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&& $$ SYNOPSIS...JC/JE NEAR TERM...JC/JM SHORT TERM...JC LONG TERM...JE AVIATION...PW MARINE...JC/JE/JM HYDROLOGY...JC/JE EQUIPMENT... is the U.S. government's official web portal to all federal, state and local government web resources and services.