Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 301959 AFDOKX Area Forecast Discussion National Weather Service New York NY 359 PM EDT SAT JUL 30 2016 .SYNOPSIS... Low pressure slowly approaches from the Ohio Valley through Sunday. This low slowly tracks along a frontal boundary that becomes nearly stationary through central New Jersey to south of Long Island Monday. Waves of low pressure track along the boundary Monday and Monday night. High pressure begins to build toward the region Tuesday, and moves over the area Wednesday. The high then slides off the northeast coast Thursday. A cold front approaches Friday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Showers and embedded thunderstorms have continued to develop, mainly west of the New York City metro. This trend will continue into the evening, although expect the activity to continue to stratify into a larger area of showers with isolated thunder as CAPE decreases. This activity is forming ahead of an approaching low pressure over the Ohio Valley with increasing low level warm advection. Have increase PoPs to categorical this evening across NE NJ and the Lower Hudson Valley, but this may need to be pushed further east across the metro and possibly western Long Island and SW CT if latest radar trends continue. The main concern is locally heavy rainfall as PWATs are running between 1.7 and 1.9 inches per latest SPC mesoanalysis. As the activity translates eastward across southern CT and eastern LI, there should be a downward trend in intensity as the airmass is more stable and the surface high just to the east. There may be a lull in showers around midnight, but there is decent agreement among latest NWP model guidance that warm advection increases further as a warm front associated with the low sets up near the region towards daybreak. Low level winds also increase which should supply deep moisture and lift for coverage of showers/isolated storms to expand. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... While not a strong low level jet, there is decent agreement among the latest NWP guidance for 25-35 kt of low level flow helping to enhance overrunning precipitation Sunday morning. The warm front will continue to lie near the region as well with the low slowly moving eastward across PA along this boundary. Showers and a few thunderstorms should be moving across the region in the morning from west to east. There is some uncertainty in where the heaviest rainfall will ultimately end up occurring, with highest confidence at this time from the city north and west. There will also be some shortwave energy moving around the broad trough over the Ohio Valley which could further support showers and a few storms. Best warm advection axis shifts to the eastern portion of the area in the afternoon, so there may be a lull in activity across the west. However, this may present an opportunity for some breaks in the clouds and some increasing instability for redevelopment of showers/storms. Likely PoPs continue into the early evening mainly along and west of the Hudson River. Locally heavy rainfall continues to be the main threat with any of the heavier showers and thunderstorms. Frontal boundary looks to set up south of Long Island Sunday night with weak waves of low pressure riding along it, so the potential for a few showers continues through the night, but coverage should be less that during the day. && .LONG TERM /MONDAY THROUGH SATURDAY/... Ridging remains over the country with a near zonal flow continuing along the Canadian/US border as shortwaves move through during the upcoming week. Monday into Monday night a trough will be moving through the northeastern and upper midwestern states, as a series of shortwaves move through the trough. At the surface a frontal boundary will be nearly stationary somewhere in the vicinity of the Ohio Valley into the mid Atlantic states and south of Long Island. The location of the frontal boundary will be affected by on going convection this weekend into the beginning of the week. Current indications are that a series of broad lows move along the boundary Monday into early Tuesday. Monday morning instability is marginal with isolated thunder possible. With daytime heating and Monday and another wave moving through chances for convection will be increasing. Moisture will be a little more limited Monday into Tuesday as moisture from the Gulf of Mexico is cutoff. However, some storms could produce locally heavy rain. The upper trough axis slides through the region early Tuesday with the last wave exiting offshore Tuesday morning. Upper ridging then builds into the east through Thursday. A rather vigorous shortwave will be coming onshore of the Pacific northwest early next week and will track along the northern US, digging a trough into the upper midwest and Great Lakes by the end of the week. Deepening surface low pressure will send a cold front toward the area Friday into Saturday as the ridge moves offshore. Humidity will be increasing for Friday and Saturday as a deep southwesterly flow sets up. && .AVIATION /20Z SATURDAY THROUGH THURSDAY/... SHRA/TSRA developing over NJ and lifting to the east. Activity will become more widespread going into this afternoon, and impacts to KNYC terminals are expected to develop between 19-21Z. Over Long Island, some SHRA/embedded TSRA developed along the sea breeze, and those should taper off in the next hour or so. Expecting SHRA/TSRA for many of the terminals through this evening, and then there should be a lull from around 04-06Z through 08-10Z. SHRA develop again towards Sunday morning, and will persist through the day. Conds will drop to MVFR with local IFR. Persistent onshore, SE flow expected into Sunday. For outlying terminals, winds will go LGT/VRB tonight. .OUTLOOK FOR 18Z SUNDAY THROUGH THURSDAY... .Sunday afternoon...MVFR or lower in SHRA. .Sunday night-Monday Night...Chance of showers and possibly a tstm. MVFR or lower conditions possible. .Tuesday...Mainly VFR, although isolated showers are possible. .Wednesday-Thursday...VFR. && .MARINE... Low pressure approaches the waters from the Ohio Valley through Sunday. The flow may increase Sunday with gusts up to 20 kt possible on the ocean waters. Otherwise...winds and seas will remain below SCA levels through Sunday night. A frontal system is expected to be in the vicinity of the forecast waters Monday with waves of low pressure tracking along a nearly stationary front Monday into early Tuesday. Winds and seas likely remain below small craft advisory levels. High pressure builds into the waters Tuesday and remains Wednesday. The high then shifts off the coast Thursday. Sub advisory conditions are expected through Thursday night. && .HYDROLOGY... Showers and embedded thunderstorms will bring an average of one half to around one inch of rainfall with the highest amounts along and west of the Hudson River. Any thunderstorms can produce localized higher amounts. Minor urban flooding will be the main threat within heavier showers/thunderstorms. A quarter to one-half inch of rainfall is possible Monday into early Tuesday morning with locally higher amounts. The greater rainfall totals are expected across the lower Hudson Valley into southern Connecticut. However, with uncertainty as to the track of low pressure confidence in the rainfall totals is low. Dry weather returns Tuesday and continues through the midweek period. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...None. NJ...None. MARINE...None. && $$ SYNOPSIS...DS/MET NEAR TERM...DS SHORT TERM...DS LONG TERM...MET AVIATION...MPS MARINE...DS/MET HYDROLOGY...DS/MET is the U.S. government's official web portal to all federal, state and local government web resources and services.