Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 171213 AFDOKX Area Forecast Discussion National Weather Service New York NY 713 AM EST Sat Feb 17 2018 .SYNOPSIS... High pressure builds retreats to the northeast as an area of low pressure tracks to near the Mid-Atlantic coast by late this afternoon, and then to a position well southeast of Cape Cod by Sunday morning. High pressure builds in on Sunday, then slides offshore through Monday. A warm front approaches from the southwest Monday night, then lifts to the north Tuesday, followed by a slow moving cold front crossing the area during the middle of next week. High pressure then builds in to close the upcoming work week. && .NEAR TERM /THROUGH TONIGHT/... Made minor changes this update to reflect the latest trends in observations and guidance. The forecast appears on track. Northern stream ridging transits the area this morning, keeping things dry and relatively cloud free, except for some high clouds moving into far W zones by late morning. A 700 hPa shortwave pushes into the area by late today, bringing with it a chance of some light precipitation across mainly the SW 1/2 of the CWA late. This will be mainly in the form of snow for areas N of Long Island Sound (including the Lower Hudson Valley) and a mix of rain and snow elsewhere. For the winter storm for tonight, the CMC and ECMWF have trended wetter, towards the NAM and SREF. The GFS is a relatively drier outlier due to its more southern low track (due to well documented issues the GFS has in dealing with damming highs east of the Appalachians). AS a result have followed a NAM/ECMWF/CMC regional/SREF blend for the forecast for tonight. A northern stream shortwave tracks into the lower Great Lakes this evening and to near the S New england Coast by Sunday morning. This shortwave will help intensify and kick a low northeast from near the mid Atlantic coast late today, to the south/east of the 40N/70W benchmark tonight. While the system is fairly fast moving, it should have fairly strong dynamics able to help maximize the production of precipitation. Thermally, it appears that areas N of Long Island Sound, including the Lower Hudson Valley, along with northern portions of NE NJ should remain all snow. Across the remainder of NE NJ, N NYC and N Long Island, any mixing should be limited to within an hour or two of the onset of precipitation (before it becomes more moderate to locally heavy in intensity). Along S parts of NYC (Staten Island, Brooklyn and S Queens) and S Long Island (the S Fork + the area S of the Long Island Expressway), mixing could persist into or through the evening hours (possibly into the morning hours over the S fork) due to weak onshore flow from the Atlantic. Given that this flow should be less than 10kt (and quite possibly 5kt or less) there is some question as to whether it will be able to overcome column cooling once the moderate precipitation begins. Snow should come to an end across the area prior to sunrise Sunday morning. There are still indications that there should be banding to the NW of the surface low - likely focused to the NW of the maximum in 850-700 Frontogenesis - which should place it from NE NJ into S Central CT this evening. However, as always there is some uncertainty as to exactly where this band will set up. Regarding models showing 1000-850 partial thickness of 1300-1304 over S portions of the area around 6z - this is misleading, because a rev of BUFKIT forecast soundings shows an isothermal layer at or just below freezing at the same time. This is consistent with heavy precipitation at this time, and supports the idea of all snow then. The possible exception to this is maybe across the S fork of Long Island, where weak onshore flow may warm the lowest part of the atmosphere sufficiently to allow for rain to still be mixed with the snow into the early morning hours. Given the above, have made the following changes to winter weather headlines: 1) Issued a Winter Storm Warning for S CT, NE NJ, N NYC, N Long Island, and the Lower Hudson Valley (except Orange and Putnam Counties) for 4-7 inches of snow 2) Issued a Winter Weather Advisory for Orange and Putnam Counties for 4-6 inches of snow. There does not appear there will be sufficient QPF to support warning level snows over at least half of these zones. 3) Continued the Winter Storm Watch for S NYC and S Long Island due to uncertainty of how much mixing with rain will occur this evening. The potential still exists for 4-7 inches of snow, with locally lower amounts near the immediate coast. In this area Staten Island is the most likely to be converted to a warning and SE Suffolk the most likely to be converted to an Advisory. At this time, did not have the required 80 percent confidence to do so. For temperatures weighed towards NAM and ECWMF 2-m temperatures with some adjustments towards wet bulb once moderate precipitation begins tonight. && .SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/... Northern stream ridging builds in Sunday and Sunday night, keeping things dry, with minimal cloud cover. With expected snow cover, did undercut guidance, towards NAM/ECMWF 2-meter temperatures. && .LONG TERM /MONDAY THROUGH FRIDAY/... Deep west/southwesterly flow aloft backs as western trough digs and eastern ridge builds early to mid week. The ridge flattens slightly late in the week as heights aloft fall. As for sensible weather, rain accompanies the warm front Monday. While the area is in the warms sector, hit or miss light rain or drizzle is possible, as is nighttime and morning fog. Some rain may accompany the cold front Wed night, with another warm front approaching Friday. Temperatures warm Monday through Wednesday. There will be quite a temp difference west of the Hudson river (warmer), and along the south facing coasts (much cooler) as you would expect this time of year. Temperatures cool slightly behind the front Thursday into Friday. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
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High pressure builds over the northeast this morning and shifts off the New England coast this aftn. Meanwhile, low pres develops near the SE coast this aftn and passes southeast of Long Island tonight. VFR through the aftn. IFR/LIFR develops between 23z and 04z as pcpn overspreads the area. Initially, a rain/snow mix is expected at NYC and Long Island terminals, but will transition over to all snow during the eve. Timing of this is a bit uncertain and could be a few hours off from current forecast. P-type should be all snow at all other terminals. The snow gradually tapers off with improving conds from west to east early Sunday morning. Snowfall accumulations of 4 to 7 inches are forecast. N winds less than 10 kt will gradually veer to the SE by aftn, then as low pres develops and passes south of Long Island tonight, will shift back to the NE. .OUTLOOK FOR 12Z SUNDAY THROUGH WEDNESDAY... .Sunday...VFR. .Monday...VFR becomes MVFR or IFR in rain and possible fog, especially late. .Tuesday...MVFR or IFR possible in the morning and again at night, otherwise VFR with SW wind G20KT possible. .Wednesday...Mainly VFR. Chance of showers/MVFR.
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&& .MARINE...
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Made minor changes this update to reflect the latest trends in observations and guidance. The forecast appears on track. Seas have fallen below 5 ft on the coastal ocean waters E of Fire Island Inlet, so the Small Craft Advisory (SCA) there has been allowed to expire. Tranquil conditions prevail on the waters as high pressure moves across the region today. Low pressure will track to the SE of Long Island tonight with the potential for a 4-6 hour period of SCA level gusts mainly on the ocean waters late tonight into Sun morning, although it will also be close on eastern LI Sound. With current headlines still up, will hold off on issuing for this which has been collaborated with surrounding offices. Sub-advsy conds are then expected through the remainder of the forecast period, although an increasing SW flow develops early next week with a tightening pres gradient in a WAA pattern. It is expected to remain too inverted over the waters to mix down higher winds that would reach advsy criteria.
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&& .HYDROLOGY... Around 1/3 to 3/4 an inch of liquid equivalent is expected Saturday night with locally higher amounts. With most, if not all of this falling in the form of snow, no hydrologic impacts are expected. No significant hydrologic impacts are expected from Sunday through the end of next week. && .EQUIPMENT... NYC NOAA Weather Radio Station KWO-35 (162.55 MHz) is off the air for an extended period of time. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...Winter Storm Warning from 5 PM this afternoon to 7 AM EST Sunday for CTZ005>012. NY...Winter Storm Warning from 5 PM this afternoon to 7 AM EST Sunday for NYZ069>073-078-079-176-177. Winter Weather Advisory from 5 PM this afternoon to 7 AM EST Sunday for NYZ067-068. Winter Storm Watch from 5 PM EST this afternoon through Sunday morning for NYZ074-075-080-081-178-179. NJ...Winter Storm Warning from 5 PM this afternoon to 7 AM EST Sunday for NJZ002-004-006-103>108. MARINE...None. && $$ SYNOPSIS...Maloit NEAR TERM...Maloit SHORT TERM...Maloit LONG TERM...Maloit/PW AVIATION...24 MARINE...24/Maloit HYDROLOGY...Maloit/PW EQUIPMENT... is the U.S. government's official web portal to all federal, state and local government web resources and services.