Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 242135 CCA AFDOKX Area Forecast Discussion...CORRECTED National Weather Service New York NY 435 PM EST Sat Feb 24 2018 .SYNOPSIS... A warm front approaches the region into tonight. A frontal system will affect the region with a low along the warm front moving across on Sunday. High pressure builds from the Midwest Sunday night through Monday night, then slides off the Mid- Atlantic coast through Wednesday night. High pressure builds to the north Thursday and Thursday night, as a warm front approaches from the southwest with low pressure riding along it. This low then passes near or just south of Long Island Friday, then slowly drifts to the east of Long Island Saturday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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Adjusted POPs near term using extrapolation from Doppler Radar and HRRR model reflectivity. An upper level jet from Baja California extending northeast through the Great Lakes will be strengthening and extending into Northern New England and the Canadian Maritimes tonight. The heights at 250mb slightly increase tonight across the local region. The 500mb heights remain nearly constant as a mid level ridge axis SE-NW orientation moves across the area. A strong shortwave pivots across the Great Lakes with increasing positive vorticity advection in the local region late tonight into Sunday. In addition, the upper level pattern shows diffluence in the jet levels for early Sunday and this conveys divergence. This increases the vertical forcing. The rain continues tonight but will be light for first half and then increases in intensity late tonight into Sunday morning, when much of the rainfall is expected. At the surface, winds become more east to southeast and strengthen as the warm front continues to approach. The parent low to this front will be deepening as it heads into the Western Great Lakes, increasing the pressure gradient. The easterly flow will increase precipitable waters to between 1.1 to 1.4 inches, which will be above the 90th percentile for February 25th according to OKX sounding climatology. This increases the moisture content available for the forcing. Raw 2 meter temperatures from NAM and GFS were weighed more heavily in the forecast for lows tonight, ranging from the upper 30s to lower 40s.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
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For Sunday, the jet core moves closer to the region, with the right front quad moving near the area by the end of the day. As previously mentioned, the diffluence and divergence aloft will be there for Sunday morning into the early afternoon. At the surface, a low forms along the warm front with signals for triple point low. Rain continues Sunday and could become heavy at times and enhanced near higher terrain with orographic lift. Rain tapers off mid to late in the afternoon. Raw 2 meter temperatures from NAM and GFS were weighed more heavily in the forecast also for Sunday without much diurnal temperature range. Forecast highs Sunday range mostly near 40 to 45. The core and jet streak maximum passes northwest of the region Sunday night. The heights at 250mb remain nearly steady Sunday and then slightly decrease late afternoon into the evening with a more rapid height fall late Sunday night. The heights at 500mb follow similarly with a slight decrease Sunday into Sunday evening followed by a greater decrease late Sunday night. Confluence occurs aloft Sunday night and the surface low moves east of the area Sunday night, giving way to high pressure building in from the Tennessee Valley. There will be a return to dry conditions and a more spatial spread of lows that will still be above normal. With moisture laden grounds and for locations that decouple with winds and hence have lighter winds, there will be some patchy fog that forms late at night.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/... A shortwave trough pushes through the area on Monday, but with dry low levels, it should move through dry. In response to low level warm advection ahead of the trough, highs on Monday should be around 10 degrees above normal. The region then remains on the top side of a deep layered ridge centered over the N Caribbean Monday night-Wednesday night. Associated subsidence should keep things dry, with minimal cloud cover. The exception on cloud cover is on Wednesday and Wednesday night, where should see a gradual increase in high then mid clouds ahead of the next system, which tracks into the mid Mississippi River Valley by Thursday morning. Temperatures Monday night-Tuesday night should run around 5-10 degrees above normal. Highs Wednesday should be around 10-15 degrees above normal and lows Wednesday night around 10 degrees above normal. The models are in good agreement that the deep layered ridge axis slides to the east on Thursday allowing a cutoff low to track into the Great Lakes Thursday then into the northeastern States Thursday night and Friday. The models then differ on how fast this cutoff low exits to the east for the remainder of the week, with the GFS/CMC more progressive than the ECMWF. This will ultimately be dependent on the location/strength of blocking downstream over the N Atlantic. As a result, while there is good agreement that rain develops over the area by late Thursday, there is not good agreement over when the precipitation ends (solutions vary from Friday night through late Saturday at this point). There also is not good agreement over how much, if any, wintry precipitation the area sees with this system, with the best chance over interior areas to the N/W of NYC and Long Island Sound. For now leaned towards a CMC/ECMWF blend, which were slower than the GFS in exiting precipitation (given downstream blocking a slower exit makes sense for now). In terms of sensible weather expect rain to develop from SW to NE Thursday. Have likely pops for mainly rain Thursday night- Friday (except for maybe a wintry mix over far N zones friday morning. For now have chance pops from Friday night into Saturday with a wintry mix across N/W mainly interior zones (though it could reach the CT coast). For now, limit chance pops Saturday to mainly the SE 1/2 of the CWA Saturday afternoon. Refer to the hydrology and coastal flooding sections of the AFD for further details on possible impacts from this storm. && .AVIATION /21Z SATURDAY THROUGH THURSDAY/... A warm front approaches late tonight into Sunday. VFR for the rest of the afternoon, bcmg MVFR this evening, then IFR late tonight thru at least Sunday morning before improving conditions. -Shra/-ra through the period. N winds mostly under 10 kt shift NE this evening, then increase from the E and become gusty late tonight and Sunday morning. .OUTLOOK FOR 18Z SUNDAY THROUGH THURSDAY... .Sunday PM...-shra/dz with IFR improving to MVFR. .Sunday night and Monday morning...Chance of rain with IFR conds in the evening mainly KBDR/KISP/KGON. Otherwise bcmg VFR with areas of fog with IFR/LIFR conds possible overnight into the morning. .Monday afternoon through Wednesday...VFR. .Thursday...Sub-vfr in rain likely. CHC E winds G30kt and LLWS. && .MARINE... An initially weak pressure gradient with sub-SCA conditions will increase late tonight in response to deepening low pressure moving into the Western Great Lakes and building high pressure in Southeast Canada. Easterly flow increases with SCA conditions becoming likely overnight into Sunday morning with gales at times for the ocean east of Fire Island Inlet. The pressure gradient weakens Sunday afternoon with the close proximity of the center of the low. Expecting the winds to decrease Sunday afternoon with gusts really starting to lower mid to late Sunday afternoon. Gales subside after 17Z Sunday and SCA winds will likely drop off soon thereafter. Aside from residual ocean swells into the SCA range of near 5 ft, expecting otherwise sub-SCA conditions with a return of high pressure for Sunday night. A relaxed pressure gradient over the waters will limit winds to around 10 kt or less Monday-Tuesday, with seas/waves 4 ft or less. The pressure gradient tightens a little Tuesday night and remains so into Wednesday night, with winds up to around 15 kt, and seas remaining 4 ft or less. The pressure gradient relaxes slightly late Wednesday night and Thursday morning, with winds falling off to around 10kt or less on all waters. The pressure gradient then begins to tighten through Thursday night with sustained winds increasing to up to 20 kt with gusts up to 30 kt possible, highest gusts over the coastal ocean waters. Seas on the coastal ocean waters build to Small Craft Advisory levels on Thursday and remain up Thursday night. && .HYDROLOGY... Rainfall late today through Sunday is expected to produce around 0.8 to 1.3 inches of rain, much of it on Sunday. There could be some locally higher amounts, especially in any areas of orographic lift. With saturated grounds, minor poor drainage flooding is possible on Sunday with the heavy rain. It should be dry From Monday through Wednesday night, with no significant hydrologic impacts expected. There is the potential for a significant rainfall (1 or more inches) from late Thursday into Saturday. At this time it is too early to specify exact impacts, other to note that at a minimum there is the potential for minor flooding of urban and poor drainage areas. && .TIDES/COASTAL FLOODING... Easterly SCA winds will likely result in 1 to 1 1/2 ft surge and 3 to 4 ft waves into Western LI Sound, which may result in some localized minor flood/wave splashover in vulnerable areas with Sunday morning high tide, particularly the southwest CT shoreline. A coastal low that could be off the Mid Atlantic coast/S to SE of Long Island from Thursday well into the weekend, could produce a prolonged period of moderate to potentially strong E-NE flow over the region. As a result, there is the potential for at least minor coastal flooding and beach erosion from late Thursday into the upcoming weekend. Stay tuned as the forecast track and strength and for this system, and hence the potential impacts become better resolved with time. && .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...None. NY...None. NJ...None. MARINE...Small Craft Advisory from 3 AM to noon EST Sunday for ANZ335- 338-345. Small Craft Advisory from 3 AM to 6 PM EST Sunday for ANZ330- 340. Gale Warning from 3 AM to noon EST Sunday for ANZ350-353. Small Craft Advisory from 3 AM Sunday to 7 AM EST Monday for ANZ355. && $$ SYNOPSIS...JM/Maloit NEAR TERM...JM SHORT TERM...JM LONG TERM...Maloit AVIATION...JC MARINE...JM/Maloit HYDROLOGY...JM/Maloit TIDES/COASTAL FLOODING...JM/Maloit EQUIPMENT...

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