Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 200809 AFDOKX Area Forecast Discussion National Weather Service New York NY 409 AM EDT Thu Jul 20 2017 .SYNOPSIS...
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Hot and humid weather will continue across the Tri-State Area today. A weak cold front will pass across the area tonight into Friday. This cold front will stall south of the area late Friday and remain around through the weekend before finally moving south as a cold front early next week. High pressure follows through mid week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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A deep WNW upper flow zonal flow will exist today around an anomalously strong polar low moving across Hudson bay. At the surface weak diurnal surface/thermal troughing will develop over the region. Although marginal to moderate instability development is expected today...there is a lack of a trigger for convection and overcoming low-level capping. Higher instability/dewpoints once again looks like it will be along the coast...where the thermal trough/sea breeze could act as focus for isolated pulse convection across NYC/LI this afternoon if any weak vort energy tracks through. Otherwise...main story will be heat today. Deep mixing and warm temps aloft should have temps rising into the lower to mid 90s for much of the region. A few upper 90s possible for NYC/NJ metro. As was the case yesterday, deep mixing and subsidence will likely promote dewpoints to mix out into at least the the lower to mid 60s in the afternoon across areas N&W of the sea breeze (NYC/NJ metro and interior). Meanwhile...south coastal areas will likely see dewpoints hold in the lower to possibly mid 70s with sea breeze development off 75 degree ocean. Based on 95-100 heat indices being reached across the NY/NJ metro area as well as coastal SW CT and the Eastern 2/3rd of LI for a second day, have continued/expanded heat advisory into these areas.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
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An active near zonal flow will develop as a anomalously strong polar low tracks through Northern Ontario/Quebec. A weak cold front approaches tonight as the first in a series of shortwaves tracks through the NE in the developing zonal flow. Main forecast challenge and concern for tonight into early Friday morning is progression of any MCS activity into the region. The shortwave energy emanating from the activity currently occurring over the Upper Mississippi River Valley/Great Lakes this morning will likely provide trigger for upstream convection (Eastern Great Lakes) this afternoon/evening which could track over the region tonight. Considerable model spread exists on the track and intensity of any activity across the region tonight. This is due to differences in the favorability of the environment (particularly mid-level lapse rates) and the unpredictability of the interaction of a yet to form convective complex with this environment. So overall the threat for a strong MCS passing through the region tonight into early Friday morning, with an associated flash flood and damaging wind threat, is low. Better chance is for scattered shower and embedded thunderstorm activity as shortwave energy moves through the region and interacts with a moist and marginally unstable airmass over the area. An isolated strong storm is possible in this scenario, with a low chance for minor urban flooding. Pretty good agreement that any shortwave energy and convective activity pushes east by Friday morning. In its wake, deep W/NW flow and subsidence will favor a hot and dry day. Temps across the coastal plain will likely rise into the lower 90s to 95, with upper 80s to lower 90s interior. Deep mixing and subsident NW flow should promote lower dewpoints area wide. At this time, it appears that heat indices could marginally reach 95 degrees across the NYC/MJ metro and LI, but not enough confidence to extend advisory at this time as dewpoints may mix out more than forecasted.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
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Although this period will start dry on Friday night, there is an increasing chance of multiple rounds of heavy showers and TSTMs from Saturday afternoon through Monday night. The weather pattern favors the development of several Meso Convective Systems/MCSs caused by short waves rotating SE arnd a canadian polar vortex. There is considerable uncertainty regarding their track and intensity, depending on where they actually develop. Adding to this complexity is where the approaching warm front stalls late Saturday - Sunday, and the subsequent development of a low pressure wave along the front late Sunday - Monday. There is the potential for heavy showers, TSTMs, and strong gusty winds during this time. Stay tuned. Weather conditions will finally improve with the passage of a cold front early Tuesday, bringing more comfortable late July weather to the area next Tuesday through Thursday.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/... A weak trough of low pressure gradually moves across eastern sections of the region overnight before a weak cold front slowly approaches Thursday. Mainly VFR conditions are forecast through Thursday. Will be monitoring for low stratus and patchy fog for KISP, KGON and KSWF overnight, but confidence of this occurring is too low so was left out of TAFS for KISP and KSWF. A general W-SW flow becomes light and variable overnight into early Thursday with W-SW flow re-developing for the late morning/afternoon. Wind speeds 10-15 kt through early evening with some gusts 15 to 20 kt with otherwise 5-10 kt flow. Therefore a few terminals may gust for a few hours for the late afternoon and early evening on Thursday. .OUTLOOK FOR 06Z FRIDAY THROUGH MONDAY... .Thursday Night...Chance of thunderstorms with sub VFR conditions, with improving conditions late. .Friday and Friday Night...VFR. .Saturday...MVFR or lower possible with a chance of showers and thunderstorms. .Sunday-Monday...MVFR or lower possible with a chance of showers and thunderstorms. && .MARINE...
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A weak pressure gradient over the region will keep sub Small Craft Advisory/SCA conditions through Friday. Although winds and seas are forecast to remain below SCA levels of 25 kt and 5 ft from Friday Night through Monday, there will be several rounds of showers and TSTMs starting Saturday afternoon through Monday. Winds and waves may be briefly higher in and around TSTMs. Stay tuned to possible Marine Weather Statements and/or Special Marine Warnings during this time. Note that areas of fog may temporarily reduce the VSBY to near 1 NM Saturday afternoon and night as a warm front moves north across the coastal waters. Depending on the exact track of the stalled front, southerly swells may begin to increase through the weekend into early next week, with a low probability for SCA conditions on the ocean by Monday. Weather conditions will improve following the passage of a cold front on Tuesday morning, when winds become offshore.
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&& .HYDROLOGY...
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There is a low threat for a convective complex to track across the area tonight into early Friday morning. The main threat if this activity materializes would be minor urban flooding, with a low threat for flash flooding. There is a low chance for additional urban flooding from Saturday afternoon through Monday night as a slow moving front interacts with a moist and unstable airmass. This will bring a threat for periods of heavy showers and thunderstorms.
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&& .OKX WATCHES/WARNINGS/ADVISORIES...
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CT...Heat Advisory until 6 PM EDT this evening for CTZ009. NY...Heat Advisory until 6 PM EDT this evening for NYZ069>071-078- 080-177-179. Heat Advisory until 8 PM EDT this evening for NYZ072>075-176- 178. NJ...Heat Advisory until 6 PM EDT this evening for NJZ004-006- 103>108. MARINE...None.
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&& $$ SYNOPSIS...GC/NV NEAR TERM...NV SHORT TERM...NV LONG TERM...GC AVIATION...JE MARINE...GC/NV HYDROLOGY...GC/NV CLIMATE...

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