Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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130 FXUS61 KPHI 071451 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 951 AM EST Wed Dec 7 2016 .SYNOPSIS... Low pressure off the mid Atlantic coast will move out to sea today while another area of low pressure drifts through southeast Canada. This low will continue to slowly drift eastward through the end of the week and move offshore of the Canadian maritimes by Friday and Saturday. With high pressure to our west, a strong west to northwest flow will remain through the end of the week. High pressure briefly affects the area Saturday night. An area of low pressure is forecast to move north of the area Sunday into Sunday night, bringing a cold front across the area. && .NEAR TERM /THROUGH TONIGHT/... 930 AM ESTF: Cleaned up the drying process a little and slowed down the temp rise this morning but back on the previous fcst this afternoon. Measurable drizzle for parts of NJ will end from west to east by midday. Eastward moving trough near 950 mb will probably help dissipate the westward inverted trough from the mid Atlc coastal low. The result: Low clouds and drizzle gradually dissipate to cigs near and above 3000 ft. Some clearing along and south of I-78 should occur this afternoon. High temperatures will generally be in the 45-50F range with cooler spots in the higher elevations of NE PA and NW NJ (mid 30s to lower 40s) and warmer spots in S DE and E MD (low 50s). Tonight...An incredible 1054 mb surface high over the northern Rockies is forecast to build southward into the Great Plains tonight. An extension from this high will ridge eastward toward the Mid- Atlantic region tonight. Meanwhile, an area of low pressure will move across James Bay and into northern Quebec tonight. This system`s cold front advances southeastward into the Great Lakes region. The pressure gradient is modest but it may still be strong enough to produce a light W-NW across most of the area. However, the sheltered valleys will likely see the boundary layer decouple and temperatures dropping rather fast after sunset. Accordingly, we`ll likely see a decent spread in temperatures tonight depending on location. High clouds associated with a strong upper-level jet streak are expected to move into the region from SW to NE overnight, but the impact on radiational cooling is questionable. A blend of the MAV/MET was used to make the min temperature forecast, but also used some of the mesoscale model guidance (RGEM, NCEP WRF ARW/NMM) to highlight the cooler valleys. && .SHORT TERM /THURSDAY/... Considerable mid and high cloudiness during the morning becoming mostly sunny and breezy during the afternoon with westerly winds gusts expected to increase to 25 mph. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... On Thursday, an area of low pressure will slowly be moving across eastern Canada, before reaching the Canadian maritimes Friday into Saturday, while high pressure remains across the central part of the country. This will keep a steady west to northwest flow across the area. A broad trough aloft will remain across the east coast, with an elongated vorticity impulse moving across the area Thursday and a weaker impulse moving across the area Friday. There is not a lot of moisture across the area Thursday as it`s forecast to remain northwest of the area, but the strength of the vort max and the surface winds may help carry some flurries/sprinkles across the area during the day. On Friday, the short wave is weaker, but there is more enhanced low level moisture and steep low-mid level lapse rates during the day, so there could be even more widespread showers or flurries/sprinkles across the area. Especially across our northwestern areas. By Saturday, another short wave/vorticity impulse is forecast to move across the area as the northwest flow begins to weaken as the low lifts farther away and high pressure begins to approach from the west. There will continue to be steep low-mid level lapse rates during the daytime combined with some enhanced moisture, especially across the northern areas. Therefore there will continue to be a chance for some isolated showers/flurries across the northern half of the area. High pressure briefly affects the area Saturday night, providing dry conditions. Thursday through Saturday will be a cold and windy period, with Friday being the windiest, and Saturday being the coolest of the 3 day period. On Sunday night, an area of low pressure is forecast to move north of the area, pulling a cold front across the region. Another low pressure is forecast to develop along the boundary, although the GFS and ECMWF differ on where they develop the low. The GFS develops this secondary low north of the area, while the ECMWF develops it to our south. Sunday night looks like the most likely period for precipitation, but it could linger into Monday if the low does develop around or south of the area as the ECMWF indicates, so we`ll keep a chance of precipitation into Monday. Temperatures will be cold enough for portions of the area to experience wintry precipitation. The exact timing and placement of wintry precipitation is still to be determined, but as usual, the most likely areas for snow would be the northern half of the area. Dry conditions are currently expected for Monday night into Tuesday as high pressure is forecast to briefly build to our south and nose its way into our area. The GFS and ECMWF are showing the potential for a coastal low to develop on the old frontal boundary to our south by next Wednesday, which could bring some precipitation to the southern half of the area. With cold temperatures, it`s possible that a period of snow could develop for the southern portions of the area. && .AVIATION /15Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today...IFR conditions in drizzle/fog many of the TAF sites at 1445z will improve from southwest to northeast late this morning, first to mvfr cigs and eventually to VFR cigs this afternoon. Wind generally northwest with gusts under 15 kt. Tonight...VFR. W-NW winds will be light (under 10 kt). Thursday...VFR multi layered cigs aoa 10000 ft during the morning becoming VFR clear by late in the day. Westerly wind with gusts 20-28 kt during the afternoon. OUTLOOK... Thursday night-Saturday...MVFR CIGS may be possible each morning before lifting to VFR during the day. Scattered showers or flurries/sprinkles are possible during the daytime of each day which may temporarily lower conditions. Gusty west to northwest winds 15- 25 knots, strongest winds Thursday and Friday. Sunday...VFR early, lowering to MVFR then IFR by the afternoon and overnight. Rain or snow possible late in the day and overnight. && .MARINE... SCA converted to SCA seas with a caveat for isolated 25 kt gusts this morning near the entrance to DE BAY. Seas are still running well above predicted in response to last evenings gale and the fcst adjustments were made. Pls see MWW or the CWF. Seas subside this evening. Thursday...anticipating the need for a westerly flow cold air advection small craft advisory for many of the waters Thursday afternoon. OUTLOOK... Thursday night-Saturday...Small Craft Advisory levels winds expected through the periods with west to northwest winds gusting 25-30 knots. Saturday night-Sunday...Winds expected to drop below Small Craft Advisory levels, but pick up again late Sunday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory for hazardous seas until 10 PM EST this evening for ANZ452>455. Small Craft Advisory for hazardous seas until 6 PM EST this evening for ANZ450-451. && $$ Synopsis...Robertson/drag 950A Near Term...Drag 950A Short Term...Drag 950A Long Term...Robertson Aviation...Drag/Robertson 950A Marine...Drag/Robertson 950A

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