Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 121440 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 940 AM EST Tue Dec 12 2017 .SYNOPSIS... A low pressure system will cross to the north of the region today, dragging a cold front through this evening. High pressure builds in for Wednesday. A weak low pressure system will cross the region on Thursday with another system moving up the coast on Friday. A brief return to high pressure for the start of the weekend with yet another system moving through our area late Sunday or Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Overall, fcst in good shape. Temperatures were adjusted upward a few degrees across the Delaware Valley with continued warm air advection ahead of the approaching front. Forecast looks to be on track given the most recent guidance. Surface low is moving rapidly into the Northeast early this morning, with warm air surging northward in our area downstream. Most precipitation is located to the north and west of the area this morning, but this will change as the attendant vort max moves through the Appalachians this morning. However, as this occurs, the vorticity will become elongated owing to downstream ridging, and large-scale ascent will be weakening. The vort max will also pivot northeastward, taking the strongest lift mostly to the north of the region. Thus, the best chances for precipitation this morning will be associated with retreating isentropic ascent in the northern CWA and with and just in advance of the main cold front. Regarding the former, convection-allowing models (CAMs) are in solid agreement, generating around 0.05-0.15 inches of liquid-equivalent precipitation thru 15Z, generally north of the I-78 corridor. The precipitation associated with the frontal passage appears to initiate late this morning as jet dynamics improve (left- front quadrant of a strong cyclonically-curved upper- level jet streak along the coast of the Southeast U.S.) and a kicking perturbation ejects eastward into the Mid-Atlantic (i.e., favorable differential cyclonic vorticity advection). Both precipitation regimes will be brief, light, and generally low- impact, but thermal profiles are favorable for a wintry mix to begin with in portions of the Lehigh Valley and all snow in the southern Poconos, even with near-surface temperatures warming above freezing. Meanwhile, precipitation will be all liquid to the south as this first frontal band moves through (generally 15Z to 20Z). However, cold air will spill into the CWA rapidly, and low-level thermodynamic profiles become favorable for snow showers, even with surface temperatures perhaps several degrees above freezing this afternoon. With strong low-level winds and residual low-level moisture, snow squalls are a distinct possibility, especially north of the Philly metro. This threat looks to be mainly confined to the afternoon hours and should end quickly after sunset. The other story will be the winds, with west/northwest surface gusts kicking up quickly after cold frontal passage. Expect gusts 25 to 35 mph, possibly a little higher, just after frontal passage. Temperatures will fall noticeably during the late afternoon, setting up a blustery period the next couple of days. Regarding temperatures, stuck close to a blend of continuity, hi-res 2-m temperatures, and the LAV. Statistical guidance will likely perform poorly today owing to the rapidly evolving synoptic and subsynoptic pattern.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... Snow showers may occasionally impact the far northern CWA tonight as cold-air advection continues in a northwest-flow regime. Lake-effect streamers look to affect at least the Poconos, where PoPs are highest, but the models look pretty dry, so kept a dry forecast after 00Z virtually everywhere else. Will need to watch the late night period, though, when another potent vort max in a never-ending series reaches the Mid-Atlantic. This may enhance lift and readily generate snow showers in the higher terrain of the far northwest CWA. Total snowfall for this area should generally be two inches or less, though most of it may fall in the showery regime this afternoon and tonight rather than early this morning. The winds and the chill will be the main story tonight, however, with west to northwest winds howling (though likely sub-advisory). With temperatures falling to near 10 in the Poconos and the lower 20s in the urban corridor and the beaches, this will be a bitterly cold night for the area. Wind chills near or below zero are forecast for the southern Poconos, and will only be about 10 degrees higher in the greater Philly metro area. Needless to say (but I will anyway), bundle up. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... A series of low pressure systems will cross the region through the extended period bringing repeated chances for some snow. Wednesday...Windy day ahead as the low continues to slowly move to the northeast. Cold air will fill in across the region and even though we will see a decent amount of sun across the area, highs will be cold and remain in the 20s to lower 30s. With the winds blowing it will feel even colder as it will feel more like it is in the single digits to teens outside. Wednesday night through Thursday...The winds start to decrease through the overnight period and that bitter chill will start to wane. Overnight lows will drop back in to the teens to lower 20s. A strong shortwave and a weak surface low will cross the region late Wednesday into Thursday. This clipper system brings a chance for snow showers across the area. The cold air looks to remain in place but enough warming may occur closer to the coast to allow for some mixing with rain to occur. The precipitation should be clearing out by the afternoon with any lingering showers clearing the coast through Thursday evening. Thursday night...High pressure will cross the region overnight and we should have a brief break between systems. Lows will fall into the teens to mid 20s across the region. Friday through Friday night... Some disagreement in the models with respect to Friday and a low pressure system developing to our south and then up the coast. Current guidance has the low staying south and moving to the northeast but just how much moisture will make it to our area is unknown as the proximity to the low will be key. Precipitation may end up being mixed across parts of the area but uncertainty as to how much warm air can make it in exits. The GFS has a deeper low and has it closer to the coast while the ECMWF continues to have a much weaker low (until it reaches the higher latitudes) and further from the coast. Will continue to mention the chance for some snow (maybe some mix along the coast) for Friday. Both models show the low pulling away fairly quickly so any precipitation should end from west to east Friday night. Saturday through Sunday...High pressure will slide across the southeastern United States on Saturday, bringing us a day of quiet weather. The weather should remain quiet through at least part of the day on Sunday. Precipitation from the next system may start to arrive by Sunday afternoon but confidence is low with respect to timing this far out. Temperatures look to be a bit more moderate over the weekend as warmer air arrives. Temperatures look to be in the 30s to lower 40s on Saturday and in the upper 30s to upper 40s on Sunday. Sunday night through Monday...Another system will approach the region sometime around Sunday night or Monday. With the warm air slated to hang around through Monday, we may see a broader area of rain and/or mix of rain and snow as the precipitation overspreads the region. Lots of uncertainty with this late period system. && .AVIATION /15Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Generally VFR, though brief sub-VFR conditions may occur with a band of showers that develops and progresses through the area later this morning and this afternoon. CIGs should generally stay in the 3500-7000 ft range today outside of precipitation. Rain/snow showers may develop this afternoon, especially north of PHL, with a snow squall not out of the question. So far, coverage and timing remain too low/uncertain for inclusion in any of the TAFs. Winds will veer from south to west today and become strong after a cold front passes the area this afternoon. Winds 5-15 kts this morning become 10 to 20 kts with gusts 30+ kts possible by evening. Low confidence in precipitation coverage/timing; medium confidence in CIGs and winds. Tonight...Generally VFR (though sub-VFR possible in the southern Poconos occasionally with snow showers). West or northwest winds 15 to 25 kts with gusts 25 to 35 kts likely. Medium confidence in CIGs/VSBYs; high confidence in winds. Outlook... Wednesday...Mainly VFR conditions expected. Gusty west to northwest winds around 15 to 25 knots with gusts around 40 knots possible. A few snow showers possible, mainly north and west of KABE/KRDG. Confidence: High Thursday through Friday...Mainly VFR conditions expected. MVFR or lower possible in snow showers. West to northwest winds around 5 to 10 kts. Confidence: Moderate Saturday...Mainly VFR conditions expected. West to southwest winds around 10 knots. Confidence: Moderate && .MARINE...
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Gale warning, effective from 6 pm this evening to 6 pm Wednesday. Until then, the small craft advisory remains in effect, as winds are now reaching criteria on the coast and on Delaware Bay.Today, winds will be primarily southwesterly in advance of a cold front that will reach the waters this afternoon. After the cold front passes, winds will switch to west or northwest and become very strong (speeds of 20 to 35 kts with gusts to 45 kts). There is a non-zero chance of storm criteria being reached in spots tonight, though the most likely outcome is a strong gale event. Outlook... Wednesday and Wednesday night...Gale force gusts expected through the day with gusts of 40 to 50 knots possible. Winds should fall below gale force late Wednesday night. Seas will also remain elevated around 5 to 7 feet on the ocean. Seas will start to subside Wednesday night. A Small Craft Advisory will be needed once the gales subside. Thursday through Friday...Small Craft Advisory conditions may continue into the early afternoon on Thursday. Winds subside below 25 knots by the afternoon and remain fairly quiet on the waters through Friday. Friday night through Saturday...Small Craft Advisory conditions possible. West to northwest winds will gusts around 25 knots possible.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ430- 431-450>455. Gale Warning from 6 PM this evening to 6 PM EST Wednesday for ANZ430-431-450>455. && $$ Synopsis...Meola Near Term...CMS/PO Short Term...CMS Long Term...Meola Aviation...CMS/Meola Marine...CMS/Meola

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