Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 190442 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1242 AM EDT Sun Mar 19 2017 .SYNOPSIS...
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Low pressure off the southern Delmarva Coast around midnight will strengthen as it passes to the southeast of Cape Cod on Sunday. High pressure builds in Sunday night and Monday. A weak disturbance is expected to move through the area Monday night before a cold front arrives Tuesday night. Expansive high pressure builds in from the northwest Wednesday before the center of the high retreats off the Mid- Atlantic coast Thursday night. A warm front lifts through the region Friday as the next low pressure system tracks into the Upper Midwest.
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&& .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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Surface low pressure was located well off the Delmarva coast around midnight. The resulting northeast surface flow was beginning to bring dry air into our region. As a result, we have removed any mention of precipitation from the Poconos, the Lehigh Valley and northern New Jersey. Cloud bases are expected to rise gradually over those areas during the balance of the night. The northeast flow may continue to bring light rain off the ocean and into parts of southern New Jersey and far southeastern Pennsylvania overnight. Low clouds will linger over those areas. Meanwhile, a mid level low will continue to drop from central Pennsylvania to eastern Virginia overnight. The feature will bring some light to moderate precipitation to the Chesapeake Bay region and to the Delmarva Peninsula. The precipitation type will be mainly rain. However, where precipitation rates increase the rain may mix with or change to snow as cold air is drawn down from aloft. We could see a very light coating of snow mainly on unpaved surfaces in parts of western and southern Chester County, northwestern Delaware and northeastern Maryland. The northeast wind is forecast to back toward the north overnight. Minimum temperatures are expected to be in the middle and upper 20s in the Poconos and in far northern New Jersey, and in the 30s elsewhere in our forecast area.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 AM MONDAY/...
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Any lingering showers across Delmarva will end shortly after sunrise. Other than that, fair weather is expected Sunday as high pressure builds into the area from the NW. Drier air will arrive on a gusty Nrly flow with gusts over 20 mph for the morning. The winds will trend lighter by evening. High temperatures Sunday will be a little below normal, reaching the low 40s north and mid 40s over southern NJ and Delmarva.
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&& .LONG TERM /MONDAY THROUGH SUNDAY/...
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Shortwave ridging aloft builds downstream into our area Sunday night and Monday as the closed upper low drifts farther off the Mid- Atlantic coast. Breezy northerly winds will linger near the coast Sunday evening before the pressure gradient weakens over the course of the night with the surface high moving in from the west. Accordingly, the coldest spots are forecast to be farther inland (west of the Fall Line) where the radiational cooling conditions will be most favorable. Lows range from the low/mid 20s in NE PA and NW NJ to low/mid 30s in the I-95 cities and along the coast. Temperatures continue to trend toward normal on Monday with highs in the 40s N/W of the Fall Line and low/mid 50s farther S/E. A weak disturbance that is currently over California trek across the CONUS along the northern periphery of the upper ridge, reaching the Mid Atlantic Monday night. A brief (3-6 hr) period of light precipitation may accompany the trough passage. Partial thicknesses show the potential for some sleet to mix in north of I-80. No accumulations are currently in the forecast. Tuesday should remain dry in between systems. The big weather story for Tuesday is the potential for temperatures to return to climo if not a couple of degrees above for the first time since March 9th. Phasing between a Pac NW disturbance and a piece of the polar vortex over Baffin Bay is forecast to occur early in the week. This will allow the polar jet to amplify and dig southward toward the Northeast states late Tuesday-Wednesday. Meanwhile, high pressure currently over Alaska will build southeastward behind the trough. Northwesterly flow around the high will advect CP air into our region Tuesday night-Wednesday. A potential interesting setup may unfold during this time as a wave of low pressure is forecast to develop along a W-E baroclinic zone on the southern periphery of this polar airmass. Guidance is currently in very good agreement with the track of the secondary low over the Carolinas. These models also shows the northern edge of the precip shield expanding toward the Delaware Bay, but the colder air arrives too late to allow for a changeover to snow. It certainly bears watching. High pressure moves into the Mid-Atlantic region Wednesday night- Thursday. After a brief respite from the late-season cold early in the week, temperatures return to below normal. High pressure progress offshore by Friday. Southerly return flow around the high will yield milder conditions to end out the week. Low chance PoPs (just rain) were added to the forecast for NE PA and NW NJ for late Friday when a disturbance moving atop the ridge passes to our north. Rain chances increase next weekend with a more organized low approaching from the west. Timing is still uncertain with the more progressive model solutions indicating Saturday is the wet day while slower solutions hold off the rain until Sunday.
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&& .AVIATION /06Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Conditions are forecast to continue lifting from IFR to MVFR during the balance of the night. Some light rain is expected around KPNE, KPHL, KILG, KMIV and KACY. Conditions are expected to improve to VFR on Sunday with a decreasing cloud cover. A northeast wind around 10 to 15 knots with gusts into the 20s overnight should become northerly for Sunday. OUTLOOK... Sunday night...MVFR CIGs w/ stratocu could possibly linger into the evening. However, clearing will occur from west to east later in the night. Nly winds 10-15 kt early will back slightly out of the NW and diminish. Monday..VFR. Light NW winds. Monday night...Mainly VFR but MVFR is still a possibility with a brief period of SHRA. Tuesday through Thursday...Predominately VFR. Low pressure passes to our south but still soon to rule out it bringing light precip to our southern terminals late Tuesday-early Wednesday if the track of the system trends northward. Winds N to NW for most of the period with the strongest winds expected to occur on Wednesday (gusts around 25 kt possible).
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&& .MARINE...
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Seas were increased a foot Sunday morning and we added lower De Bay to the gale warning. While there is a little uncertainty regarding the strength of the northeast wind gusts that will arise near 08z along the De Coast, there appears to be enough information to expect a period of 35 kt to possibly 40 kt gusts for 3 to 6 hours in the 08-14z time frame. This in part to weak caa in the strengthening ne gradient, as well as pres rises associated with the lobe of trailing low pressure moving east southeast out of VA into the ocean (that low vertically slanted westward to the esewd moving oncoming cyclonic circulation system aloft). Scattered areas of light rain are expected tonight and fair weather for Sunday. OUTLOOK... Sunday night...SCA will likely be needed once GLW headline is dropped since northerly winds are forecast to gust 25-30 kt and seas in our coastal waters are between 5-8 ft. Monday...Winds weaken but seas may take some time to subside to below 5 ft so a SCA may need to be extended for our coastal waters through at least the morning, if not later. Eventually, conditions fall to below SCA criteria. Tuesday...No marine hazards anticipated. Wednesday...NW winds increase in conjunction with a steady pressure rises. A SCA may be needed with GFS soundings indicating the potential for gusts right around 25 kt starting early in the morning Wednesday and continuing into the evening. Thursday...Wind and seas below SCA criteria.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Warning until 2 PM EDT this afternoon for ANZ431-450>455. Small Craft Advisory until 2 PM EDT this afternoon for ANZ430.
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&& $$ Synopsis...Klein Near Term...Iovino Short Term...O`Hara Long Term...Klein Aviation...Iovino/Klein Marine...CMS/Klein/O`Hara

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