Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 231951 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 351 PM EDT Mon Apr 23 2018 .SYNOPSIS... Large area of high pressure will continue to build away from our region. Meanwhile, a low in the lower Mississippi Valley will slowly lift into the Mid Atlantic states by Wednesday. Another low pressure system passing to the south of our region, could bring another chance of rain late this week. By late in the weekend, high pressure begins to build south towards the region. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Quiet conditions will continue tonight across the area as high pressure remains offshore of the New England coast. Our area will remain under the influence of southeast return flow, which will allow moisture to increase some overnight. Overnight lows will drop through the night as winds diminish and mostly clear skies will begin the night. However, with the flow being more from the southeast, and the fact some clouds may begin moving in late in the night, overnight lows are not expected to be as cool as Sunday night. Most places should drop close to normal, except portions of the Pine Barrens which might drop a few degrees lower and have some patchy frost develop. We will include patchy frost in the forecast, but not issue a Frost Advisory yet as there is still uncertainty as to how low the temperatures will get. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... Dry conditions will begin the day Tuesday, before rain chances increase through the day. An occluding low pressure system will continue moving across the southern Appalachians early in the day, before slowing moving into the Carolinas by the afternoon. Our area will be well north of the low pressure system and the slowly lifting warm front through the daytime hours. However, south-southwest flow aloft will lead to increasing moisture through the day. A couple of short wave/vorticity impulses will lift across the area through the day as well. Rain chances increase, especially during the afternoon as the second of the short waves moves through and the best moisture/lift combination takes place. There is little to no instability forecast, so no thunderstorms are expected at this time. Winds will become gusty through the morning and into the afternoon with gusts reaching 20 to 25 mph at times. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Overview: The main focus through this period remains on the Tuesday night and Wednesday time frame and the potential for heavy rain (though this threat appears to be decreasing). After that, the next chance for rain looks to be primarily on Saturday as our region will be in between a low off the coast and a cold front approaching from the west. Details: The low continues to very slowly progress northeast, reaching the Mid Atlantic by Wednesday. The window for heavy rain has narrowed a bit, now looks to be confined mainly late Tuesday night, though lingering light rain is possible into Wednesday evening. As for the heavy rain threat, precipitable water values on Tuesday night are still expected to be well above normal, possibly in the 90th percentile for this time of year. However, there are a few factors working against the heavy rain threat. The warm cloud layer is relatively shallow (generally 8000 to 9000 ft), thanks to continued below normal temperatures. Additionally, storm motions aren`t expected to be particularly slow, around 20 mph, although training storms could make this point moot. In general, it looks like the highest threat for heavy rain will be over the coastal plains of Delmarva and far southern NJ. There is some very limited elevated instability Tuesday night, but it is very elevated (generally for parcels at or above 850 mb). Thus, have not mentioned thunderstorms in the forecast for now, but will continue to monitor this potential as we get closer. For the late week period, there remains some uncertainty in how two low pressure systems, one lifting out of the southeast and becoming a coastal low and one digging out of the north central U.S., will interact and what the impact will be for our region. However, the models today are in better agreement than what we have seen the last few days, with a brief chance for rain on the back side of the coastal low on Friday night or Saturday, and a limited impact from the cold front moving in from the west. && .AVIATION /20Z MONDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR conditions will continue through tonight. Winds will be from the south-southeast around 5-10 knots through this evening, but will become light and variable most places overnight. Tuesday...VFR conditions will begin the day across the TAF sites through at least midday Tuesday. However, clouds will thicken and lower through the afternoon, and could eventually reach MVFR by late in the afternoon or early evening. Rain will also be moving from the south during the afternoon hours. Outlook... Tuesday night and Wednesday...MVFR and even localized IFR conditions will be possible especially through Wednesday morning as periods of rain are likely across the region. Easterly wind gusts up to 20 kt are possible. Moderate confidence on flight categories, but low confidence on the timing of any flight category changes. Thursday...VFR conditions expected with light westerly winds. High confidence. Friday and Saturday...Mostly VFR conditions are likely, though there is a chance that rain and low clouds could lower conditions to MVFR at times. Wind is forecast to be westerly or southwesterly near or below 10 kt. Moderate confidence on flight categories and wind. && .MARINE... Conditions will remain below advisory levels tonight and into the first half of Tuesday. However, winds could begin gusting around 25 knots by the afternoon, while seas are expected to build to 5 feet or higher by the afternoon as well. Therefore the Small Craft Advisory remain in place starting noon Tuesday. Outlook... Tuesday night and Wednesday...Easterly and southeasterly winds increase to 15-20 kt with gusts up to 30 kt, and seas build up to 10 feet on the Atlantic coastal waters. Wind gusts near 25 kt are also expected on the Delaware Bay primarily Tuesday night. Wednesday night and Thursday...Winds shift to northwesterly and should decrease. The main question will be how long seas remain above 5 feet on the coastal waters. Current forecast is for elevated seas into late Thursday, but there is still considerable uncertainty with this. Friday and Saturday...Winds and seas are currently forecast to stay below SCA criteria on all waters. However, there remains some uncertainty with if a coastal low develops and intensifies. If this happens, there is a slight chance for SCA criteria, primarily on Saturday. && .FIRE WEATHER... Relative humidity values dropped once again today, similarly to Sunday, into the 20 and even teens in some areas. However, winds have not been very windy except occasional gusts in the teens to around 20 mph in a few spots. The low humidity values will continue into sunset, while winds may increase slightly. However, since winds are not gusting frequently or widespread, we are not expecting any enhanced statements at this time. While it will be a bit more breezy and gusty Tuesday, relative humidity values will be higher on Tuesday than today. So no enhanced statements are expected on Tuesday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from noon Tuesday to 6 PM EDT Wednesday for ANZ450>455. Small Craft Advisory from noon Tuesday to 2 AM EDT Wednesday for ANZ430-431. && $$ Synopsis...Johnson Near Term...Robertson Short Term...Robertson Long Term...Johnson Aviation...Johnson/Robertson Marine...Johnson/Robertson Fire Weather...Robertson

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