Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 200855 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 355 AM EST Fri Jan 20 2017 .SYNOPSIS... High pressure over the Mid-Atlantic will move offshore early today. Weakening low pressure to our west will lift northeast through the region this afternoon and evening. High pressure returns briefly for Saturday. A strong and complex area of low pressure will impact the Mid-Atlantic from Sunday through Tuesday, with dry weather for the remainder of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Early morning surface analysis shows a 1018 mb high centered along the Mid-Atlantic coast. Looking upstream over our area, a weak wave of low pressure was evident over the Midwest (central IL-IN) with a warm front extending southeastward from the low through the southern Appalachians and along the GA-SC coast. Water vapor satellite loop shows a well-defined upper low spinning somewhere near the Upper Mississippi and Red River basins. A low-level jet continues to draw deeper moisture from the Gulf of Mexico ahead of the system, resulting in an organized band of precipitation over the Southeast and Midwest States overnight. The leading edge of the denser high clouds from the upstream low was overspreading the area early this morning. Additional low clouds were persistent across northeastern PA and northwestern NJ for much of the night. Fog development have been hindered by the cloud cover thus far. Not much more than patchy ground fog is anticipated near daybreak across the forecast area. Hi-res models are in good agreement with the timing of precipitation across the area. Light rain is forecast to arrive in our far southwestern zones (eastern MD shore of the Chesapeake Bay and southeastern PA) during the late morning, then move northeastward into the Delaware Valley closer to midday and finally our far northeastern zones (northwestern NJ and coastal central NJ) during the early to mid afternoon. The rain doesn`t last that long and should end quickly from west to east during the late afternoon. The back edge of the precipitation should already be near the coast and the I-80 corridor shortly after sunset. The upper shortwave trough that will provide the lift for precipitation will become increasingly sheared as it lifts northeastward toward the downstream ridge and while the the upper low reorganizes farther upstream over the Great Plains. Accordingly, light rainfall amounts are favored across our region owing to the weakening dynamics by the time it reaches our area. QPF ranges from one-tenth to one-quarter inch. Several days out from this event, there was a concern for freezing rain in the southern Poconos and far northwestern NJ today. However, the icing threat has decreased as models have trended warmer with temperatures. The stratocu that developed across the northeastern PA and northwestern NJ overnight has kept temperatures up across the far north and ultimately may be our saving grace. Nonetheless, cannot rule out a few icy spots along the ridges of Carbon and Monroe County (mainly at elevations above 1700 ft) late this afternoon and early this evening where wet-bulb temperatures are forecast to be near 32F. Forecast high temperatures range from the mid 30s in the Poconos to around 50F in the coastal plain of NJ and Delmarva. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/... The bulk of the precipitation is expected to have already moved north and east of the area into southern NY and off the coast. Boundary-layer winds are forecast to be light tonight so even with veering flow, low-level moisture and clouds are expected to remain over the area for much of the night. The question is if we see any breaks in the clouds overnight. Dense fog could develop where we see partial clearing. For now, will advertise patchy coverage with the stratus already established. Low temperatures range from the mid 30s across northeastern PA and northwestern NJ to around 40F along and east of I-95. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Sat will be dry with wk high pres over the area. The dry conds will continue into Sat night and erly Sun. Then, low pres over the srn plains will move newd on Sun. Guid has come into better agreement wrt a slower onset time. It now appears that the first part of Sun will remain dry, with precip holding off until aftn. The further n you are the better chc you are of staying dry thru the day. But even the srn Delmarva will stay dry thru the mrng hours based on current guid. Precip will overspread the area thru the aftn hours. It looks like there will be a pd of steady, heavy rain Sunday night, especially over srn areas, though the guid still differs on where the heaviest rain will fall and when. The GFS wants to bring in a lull in the precip area wide on Mon mrng while the ECMWF keeps it wet. The EC has perhaps a lull later Mon. But it seems there will be a second wave at some point on Mon, again, exactly where and when is difficult to pinpoint attm. Yet a third wave looks to move thru Mon night before the low pulls away from the region and things dry out durg the second half of Tue. Temps look to be above nrml durg this whole event and the vast majority of the region will see plain rain durg the whole time frame. However, temps could be marginally cold enough in the Poconos and portions of Sussex County NJ Sun night for some light amounts of freezing rain and there could be some light amounts of snow as the cold air wraps around the low Monday night. Neither of these look to be significant events, but obviously bear watching. High pres builds in behind the low for a dry Tue night into Wed, though there cud be a passing shower in the nw, with a wk cdfnt. More genly dry wx is expected for Thu. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Mainly VFR this morning with high clouds streaming in. However, patchy ground fog may develop briefly toward sunrise in the typical fog-prone rural terminals. The TAFs have trended away from fog and low clouds early this morning. Light and variable winds under 5 kt early this morning. Rain will overspread the area from SW to NE between 15-18Z. After a few hours, the rain will quickly end from W to E between 20-23Z. CIGs/VSBYs will lower to MVFR after the rain arrives. Think LAMP guidance is too pessimistic for this afternoon and even into tonight, showing an extended period of IFR conditions this afternoon and LIFR tonight. The current thinking is we will eventually get to IFR and LIFR in spots but it will take longer (maybe not until after sunset in spots) than indicated by guidance. OUTLOOK... Sat thru Sat night...Some MVFR or IFR in low clouds or fog erly, otherwise VFR. High confidence. Sun...Conds deteriorating from S to N during the aftn with IFR conditions psbl. Moderate confidence. Sun night thru Mon night...IFR and lower conds psbl in RA. E winds 15-20 KT with 20-30 KT gusts at terminals north and west of I- 95, and 20-30 KT with 30-45 KT south and east of I-95 psbl on Mon. Decreasing wind Mon night. Moderate confidence. Tue...IFR/MVFR erly with RA then improving to VFR by eve from s to n. Moderate confidence. && .MARINE... No hazards are anticipated on the waters today and tonight. Wave heights in our coastal zones continue to hold steady around 4 ft with longer period easterly swells. Seas will gradually subside to 2- 3 ft by tonight. OUTLOOK... Sat through Sun morning...No marine headlines anticipated during this time. Seas will genly be around 2 to 3 ft with wind 10 kts or less. Sun aftn thru Mon night...Easterly flow increases to 15-20 KT with 25-30 KT gusts starting Sun aftn, and then gale force gusts of 35-40 KT likely Monday and Monday night. Pds of mdt to hvy rain with low VSBY expected. Storm force winds possible, especially over the nrn waters on Monday. Conditions begin to improve late Monday night, and winds decrease to SCA levels. However, seas on the ocean should remain above SCA criteria. Mon night thru Wed...wind decreases below headline criteria, but ocean seas will remain above SCA criteria. It is possible that some areas, especially across southern waters, may drop below SCA seas on Wed. && .TIDES/COASTAL FLOODING... Due to the persistent and increasing easterly flow around a coastal low Sunday-Monday, there could be several tidal cycles of coastal flooding, but this will also be dependent on the track of the low and will become more evident as we approach Sunday night and Monday. Latest guidance indicates the potential for widespread minor coastal flooding, but that will come into better focus this weekend. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Nierenberg Near Term...Klein Short Term...Klein Long Term...Nierenberg Aviation...Klein/Nierenberg Marine...Klein/Nierenberg Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.