Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 141731 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 131 PM EDT Mon Aug 14 2017 .SYNOPSIS... High pressure will influence our weather today. A weak warm front forms tonight along the mid Atlantic coast. Tropical Storm Gert is expected to pass well to our southeast and east on Tuesday and Tuesday night. A frontal boundary is anticipated to arrive from the north during the mid week period, then retreat north by Friday. Eventually this front should move off the mid Atlantic coast during this weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Updated the forecast again based on rapidly improving consensus among the high-resolution guidance. The past three HRRR runs have consistently produced a band of showers moving along and northwest of the I-95 corridor from late this afternoon through this evening, stemming from the shield of light rain/showers in the central Appalachians at this time. There is little to no buoyancy available for convection (as is apparent with the character of the precipitation this morning), but the 12Z NAM Nest (which has come around much more similarly to the HRRR) does generate some isolated convective cells this evening as the band moves through. Will reevaluate the potential for storms before this afternoon`s forecast package is released. Nevertheless, large-scale ascent appears to be sufficient with the vort max moving through this afternoon for a corridor of precipitation. With the evolution/location of the precipitation becoming clearer, raised PoPs along and northwest of I-95 during the 21Z to 06Z time frame (spilling into the short term period). Some further refinements to the forecast may be necessary as the showers approach, but for now, expect little to no thunder and precip amounts generally a trace to a tenth of an inch. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... Updated the forecast again to increase PoPs this evening along and northwest of I-95 based on improved consensus among the hi- res model suite for the vort max moving through the area late today to generate sufficient lift to produce/maintain light showers in a band or bands in this region during the evening hours. The rest of the discussion below remains relevant. For the 930 am update, biggest change was to reduce PoPs for most of the area, except for the Lehigh Valley northwestward, where there is a somewhat stronger signal for meaningful QPF at some point during the night. This will be in association with a second vorticity maximum moving through the region during this period. There is not particularly good agreement within the high-resolution model suite regarding timing of any showers during this period, and given the low-amplitude/weak nature of the associated vort max, this is not surprising. The latest HRRR shows an area of showers moving southwest-to-northeast through the I-76/I-78 corridors this evening into the early overnight period. The 06Z NAM Nest and 00Z WRF-ARW/NMM simulations de- emphasize this band of showers and instead produce a band farther to the northwest later in the night (generally after midnight). Given the uncertainty, think slight-chance PoPs are warranted in most of the region, especially after midnight as the lift associated with the aforementioned vort max moves through. Nevertheless, the variable nature of the past few HRRR simulations and its disparate evolution of the upper-level shortwave trough do not provide much confidence in the forecast. Made few changes to temps/dews/winds, with lows in the lower 60s in the Poconos, near 70 in the urban corridor, and in between elsewhere. Light winds and mostly cloudy skies should be expected. && .LONG TERM /TUESDAY THROUGH SUNDAY/... 500 MB: A short wave trough in southeast Canada grazes the northeast USA midweek followed by weak ridging by Friday with another short wave trough moving through the Great Lakes and Ohio Valley next weekend. Temperatures: So far the month through the first 13 days, is averaging about 1-4 degrees below normal. The GFS is warmer than the EC this 6 day period but both with the idea that it will average about 5 degrees warmer than normal Wed-Thu-Fri, with the EC cooling down to near normal next weekend while the GFS proceeds with above normal temps. Forecast basis: a 50 50 blend of the 00z/14 GFS/NAM MOS was applied Tue-Wed, then the 00z/14 GFS MEX MOS was used for Wednesday night with the 05z/14 WPC gridded elements applied for the Thu-next Sun, except Thu/Fri when the 00z/14 EC 2m temps suggested using the warmer GFS MEXMOS for a better operational model fit. The dailies... Tuesday...Leftover morning showers with late day clearing? Light east during the morning becoming light southwest during the afternoon. Dewpoint 62-63PHl, but 70-75 DELMARVA. 15C at 850MB. 900J MLC Tuesday night...Partly cloudy. Patchy dense fog possible in the countryside. Light west - northwest. Wednesday...Partly to mostly sunny. Light northwest becoming light south to southwest late except a cfp to I-78 late in the day with a windshift to northwest. One or two thunderstorms may form near I-78 late in the day? Dewpoint noticeably warmer (mid- upper 60s PHL and generally 70 elsewhere, except 75 DELMARVA. 2m T 88PHl with BL T28C and 850 16C. Less MLC so we`re not sure if any convection can develop...but with a boundary moving south...need to be alert for possible convection. Wednesday night...wind shift progresses southward with a wshift to north or northeast everywhere. If convection develops toward 6 PM near I-78, it might last through midnight. Thursday...Patchy fog early? More clouds then Wednesday...especially late, call it partly sunny. Light southeast to south. MLC 900J Delmarva. Chance of a thunderstorm as the boundary retreats slowly north. Uncertainty exists and confidence on qpf occurrence is below avg. Friday...Patchy early morning fog? Scattered showers and thunderstorms. 1000-1800J MLC. Dewpoints in the low to mid 70s. Heat index in NJ/PA in the upper 90s and up to between 100 and 105 on the Delmarva. Small chance we will need a heat advisory. Tides are astronomically high near days end...~6-7PM each day through the weekend and we may eventually need to monitor for approaching advisory threshold. Friday night...P/C or Mo cloudy. Foggy late if P/C. Any showers and tstorms should end early. Weekend..Becoming Partly to mostly sunny both days with a west wind. More comfortable. && .AVIATION /18Z MONDAY THROUGH SATURDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 18Z TAFs...Major changes to the TAFs this afternoon. Increasing confidence that a band of light rain or showers will move through most of the terminals late this afternoon and evening. Exact timing/coverage not completely clear, so VCSH and TEMPO groups included for now along/northwest of I-95. General timing is 20Z to 06Z. Mostly VFR expected, even with the precipitation, but CIGs will lower to 5-10 kft during this period. Sub-VFR CIGs are expected near/after 06Z as somewhat variable winds become more easterly. Sub-VFR conditions may prevail through Tuesday morning. Winds below 10 kts through the period. OUTLOOK... Tuesday...Early morning low clouds and visibility restrictions are possible. A chance or slight chance of showers and isolated thunderstorms. Tuesday night through Wednesday night...Mainly VFR. Late night and early morning low clouds and visibility restrictions are possible. small chance of a late Wednesday afternoon or evening Thunderstorm east central PA through central NJ. Thursday through Friday...Mainly VFR with a pretty good chance for a period or two of showers and thunderstorms. Late night and early morning low clouds and visibility restrictions are possible.
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&& .MARINE... Rather quiet conditions will continue on the waters today and tonight. Any swells from tropical storm Gert are not expected to arrive on the waters in the near or short term. Winds today will be mostly onshore and around or below 10 knots. A chance of showers exists tonight, with perhaps a stronger gust or two. A lightning strike cannot be ruled out completely, but the environment is not particularly favorable for storms. Outlook... SCA seas possible from Gert swells Tue eve DE waters but otherwise sub SCA conditions expected through this week. RIP CURRENTS... The rip current risk has been upgraded to moderate based on reports from the beaches yesterday of several rescues in similar wind/wave/swell conditions. With increased-period swell likely during the next 24-48 hours via Tropical Storm Gert and low tide during the evening hours, there is no reason to suspect conditions will be better today; instead, they will likely be a little worse. Swim only in the presence of lifeguards where safety prevails. Otherwise, you`re on your own with any rescue potentially critically delayed. There have been numerous drowning fatalities this year along the NJ coast. Tropical Storm Gert is forecast to pass well to our southeast and east on Tuesday and Tuesday night. Swells from the system will likely bring at least a moderate risk for the development of dangerous rip currents Tuesday and Tuesday night. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Drag Near Term...CMS/O`Hara Short Term...CMS/O`Hara Long Term...Drag Aviation...CMS/Drag/O`Hara Marine...CMS/Drag/O`Hara

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