Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 231032 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 632 AM EDT Mon Oct 23 2017 .SYNOPSIS... Strong high pressure is south of Nova Scotia today. A cold front will move slowly through our region Tuesday afternoon and night. Low pressure is expected to develop along the front and pass over coastal New England Wednesday. High pressure follows for Thursday and Friday. Another cold front should cross our region this coming weekend as low pressure develops northward along it. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Today will be transitional from the spectacular past few days to what awaits Tuesday. In between, today, we will have an increase in clouds across the area with perhaps a few stray showers up north this afternoon. The low clouds across the area early this morning will likely mix out as an increasing SE flow develops across the area. There is still plenty of drier air at the mid-levels, so support for organized precip is not there. A couple showers probably near the higher elevations across the N/W could form later on. Temperatures today will be mild, but with the increasing clouds, readings will not reach the highs that were over the area Sunday. Mostly low/mid 70s are expected. Dew points will be on an upward trend too, trending up through the 60s by afternoon. Winds will increase from the SE at 10 to 15 mph by afternoon. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... A slow moving cold front and a wave of low pressure moving along it will affect the weather tonight across the area. Pops will increase overnight with the steady precip reaching the wrn areas first, probably after midnight, then spreading towards the Lehigh and Delaware Valleys by dawn. Tstms are possible with the activity and gusty winds are also possible with the strengthening low level winds. A few gusts across the srn Poconos may be over 40 mph tonight, but probably the stronger winds will arrive later Tue. Probably not enough for a wind advisory attm, but something to consider later today. Temperatures will remain very mild for late October with mostly 60s over the area. && .LONG TERM /TUESDAY THROUGH SUNDAY/...
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Hazards: Tuesday as in the HWO...poor drainage street flooding from periodic heavy showers (see WPC QPF), and potential for damaging thunderstorms (see SPC products) if there are organized short lines in the HSLC unidirectional southerly flow environment. Tuesday night...dense fog potential e PA? Not in the grids attm since there was a lot to consider. Thursday night ...Frost potential I-78 region and the Pine Barrens of NJ. Not yet in the HWO. Looking just beyond Day 7...frost freeze potential Halloween morning along and northwest of I-95 (not Philadelphia urban corridor). 500 MB: A -2SD trough in the Great Lakes-Ohio Valley Tuesday weakens negatively tilted across the mid Atlantic states Thursday. Heights rise Fri leading to surface temp warming, ahead of another strong -2sd trough developing down into the Great Lakes and Ohio Valley next weekend. Temperatures: Calendar day averages near 15F above normal Tuesday, around 5F above normal Wednesday, near normal Thursday and Friday, then about 5F above normal Saturday and uncertainty Sunday whether its normal or above. Forecast basis: Unless otherwise noted a 50 50 blend of the 00z/23 GFS/NAM MOS was used for Tue-Wed, the 00z/23 GFS MEXMOS Wed night and Thu, then the 05z/23 WPC gridded elements D4-8 including 12 hr POPS-max/min temps, and 6 hrly wind/sky/dewpoint. The dailies... Tue and Tue evening...HSLC QLCS potential svr storms within the bands of southerly gusty wind showery rains, some of which will be heavy in the 1.8" PWAT within a deep generally unidirectional flow (storm motion around 200 degs at 40-45kt). Subtle sfc waves in diffluent thickness ahead of slow moving cold front may allow potential Maddox Synoptic scale FF situation but not in the forecast attm. Note the 00z/23 NAM fcst a narrow sliver of 5-7" rainfall over W NJ 09z Tue-06z Wed. This may happen but think it will be a little further east if it does. Humid with dewpoints 65-70 much of the area for awhile Tuesday. Minimum temps about 20F above normal and potentially record breaking at a number of locations including ACY PHL and long shot ABE...pls see the CLI section. We will probably have to wait until 1 AM Wednesday to know Tuesdays calendar day low. Tue night...Showery rains end from west to east except maybe along the Atlantic coasts. Wind shifts to light west but PWAT remains high and so too considerable cloud cover. BUT, if there is clearing in e PA/e MD...patchy dense fog could form in the still moist boundary layer. Wednesday...The cold front and its associated cloud cover and rain will continue to move slowly eastward as a final wave of low pressure develops along the boundary and moves up into coastal New England. PWAT still modeled 1.1" along the I-95 corridor 12z Wed. Becoming partly sunny with a west to southwest wind. Wednesday night...Chance of showers late as the negative tilted but weakening trough axis approaches from the west. The associated clouds should minimize the potential for frost. Thursday...The axis of the mid level long wave trough is forecast to pass overhead Thursday morning and it should finally kick the surface system well to our east. Coolest daytime high temperatures of this work week. Residual sprinkles possible in the morning- midday hours. Thursday night...frost-freeze potential I-78 under mostly clear skies and decoupling winds Friday...Sunny and becoming milder during the afternoon. This weekend... didn`t spend alot of energy here. Lots of variability but suffice to say with a high amplitude trough to our west... it will rain, but timing is uncertain so we used straight WPC POPS. Best estimate is that it rains Saturday night into Sunday. Confidence on details below average.
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Plenty of questions with regards to the TAFS today. The low ST has quickly swept up across the region overnight. Fog has developed too in some areas with VSBYS below 1/2 mile at a few spots. Some of the questions for the day are how much will the low clouds break up and when. Probably a slow return to MVFR then VFR later this morning, but uncertainties remain. More low clouds and rain arrive tonight as low pressure and a slow moving front track towards the region. Winds will increase tonight to 15-20 knots mostly from the SE. OUTLOOK... Tuesday through Tuesday evening...Occasional MVFR and IFR conditions in showers. Scattered thunderstorms and heavy rain likely, especially NJ and DE. South wind gusts around 25 to 35 knots in the afternoon. Gusts in a tstm could reach 45 kt. Low level winds just above the sfc near 1500 feet southerly near 50 kt Tuesday. Tuesday night...Conditions improve from west to east at least temporarily as showers end and wind becomes light west. This may set up the potential for IFR st/fog late at night? Wednesday...VFR. West to southwest wind. Wednesday night...VFR, chance of a short period of MVFR showers late. Wind shift northwest. Thursday...VFR. Chance of morning sprinkles. Friday...VFR.
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&& .MARINE... Sub-SCA conditions today and into the evening, then SCA flag will fly with increasing winds and seas developing ahead of a cold front. We will make a change to the start time, moving it up by a few hours to better align with our neighboring offices. Today, the weather will be fair with only some patchy fog this morning. Tonight, showers will overspread the waters late and a tstm is possible towards dawn. Winds will gust over 25 knots after midnight. Seas will increase to 5 to 7 ft on the ocean by dawn. OUTLOOK... Tuesday...A Small Craft Advisory for southerly wind gusts near 30 knots. Localized gale force gusts are possible on Tuesday. Waves on our ocean waters will build to 7 to 10 feet. Tuesday evening...A Small Craft Advisory extension may be needed for southerly wind gusts near 30 knots in the evening then the wind decreases as it shifts to west. Waves on our ocean waters are forecast to be 7 to 10 feet early, then slowly subside. Late Tuesday night through Thursday night...A Small Craft Advisory for hazardous Seas will be needed on our ocean waters for wave heights of 5 to 6 feet, even as the wind becomes west to northwest around 10 to 20 knots. Friday...No marine headlines are anticipated. && .HYDROLOGY... Hydro: storm total rainfall Tue-Tue night, generally 1-2" with in excess of 2 inches most favored over NJ. Leaf- clogged drains will enhance the possibility of ponding of water on roadways. NOTE: 00z/23 NAM has 6-7" of rain near I-95 between 09z/24 and 06z/25. We are not forecasting that... but in the small chance it would occur, then the recent dry spell will not be able to prevent small pockets of flash flooding. We will monitor NAM high resolution model trends the next 24 hours but for now...we are conservative but not discounting the potential. && .CLIMATE... Vulnerable record high minimums Tuesday the 24th. As of this 330 am forecast...we are forecasting records here. However, if the cold front speeds up and a little clearing ensues then the min temp would probably fall just short. Right now the mid shift forecaster has odds favoring records. Elsewhere for other records anticipated. We`ll add locations if we forecast warmer in future forecasts. Allentown 58 1975 Atlantic City 63 2001 Philadelphia 63 1900 && .EQUIPMENT... 44091 buoy drifted away from its mooring and has since been recovered. Its return to service date is still unknown, though we`ll try to have an answer Monday afternoon. Weather observations at KVAY should be incomplete through Monday morning. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 10 PM this evening to 6 PM EDT Tuesday for ANZ430-431-450>455. && $$ Synopsis...Drag Near Term...O`Hara Short Term...O`Hara Long Term...Drag 633 Aviation...Drag/O`Hara Marine...Drag/O`Hara Hydrology... Climate... Equipment... is the U.S. government's official web portal to all federal, state and local government web resources and services.