Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 262000 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 400 PM EDT Sun Mar 26 2017 .SYNOPSIS...
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Low pressure moving from the Mississippi Valley into the Great Lakes will lift a warm front northward through our region overnight into Monday morning, followed by a cold frontal passage Tuesday night. Canadian high pressure will build into the area Wednesday through Friday. Low pressure and its associated fronts in the Ohio Valley on Friday will move through our region on Saturday. A weak area of high pressure is expected to move into the area from the west on Sunday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... 930 am update: No changes to the forecast this morning. Onshore flow to the north of a backdoor cold front is inundating the region this morning with low clouds and occasional drizzle. Mount Pocono remains below freezing, so a light glaze may occur on trees, but no societal impacts have been observed or reported to this point. Conditions will change little through the day, with temperatures unable to rise in the low clouds and maritime origins of the low-level flow. Expect little to no measurable precipitation through the daytime hours as the midlevels should remain relatively dry. Previous discussion... High pressure across Quebec/Maine will move offshore today while weak low pressure slowly moves to near Chicago by evening. A steady onshore flow will continue across our area thru the day. This will promote an abundance of low clouds and fog along with some light rain and drizzle at times. Pops will be greatest across the northern areas where we have hi chc pops and the pops taper off s/e to slgt chc across srn DE. Unlike Saturday, temperatures will be below normal, with highs only in the 40s in most areas, and a few low 50s over Delmarva. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... The overall pattern will not change much tonight with high pressure north/east and weak low pressure moving closer across the Great Lakes. The onshore flow will continue and the chances for more measurable rains will increase tonight. The best chances will again be across the southern Poconos, Lehigh valley and north NJ where we will have categorical pops. The pops decrease back to likely over the Del Valley and cntrl NJ and the chc pops over south NJ and srn DE. Patchy fog and drizzle will occur between the bouts of rain. Overall qpf will not be that great, a few hundredths across the south and up to 1/4 inch up north. && .LONG TERM /MONDAY THROUGH SUNDAY/...
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A split flow regime will continue across the conus during this period. While the northern stream remains displaced to the north, our primary weather makers will be in the southern stream. A closed low along the lee of the Rockies will gradually open and eject northeastward. Its associated weak surface reflection and cold front will traverse the middle Atlantic, impacting our sensible weather Monday night into Tuesday night. A cyclonic flow aloft on Wednesday will gradually give way to ridging on Thursday. The next southern stream system in the pipeline will impact our region Friday into Saturday. With good run to run model consistency in terms of the timing of the cold frontal passage Tuesday night, the next challenge will be how the next southern stream system is handled during the Friday and Saturday period, including the extent of phasing with the northern stream. In particular, the track of this system and the extent of the cold air to the north will have implications on any potential p-type issues. Temperature-wise, around 10 to 15 degrees above normal on Tuesday, then generally normal to several degrees above normal from Wednesday through next weekend. Another round of showers is expected mainly after midnight Monday night into Tuesday night across the entire region. With the cold frontal passage Tuesday night, showers should move offshore prior to Wednesday morning. We do not anticipate any p-type issues during this period. We expect patchy fog Monday night, which may linger into Tuesday afternoon north of a PA Turnpike to I-195 line. Within the warm sector in advance of the cold front, we expect sunshine to break out, especially south of the aforementioned line with temperatures reaching the upper 60s to lower 70s. A low-level moist tongue will also be in place across this area, with Dew Points well into the 50s. Models indicate some weak ML Cape, along with negative Lifted and Showalter Indices, within an environment characterized by poor lapse rates and weak shear. We have included a chance of thunder in the forecast, and there may be some localized heavier downpours as well. Given low Precipitable Water values around one inch, urban and small stream flooding is not a concern. In the wake of the cold front, expect a return to fair weather for Wednesday and Thursday. Beyond day four, the models have come into better agreement on the Friday-Saturday system, but there still remains uncertainty regarding the degree of phasing between the northern and southern stream, including p-type implications. The GFS, ECMWF, and Canadian take low pressure from the MS Valley into the Great Lakes, with varying degrees of secondary development over the middle Atlantic. The UKMET is an outlier, maintaining a purely southern stream system, with the primary low moving through the middle Atlantic. In fact, the UKMET represents yesterdays GFS solution, as it was previously in the former camp. The forecast brings in chance PoPs Friday into Saturday. Also, given the orientation of the surface high, cold air damming to some extent is likely. Thickness values indicate the cold air is right on our door step, so p-type could be an issue, especially across the northern half of our CWA.
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&& .AVIATION /20Z SUNDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 18Z TAFs...General MVFR conditions will continue through early this evening, likely deteriorating to IFR or possibly LIFR during the overnight hours. Moderate confidence that light rain will move in between 03Z and 06Z at KRDG/KABE and by 09Z at KTTN/KPNE/KPHL/KILG. There may be slow improvement to MVFR after sunrise, but scattered showers may continue through the morning hours. Less confidence of precip at KACY/KMIV. Winds generally east 10-15 kts through early evening with a trend toward S or SSW late tonight through the morning hours tomorrow. OUTLOOK... Monday night...Periods of MVFR/IFR likely in low clouds and fog across all TAF sites. Tuesday...MVFR likely Tuesday morning at all TAF sites, with improvement to VFR by afternoon at all but ABE, RDG, and TTN. Tuesday night...MVFR possible at ABE, RDG, and TTN early, otherwise, VFR. Wednesday thru Thursday...Predominantly VFR conditions expected. Friday...VFR conditions may deteriorate to MVFR with the arrival of the next weather system. There is the potential for northwest wind gusts up to around 25 knots Tuesday night into Wednesday.
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&& .MARINE...
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We will keep the SCA flags as they are on the waters and just raise the next segment with the 330 am issuance, instead of waiting until 6 a.m. The onshore flow will continue today and a gradual building of seas will continue. Scattered showers and patchy fog expected. OUTLOOK... Seas may remain elevated into Monday night, and the SCA may need to extended into this period. A period of northwesterly wind gusts may reach SCA criteria on Wednesday. Sub-SCA conditions are expected on Thursday. The approach of the next system may lead to a return to SCA conditions by Friday.
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&& .TIDES/COASTAL FLOODING...
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Onshore flow is expected through Monday morning. Astronomical tides are also increasing, coincident with the New Moon this Monday, March 27. The ESTOFS remains most aggressive of the guidance suite for the sunrise Monday high tide along the DE and NJ oceanfront. Although the ESTOFS has outperformed the other guidance in recent past events, its trends for positive departures (surge) appear to be lessening. Both the SIT multi model review and GFS ETSS are more conservative, and verifying better as of Sunday afternoon. The wind will also be trending more parallel (southerly) to the shore by sunrise Monday, so any minor tidal inundation flood risk remains a low potential with no action at this time.
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&& .CLIMATE... March as a whole for PHL, is still on track to average one half to 1 degree below normal, despite the warmth of ydy through Wednesday. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 7 PM EDT Monday for ANZ450>453.
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&& $$ Synopsis...Franck Near Term...CMS/O`Hara Short Term...O`Hara Long Term...Franck Aviation...CMS/Franck/O`Hara Marine...Franck/O`Hara Tides/Coastal Flooding... Climate...

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