Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 262026 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 426 PM EDT Sun Mar 26 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Vorticity maximum is moving northeastward through the Midwest this afternoon into the Great Lakes region, with attendant surface low in northern Illinois. Quasi-stationary front extends east from the surface low into northwest PA before extending southeastward into the southern Mid-Atlantic region, with a cold air damming regime in place east of the Appalachians in the Northeast. Persistent east-northeast flow has brought increased low-level moisture underneath midlevel subsidence via downstream ridging relative to the aforementioned vorticity maximum. As the vorticity maximum moves northeastward tonight, the backdoor cold front in the southern Mid-Atlantic should lose its southward progress and move poleward as a warm front. However, models often overdo the progress of such fronts, especially at night, and suspect this bias does exist to some degree with today`s simulations. I generally undercut guidance temperatures tonight by a few degrees, and this may not be enough, particularly in the notoriously stubborn southern Poconos, where temperatures have been running under guidance today by about 3-5 degrees. With that in mind, temperatures will remain near freezing early this evening at elevations around/above 1000 feet, so light glazing may continue on elevated surfaces via a thermal/moisture profile suggestive of freezing drizzle. No societal impacts have been noted today with the frozen precipitation, and with temperatures gradually warming overnight, am not expecting much if any impact going forward. By late evening/overnight, precipitation is expected to be all liquid (including after contact with surfaces), and the forecast features all rain tonight. High-resolution models have been consistent in showing large- scale ascent increasing in the northern Mid-Atlantic downstream of the Great Lakes vort max late this evening and overnight, with a batch of rain moving into NY/PA/MD/WV this evening. Given the northeastward motion of the vort max, our CWA will be on the southern fringe of the strongest ascent, with good agreement among hi-res guidance that chances for precipitation drop off considerably south of the Mason-Dixon Line. As such, have the highest PoPs in the southern Poconos/northern New Jersey (65-90 percent), moderate chances (30-60 percent) in the I-95 corridor, with slight or unmentionable PoPs generally south of the Mason- Dixon Line. One other question mark tonight is the extent/severity of fog. Currently, thinking that winds will be high enough to preclude more substantial/widespread fog formation (especially with the approach of the shortwave trough, which tends to also mitigate widespread fog). However, fog formation may be more substantial near and off the coast, where dew points will exceed sea surface temperatures late tonight. Thicker fog, should it develop over the ocean, may advect onto the New Jersey coast and create local visibilities below a half mile.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
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By tomorrow morning, the shortwave trough in the Great Lakes is expected to continue northeastward into the St. Lawrence Valley, with the organized large-scale ascent moving into New England and adjacent southeast Canada. Precipitation will likely become more showery by late morning, with perhaps even hints of blue sky in portions of Delmarva during the afternoon. The warm front will sweep northward well into New England by this point, and with the warm southerly fetch, temperatures will warm substantially above today`s values. Forecast highs are 15-25 degrees above today`s values. Forecast temperatures may be on the low side if partial sunshine occurs, especially considering the general negative bias of guidance in warm sector regimes this winter. There is some question if localized lift can generate more convective showers during the afternoon tomorrow. Forecast soundings are at least marginally supportive of this, particularly northwest of I-95, where residual colder air at midlevels combined with a well-mixed boundary layer may permit development of isolated/scattered showers. Felt compelled to include a mention of isolated thunder during the afternoon given the indications of positive buoyancy during peak heating. This is conditional, however, as transient ridging upstream of the departing shortwave trough may preclude sufficient lift necessary for the development of any convection.
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&& .LONG TERM /MONDAY THROUGH SUNDAY/... 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. && .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. General MVFR conditions will gradually lower to IFR this evening. Light rain will move in between 03Z and 06Z at KRDG/KABE and by 09Z at KTTN/KPNE/KPHL/KILG and continue through around daybreak. There may be slow improvement to MVFR after sunrise and even VFR during the afternoon, but scattered showers remain possible through the day. A lightning strike or two may occur with showers during the afternoon tomorrow in SE PA/N NJ, but confidence is too low to include in TAFs. Chances of precip are lower 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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Marginal but persistent small craft advisory conditions will continue through Monday. East to southeast winds 10-20 kts with higher gusts will occur, especially this evening and again tomorrow afternoon. Seas will likely remain elevated (above 5 feet) offshore the New Jersey coast through at least tomorrow afternoon. As a result, extended the small craft advisory through 7 pm Monday. Another concern is fog tonight and Monday, with dew points expected to surge to values above sea surface temperatures late tonight and tomorrow. Visibilities will likely lower to around a mile at times late tonight and tomorrow and may become less than a mile locally. At this time, not confident enough in issuing an advisory, but will monitor closely tonight for potential need for statements or advisories. 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... 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. && .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... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 7 PM EDT Monday for ANZ450>453. && $$ Synopsis...Franck Near Term...CMS Short Term...CMS Long Term...Franck Aviation...CMS/Franck Marine...CMS/Franck Tides/Coastal Flooding...Drag Climate...Drag

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