Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 270134 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 934 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 front 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 our region from the west on Sunday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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The marine influence, courtesy of a cold air damming setup and onshore, will maintain stratus and some fog across the area. The extent of the fog may worsen for a time overnight as warmer air starts to move northward, and the forecast soundings indicate the inversion lowers some. The extent of any dense fog is less certain, however the higher terrain of the northwest zones and closer to the coast may have the highest chance of occurrence. We went with areas of fog with the drizzle until PoPs increase enough to include a shower mention. Temperatures remain around freezing at elevations mainly at and above 1400 feet in the southern Poconos over to northwestern Sussex County in New Jersey, so light glazing is probably continuing on elevated surfaces. This is mainly from freezing fog/drizzle so far. There may be a little freezing rain as the showers arrive later on, however temperatures should warm ever so slowly overnight. No societal impacts have been noted with the frozen precipitation and since it is confined to the higher elevations we are highlighting this with a Special Weather Statement. A short wave and associated surface low will track into the eastern Great Lakes overnight. The associated ascent will mostly affect our western zones, and therefore showers are expected to increase and become more numerous across these areas. The highest PoPs therefore remain for these areas, with these decreasing with an eastern and southern extent. It appears that there should not be enough elevated instability, therefore no thunder was included at this time. The hourly temperature and dew point grids were adjusted based on the latest observations, then some high-res guidance was blended in to try and better capture the colder air in the Poconos. Low temperatures were lowered a little for several areas given a slight downward adjustment to the current temperatures.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
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By Monday 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 Monday afternoon. 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 NIGHT 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 /01Z MONDAY THROUGH FRIDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Overnight...IFR ceilings, lowering to LIFR at times, along with areas of MVFR/IFR visibility due to fog and some drizzle. The extent of the visibilities due to fog is less certain. Some showers are expected to develop between through 06Z at KRDG and KABE and toward 08Z at KTTN, KPNE, KPHL and KILG. East-northeast winds mainly 10 knots or less, becoming southeast late. Monday...IFR conditions (ceilings and fog) should improve to MVFR during the morning, then VFR during the afternoon. Some showers should continue through the day (most coverage in the morning north and west of KPHL). A thunderstorm may occur during the afternoon mainly north and west of KPHL, however coverage should be isolated. The chance of showers is lower at KACY amd KMIV, although there may be some drizzle in the morning. Southeast winds up to 10 knots, turning south during the morning then southwest in the afternoon. 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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A Dense Fog Advisory (visibility 1 NM or less) remains in effect for all zones through 8 AM Monday. Coastal observations along with some web cams indicate fog development continues especially across the southern areas, and this should thicken and expand northward through the night as warming occurs aloft and dew points increase some. Marginal but persistent small craft advisory conditions will continue through Monday. East to southeast winds 10-20 kts with higher gusts will occur. Seas will likely remain elevated (at or above 5 feet) offshore the New Jersey coast through at least Monday afternoon. As a result, the small craft advisory goes through 7 PM Monday. 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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&& .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...Dense Fog Advisory until 8 AM EDT Monday for ANZ430-431- 450>455. Small Craft Advisory until 7 PM EDT Monday for ANZ450>453. && $$ Synopsis...Franck Near Term...CMS/Gorse Short Term...CMS Long Term...Franck Aviation...Franck/Gorse Marine...CMS/Franck/Gorse Climate...

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