Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 231649 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1249 PM EDT SAT JUL 23 2016 .SYNOPSIS... Frontal boundary sinks through the region today. Then high pressure builds over the region through the weekend. By Monday, a cold front will approach from the west, but may stall over the region through mid week. High pressure may build in from the north for next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Forecast seems on track thus far so only minor adjustments were made to the forecast for this afternoon and tonight with the 1230 PM ESTF update. Added enhanced thunderstorm wording (gusty winds and hail) to the grids for far southern PA and NJ as well as eastern MD and DE for later this afternoon and evening. Midday mesoanalysis shows a moderately unstable airmass along and southeast of the surface trough with MLCAPE values already 1500-2000 J/kg from about Phila southward where temps are in the 90s and dewpoints in the 70s. High values of DCAPE off SPC mesoanalysis indicate a potential for strong convective wind gusts if storms to develop. Overall, the environment is very favorable for wet microbursts given how moist the boundary layer is and dry the column is above it. Previous Discussion from 10 AM... Mid morning surface analysis shows a surface low over the Canadian Maritimes. A cold front extended to the west-southwest along the Northeast U.S.-Eastern Canada border. Meanwhile, a pre- frontal trough in the Gulf of Maine extended south-southwestward into the mid-Atlantic region. Over our area, the surface trough was positioned several miles to the south/east of the I-95 but oriented parallel to the major interstate. Although not quite noticeable right now, a contrast in dewpoints on either side of this boundary will become evident this afternoon as the surface trough sharpens and drier air aloft mixes down to the surface. Dewpoints that are currently in the upper 60s and low 70s with a west to northwest wind behind (i.e., to the north and west) the boundary are expected to fall into the 50s and low 60s later today. Farther south into the Delmarva and far southern NJ, conditions will remain humid with dewpoints steady in the upper 60s and low 70s. The Excessive Heat Warning will go into affect late this morning for the urban I-95 centers including Wilmington, Philadelphia and Trenton metro where heat indices are forecast to be in the 97-103F range this afternoon. We will also keep the Heat Advisory in place farther east into central/ southern NJ where heat indices are forecast to be in the 98-104F range and farther south into eastern MD and DE where it will be much more humid and thus heat indices are forecast peak in the 103-109F range. Farther north across northeastern PA and northwestern NJ, heat indices away from of the cooler mountains will be in the 90-95 range with much drier air in place. Hourly PoPs and Weather grids were updated through tonight with the mid morning ESTF update. The hi-res convective-allowing model guidance are all in pretty good agreement showing convection initiating just south of the front near the Mason-Dixon line and southern NJ by about 3 PM. Showers and thunderstorm development expanding southward into the Delmarva late this afternoon. Coverage of showers and storms should mainly be isolated. Better coverage is expected this evening across our southern zones with additional lift arriving with a mid-level shortwave trough.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... As the front continues to sink southward through the area and the short wave/vorticity impulse continues southward as well, showers and thunderstorms could continue into the evening across the southern portions of the area. So we will keep the slight chance/chance POPS across the southern half of the area through the evening hours. Once the front and short wave move south of the area around or shortly after midnight, any precipitation will come to an end. With lowering dewpoints and lightening winds, some areas could be several degrees cooler tonight compared to Friday night. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Sunday...Surface high entrenched north of our region along with and upper level short wave ridge crossing the NE US at this time should keep our region dry on Sunday. With weak surface flow, expect another day of very low relative humidity values. However, unlike today, tomorrow winds should be light. Monday...low level southwesterly flow ahead of the next cold front develops. This, combined with increasing 1000-500mb thicknesses thanks to the previously mentioned short wave ridge will lead to the hottest day of the next week, and possibly the hottest day we`ve seen so far this year. Additionally, the southwesterly flow will also be responsible for moisture advection, leading to increasing dew points and heat index values above 100 for most of the area. As the front, and an upper level short wave trough, approaches late in the day, could see scattered thunderstorms develop. Coverage may be limited thanks to limited shear and warm mid levels, but if any storms develop, they could have downburst potential with gusty winds. As a result, there is a marginal risk for severe storms for some of our region. Will hold off on mentioning in the HWO for now. Tuesday and Wednesday...relief behind the cold front will be more in the form of lower relative humidity values, while temperature wise, we should only be a few degrees lower as compared to Monday. depending on where the front stalls, we could see showers and thunderstorms develop along the stalled front each afternoon and evening. Thursday and the cooler air mass continues to slide south, a modest cooling trend should continue. In the upper levels, expect mostly a zonal flow pattern. As such, could see a typical summertime convection pattern with isolated to scattered showers and thunderstorms each afternoon and evening. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions are expected across the area today and tonight for most locations. There is a chance of isolated/scattered showers or thunderstorms across the southern half of the area today into this evening. However, being they are forecast to be isolated/scattered in nature, we have left them out of the TAFS for now, although we may need to include the in a TEMPO group in the future. If a shower or thunderstorm did develop, they could briefly lower VSBYS. Some patchy fog may develop across mainly the southern portions of the area later tonight, especially if any precipitation falls across the area. Southwest-west winds will become more west-northwest through the morning and into the afternoon as a frontal boundary moves across the area. Winds are expected to become gusty around 15-20 knots through today as well. The gusts will drop off this evening, and eventually winds will become light and variable most places overnight. Any directions overnight would be more northwest, become northeast by daybreak. Outlook... Sunday...VFR conditions expected Monday...MVFR or even brief IFR conditions possible with scattered thunderstorms...primarily in the afternoon and evening. Tuesday and Wednesday...mostly VFR conditions expected. A small chance of thunderstorms on the Coastal Plains and southern Delaware Valley (including KACY...KMIV...KILG...and KPHL) on Wednesday. && .MARINE... W-NW winds are currently 10-15 kt in our NJ coastal waters from ANZ452 northward and under 10 kt farther south in our coastal waters and DE Bay. We expect the winds to back out of the SW or even S-SE in spots this afternoon once the sea-breeze develops a few miles offshore. S-SWly winds are forecast to be strongest late this afternoon and this evening in our coastal waters off S NJ and DE. Winds could be sustained near 20 kt but the potential to mix down higher gusts is minimal with a hot airmass residing over the relatively cooler waters. While there could be a few gusts near 25 kt late in the day nearshore, do not expect conditions will warrant a SCA. Medium-period sly swells will persist today. Wave heights should subside to 2-3 ft this afternoon after starting 3-4 ft this morning. Sub-Small Craft Advisory conditions are expected today and tonight. However, winds could gust around 20 knots today. A frontal boundary will sink southward across the waters later today and into the overnight hours. It is possible that isolated/scattered showers or thunderstorms may affect the waters later today into this evening. Outlook... Sunday...winds and seas should remain below SCA criteria. Monday...winds and seas on the coastal waters may approach or even reach SCA criteria primarily from the afternoon through Monday night. Tuesday through Wednesday...winds and seas should remain below SCA criteria. Rip Currents...Offshore winds today will make it unfavorable for the sea-breeze front to progress inland this afternoon, except may at the barrier islands. The W-NW winds that are currently occurring this morning should back more out of the SW or even S this afternoon and increase ahead of a pressure trough from about LBI southward. This would enhance the potential for the formation of dangerous rip currents at the south-facing beaches in Cape May. After coordinating with local beach patrol on our daily surf conference call, we opted to go with a moderate risk for rip currents in Cape May County. Lifeguards at Cape May had rip current issues yesterday and expect the same today with the setup being similar to yesterday. Elsewhere, kept the risk low, but it will likely be at the upper end of it later this afternoon as winds increase and become oriented parallel to the shore. && .FIRE WEATHER... With rising temperatures and lowering dewpoints, relative humidity values are forecast to drop near 30 percent later today across the northern two-thirds of NJ and eastern PA. Meanwhile, west- northwest winds will increase to 10-15 mph and could gust in the low 20s. Summertime is not usually a period when we are concerned about fire weather, but contacted our fire weather partners in NJ and PA to determine how dry the fuels are and if any enhanced statements should be issued. A statement does not appear necessary at this time unless the wind speeds become much higher than currently forecast. Farther south across Delaware and Maryland, RH values are not expected to drop below 35-40 percent, so we have less concerns that far south. && .CLIMATE... Record high temperatures today through Thursday are below. Site 23rd 24th 25th 26th 27th 28th ---- ---- ---- ---- ---- ---- ---- PHL... 101-2011 98-2011 96-1899 101-1892 101-1940 100-1941 ABE... 99-1955 95-1999 95-1999 98-1940 98-1955 97-1949 ACY... 105-2011 100-2011 99-2010 96-2011 99-2005 98-1999 ILG... 100-2011 98-2011 96-1987 99-1894 100-1894 101-1894 TTN... 104-2011 98-1910 97-1999 99-1892 100-1894 101-1894 GED... 104-2011 99-2011 99-2010 97-2012 98-2005 99-1949 RDG... 100-2011 96-2010 96-1999 99-1940 98-1955 99-1941 MPO... 91-1955 91-1914 90-1999 89-1949 91-1955 93-1949 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Excessive Heat Watch from Monday morning through Monday afternoon for PAZ060>062-101-103-105. Excessive Heat Warning until 6 PM EDT Monday for PAZ070-071- 102-104-106. NJ...Excessive Heat Watch from Monday morning through Monday afternoon for NJZ007>010-012>014-016-020>027. Heat Advisory until 8 PM EDT this evening for NJZ012>014-016- 020>027. Excessive Heat Warning until 6 PM EDT Monday for NJZ015- 017>019. DE...Excessive Heat Watch from Monday morning through Monday afternoon for DEZ002>004. Heat Advisory until 8 PM EDT this evening for DEZ002>004. Excessive Heat Warning until 6 PM EDT Monday for DEZ001. MD...Excessive Heat Watch from Monday morning through Monday afternoon for MDZ008-012-015-019-020. Heat Advisory until 8 PM EDT this evening for MDZ008-012-015- 019-020. MARINE...None. && $$ Synopsis...Johnson Near Term...Klein Short Term...Robertson Long Term...Johnson Aviation...Johnson/Robertson Marine...Johnson/Klein/Robertson Fire Weather... Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.