Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 251918 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 218 PM EST Sat Feb 25 2017 .SYNOPSIS... In the wake of the strong cold front moving through our region Saturday afternoon, cooler air will settle in over the region. High pressure then builds south of the region on Sunday before moving offshore on Monday. Low pressure approaching from the west will lift a warm front through the region on Tuesday. The aforementioned low will propagate from the Great Lakes Region into eastern Canada. The associated cold front will move through Wednesday night or Thursday. A quick moving low pressure system will slide over the Mid Atlantic at the end of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 130 pm update: Forecast updated to include severe thunderstorm watch 39 for portions of southeast PA, southwest NJ, the Eastern Shore of MD, and northern/central DE until 8 pm. Grids updated to include isolated damaging wind gusts. Environment continues to become more favorable for severe storms, with storms rapidly intensifying from Harrisburg, PA, southwestward to Charlottesville, VA. These storms are moving into an increasingly unstable air mass, with MLCAPE 500-1000 J/kg and effective bulk shear 40-60 kts. With continued heating in the low levels, near-surface lapse rates are continuing to increase, with 7-7.5 C/km readings reaching the I-76 corridor. Though thermodynamic profiles do not appear favorable for large hail, some subsevere hail is possible with the strongest updrafts. 1230 pm update: Environment is becoming more unstable this afternoon as diabatic heating has increased with the partial clearing observed in much of the area this morning. Model analyses indicate MLCAPE 500-1000 J/kg generally south of I-76 (and away from the coast) with effective bulk shear 40-60 kts. Though low-level lapse rates have been slow to improve this morning, some 7+ C/km readings are now present in southeast PA. As the cold front nears the region this afternoon, scattered storms should continue to intensify in the increasingly unstable air downstream. Already have showers developing in the southern Poconos, with a clear uptick in convection in central PA during the past hour. With fast unidirectional flow oriented parallel to the front, one or more lines of storms should quickly organize as it/they move east into the area along/ahead of the front. Downward momentum transfer of this fast flow remains possible with the stronger updraft cores, at least in isolated instances, especially if surface heating continues this afternoon (improving low-level lapse rates). Made substantial modifications to the forecast this afternoon. Increased temps across the area a bit more, changed PoPs to account for latest trends, and kept mention of fog in coastal NJ, where southeast winds will likely keep patches/areas of fog around before the storms move in. 930 am update: Boundary layer mixing has successfully eroded most of the lower clouds and fog in far southeast PA and much of inland NJ; however, patchy dense fog continues across coastal NJ. Coastal webcams showing Seaside Heights shrouded in fog, with at least patchy fog also visible near Atlantic City, Belmar, Tom`s River, and Cape May. Though improvement is expected through the morning hours, nudged temperatures downward in coastal areas just a touch through the day as the consistent southeast fetch will likely advect cooler air over the ocean waters inland and may keep low clouds in place through much of the day. Temperatures were running warmer than forecast in areas where clouds have eroded, but suspect the warming trend will be brief given the increasing clouds moving in from the west. Tweaked hourly temperature grids to account for the current trends, but only modified max temps a degree or two in most locations, as the expected slowing trend of the warming will likely preclude temperatures from exceeding guidance excessively, as has been observed several times in the past week. Regarding the convective potential this afternoon, not much has changed with the forecast thinking. High-resolution guidance has been quite consistent with the timing of the main line of convection generally from 20Z to 01Z from west to east. Instability continues to look marginal but sufficient for updraft maintenance as the cold front approaches the region this afternoon. Within an environment of favorable deep-layer shear and unidirectional wind profiles with directions mostly parallel to the front, a line of convection will likely progress through the area. The line may contain little lightning given the marginal/thin CAPE profile, but with steep low-level lapse rates underneath fast midlevel flow, downward momentum transfer of these strong winds should occur with relative ease during the afternoon hours. With favorable low-level helicity in vicinity of the front, bowing structures and even an embedded rotating storm/mesovortex cannot be ruled out. Previous discussion below... Another rather mild early morning across the area. Several items to discuss today as we have more warmth ahead of a strong cold front. Some varying amounts of low clouds with patchy fog this morning, with much of this (at least low clouds) across portions of eastern Pennsylvania and northern New Jersey due to more of an upslope component of the low-level flow. Some thicker fog/stratus has been noted along portions of New Jersey coast and near the far upper portion of Delaware Bay near Wilmington. Some guidance keeps low clouds across much of the area all day, however the thinking is that enough mixing and boundary layer heating occurs to erode this by early afternoon. Some fog was kept from northern Delaware eastward to the New Jersey coast. We now switch to the convective potential with an incoming strong cold front this afternoon. A strong upper-level trough will continue to move from the Midwest to across the Great Lakes region today. This trough is forecast to start taking on a negative tilt as it reaches the eastern Great Lakes later today. This will drive strong height falls eastward this afternoon along with a strengthening wind field. The forecast soundings indicate deep unidirectional flow with some veering below about 900 mb. There should be enough instability to produce some thunder this afternoon, however strong forcing along the leading edge of the height falls along with robust short wave energy should result in a convective line. This line will probably be more low-topped, however enough instability is expected inland to produce some lightning. Given the deep unidirectional flow contributing to strengthening shear, some bowing segments are possible within the line. Momentum transfer of stronger winds from aloft down to the surface can result in locally damaging gusts, especially with any bowing segments along with stronger cores due to water loading of the downdrafts. While some smaller hail cannot be ruled out, instability should be limited as soundings show more of a thin CAPE profile. We will continue to carry gusty winds in the forecast with thunder (may need to add damaging winds later), with the convective line generally moving from west to east across the area from about 20-00z. The intensity of the convection should lessen as it reaches the coast due to the cooler ocean influence. Southerly flow ahead of the cold front will result in another very warm day, with several areas already starting rather warm. Highs will be able to over achieve expectations if more clearing occurs ahead of the cold front during peak heating. High temperatures were a MOS/continuity blend but then raised some away from the coast given a warmer start. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... A negative tilt upper-level trough will be lifting across New England tonight, with low pressure tracking across eastern Canada. This will push a strong cold front offshore early in the evening with and showers and thunder quickly coming to an end. Strong cold air advection occurs immediately behind the cold front through about the first half of the night. This combined with stronger flow should create a gusty west to northwest for a time. As the colder air advects into the area, it will also be drying out however some forecast soundings indicate that as a subsidence inversion develops, there will be some lingering moisture trapped for a time. This may result in bands or areas of stratocumulus for awhile especially across the northern zones. It will be noticeably colder tonight with an added wind chill to the mix, reminding us that it is still winter. Low temperatures were mostly a MOS/continuity blend. && .LONG TERM /SUNDAY THROUGH SATURDAY/... Few changes made in the long term so as to focus on current hazards this evening. Unsettled weather pattern is on tap through the week. Monday and Tuesday are very low confidence especially with respect to precip chances. As long as the high isn`t more progressive in shifting off shore, during the day Monday should stay dry. Monday night into Tuesday there is low confidence as to the extent of rain ahead of the arrival of the warm front and with the timing of the warm front. Models have continued their run-to-run jumpiness with the timing of this front. Wednesday and Wednesday night looks to be the highest chance of precip with the region solidly in the warm sector. Thunderstorms may be possible as well if we can warm up enough to realize some modest surface based instability. Cold front still on track to move through late Wedensday night or Thursday. Once it moves through, we should see a lull in the precip, albeit a brief one. The next question will be the timing and track of a quick moving low which could move across the region Friday or Friday night. Moisture may be limited as there will be strong dry air advection behind the cold front, but there is some chance for precip around the area. && .AVIATION /19Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 18Z TAFs...Mostly VFR before storms move in this afternoon. Timing looks to be 19-23Z for KRDG/KABE, 20-00Z for KPHL/KPNE/KTTN/KILG, and 22-02Z for KMIV and KACY. Sub-VFR VSBYs/CIGs expected as the precipitation moves through, with gusty/erratic winds 30-45 kts possible in proximity to the stronger storms. Winds will quickly veer to westerly or northwesterly after shower/storm passage late this afternoon/evening, with winds gusting 20-30 kts through tomorrow afternoon before diminishing. Outlook... Sunday night and Monday...VFR conditions expected. Monday night through Thursday...periods of rain and showers are possible. When/if this does occur, MVFR ceilings or lower are possible. Highest chance is Wednesday and Wednesday night. There is considerable forecast uncertainty in this period. && .MARINE... 1230 pm update: Dense fog continues to plague the waters off the coast of New Jersey. Extended the dense fog advisory through 6 pm. Winds are slowly increasing over the waters as well, so current SCA looks good. Rest of forecast unchanged. 930 am update: Webcams continue to show dense fog over the coastal waters of New Jersey. Expanded the dense fog advisory to cover all marine zones adjacent to the coast through noon. Otherwise, forecast is in good shape. Previous discussion... A narrow area of stratus and/or fog continues right near the coast of New Jersey with this mostly confined from portions of Ocean County northward. Farther south, the low-level trajectory suggests less development however some fog has been reported. The dense fog advisory this morning remains in effect for just the northern two Atlantic coastal waters zones. Otherwise, a southerly flow will continue today and the increase may be slowed due to much warmer air over cooler water. This will increase especially later today as a strong cold front arrives, with colder air quickly arriving tonight in its wake. It is tonight when stronger winds are expected as mixing becomes much more efficient. There could be some low-end gale force gusts tonight, however it looks marginal and potentially short in duration therefore we will maintain the Small Craft Advisory. Seas will also respond to the increasing wind, however this was slowed a bit for this morning. A line of showers and embedded thunderstorms will move across the area late this afternoon and especially early this evening. These may weaken some as they encounter the marine influence, however some local gusts of 30-40 knots are possible along portions of the line. Outlook... Sunday night and Monday...wind gusts to 25 kt may linger for a few hours on Sunday evening, but will be below SCA criteria for the remainder of this period. Tuesday through Wednesday...winds and seas should generally stay below SCA criteria, though wind gusts above 20kt will be possible on Wednesday. Periods of rain/showers possible, especially on Wednesday. Wednesday night and Thursday...Northwesterly wind gusts around 25 kt are possible && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Dense Fog Advisory until 6 PM EST this evening for ANZ450>453. Small Craft Advisory until 6 PM EST Sunday for ANZ450>455. Small Craft Advisory from 6 PM this evening to 6 PM EST Sunday for ANZ430-431. && $$ Synopsis...Johnson Near Term...CMS/Gorse Short Term...Gorse Long Term...Johnson Aviation...CMS/Gorse/Johnson Marine...CMS/Gorse/Johnson is the U.S. government's official web portal to all federal, state and local government web resources and services.