Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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581 FXUS61 KPHI 251644 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1244 PM EDT Thu May 25 2017 .SYNOPSIS... Low pressure will move from the Ohio Valley to the eastern Great Lakes region today. Meanwhile, a warm front will lift northward into parts of our region today into tonight while new low pressure develops along it. This low pressure system then strengthens as it tracks away from our area tonight and Friday. Weak high pressure may briefly build in for Saturday, then low pressure and its associated fronts move through later Sunday. A secondary cold front moves through during Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 945 am update: Made some modifications to the Wx/PoPs grids today based on latest simulations/trends from convection- allowing model guidance. Steadier precipitation associated with isentropic lift poleward of a surface warm front located in the southern Mid-Atlantic region was progressing northward past the Mason-Dixon Line within the past half hour or so. This will continue to lift northward, likely north of the I-78/I-195 corridor by late this morning. Near and south of the warm front, precipitation will be showery/spotty, so felt that PoPs were too high in these areas during the late morning and early afternoon. There is a strong signal for low-topped convection to develop this afternoon in the vicinity of the surface trough as it lifts slowly northward into the region, aided by the approach of a midlevel low/vorticity maximum, which should pivot northeastward from the Lake Erie vicinity southeastward to the North Carolina coast by 00Z Friday. The large-scale lift via differential cyclonic vorticity advection and substantial upper-level divergence on the northwest side of a meridionally-oriented 140+ kt 250-mb jet will permit the development of sufficient midlevel instability (via midlevel cooling). Surface heating to the south of the warm front should generate convection fairly quickly this afternoon, probably to the south and west of the CWA, before entering the region by late afternoon or early this evening. Though instability will be generally weak, upper-level winds are sufficiently strong for some storm organization. Marginally severe hail is a possibility if storms can remain somewhat discrete, given the cold midlevel temperatures and sufficient deep-layer shear. Strong straightline winds are also possible given the increasingly steep low- and midlevel lapse rates. There is even a low risk of an isolated brief tornado for storms taking advantage of the enhanced low-level helicity near the warm front/surface low. Previous discussion... A pair of mid level lows were located over the Ohio and Tennessee River Valleys early this morning. The feature will progress eastward with the two centers reaching western Pennsylvania and Virginia early this evening. There were two surface lows early this morning, as well. One was located in western Ohio and the other in southwestern Virginia. A warm front extended eastward from the Virginia low. The two low pressure systems are forecast to merge into a broad surface low over Pennsylvania and Maryland early this evening. The warm front is anticipated to lift slow northward into northeastern Maryland, Delaware, extreme southeastern Pennsylvania and southern New Jersey today. The initial band of rain associated with the warm advection will continue to lift northward through our region this morning. The back edge of the steady rain was into the far upper Delmarva and southern New Jersey around 6:00 AM. It is expected to work its way from south to north through the remainder of our forecast area before 9:00 AM. Rainfall totals in the steady precipitation should range from about a third to three quarters of an inch. A mostly cloudy sky and showers are anticipated for the balance of the day to the north of the warm front. As the front reaches our southern counties there should be some substantial breaks in the cloud cover as those location get into the warm sector. We will continue to mention a chance of thunderstorms for this afternoon. Locations in the warm sector will become unstable with mixed layer CAPE values rising into the 600 to 1000 J/Kg range. The approach of the mid level low and some elevated instability will spread the potential for thunder to the north of the warm front. The Storm Prediction Center has southeastern Pennsylvania, southern New Jersey, Delaware and northeastern Maryland under a marginal risk for severe weather for today. Today`s temperature forecast is one of low confidence. The actual temperatures will depend upon where the warm front settles and how much clearing takes place over our southern counties. Generally, highs should range from around 60 in the Poconos to near 80 in southern Delaware. The wind is forecast to remain from the east to the north of the warm front. It should veer to the southeast and south once the front passes. Speeds are expected to favor the 8 to 12 MPH range with some gusts of 15 to 20 MPH possible. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... 945 am update: Expecting storms to linger through the evening hours as the midlevel low moves through the region, especially north of the Mason-Dixon Line. Kept storm chances longer, and extended them through much of the night in far east-central PA and northern NJ. High-res guidance has the main window of convection in the 22Z to 04Z time frame (6 pm to midnight), so this may end up being more of an evening event for the area. With degree of cold air aloft, suspect thunder will remain a threat through much of the night where precipitation occurs. Previous discussion... The mid level low is forecast to pass over our region tonight, reaching southern New England toward daybreak on Friday. The surface low will likely follow a similar pattern. We are expecting a mostly cloudy sky and showers for tonight. Thunderstorms may linger into the evening. The wind is anticipated to become light and variable tonight in much of eastern Pennsylvania and northern and central New Jersey before settling into the west around 5 to 10 MPH. A southwest wind around 10 MPH is anticipated to shift to the west on the upper Delmarva and in southern New Jersey. Low temperatures are expected to favor the 50s for tonight. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Summary...Unsettled at times, however warmer temperatures are expected. Synoptic Setup...A significant closed low is forecast to shift up into New England Friday then move out Friday night into Saturday. Some brief weak ridging should slide into the east Saturday, however the next trough amplifies from the Midwest and Great Lakes later Saturday and Sunday. This trough looks to close off and be slow to shift eastward through Wednesday, sending a slow moving low pressure system well to our north while a series of cold fronts cross our area. Timing and therefore the details are a bit less certain given an amplified pattern with the idea of a closed low involved. We used a model/continuity blend for Friday through Saturday night, then blended in the 00z WPC Guidance thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Friday...Strengthening low pressure moves away during Friday, and therefore after some possible low clouds and a shower to start improving conditions are expected. However, cyclonic flow combined with another short wave and a surface trough reflection should result in some showers. The northern areas have a higher chance for some showers than farther south, but overall coverage looks to be isolated to scattered. A west- northwest wind will increase and become gusty for a time as the pressure gradient tightens and cooling occurs aloft allowing for better mixing. The winds diminish at night along with any showers dissipating early, and lingering clouds should thin out. For Saturday and Sunday...The flow turns generally more zonal with a weak but brief ridge in the east by later Saturday. The model guidance overall shows a short wave within this flow although the timing is a little uncertain, however this may ultimately track just to our south where a frontal zone will be located. A closed low centered north of the Great Lakes is forecast to amplify south and eastward gradually by late in the weekend. This will drive a cold front eastward and across our area later Sunday, however the model guidance overall suggests low pressure develops along a triple point as the main surface low to our west and north occludes. This may end up enhancing the showers and some thunder during Sunday, especially during the afternoon and evening. Backing up some, Saturday may end up being dry as enough ridging is in place, and short wave energy runs to our west and southwest. We will carry some more clouds Saturday but keep it basically dry, and therefore Saturday looks like the better of the two weekend days. The flow looks to turn more southeasterly in the lower levels Sunday, and therefore while still mild it will turn cooler along the coast. For Monday and Tuesday...A closed low is forecast to expand across the Great Lakes, Midwest to the Ohio Valley and gradually into the Northeast. This will drive surface low pressure eastward mainly north of the Great Lakes. An initial surface low should be moving away from the Mid Atlantic coast early Monday as a frontal zone sits offshore. The main cold front is forecast to move across our area Tuesday and shift offshore Tuesday night. The second half of Tuesday may be convectively active as a cold front arrives, however this will depend on short wave timing and potential convective development to our west. The timing overall is less certain given the scope of the closed low to our north-northwest and plenty of short wave energy revolving around it. Kept chance PoPs for Monday then slight chance to chance PoPs on Tuesday. We are anticipating warmer temperatures ahead of the cold front. For Wednesday...As the center of the closed low gradually shifts across eastern Canada, the overall trough becomes more established across the Northeast and into the Mid Atlantic. The main surface low is forecast to be east of James Bay Canada with its trailing cold front east of our coast to start Wednesday. A cyclonic flow will be in place and a surface trough should be sliding across the Great Lakes region. While less likely at this time, a few showers still cannot be ruled out given the synoptic setup. For now, kept some slight chance PoPs across the northern and western areas. Given the core of the closed low well to our north, keeping the colder air aloft there, daytime temperatures look to be on the warmer side. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Deteriorating conditions are expected for early this morning in rain. IFR conditions are likely to prevail. The main band of rain should lift to the north of our TAF sites this morning. However, conditions should improve only into the low end of the MVFR category with showers. KMIV and KACY could improve to VFR but it is a low confidence forecast. There is a chance of thunderstorms for this afternoon and evening. Conditions are forecast to remain in the MVFR and perhaps the IFR range for tonight with additional showers. An improvement to VFR is possible at KACY, KMIV, KILG and perhaps at KPHL and KPNE. An east wind around 8 to 12 knots this morning is expected to veer toward the southeast and south at KACY, KMIV, KILG, KPHL and KPNE as the day progresses. The wind may become light and variable for a time tonight before settling into the west at 5 to 10 knots. OUTLOOK... Friday...Possible MVFR/IFR ceilings early, otherwise improving to VFR. Isolated to scattered showers should develop during the day before dissipating in the evening. West-northwest winds increase with gusts up to 25 knots, then diminishing in the evening. Saturday...VFR overall. Light northwest winds turning southerly toward evening. Clouds increase at night and possibly lower. Sunday...MVFR/IFR conditions possible with showers and some possible thunderstorms, especially in the afternoon and evening. Monday...Sub-VFR conditions possible at times with a chance of showers and afternoon/evening thunderstorms. && .MARINE... A warm front is expected to lift slowly over the waters of Delaware and New Jersey today. Low pressure from the west and southwest should pass across the waters tonight. Wave heights on our ocean waters are expected to remain in the 4 to 7 foot range for today and tonight. As a result, the Small Craft Advisory will stay in effect. The wind is anticipated to work its way gradually around from the east to the south to the west during the period. Speeds should favor the 10 to 20 knot range with some gusts around 25 knots on our ocean waters. RIP CURRENTS... The flow is expected to veer from the east to the south today along the coasts of Delaware and New Jersey with wind speeds mainly in the 10 to 20 knot range. As a result, we will keep the moderate risk for the development of dangerous rip currents. The wind flow is forecast to become westerly for Friday. As a result, the rip current risk is anticipated to transition to low at that time. OUTLOOK... Friday...Small Craft Advisory extended through 22z for the ocean zones as elevated seas gradually subside with an offshore flow. Some wind gusts may reach 25 knots during the day especially nearshore, however confidence is not high enough to issue an advisory for Delaware Bay. Saturday through Monday...Conditions are forecast to be below advisory criteria, however winds may gust to around 20 knots at times especially on Sunday. && .TIDES/COASTAL FLOODING... A Coastal Flood Advisory was issued for portions of the Eastern Shore of Maryland. Levels on the Chesapeake Bay continue to rise at Cambridge and departures are already 2 feet and rising. They should continue to rise, before slowly lowering through high tide. But with the expected departure of near 2 feet around high tide, levels are expected to reach advisory levels. The astronomical tides with the new moon are particularly high with the evening high tide cycle. Positive tidal departures near 1 foot are anticipated again for today. As a result, we expect to see widespread coastal flooding expected with this evening`s high tide. We will continue the Coastal Flood Advisory for this evening in the coastal counties of New Jersey and Delaware, as well as for the counties along Delaware Bay and Raritan Bay. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Friday for NJZ012>014-020>027. DE...Coastal Flood Advisory from 7 PM this evening to 1 AM EDT Friday for DEZ002>004. MD...Coastal Flood Advisory until 8 PM EDT this evening for MDZ015- 019-020. MARINE...Small Craft Advisory until 6 PM EDT Friday for ANZ450>455. && $$ Synopsis...Gorse Near Term...CMS/Iovino Short Term...CMS/Iovino Long Term...Gorse Aviation...Gorse/Iovino Marine...Gorse/Iovino Tides/Coastal Flooding...Robertson/Iovino

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