Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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413 FXUS61 KPHI 281436 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 936 AM EST Tue Feb 28 2017 .SYNOPSIS... Low pressure organizing in the Central Plains today moves into the Great Lakes tonight as it strengthens, then tracks across northern New England later Wednesday. This storm may continue to strengthen as it moves into the Canadian Maritimes Thursday. An associated warm front lifts to our north today, then a strong cold front sweeps through later Wednesday. A clipper system may move through Friday, otherwise high pressure begins to build in from the west later Friday before sliding to our south Saturday. Low pressure in the Midwest Sunday moves into the Great Lakes Monday with a warm front to our north. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Clouds have cleared out quickly this morning. However, another area of mid level clouds is approaching from the west. Thus, expect mostly cloudy conditions by mid afternoon. Given this early break in the clouds, adjusted the highs up slightly above guidance as winds should be shifting southerly soon resulting in the beginning of warm air advection. Surface low pressure takes a northeast track from the Midwest to the Great Lakes, and this will lift another warm front into the Mid- Atlantic and Northeast by late afternoon. Precip chances ramp up over the Lehigh Valley, SE PA, and western portions of the Delmarva after 4 pm. Will keep thunder out of the forecast for today due to marginal instability. Think record high temps should be safe, for the most part. However, if more sunshine is able to break out today, then temps could be warmer than forecast, resulting in some new high temperature records. See Climate section below for high temperature records for today and Wednesday.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... Warm front continues to lift north tonight. Strong WAA will be underway, and low level moisture will increase across region. As upper trough digs through the Great Lakes, several shortwaves will eject ahead of that trough and towards the Mid-Atlantic and Northeast, and this will provide the forcing needed for showers to develop. Will carry categorical/likely PoPs early Wednesday night, as both NAM and GFS have a slug of precip in the 00-06Z Wednesday timeframe. PWATs during this time look to range from 1.25-1.4", so it is possible for a brief period of locally heavy rain with the passage of the warm front. Lingering showers possible from 06-12Z Wednesday, but any additional rainfall will be light. With abundant low-level moisture, will expect at least patchy fog to develop, but would not rule out some areas, especially along the coasts and areas where rainfall was heaviest, for locally dense fog to develop. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Summary...Record warmth potential Wednesday afternoon, and severe thunderstorms possible Wednesday afternoon and evening. Cooler Thursday and then colder Friday and Saturday followed by another warm up Sunday and Monday. The synoptic setup is comprised of a sharpening upper-level trough moving across the Midwest and Great Lakes Wednesday then across the Northeast/Mid Atlantic Wednesday night and Thursday. This trough looks to sharpen again Friday before shifting eastward with the flow backing in its wake. Some ridging arrives early next week ahead of the next sharpening trough in the Midwest and Great Lakes. This all results in a temperature roller coaster. We used a model and continuity blend for Wednesday into Friday, then blended in the 00z WPC Guidance thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Wednesday...A sharpening upper-level trough from the Great Lakes region will drive a strengthening surface low to our northwest. Our region will be within the warm sector, and with increasing flow and instability the ingredients are coming together for the potential for severe thunderstorm development. Record warmth possible for several areas especially with enough sunshine. The model guidance indicates an initial burst of showers (perhaps some thunder) to start the morning as a lead impulse/warm air advection burst moves east-northeastward. Once this feature clears the area, at least partial clearing is anticipated with probably a lull in showers. An inversion is forecast to be in place due to a warm layer aloft, however this should erode with adequate boundary layer heating and mixing. Farther south for Maryland and Delaware and even into southern New Jersey, the inversion may hold until late afternoon or early evening. Model guidance is in good agreement in developing convection ahead of a cold front, potentially initially tied to a pre-frontal trough however the overall convective evolution may be complicated by the extent of some clearing. The development will be within a modestly unstable airmass with CAPE values on the order of 1000 j/kg. The airmass is characterized by strengthening flow as a low-level jet of 60 knots moves across the area and mid level flow of around 80 knots. This flow will allow for strong deep-layer shear, however there is some veering noted in the lowest levels of some of the soundings. Given the deep unidirectional flow, storms should favor a linear mode with embedded bowing segments. Given the strong shear however, transient rotation cannot be ruled especially if the low-level flow backs a bit and some circulations can develop within any QLCS areas. An elevated mixed layer (EML) has been indicated at least in some of the soundings, which would favor more instability and heighten the hail and wind threat. If enough instability combined with a well mixed boundary layer coincides, DCAPE would increase resulting in a higher damaging wind threat. Much of the area has the potential for some severe thunderstorm development mainly Wednesday afternoon and evening, however about the southern half of the area may have a higher chance given the cap holding longer allowing the instability to build further. A challenge is the extent of cloud cover (early activity exiting soon enough) as this would impact the amount of instability realized, however it appears there will be enough boundary layer mixing to allow for adequate heating. PoPs decrease for a time through the morning, then ramp up in the afternoon and evening as convection traverses the area from west to east. There could very well be enough upscale growth and cold pool development to organize the convection into a solid squall line as it shifts eastward. Given the potential along with the new Day 2 SPC Outlook, damaging winds has been added to the forecast for the afternoon and early evening. Convection will produce a period of heavy rain as the PW values are forecast to near 1.5 inches, however storm motion should be quick enough and therefore limit the flood potential. Improving conditions during Wednesday night as a strong cold front shifts offshore, however cold air advection arrives in its wake setting the stage for a return to colder conditions. For Thursday and Friday...Cold air advection continues, and an upper- level trough is forecast to sharpen from the west later Thursday night and Friday into the Northeast. Most of the model guidance indicates a clipper accompanying this with a reinforcing shot of colder air in its wake. Moisture looks more limited with this clipper, however a band of precipitation may accompany it especially near and just north of the surface low track. This could be a compact feature, however moisture may be the limiting factor. We will indicate low chance PoPs for now later Thursday night and Friday. Thermal profiles support mostly snow up north to a rain/snow mix southward and possibly all rain close to the coast and far southern areas. A gusty northwesterly wind will occur both days, with peak gusts to about 35 mph possible. High pressure starts to approach from the west late Friday. For Saturday through Monday...High pressure slides to our south Saturday before shifting offshore. The airmass will start to moderate Saturday aloft, however a northwest flow will occur although lighter. This should result in another chilly day, before a return flow starts to take hold Sunday along with an increase in warm air advection. As a result, a warming trend is expected once again Sunday and Monday. && .AVIATION /15Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. This morning (12Z-18Z)...VFR. LGT/VRB winds. This afternoon (18Z-00Z)...MVFR conditions possible in -SHRA, mainly at KABE/KRDG/KPHL/KILG. Otherwise, VFR. S winds 5-10 KT. Tonight (00Z-06Z Wednesday)...IFR and lower conditions in SHRA/BR. S winds 5-10 KT. Late tonight (06-12Z Wednesday)...Lingering SHRA taper off. IFR and lower in fog. LLWS with SW winds 35-45 KT at 2000 ft possible. Outlook... Wednesday...MVFR/IFR possible to start with some showers, then conditions should improve to VFR for a time. A period of MVFR/IFR conditions then as a strong cold front arrives in the afternoon with showers and thunderstorms through the evening. Some of the thunderstorms will be capable of producing gusts to 40 knots or greater. Southwesterly winds increasing to 15-20 knots with gusts to 30 knots, becoming westerly at night in the wake of the cold front and possibly diminish some. Conditions improve to VFR later Wednesday night as the strong cold front moves offshore. Thursday and Friday...VFR overall, however there is a low chance for some snow/rain late Thursday night and Friday as a weak system moves through. Northwesterly winds 15-20 knots with gusts up to 30 knots, diminishing Thursday evening before increasing some again on Friday. Saturday...VFR overall with less wind. && .MARINE... Today...Sub-SCA conditions. No headlines. S-SW winds 5-10 KT with seas 2-4 feet. Tonight...VSBY restrictions expected in showers and fog. Widespread fog, possibly dense, expected late tonight on all waters. SW winds increase to 15-20 KT with 25 KT gusts on the ocean waters, so a SCA will be issued starting at 5 AM. Outlook... Wednesday...Small Craft Advisory issued through early Thursday. Southwesterly wind gusts at or above 25 knots, however mixing may be more limited away from the coast due to much warmer air moving over the chilly ocean water. Some fog in the morning, however the extent of this will depend on the strength of the winds. Showers and thunderstorms especially late in the afternoon and at night, and some thunderstorms with a possible squall line may produce wind gusts to 40 knots. Winds shift to west and northwest at night as a strong cold front moves through. Thursday and Friday...Small Craft Advisory conditions are expected within a cold air advection regime resulting in more efficient mixing. There is the potential for periods of gale force gusts, however held off on a watch at this time. Saturday...The winds and seas dropping below Small Craft Advisory criteria through the day. && .CLIMATE... Record high temperatures are listed below. Location Tuesday 2/28 Wednesday 3/1 ABE 67 in 1976 67 in 1972 ACY 70 in 2011 72 in 1972 GED 74 in 1997 73 in 1976 ILG 66 in 2011 75 in 1972 MPO 64 in 1997 67 in 1972 PHL 68 in 1976 76 in 1972 RDG 70 in 1976 74 in 1972 TTN 69 in 1903 74 in 1972 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 4 AM Wednesday to 6 AM EST Thursday for ANZ450>455. Small Craft Advisory from 10 AM Wednesday to 6 AM EST Thursday for ANZ430-431. && $$ Synopsis...Gorse Near Term...Johnson/MPS Short Term...MPS Long Term...Gorse Aviation...Gorse/MPS Marine...Gorse/MPS Climate... is the U.S. government's official web portal to all federal, state and local government web resources and services.