Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 291014 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 514 AM EST Tue Nov 29 2016 .SYNOPSIS... Strong low pressure in the upper Midwest will send a warm front north through our area this afternoon. The low moves through the Great Lakes region Wednesday pushing a cold front eastward across the mid Atlantic states Wednesday night. The low redevelops to the Canadian maritimes Thursday and Friday. High pressure moves to the mid Atlantic states this weekend. Low pressure could affect our weather late Sunday or more likely early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A powerful upper-level trough will remain across Plains today, however a strong short wave trough rotating northward up across the eastern Great Lakes early this morning will help drive periods of rain into our area. Today will be part one of much needed rainfall. As the aforementioned system lifts into the Great Lakes, a southerly low-level jet on the order of 50-65 knots overspreads the area this morning. This will then shift east during the afternoon. This feature will drive increasing warm air and moisture advections across our area. The moisture advection is evident in the low-level theta-e fields as well as PW values increasing to 1.5 inches by later this morning. This will result in a corridor of enhanced lift across much of the area, therefore periods of rain will occur from southwest to northeast, some of which can be heavy at times. Given the soaring PW values, rainfall amounts should average in the 0.50- 1.50 inch range. For more details on the rainfall, see the hydrology section below. While some elevated instability may develop, the profiles via forecast soundings are rather saturated therefore we opted to not add in thunder at this time. We continued with a rapid increase in the PoPs this morning, and the steadier/heavier rain may be centered from the I-95 corridor on north and westward. As a surface warm front lifts northward during the course of the day, pushed by the robust low-level jet, boundary layer warming will allow for turbulent mixing and therefore breezy conditions are expected to develop for much of the area especially closer to the coast. The gusty winds will then diminish toward evening. High temperatures are mostly a blend of continuity and MOS. The very near term hourly temperature and dew point grids were adjusted with the latest obs, then the LAMP/LAV guidance was blended in. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... A powerful upper-level trough remains anchored across the Plains tonight. An initial impulse continues to track to our north pulling a warm front with it. A robust low-level jet will be shifting offshore early this evening, with much if not all of the lift going with it. This should rapidly diminish any rain as it tapers to some showers. There is some drying forecast to overspread our area, especially in the mid levels, however the wind field is forecast to be much lighter. This should result in lots of lower level moisture remaining in place, and at least some forecast soundings indicate that an inversion develops which helps to hold in the low-level moisture. It is possible that enough drying occurs especially across the western zones where the low clouds partially clear out for a time. Given the light winds and moisture left around, some fog is expected to develop. This could become more widespread especially if the lower cloud cover thins or clears for a time. Given the uncertainty regarding the extent of the low clouds, not anticipating widespread dense fog at this time. The next impulse rotating around the parent closed low will start to approach from the southwest toward morning. This will increase some warm air advection as another low-level jet begins to approach. The ascent associated with this should result in some rain developing once again. As of now, the bulk of this looks to hold off until during the day Wednesday. Low temperatures were a MOS/continuity blend and this results in mainly a milder night. && .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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500MB: A ridge in the northeast USA Wednesday gives way briefly to a trough Thursday. Thereafter, there are major model discrepancies regarding the evolution early next week. The 00z/29 GEFS develops a ridge in the east central USA while the operational ECMWF and yesterdays 12z/ecefs lifted a significant southern stream short wave northeastward from Mexico into the northeast states. Temperatures: November as whole will average 1 to 3 degrees above normal (see climate section at 5 am for more details). Calendar day averages Wednesday will range from about 13 degrees above normal far north FWN-MPO) to as much as 23 degrees above normal far south (GED), cooling to near 10 degrees above normal Thursday, still around 5 degrees above normal Friday and 1 to 4 degrees above normal Saturday, Sunday and Monday. Forecast basis: Unless otherwise noted, a 50 50 blend of the 00z/29 GFS/NAM MOS guidance Wednesday-Thursday, thereafter the 00z/29 GFS MEXMOS Thursday night, and then the 04z/29 WPC guidance Friday-Monday, at times blended with the 00z/29 GFS MEXMOS guidance or the long term temps were adjusted because of the 00z/29 ECMWF 18z 2m temp fields. The WPC POP Sunday into Monday was cut 30% each period to indicate a slight delay in the southern stream qpf threat and hold onto the drier GEFS and GGEM trends which reflects a slower arrival in the 00z/29 NAEFS mean qpf fields. The dailies... Wednesday...becoming breezy and rather warm along and southeast of I-95 while its mild northwest of I-95. Periods of showery rains with PWAT increasing to 1.6 inches by Wednesday evening near the I-95 corridor. SWI -2 , TT 52 and KI near 35 should result in isolated strong tstms southeast of I-95 (please see SWODY2). Thunder was trimmed a little to the southeast based on the more positively tilted EC and its projected instability fields. Heavy rain potential continues and probably near and nw of I-95. FLS issuance for poor drainage street flooding rains is anticipated. Northwest of I-95...rains are probably a little more persistent through the day and fog may be a problem, especially high terrain. Additional rainfall totals of 0.5 to 1.5 inches are expected across parts of eastern Pennsylvania and northern New Jersey with lesser amounts southern Delaware. See climate section for daily rain records. South to southwest winds will probably briefly gust 30-40 mph in southern Delaware during Wednesday afternoon. Confidence: Above average. Wednesday night...heavy showery rains end from west to east with a wind shift from south to west toward midnight. Still, Thursday morning min temps at least 20 degrees above normal (Calendar day mins for December 1 will be lower due to cold air advection Thursday evening). Confidence: Above average. Thursday: Dry and breezy, but still relatively mild. Gusty west northwest wind to 30 mph. Confidence: Above average. Friday-Saturday...Dry air and a return to near normal temperatures. A west northwest flow may bring some cold advection stratocumulus at times, especially to our northwestern counties. Winds will gust 20 to 25 mph at times, especially afternoons. small chance of a flurry or sprinkle in the Poconos Saturday. Confidence: Above average. Sunday-Monday...chance of precipitation, including snow in the northwest portion of our forecast area. Large spread in the guidance makes this a low confidence forecast. The GFS is suggesting dry high pressure while the ECMWF is continuing to indicate that low pressure will influence our weather at that time. WPC POPS were cut Sunday into Monday due to the uncertainty seen in the GEFS 6 hr pops for .05 and the possibility that the short wave ejection from Mexico is delayed a day.
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&& .AVIATION /10Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Clouds lower through this morning. Rain overspreading the area from southwest to northeast mainly between 10-15Z, then periods of rain continues for much of the day. The ceilings quickly lower to MVFR after the rain begins, then periods of IFR for much of the day. The visibilities will lower to MVFR/IFR during heavier rain. Low-level wind shear is included this morning as a southerly low- level jet (at 2,000 feet) increases to 45-60 knots. This occurs initially when surface winds are southeasterly at 10 knots or less. The surface winds increase after a warm front lifts north later in the morning and the low-level jet gradually shifts east, therefore low-level wind shear concerns generally decrease. Southerly surface winds will increase to 12-18 knots with gusts to around 25 knots across much of the area from later this morning through mid/late afternoon. Less confidence regarding gusts at RDG and ABE. Tonight...Any lingering showers end early with MVFR/IFR ceilings continuing, however areas of fog should develop as clouds may thin some in combination with light winds and lots of leftover moisture. Confidence regarding low ceilings is lower than average as they may partially clear for a time especially in the RDG to ABE corridor. OUTLOOK... Wednesday...KPHL KPNE KILG KMIV KACY: IFR conditions with low ceilings and fog should improve to MVFR during the day where a gusty south to southwest wind of 20-30 kt develops during the afternoon. Periods of showery heavy rains will produce brief IFR conditions during the afternoon and evening and there could be a gusty tstm (please see SWODY2). KABE KTTN KRDG: Generally IFR conditions in st/fog/periods of showery heavy rains. Small chc of an afternoon tstm. LLWS possible in the afternoon with a 45 to 50 knot low level southwesterly jet at that time. Wednesday night...MVFR and IFR conditions in moderate to heavy rain during the evening with a chance of thunder. A cold front should arrive around midnight followed by clearing and improvement to VFR. Low level wind shear is possible in the evening due the expectation of a 45 to 50 knot low level southwesterly jet at that time. Thursday through Saturday...Mainly VFR sct-bkn aoa 4000 ft. West to northwest wind gusts 25-30 kt Thursday afternoon and 20-25 kt both Friday and Saturday afternoons. && .MARINE... A Small Craft Advisory is in effect for all zones today and the Atlantic Coastal water zones tonight. A southerly low-level jet on the order of 50-65 knots will move across the region today. This will allow warmer air to stream northward along with a surface warm front. Periods of rain will also occur and this should keep the low-levels from mixing as efficiently (poor lapse rates). As a result, kept a strong advisory in place and held off on upgrading to a Gale Warning however there still is a possibility for a few local gale force gusts by later in the day especially with any heavier downpours. The winds diminish tonight as the low-level jet shifts offshore, and with higher dew points remaining for awhile this could allow for some fog to develop. The seas will build into the 5-8 foot range today, then subside tonight. OUTLOOK... Wednesday...A Small Craft Advisory is in effect for our ocean waters for wave heights of 5 to 8 feet. Also, wind gusts of 30-35 knots are possible on Wednesday afternoon along with a possible strong tstm over the DE and s NJ waters. A Special Marine Warning may need to be issued to highlight gale force gusty showers/tstms, especially if convection can become organized. Wednesday night...Wind gusts of 25 to 30 knots are expected initially from the southwest then from the west following the passage of a cold front after midnight. There could be a brief surge of gale force gusts on either side of the frontal passage. Thursday through Saturday...West to northwest wind gusts of 20 to 30 knots are expected through the period with an SCA probable for Thursday and a lesser chance of an SCA Friday and Saturday. && .HYDROLOGY... Much needed rain across the region the next couple of days. For today and tonight...A robust low-level jet will drive increasing moisture advection during the day today. The PW values are forecast to increase rapidly to 1.5 inches by Midday. As enhanced ascent occurs from the low-level jet periods of rain will result, some of which can be heavy at times. It appears that 0.50-1.50 inches of rain should be common with the highest amounts along and north/west of the I-95 corridor. The placement of this though will be determined primarily by the advancement of a warm front today. It has been rather dry, therefore just some ponding of water on roads during heavier rainfall rates is anticipated along with some possible poor drainage flooding. Some poor drainage flooding though could be locally enhanced where fallen leaves clog storm drains. The rain/showers taper later this afternoon and early this evening, with much of tonight tending to feature a lull before the next batch of rain arrives Wednesday. For Wednesday...Storm total rainfall (even including Tuesday`s amounts) will average 1-3 inches but isolated higher amounts of 3+ inches are certainly possible if the heaviest rain from both waves of rain falls over the same areas. Conversely, some areas on the coastal plain may receive less than an inch of rain, particularly southern Delaware. Through late Wednesday, we are not expecting main stem flooding or even smaller basin flooding since these rainfall amounts will be spread out over a 48 hour period (with a lull in between) and the antecedent conditions are rather dry. Poor drainage flooding is always a possibility especially during heavier rainfall rates but impacts from this type of flooding are generally minor, except for travel during the morning and evening commutes. && .CLIMATE...
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PHL should average 50 degrees for the month or 2.4 above the average of 47.6. It is estimated that the large positive departures (warmth) of today and Wednesday will warm the monthly departure by an estimated 0.3 to 0.9 degrees from the values calculated through the 28th. Meanwhile... Daily record High temperatures for Wednesday 11/30 where they appear vulnerable to being within 2 degrees of record. GED 74 - 2006 Daily record rainfall values for Nov 29 and 30 (less confidence on approaching records today, whereas on Wednesday, several sites may establish a new record rainfall for the date.) NOV 29 NOV 30 ACY 1.93-1963 1.25 1923 PHL 1.68-1971 0.96-1972 ILG 2.25-1945 1.28-1934 ABE 1.70-1935 0.86-1923 TTN 1.40-1987 0.76-1898 GED 1.47-1982 1.71-1987 RDG 1.69-1984 1.52-1987 MPO 2.71-2005 3.00-1923
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST Wednesday for ANZ450>455. Small Craft Advisory until 6 PM EST this evening for ANZ430- 431. && $$ Synopsis...Drag Near Term...Gorse Short Term...Gorse Long Term...Drag 513 Aviation...Drag/Gorse Marine...Drag/Gorse Hydrology... Climate...513

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