Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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639 FXUS61 KPHI 240217 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 917 PM EST Wed Jan 23 2019 .SYNOPSIS... Low pressure will pass through the Great Lakes and into Ontario and Quebec tonight. Areas of rain, some heavy, will develop late tonight and particularly tomorrow ahead of a cold frontal passage later tomorrow. High pressure and cooler but drier weather builds in for Friday and Saturday. A cold front will cross the region Sunday associated with a low pressure to our north which may bring some snow showers. Another period of unsettled weather is possible early next week, but its evolution is uncertain. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... Evening update: Hourly temperature forecasts have been a bit of a challenge this evening as periodic breaks in the lower clouds have allowed certain (primarily) inland locations too cool more than expected. However, an increasing southerly gradient combined with more robust cloud cover should result in a general warming trend as we head into the overnight. Also reduced PoPs NW of the I-95 corridor for the evening period, as hi-res guidance has continued to slow the onset of prefrontal precipitation (generally after 06Z with some guidance including the HRRR/RAP holding off until around 09Z). Southerly winds have also begun to increase along coastal areas this evening with ACY already gusting to around 30 kts. These winds will further increase through the overnight into Thursday morning, with speeds also increasing inland (albeit to a lesser extent). Going forecast/headlines in good shape...see discussion below: Low pressure will track across Quebec Canada tonight with a cold front arriving in our area toward daybreak. The overall system is slower and therefore a delay in the precipitation has resulted. This has allowed temperatures to warm even more though especially the southern areas with around 60 degrees across southern Delaware this afternoon (up to near freezing in the Poconos). A look at mesonet data shows much of Carbon County is above freezing and therefore the Winter Weather Advisory has been cancelled as additional warming is expected. For Monroe County, opted to extend the Winter Weather Advisory through 7 PM given several locations closer to freezing still. Any icing that occurs is expected to be light, and additional warming through the night will allow just plain rain to occur. A robust low-level jet will continue to maintain warm air advection through the night. As this occurs, southerly flow will likely increase especially in the coastal plain and the higher terrain such as the Poconos. The combination of a southerly breeze and warm air advection should result in temperatures rising through the night. There will also be an increase in the moisture advection as deep moisture overspreads our area. The main forcing looks to arrive later tonight and especially during Thursday, therefore much of the rain that arrives from the west tonight should be on the lighter side. Given the slower trends, significantly slowed the eastward increase in the PoPs especially through this evening. Low temperatures should occur early this evening, then as mentioned should tend to rise. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... An upper-level trough arriving from the west has a substantial wind field associated with it. Low pressure will track well to our north during Thursday, however its cold front will be working its way across the region. Very strong southerly low-level flow in advance of the front will continue to advect warm and unusually moist air into the area. The precipitable water values are forecast to near 1.5 inches ahead of the front, which is very impressive for this time of the year. This will result in an area of heavier rain from west to east. A low-level jet of 50-80 knots (500 mb jet streak of around 100 knots), moves across much of the area. In addition, a 170-180 knot 250 mb jet arriving from the west places our region in the very favorable right entrance region. This all points to a corridor of strong and deep ascent, which will be enhanced by a band of strong 850-700 mb frontogenesis. There are also notable pressure rises behind the front. The forecast soundings show meager instability in advance of the front, therefore despite a narrow convective-looking line with the front little or no lightning is expected (perhaps a better chance for some lightning near or off the coast). For now, did not include thunder given the meager instability reducing the updraft strength. The greatest rainfall rates should be mostly with the strongly forced line as it quickly moves eastward across the area. The QPF is generally 1 to 2 inches during the day Thursday across the area, with most of this occurring in a 3-6 hour period at any particular location. For the flooding risk, see the hydrology section below. Winds will also be gusty in advance of the front, especially where surface temperatures are into the 50s as mixing will be more efficient within the presence of a low-level jet. The strongest wind field is especially across our eastern to southern zones, and given the mixing potential (depth and duration) opted to issue a Wind Advisory for Thursday morning for Monmouth County south to Cape May and Cumberland counties then much of our eastern Maryland Shore and Delaware counties. The strongest winds may be just inland from the coasts as the cooler waters may reduce the mixing some at the land- water interface. Elsewhere, gusty winds should occur, however there is less confidence in 40+ knot wind gusts. In addition, any convectively-induced precipitation may at least partially mix down some of the very strong winds from aloft for a brief time. High-res guidance generally shows the low-topped convective line strengthening into the I-95 corridor eastward as it encounters warmer air and the stronger wind field. The front quickly moves offshore during the afternoon with precipitation ending quickly behind it, and colder air will filter in. Depending on how quickly the precipitation ends in the wake of the front, may see a brief changeover to snow showers before ending especially in the far northwest zones. As of now, the thinking is that the bulk of the precipitation ends before a changeover to snow can occur. Temperatures will surge in advance of the front, with readings in the 50s to near 60 southeast of mostly the Fall Line. However, temperatures will fall behind the front especially toward later in the afternoon and evening, with lows below freezing virtually everywhere Thursday night. Skies should clear Thursday evening/night, although some lingering moisture may keep the lower clouds present in the far northwestern zones. Some flurries or a few snow showers are possible especially in the Poconos as low-level northwesterly flow occurs along with cold air advection. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Overview... Most if not all of this period will be driven by the presence of a broad, deep trough over the central and eastern US. The axis of the longwave trough will move little for several days, but the flow within the trough is progressive and will allow for multiple shortwaves to rotate through it. The tracks and timing of these features will dictate our precipitation opportunities for this weekend and early next week. With the depth of the trough, appears we are largely shut out from any southern stream moisture this period, which would favor precipitation threats to be on the lighter side. However, given multiple northern stream disturbances moving through, it will have to be monitored as to whether we can tap into any of the better moisture to the south and potentially create a larger precip event. Present indications suggest not, but still several days away. Plenty of bitterly cold air will be present well to our north over this stretch, but despite the troughing pattern the core of the cold remains largely bottled up in Canada. Thus, temperatures should not stray too far from normal for most of this period. Dailies... Friday-Saturday... This looks like the quiet portion of the long term. Cold front will be well offshore by Friday morning with high pressure gradually building in from the west. Temperatures trend cooler but not especially cold. A shortwave passing through the Great Lakes and into the Northeast on Friday is largely moisture starved, but could produce some light snow showers in western and northwestern areas. Saturday night-Sunday... First potential unsettled period of the long term comes here as a potent shortwave and closed H5 low dives into Ontario. However, this system remains well to our north, and appears unlikely to connect with better moisture well to our south and east. This means the most likely outcome is just some scattered snow showers late Saturday night and Sunday. Winds turn out of the south before a cold frontal passage late Sunday, so it is possible these showers may mix with rain in southern areas given decent WAA ahead of the front. Sunday night-Monday... High pressure likely builds in to our north, at least briefly. Guidance does begin to diverge quite a bit by this period, not surprising given models often struggle with small, fast moving features like we will be dealing with. But feel this period should be mostly dry. A lot of spread on temperatures at this range as baroclinicity looks strong over the East, with very cold air not far to the north but much warmer not far south and our region likely caught in between. Stayed close to the consensus which leaves us near average. Monday night-Wednesday... Confidence decreases further here. Decent agreement on a clipper low moving through the lingering trough and approaching Monday night or Tuesday, with some discrepancies on its exact track. We are probably in line for a precipitation event centered around Tuesday, but several scenarios exist. This includes a warm solution of a single parent low passing to our west, which is favored by most of the 23.12z guidance. However, some form of coastal redevelopment could also occur and introduce more complications, as some of the 23.0z guidance and 23.12z EC suggested. Plenty of time to iron this out and as usual for this range keeping details limited for now. Drier and colder weather likely returns by Wednesday though can`t rule out some snow showers here either. && .AVIATION /02Z THURSDAY THROUGH MONDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...Conditions will deteriorate to MVFR overnight. Rain chances increase from northwest to southeast, especially after midnight-3AM. South winds around 10 to 15 kts, gusty winds expected near/southeast of PHL especially ACY. Southwesterly LLWS anticipated through the night. Thursday...IFR restrictions will continue through at least early afternoon, with rain (heavy at times). Southwesterly LLWS will also continue through the morning. Conditions are expected to improve rapidly during the afternoon with the passage of a cold front. VFR is possible late. Southwest winds 10 to 20 kts with higher gusts before the frontal passage, becoming northwest afterward. Outlook... Thursday night...Mainly VFR with west to northwest winds around or below 10 kts. Friday and Saturday...Mainly VFR conditions expected. Winds generally below 20 knots. Saturday night through Sunday night...A cold front may bring rain and/or snow showers, thus possibly lowering visibilities and bringing restrictions, especially at KRDG and KABE. Monday...Mainly VFR. Some MVFR possible in rain and/or snow showers. && .MARINE... Small Craft Advisory conditions through tonight, then upgraded to a Gale Warning for Thursday morning to early Thursday afternoon. An intense wind field will accompany a cold front that moves across our area on Thursday. Ahead of this front, very strong south to southwest winds will overspread the area with this peaking mainly Thursday morning. It is challenging regarding how much of the wind is able to mix down to the surface given strong warm air advection over the much cooler waters. The forecast soundings indicate a rather limited mix layer, however very strong winds are just above the surface. In addition, a narrow line of low-topped convection (probably little or no lightning) will accompany the front later Thursday morning. Stronger winds could mix down with this. Given the intense wind field forecast, opted to go with a Gale Warning for all waters for a portion of Thursday. Small Craft Advisory conditions will then follow through Thursday night as much colder air follows (although not as cold as the previous cold front). Outlook... Thursday night...The rain moves out and northwest winds diminish, but seas will likely remain above advisory criteria through the night. Friday-Saturday...VFR. Winds mainly out of the west and northwest with gusts at or below 20 kt. High confidence. Saturday night-Sunday...Rain or snow showers possible ahead of a cold front. Localized ceiling and visibility restrictions possible with mainly MVFR conditions but brief periods of IFR possible. Winds shift to S then SE but gusts remain under 20 kt. Moderate confidence. Sunday night-Monday...Mainly VFR or MVFR conditions currently expected with isolated rain or snow showers possible. Moderate confidence. && .HYDROLOGY... A flood watch remains in place for late tonight through Thursday afternoon from the Lehigh Valley and northwest New Jersey southeastward to the I-95/295 corridors. A cold front will move through the area on Thursday. A round of rain, moderate to heavy at times, is expected in advance of the front late tonight into tomorrow morning. Expected storm totals are generally from 1 to 1.5 inches, with locally higher amounts possible. Normally, these totals would not be forecast to produce much flooding. However, a number of factors suggest an elevated threat exists... 1) Antecedent wet conditions, including from the rainfall this past weekend. 2) Relatively cold soils, aided by the arctic air that spread across the region the past couple of days. 3) The expectation of most of this rain occurring in a three-to-six hour period. With higher rain rates expected during a portion of the event, especially just in advance of the frontal passage on Thursday, additional runoff will be generated, which could lead to flooding of particularly sensitive spots (e.g., urban areas and quickly-responding creeks and streams). 4) Light to moderate rain occurring for a few hours before the heaviest rainfall occurs, which may act to saturate soils sufficiently for efficient runoff during the most intense rainfall. An isolated instance of flash flooding is possible, especially if the heaviest rainfall (and high rainfall rates) occurs along the urban corridor or atop smaller stream/creek basins that respond rapidly to runoff. River flooding cannot be ruled out, especially on the Passaic, Rockaway, North Branch Rancocas, and Millstone Rivers, if the axis of heaviest rainfall intersects these basins. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flood Watch from 4 AM EST Thursday through Thursday afternoon for PAZ061-062-070-071-101>106. NJ...Flood Watch from 4 AM EST Thursday through Thursday afternoon for NJZ001-007>010-012>020-026-027. Wind Advisory from 6 AM to noon EST Thursday for NJZ013-014- 020>027. DE...Flood Watch from 4 AM EST Thursday through Thursday afternoon for DEZ001. Wind Advisory from 6 AM to noon EST Thursday for DEZ002>004. MD...Flood Watch from 4 AM EST Thursday through Thursday afternoon for MDZ008. Wind Advisory from 6 AM to noon EST Thursday for MDZ012-015- 019-020. MARINE...Small Craft Advisory until 6 AM EST Thursday for ANZ430-431- 450>455. Gale Warning from 6 AM to 1 PM EST Thursday for ANZ430-431- 450>455. && $$ Synopsis...O`Brien Near Term...Carr/Gorse Short Term...Gorse Long Term...O`Brien Aviation...Kruzdlo/Carr/O`Brien Marine...Kruzdlo/O`Brien Hydrology...

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