Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 201044 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 644 AM EDT Tue Apr 20 2021 .SYNOPSIS... As a surface low lifts northeastward through eastern Canada, a cold front will approach the region today. Another wave of low pressure will develop along the front in the middle Mississippi Valley, lifting northeastward through the Northeast on Wednesday. The cold front will sweep off the coast by Wednesday afternoon, with a strong high building into much of the central and eastern U.S. through the end of the week. Another system will affect the region this weekend. && .NEAR TERM /THROUGH TONIGHT/... Objective 00z upper-air analysis indicates continued troughing across much of North America, with fast 250-mb flow confined to the far southern/southeastern U.S. Two midlevel vorticity maxima were noteworthy at 500 mb: one in southeastern Canada and the Great Lakes region and another now entering the Great Plains. An 850-mb baroclinic zone was oriented northeast-to-southwest from southeastern Ontario to the central Rockies, with strong cold advection in the northern/central Plains in advance of a developing surface high that will bring unusually cold air to this region the next few days. This will also aid in the development and intensification of a surface low along the baroclinic zone today, which will be our main player for Wednesday and Thursday. Meanwhile, as the central U.S. storm evolution plays out today, downstream ridge amplification will occur in the eastern U.S. Aiding in this process will be increasing warm advection on the upstream side of a weak high in the western Atlantic. Considerable surface warming will occur across the region as a result, with southwesterly flow enhancing low-level mixing. Temperatures will soar in this pattern, with highs likely in the 70s for much of the area. Skies should be mostly sunny this morning, but there may be some increasing cloudiness later today as the system out west increases its influence on our region, especially for the northwest CWA. The only issue of concern today is fire weather; see the discussion below for more information. The central U.S. shortwave trough will pivot to a neutral orientation tonight in the Midwest, as a kicker perturbation digs into the northern/central Plains and aided by the continued ridge amplification along the East Coast. A surface low will intensify along the baroclinic zone stretching northeastward to the Northeast. Southerly flow will continue overnight as a response to the developing low, and warm-advection precipitation should develop to our northwest, perhaps affecting areas generally northwest of the Fall Line overnight. Was fairly skittish with PoPs, as models tend to be too aggressive with the southeastward extent of precipitation into the developing warm sector. Nevertheless, think there will be some showers in the Poconos and Lehigh Valley at least during the overnight hours. Temperatures will be mild, with lows in the 50s for all but the Poconos and far northwest New Jersey. && .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... Main concern remains with severe potential on Wednesday and winds Wednesday night and Thursday. The shortwave trough advancing into the Northeast on Wednesday will begin to take on a negative tilt as a 500-mb jet streak surges eastward into the central/southern Appalachians and adjacent Mid-Atlantic. Models are converging on the timing of the low and attendant cold front, with the GFS trending slower and ECMWF trending faster. The timing of the front will be critical, as its current consensus is generally in the 17z-21z time window across the CWA. If the timing is a little faster, sufficient instability may not develop to produce vigorous convection for at least western portions of the area. However, convection-allowing models seem to suggest favorable timing for a strongly-forced squall line (or at least a broken convective line) to develop along the surging front. Model soundings indicate a strongly sheared environment, with deep-layer bulk shear of 40-50 kt and 0-1 km of 150-250 J/kg. CAPE will be modest at best (probably around or below 750 J/kg), but this has often proved sufficient in the past for strong/damaging gusts given similar-looking wind profiles. Given the thermodynamic profile, think this event would be similar to 28 March in terms of lightning. There may be very little to speak of given the shallow nature of CAPE and its overall thin appearance through the profile. One caveat to this potential is extent of clouds and precipitation in the morning, especially in the northern CWA. It is quite possible temperatures struggle to reach the 60s in the Poconos and far northern New Jersey. Should precipitation be more extensive, these cooler temperatures may linger toward the I-78 corridor as well. For this reason, have sharpened the high-temperature gradient on Thursday, with temperatures near or above 70 southeast of the I-95 corridor (slightly above consensus) and in the mid 50s in the Poconos (slightly below consensus). Wednesday will be breezy, as well, as mixing in advance of the front will allow for speeds of 10-20 mph and gusts to 25+ mph. Then, in the immediate post-frontal environment, a surge of stronger gusts may occur as winds become west or northwest. Conditions will be breezy on Wednesday night (especially during the evening, with gusts of 25-35+ mph possible), but winds will likely reach a nocturnal minimum. By Thursday, speeds will pick up again, with model soundings indicating mixing up to the 800-700 mb layer. Widespread gusts of 30-40+ mph are likely, and an advisory may be needed if models trend more aggressively. The other issue of concern for the Wednesday night through Thursday night period will be the cold air seeping into the area. Temperatures will plunge behind the front, likely reaching near-freezing or below northwest of the urban corridor. Given the elevated winds Wednesday night, this continues to look like a freeze/no-freeze forecast, and confidence is not overly high given that the winds may keep temperatures somewhat higher than model consensus. However, we will likely need to issue freeze products for portions of the area Wednesday night. Highs on Thursday will likely be around 15 degrees colder than Wednesday, and winds will diminish somewhat on Thursday night (but probably not enough for frost development). Another night of freeze potential, though, especially north of I-78. && .LONG TERM /FRIDAY THROUGH MONDAY/... As the low in the Northeast lifts into the Canadian Maritimes by the end of the week, the pressure gradient across the Mid- Atlantic will begin to relax. Though breezy northwest winds appear likely to continue on Friday, the day should be much more tolerable than its predecessor. This will be especially true since midlevel ridging will approach the East Coast, so column warming will greatly increase temperatures from the previous day. Expect highs about 10 degrees warmer. However, that midlevel ridging comes in response to another strong perturbation moving through the central U.S. Additionally, a northern-stream counterpart will aid in some phasing into a more-amplified/stronger trough as it approaches the East during the weekend. Deterministic output is fairly agreeable given the setup, suggesting that a surface low will trek along or near the coast Saturday night and Sunday, bringing widespread precipitation to the region. Strong winds will likely accompany the low to its north and west, making for a rather unpleasant few hours as the heavy rain moves through. A soaking 0.5-1.5 inches QPF seems to be the consensus, which will likely mostly fall in the beneficial range of precipitation totals. However, we will need to watch the system given its healthy and dynamic look. Breezy northwest winds and cooler temperatures are expected following the system into the beginning of next week. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Early this morning...Mainly VFR, but patchy/transient fog is possible through 14z, especially at TTN/PNE/ILG. Light and variable winds. Moderate confidence. Rest of today...VFR with southwest winds increasing to 7 to 15 kt with higher gusts by this afternoon. High confidence. Tonight...Mainly VFR, especially during the evening, but some restrictions may creep into areas northwest of the Philly terminals overnight as the potential for showers increases. Winds generally becoming south up to 10 kt. There is a chance for south to southwest LLWS overnight. Have not placed in the TAFs yet, but confidence is beginning to increase in the need for mention. Moderate confidence. Outlook... Wednesday...Showers and isolated storms are likely during the late morning into the middle afternoon, as a convective line will likely move through the region. Strong/erratic winds and brief restrictions are possible with the passage of the line. Otherwise, mainly VFR is expected, with south to southwest winds 10 to 15 kt with gusts to 25 kt becoming west to northwest 10 to 20 kt with gusts to 30+ kt by late afternoon into the evening. Moderate confidence. Wednesday night...Mainly VFR with northwest winds 10 to 20 kt with higher gusts possible. Moderate confidence. Thursday...Mainly VFR with northwest winds 15 to 25 kt with gusts to 35+ kt. High confidence. Thursday night and Friday...VFR with west to northwest winds 10 to 20 kt with higher gusts. High confidence. Friday night and Saturday...Mainly VFR with west to southwest winds 5 to 10 kt. && .MARINE... Sub-advisory conditions are expected through tonight. However, southwest to south winds of 10 to 20 kt may occasionally gust close to 25 kt at times. Seas will be increasing to 2 to 4 feet by tonight. Outlook... Wednesday...Advisory conditions will likely develop as south to southwest winds increase to 15 to 25 kt with higher gusts. Seas nearing 5 feet. A small craft advisory is in effect beginning at noon. Storms will likely move through during the afternoon, with gusty/erratic winds and higher waves in their proximity. Wednesday night and Thursday...Winds becoming west or northwest and increasing to 20 to 30 kt with gusts to 40 kt. A gale watch is in effect. Thursday night...Lingering gales possible in the evening; otherwise, mainly advisory conditions expected. Friday...Lingering advisory conditions expected, though winds and seas should be subsiding during this time. Friday night and Saturday...Sub-advisory conditions expected. && .FIRE WEATHER... Winds will increase out of the southwest today, becoming around 10 to 15 mph with occasional gusts to 20 mph or so. Though dew points are expected to be increasing with time, the strong heating and increasing winds may act to mix out the higher moisture-content of the air. Minimum relative humidity values may approach or even fall below 30 percent this afternoon, especially for areas southeast of Interstate 95. We will coordinate with fire-weather partners later this morning to determine if a special weather statement will be required for portions of the area today. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Wednesday evening through Thursday afternoon for ANZ430-431-450>455. Small Craft Advisory from noon to 6 PM EDT Wednesday for ANZ430-431-450>455. && $$ Synopsis...CMS Near Term...CMS Short Term...CMS Long Term...CMS Aviation...CMS Marine...CMS Fire Weather...CMS is the U.S. government's official web portal to all federal, state and local government web resources and services.