499 FXUS61 KPHI 041505 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1005 AM EST Fri Dec 4 2020 .SYNOPSIS... A large low pressure system will develop over the Tennessee Valley later today. As it progresses further northeast, it will affect our region starting tonight through tomorrow. Another low pressure system will track close to our region, but stay mostly off shore Monday into Tuesday. After then, a large area of high pressure builds in from the south. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The main weather story over the next couple days will be the storm system due to affect the region mainly tonight into Saturday. This system is still in its developing stages near the Gulf Coast with a second piece of energy with the northern stream pivoting SE through the Great Lakes region. Moisture in the mid and upper levels has been streaming into the region and some returns can be seen on radar. However most of this is not reaching the ground as the low levels remain fairly dry. We are beginning to see rain reaching the surface this morning as the atmosphere moistens. We had a fairly extensive area of virga earlier as the lower atmosphere has been quite dry with dew points in the 20s and 30s. We are starting to see these rise into the upper 30s to 40s now. Lift will be provided by some weak impulses aloft well ahead of the system plus the low level warm advection pattern. The first of these has helped initiate a band of rain across parts of northeast MD and northern DE. This will gradually lift across southeast PA and spread into portions of NJ through the morning. It shouldn't be too heavy during the day today with amounts mostly under a half inch. With overcast skies and the rain developing it will be cooler than Thursday with highs mainly in the 40s over eastern PA and northern NJ with 50s over Delmarva and southern NJ. && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... By early this evening, developing low pressure will be over the Carolinas associated with a southern stream shortwave. Meanwhile energy with the northern stream will be diving south into the southern Great Lakes. These pieces of energy look to phase through tonight resulting in the low rapidly deepening as it moves northeast through Virginia and eventually to a point east of Delmarva by 12z Saturday. As a result, rain will be widespread across the region through tonight and become heavy at times by the overnight period over portions of southern Delmarva into southern NJ...especially from around Sussex Co. Delaware towards Cape May. This will occur as PWATs over 1 inch are advected in with a strong LLJ. Strong, deep lift will be aided by the phasing of the upper level waves and overlapping left jet exit and right jet entrance dynamics. The good news is that the very heaviest of the rain looks to stay south of the I-95 corridor limiting the flood threat there. Also, it should be mostly a liquid event except over the higher elevations of the southern Poconos where some snow is likely to mix in at times. Heading into the day Saturday the low will continue to rapidly deepen while moving fairly quickly north and east towards Cape Cod. As a result, heavy rain during the first part of the morning should actually end fairly quickly from southwest to northeast by around the late morning into the early afternoon time frame. Around this time though strong N/NW winds look to quickly pick up in the system's wake potentially resulting in gusts upwards of 35 to 45 mph...strongest near the coast. This borders on advisory levels and it could result in some power outages. Also, the rain/snow mixed over the southern Poconos could end briefly as a period of all snow here as well as over northwestern NJ. At this time we are not forecasting significant accumulation as it should stay under an inch but we'll have to watch this closely as sometimes these snow bands on the back side of systems can ""over perform"". As mentioned though the good news is that the storm will be pulling away quickly by Saturday afternoon. Total rain amounts should be around an inch for the urban corridor with upwards of 1.5 to 2.5 inches over southern Delmarva and coastal NJ. This could result in some flooding near the coast especially since the heaviest rain will be occuring or having just occurred around the time of Saturday morning's high tide. This will need to be watched closely. Otherwise it remains windy and raw through Saturday afternoon in the wake of the system with temperatures not budging too much. Tonight's lows should be mainly in the 40s (except 30s over the southern Poconos and NW NJ) and temps won't rise much from this Saturday and will even be falling by mid afternoon. Strong low pressure continues moving northeast Saturday night as it moves from near Cape Cod towards the Canadian Maritimes. As a result, gusty winds only slowly diminish with time as the gradient remains strong in its wake. Otherwise it should be dry but colder with lows mostly in the 20s to low 30s. && .LONG TERM /SUNDAY THROUGH THURSDAY/... Overall, it still looks like a mostly quiet weather period once we get through the early weekend system. There are two potential caveats to that however. First, with the low developing over the southeast and then shifting out to sea Monday into Tuesday. Some guidance, most notably the ECMWF, has trended northward with the track of this low, which could result in the northern part of the precip shield affecting northern Delmarva and southeastern NJ. I've kept the forecast dry for now, and am favoring a further south track, as the sub tropical jet should generally stay south of the mid and upper level ridge axis, while the polar jet should be well north of this system. With this pattern, it would tend to favor the low digging southward or at least tracking straight east rather than lifting to the northeast faster. Secondly, During the Wednesday into Thursday time frame, a low and mid level shortwave trough digging from Ontario to Maine, could set the stage for strong northwesterly flow behind the axis which could result in some lake effect showers. However, once again, kept the forecast dry through this period as it is hard to resolve critical details for this type of system this far out. Temperatures through this period should stay near to slightly below normal. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Through 18Z...Mostly VFR conditions with ceilings gradually lowering below 10000 ft AGL through the morning. Southwesterly wind generally less than 10 kt. High confidence. Today after 18Z...Mostly VFR conditions, though some MVFR ceilings and even some MVFR visibility restrictions are possible as rain starts to nudge in to the region. Some locations, especially KRDG and KABE could drop to IFR after 21Z. Southwesterly wind less than 10 kt. Moderate confidence on most aspects of the forecast, but low confidence on the timing and extent of MVFR and IFR conditions. Tonight...Ceilings should be mostly MVFR at the beginning of the evening, but will likely drop to IFR for most, if not all TAF sites by 06Z. Visibility restrictions are possible too with moderate to heavy rain developing. It should be all rain, though a rain snow mix will be possible just northwest of KABE. Light and variable winds, becoming northerly around 10kt after 06Z. Moderate confidence on the overall pattern, but low confidence on timing details. Outlook... Saturday...IFR conditions (which will start late tonight), will likely continue through much of, if not the entire, morning. Both low ceilings and visibility restrictions in rain are likely. The rain should begin to move out around mid day, and with it, we will likely see quick improvement in visibilities, and gradual improvement in ceilings. Many sites could return to VFR by late afternoon. Northwesterly winds 10 to 20 kt, with gusts up to 30 kt possible. Stronger gusts, up to 35 kt, will be possible at KACY. High confidence on the wind forecast. Moderate confidence on the overall pattern. Low confidence on the timing of the return to VFR conditions. Saturday night through Tuesday...Mostly VFR conditions are expected through this period. Northwesterly winds generally less than 10 kt for the overnight periods, and 5 to 15 kt with gusts up to 25 kt during the day time hours. Moderate confidence. && .MARINE... Marginal Small Craft Advisory conditions with gusts right around 25 kt are expected to continue on the Atlantic coastal waters through the day today with an SCA in effect until 6 PM. For the Delaware Bay, winds and seas should remain below SCA criteria through the day. For tonight, winds will subside for a time this evening before increasing later at night ahead of the approaching storm. NE winds gusting 20 to 25 knots can be expected by morning. For Saturday, the Gale Watch has been upgraded to a warning as rapidly deepening low pressure moves northeast right over the waters and then off toward Cape Cod. Northeast winds will shift to northwest through the morning with gusts up to 45 knots expected over the Atlantic Coastal waters and gusts to at least 35 knots over Delaware Bay. Also, seas of 6 to 9 feet can be expected over the coastal waters. Winds and seas will only slowly come down with time through Saturday night as the low continues moving away. Outlook... Sunday through Tuesday...Once winds subside below 25kt and seas subside below 5 ft on Sunday, we should stay below SCA criteria through the remainder of this period. However, there is a small chance that winds gusts could exceed 25 kt on the Delaware Atlantic coastal waters Monday into Tuesday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST this evening for ANZ450>455. Gale Warning from 6 AM Saturday to 6 AM EST Sunday for ANZ450>455. Gale Warning from 6 AM to 6 PM EST Saturday for ANZ430-431. && $$ Synopsis...Johnson Near Term...Davis/Fitzsimmons Short Term...Fitzsimmons Long Term...Johnson Aviation...Gorse/Johnson Marine...Davis/Fitzsimmons/Johnson