Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 202201 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 601 PM EDT Mon Mar 20 2017 .SYNOPSIS... A weak low pressure system will slide over the Mid Atlantic tonight into tomorrow. A cold front is expected to arrive in the region tomorrow night. A large area of high pressure is then expected to build south and then slide off shore through the remainder of the work week. A warm front is forecast to lift through our region on Friday. Low pressure is expected to approach from the west over the weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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600 pm update: Agree with previous shift that the trends for precip in our area overnight are downward. Most mesoscale/storm-scale model simulations paint an awfully dry night for the CWA, especially for the northeastern half of the region. Weak perturbation in northwesterly flow presently in the Midwest will progress fairly quickly ESE overnight into the Mid-Atlantic region. The perturbation is generating a mesoscale convective system (MCS) in portions of Indiana and Ohio at this time. The remnants of this MCS are expected to move through the Appalachians this evening and into the Mid-Atlantic overnight. However, the evolution of this precipitation is the main question, with model variability quite high in the handling of this precipitation over the past couple of days. The general trend has been farther south and west and with a noticeable diminishing trend to the lee of the Appalachians. Both of these trends make sense owing to the digging of a stronger (kicker) vort max in southern Canada and the progression of the perturbation into a transient ridge straddling the East Coast this evening. As a result, reduced PoPs even more across the area overnight, with the highest probabilities remaining in Delmarva. There is some indication from convection-allowing models (CAMs) that isolated/scattered light showers may move into southeast Pennsylvania and New Jersey near daybreak, which is at least hinted at by the 18Z GFS and 12Z ECMWF. Will look at the next few cycles from these CAMs before biting, however. Also modified temps/dew points rather substantially this evening given another day of guidance too low with the former and too high with the latter. Temperature forecast this evening is low confidence given the small window of favorable radiational cooling before the increasing influence of cloud cover later. Previous discussion... Increasing clouds are expected overnight as a weak disturbance to the west arrives overnight. The trend in the latest models is to keep much of the precipitation to our S/W, so we have lowered pops across the area. We still have chc pops over Delmarva and into Chester county, PA. Feeling is that whatever arrives later tonight will be weakening and very light. Low temperatures tonight will be in the low 30s north and mid to upper 30 south/east. Winds will be light.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... The disturbance to the south will move offshore Tuesday and weak high pressure will be across our area. A dry day is expected, but there will be a good deal of cloudiness during the morning. The clouds will tend to diminish from NW to SE during the afternoon. It will be mild on Tue with highs in the low/mid 50s North and mid/upper 50s South. Winds will be mostly W or NW at 5 to 10 mph. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Wednesday through Thursday...The long term period will start off seasonably cool thanks to the passage of a cold front Tuesday night. Temperatures both days will be 10 to 20 degrees below normal. On Wednesday, windy conditions are possible thanks to a tight pressure gradient behind the cold front. However, the pressure gradient will relax quickly Wednesday night as the high approaches the region and by Thursday winds will turn light and variable as the high slides over the Mid Atlantic. Friday...As the high slides further off shore, a warm front is expected to lift through the region. Precipitation is expected with and perhaps just ahead of the front. If we do have precipitation ahead of the front, it could be a mix of freezing rain and sleet from the Lehigh Valley to the north and west. A warm layer in the mid levels is expected, and it looks like there will be some weak lift ahead of the front. A limiting factor however, will be a dry boundary layer already in place. For now, will mention the possibility in the HWO. Saturday...Should remain in the warm sector with continued southerly flow. Have chosen to stay close to the previous forecast which is slightly above most guidance for highs on Saturday. There have been several recent cases of southerly flow/warm air advection events that the MOS guidance and especially operational models have underestimated the warm air advection. This looks to be a similar pattern to those cases, so will go with a forecast that is 10 to 15 degrees above normal. Sunday and Monday...As the next low pressure system approaches the region, there is considerable uncertainty in how far south the cold air will get across the region. This of course, means considerable uncertainty in what type of precipitation we might have. For now, it looks like most of the region should see rain, with the possible exception of the southern Poconos and NW NJ which might again see a wintry mix especially Sunday night into Monday morning. && .AVIATION /22Z MONDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Fair weather is expected the rest of today as a ridge of high pressure weakens over the area. Only Ci/Cs clouds expected through dusk, then some Ac/As clouds will arrive. Winds will be NW/W at less than 10 knots overall. Tonight, we expect mostly a VFR fcst, but a disturbance passing by to the W/S may bring a few lower clouds perhaps a passing shower. We therefore have some MVFR cigs mentioned at KRDG/KABE. The trend in the latest tafs is to be more optimistic with regards to conditions overnight. The latest models have backed off on Pops/QPF, keeping it more to the S/W. Tuesday, you can expected VFR but cloudy conditions and NW/W winds up to 10 knots. Outlook... Wednesday through Friday...Mostly VFR conditions are expected. Northwesterly winds gusting near or above 25 kt are possible on Wednesday. Saturday...Starting VFR, but may lower to MVFR or lower late in the day as precipitation moves in. && .MARINE...
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Seas continue to lower over the coastal waters, and the small craft advisory will be allowed to expire as scheduled. Previous discussion... We will continue with the SCA flag until the 6 p.m. expiration time. Winds have already diminished below criteria and seas are still a bit over 5 ft attm. Once the flag comes down, sub-SCA conditions for the rest of the night and Tuesday. The weather will be fair expect for perhaps a sprinkle towards dawn across Delaware Bay or the southern coastal waters. Outlook... Wednesday...Gale force winds are possible on all waters, but most likely on the New Jersey Atlantic Coastal waters. Wednesday night...winds will drop off quickly after midnight, though SCA conditions may linger especially on the Atlantic Coastal waters for much of the night. Light freezing spray will be possible. Thursday...winds and seas are expected to be below SCA criteria. Friday and Saturday...winds will begin to increase Friday and seas will subsequently increase as well. SCA conditions are possible especially Friday night into Saturday.
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&& .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Gale Watch from Wednesday morning through Wednesday evening for ANZ450>453. Small Craft Advisory until 6 PM EDT this evening for ANZ450>455. && $$ Synopsis...Iovino/Johnson Near Term...CMS/O`Hara Short Term...O`Hara Long Term...Johnson Aviation...Johnson/O`Hara Marine...CMS/Johnson/O`Hara

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