Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 221442 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 942 AM EST Tue Jan 22 2019 .SYNOPSIS... Arctic high pressure across the Mid-Atlantic region will move offshore by tonight. Low pressure will move into the Great Lakes region Wednesday and then down the Saint Lawrence Valley Thursday. Its associated fronts will cross the area tonight and Thursday. High pressure will return for Friday and Saturday. A cold front will cross the area Sunday. Next week, a coastal system may affect the area later Monday and Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The center of strong arctic high pressure is over the Mid-Atlantic region this morning, however it will be shifting eastward through the afternoon. This has allowed the pressure gradient to relax quite a bit so far and it will do so even more, resulting in less wind through the day. A mostly sunny sky today, although some increasing high clouds will occur during the afternoon as a jet streak remains overhead and warm air advection begins to spread into the region. While it will still be cold today, it will not be as harsh has yesterday due to much less wind and some warm air advection occurring. For the 930 AM update, no significant changes were needed as the forecast is in good shape. Just tweaked the hourly grids over the next few hours to keep them current with the latest observations and trends. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... The surface high will move offshore tonight, with a midlevel ridge axis moving through the region during the overnight hours. A potent vort max will be be moving through the central plains, with several smaller perturbations ejecting from the main trough into the Ohio Valley and downstream. Warm advection will increase overnight, which will act to continue the increasing cloudiness and to stunt diurnal cooling, particularly overnight as the cloud cover begins to thicken/lower and surface winds become predominantly southerly. However, the temperature forecast is quite tricky tonight, because winds will be light during the evening, and cloud cover may be too thin and too high to prevent a sharp nocturnal drop during the evening. Consider the MOS guidance for Millville: 13 from the MAV, 17 from the MET, and 24 from the ECS. Results will be highly sensitive to the above-mentioned factors, with locally large errors probable as meso- to microscale effects combat the larger-scale evolution. In general, went on the cold side of guidance and especially so in the susceptible Pine Barrens and valley locations northwest of Philadelphia. Confidence is rather low, since the timing of increased cloud cover is quite uncertain. Forecast lows are generally in the teens outside of the urban corridor and the immediate coast, with temperatures around 20 or even in the lower 20s in Delmarva. Precipitation associated with the central U.S. trough and ejecting perturbations should remain well north and west of the area through the night. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... On Wednesday, the next storm will be moving into the area. The recent trends are pointing a slower moving system, so the pops have been lowered in Wed somewhat. The warming trend will have more time to take hold, so the threat for significant amounts of sleet or freezing rain seem to be lessening with time. Still, we do have marginal temperatures Wed morning across the southern Poconos and adjacent areas S of that. For that reason, we have not eliminated the chc for some sleet and freezing rain from the grids yet. Elsewhere, pops steadily increase thru the day with occasional rain Wed afternoon and into Thu. Precipitation totals will be 1 to 2 inches across the region. High pressure will build back across the area late Fri and into Saturday, so a dry fcst is continued. It will get cold again Fri/Sat with readings below normal, but not the arctic chill of the past few days. Highs Fri/Sat in the upper 20s to low 30s across the Delaware Valley and some 5 to 10 degrees cooler across the southern Poconos and north NJ. Model agreement begins to become a factor later this weekend with some indicating a cold front and some showers Sunday and other not. We have just continued with chc pops for the northern areas for now and kept slgt chc pops elsewhere. Temperatures will favor snow N/W and rain across the Delaware Valley and S/E. Next week, the operational models become all over the place with regards to various systems affecting the region. The 00Z CMC was trending like the 12Z EC with a coastal system coming near our waters next Tue, but then the 00Z EC came in without this low. The GFS has a front crossing the area next Tue with a weak wave forming SW of our area then moving thru Tue night with (probably) snow. The EC and CMC don`t have this at all. So the rather high amounts of uncertainty will lead me to keep some slgt chc pops for the Mon/Tue periods for now. Temperatures should remain a few degrees below normal. && .AVIATION /15Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of today...VFR. Northwest winds around 10 knots, becoming light and variable during the afternoon. Tonight...VFR with increasing mid and high clouds. Winds generally light but may become predominantly southerly. Outlook... Wed thru Thu...Lower CIGS and VSBYS with rain and fog. IFR/LIFR possible. Gusty winds and LLWS possible Thu morning. Thu night...Improving conditions. Few showers psbl. Patchy fog. Fri thru Sat...Mostly VFR expected. && .MARINE... The Small Craft Advisory has been cancelled as the winds continue to diminish and seas are mainly in the 3-4 foot range. Light freezing spray will continue for awhile today, however the rates have significantly decreased and therefore the Freezing Spray Advisory has been cancelled. A Special Weather Statement remains in effect to address the low water levels along the tidal Delaware River. The next low tide will occur during the afternoon, but it looks as if the combination of slackening winds and increased time from the full moon will prevent a third round of widespread blowouts. However, some models (e.g., ESTOFS; Stevens Institute ensembles) come quite close. We will continue to monitor. Outlook... Wed thru Thu...SCA conditions expected. Rain and fog. Thu night thru Fri night...SCA expected on the ocean and mostly Sub-SCA on Delaware Bay. Fair. Sat/Sat night...Sub-SCA. Fair. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...O`Hara Near Term...CMS/Gorse Short Term...CMS Long Term...O`Hara Aviation...CMS/Gorse/O`Hara Marine...CMS/Gorse/O`Hara

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