Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 201322 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 922 AM EDT Mon Aug 20 2018 .SYNOPSIS... A stationary front remains situated south of Delmarva as high pressure noses in from the north. The high moves offshore on Tuesday as that stationary front returns north as a warm front Tuesday night. A cold front moves across the region on Wednesday. High pressure builds in from the north and west to close out the work week. The high then moves offshore early in the weekend as a cold front approaches from the west late in the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 915 am update: No major changes to the forecast at this time. Considerable low cloud cover persists with some areas of clear breaks developing. Overall though, still expect a partly to mostly cloudy day with some showers and storms possible this afternoon across the southern Delmarva. 630 am update: Have modified temperatures and sky cover to account for current observations. As usual, models are too aggressive thinning out the clouds in northeast low-level flow, so have slowed the improving sky cover considerably with this update. Otherwise, forecast is in good shape. Previous discussion... A rather tricky forecast today, as the region will be downstream of several perturbations ejecting from a fairly potent vort max moving slowly through the central plains of the U.S. The strongest of these perturbations will be approaching the central Mid-Atlantic during peak heating, and high- resolution guidance is suggesting development of diurnally- enhanced convection just downstream. The trek of the perturbation is mainly to the south of the CWA, but large-scale lift will brush the southern portions of the area today. Will be interesting to see if central/southern Delmarva are able to tap into some of the instability (albeit elevated) looming to the south of the front in the central/southern Mid-Atlantic. Several recent simulations of the HRRR suggest this potential exists, but the NAM Nest and the WRF simulations are a little less aggressive and/or farther south with this convective development. Nevertheless, given the decent lift in advance of the perturbation, I increased PoPs south of Dover today and included a slight chance of thunderstorms based on potential for some elevated instability to develop. Meanwhile, the current light rain/drizzle in much of the southern half of the area is not well forecast among the models (nor is it well-sampled from radar owing to its rather low generation region). Currently, think the continued advection from the east- northeast will shunt this area of precip (slowly) southward as it gradually shrinks in size. This is fairly low confidence, however, and I think keeping a slight chance of showers (or at least sprinkles) today is prudent, especially near the coast and south of I-76 (in closer proximity to larger-scale lift and farther from the growing influence of the surface high nosing southward). Otherwise, the main story today will be the continued east- northeast low-level flow in most of the area. There are some indications (via model soundings) that some drying will occur in the tropospheric profiles, but the signal is mixed at best. With this in mind, I was a little more pessimistic than the consensus sky forecast today, but somewhat more optimistic than our previous forecast. Think the current overcast will become more of a partly-to-mostly cloudy sky during the day, with max temperatures near or slightly below average. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM TUESDAY/... Tonight, the perturbation moving through the Mid-Atlantic will be shifting slowly offshore, with shortwave ridging likely to enter the region just upstream. However, onshore flow will continue in this regime, and residual moisture in the low levels will likely keep chances for low clouds and drizzle/sprinkles/light showers around. As a result, I was hard- pressed to keep PoPs out of the forecast (especially given what has transpired early this morning). So the forecast includes slight chances of light showers for most of the area tonight, though given model soundings, would not be surprised if little or nothing occurs...and if anything does occur, it might be in the form of drizzle or light sprinkles rather than showers. Of higher confidence is the increase and lowering of clouds with time, and this will keep temperatures from falling too much. Given observed temperatures early this morning, I thought MAV/MET MOS looked a little too warm, so I went a touch below these numbers, but not by much given the above-mentioned modifying effects of the increasing/lowering clouds. && .LONG TERM /TUESDAY THROUGH SUNDAY/... High pressure centered over New England and the Gulf of Maine slowly lifts to the north and east on Tuesday. Meanwhile, low pressure moving from the Midwest and into the Great Lakes will lift the stationary front south of the region north as a warm front late in the day and at night. Surface dewpoints climb into the 70s, and this results in PWATs increasing to over 2 inches as most of the region will be in the warm sector ahead of an approaching cold front. A pre-frontal trough moves into the region late Tuesday afternoon and Tuesday evening. Showers and thunderstorms will develop from west to east, and thunderstorms may produce locally heavy rain Tuesday night. The cold front will be through western portions of the forecast area Wednesday morning, and then the front works its way east during the day. Meanwhile, a strong H5 trough/shortwave combo moves through the region Wednesday afternoon and evening behind that front. This will keep at least scattered showers in the region through the day. High pressure then builds in from the north and west with a cooler and much dryer airmass on Thursday. Surface dewpoints fall into the 50s, and 1000-500 mb thicknesses will fall to 555-560 dam. With highs generally in the 70s to low 80s, it will feel quite pleasant, especially compared to the warmth and humidity of most of August so far. Even lows Wednesday night and Thursday night will fall into the 50s and low 60s, which will be a welcome relief. High pressure moves over the area Friday, then offshore on Saturday. Temperatures and humidity levels begin to creep back up over the weekend, then there may be a return to some unsettled weather to start of next week. && .AVIATION /13Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Mainly VFR expected with MVFR cigs at MIV/ACY as of mid morning expected to improve to VFR by this afternoon. Winds east/northeast 5 to 15 kts. Moderate confidence. Tonight...CIGs are expected to deteriorate to MVFR with time, though exact timing of this is uncertain. VSBYs may stay somewhat elevated given the thickening cloud cover, however. Winds east- northeast 5 to 10 kts. Low confidence. OUTLOOK... Tuesday and Tuesday night...Generally VFR. Showers and thunderstorms will result in MVFR/IFR CIGs/VSBYs late Tuesday afternoon and Tuesday night with locally heavy rain. Patchy fog possible Tuesday night. SE winds 10 kt or less Tuesday will become S Tuesday evening, then SW late Tuesday night as a cold front begins to move through the region. Wednesday...Lingering showers into the afternoon, then conditions will improve to VFR. SW winds 10 kt or less will become NW 10-15 kt with gusts up to 20 kt in the afternoon. Thursday through Friday...VFR. Localized late night and early morning VSBY restrictions in fog are possible. Wind less than 10 knots veering gradually from north to southeast. && .MARINE... 630 am update: Seas have lowered below criteria on the northern New Jersey Atlantic coastal waters, so have cancelled the advisory there. Seas remain at or above five feet at buoy 44091 and buoy 44009, so the advisory remains in effect everywhere else at this time. Previous discussion... Winds have diminished below advisory criteria on the Atlantic waters early this morning, but seas remain elevated (generally 5-6 feet). Wave guidance has done poorly with the wave forecast the past 24 hours, underplaying the elevated seas, especially off the Delaware beaches. As such, I increased the seas forecast by 0.5-1 ft today. This also meant keeping the advisory in place as is, since I suspect the models are too quickly lowering seas below criteria. Northeast winds will continue today around 10 to 20 kts with gusts expected to remain below criteria. Winds are expected to become more easterly tonight as they diminish to 5 to 15 kts. Seas should be below advisory criteria late this afternoon into tonight. There is a slight chance of showers (and even a storm off the Delaware coast) today and tonight, with potential visibility restrictions and locally higher waves/winds in their proximity. OUTLOOK... Tuesday through Friday...There is a chance for 25 kt wind gusts on the ocean waters Tuesday night and Wednesday, along with seas building close to 5 feet on the ocean waters. Otherwise, sub-SCA conditions expected. RIP CURRENTS... Upgraded the risk of dangerous rip currents to moderate for the Delaware coast and have maintained the moderate risk for the New Jersey coast. With east-northeast winds 10 to 20 kts and medium-period swell of three or more feet, the rip current risk appears elevated. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 4 PM EDT this afternoon for ANZ451>455. && $$ Synopsis...MPS Near Term...CMS/Fitzsimmons Short Term...CMS Long Term...MPS Aviation...CMS/Fitzsimmons/MPS Marine...CMS/MPS

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