Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 241343 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 943 AM EDT Wed May 24 2017 .SYNOPSIS... Low pressure will move out to sea today, meanwhile another low pressure system will track up the Ohio Valley before moving into the eastern Great Lakes tonight and early Thursday. A warm front will lift northward into portions of our area Thursday, with another low pressure system developing along the cold front. As the low strengthens it moves away from our area during Friday. Weak high pressure may briefly build in for Saturday, then low pressure and its associated fronts move through Sunday night and Monday with the main cold front scheduled to shift offshore Tuesday morning. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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9 AM surface analysis shows a frontal boundary positioned south of the OH Valley, stretching eastward across NC and then northeastward off the east coast. Two waves of low pressure were noted along the boundary: 1) A 1000 mb low about 200 miles off the Delmarva coast and 2) a 1009 mb low near the KY-IN border. In between these two low pressure systems, weak high pressure was ridging down along the New England and northern Mid-Atlantic coastline. Only a few light showers associated with the offshore low were lingering over the forecast area as of mid morning, but the last of these showers should end late this morning. The drying trend can be linked to mid-level shortwave ridging that builds in this afternoon. Although there will likely be partial breaks in the cloud cover today (one such break already occurring in C and SE PA mid morning) but do not foresee the sun peaking out for an extended period of time today. Soundings show quite a bit of moisture trapped underneath the mid-level subsidence inversion throughout the day, so pockets of stronger heating from the breaks in the clouds should quickly lead to the development of stratocu as boundary-layer mixing deepens. Below normal temperatures can be expected today owing to the clouds and onshore flow around the surface high. Only minor adjustments were made to the high temperature forecast with the 930 AM ESTF update, mainly to highlight low 70s for inland (e.g., E PA) and northern (e.g., I-70 and I-80 corridors) areas except at higher elevations. Highs closer to the coast should generally be in the mid to upper 60s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM THURSDAY/...
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930 AM ESTF...Updated hourly PoPs/Weather grids for tonight to slow down the onset of the steadier precipitation from south to north tonight. Although showers will likely remain upstream of our area this evening, will keep chance PoPs for Delmarva late this evening to account for the possibility of precip to move in a bit earlier than guidance is currently indicating (a common model bias in overrunning setups). Previous Discussion... The warm front associated with a low pressure center over the Ohio Valley will propagate north into our region late tonight. At the same time, a mid level vorticity maxima will develop over eastern PA in advance of the mid level short wave trough. The vort max may develop before the front arrives, but still, expect enough lift with either feature to result in widespread rain across the region, especially after midnight. There remains some signal for heavy rain (precipitable water values well above normal and a deep warm cloud layer). However, the window for very high pwat values is rather narrow, and storm motions aren`t particularly slow. Thus, expect the flooding threat to be limited. There is also some meager elevated instability depicted in the model soundings, generally for locations south of a PHL to ACY line, and generally after 09Z. Thus, kept mention of slight chance of thunderstorms across Delmarva very late tonight, but expect mostly rain showers.
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&& .LONG TERM /THURSDAY THROUGH TUESDAY/... Summary...Unsettled at times however some warmer temperatures are expected. Synoptic Setup...A significant upper-level through/closed low is forecast to be shifting east Thursday from the Ohio Valley and southeastern states. A potent short wave on the south and east side along with a speed maximum should allow for the trough to take on a negative tilt later Thursday, then the entire system shifts up into New England Friday. Some brief weak ridging may slide into the east Saturday, however the next trough amplifies from the Midwest and Great Lakes later Saturday and Sunday. This trough looks to close off and be slow to shift eastward through Tuesday, sending a slow moving low pressure system to our north while its fronts cross our area. Timing and therefore the details are less certain given an amplified pattern with the idea of some closed lows involved. We used a model/continuity blend for Thursday through Friday night, then blended in the 00z WPC Guidance thereafter. Some adjustments were then made following additional collaboration with our neighboring offices. For Thursday and Friday...An amplified flow will maintain a significant upper-level trough/closed low as it migrates across the East. The model guidance is in good agreement with a potent short wave rotating around the base of this trough during Thursday, then swinging to our east in the evening. This energy is accompanied by a strong 500 mb jet streak, which assists in making the trough take on a negative tilt. The stronger flow however is forecast to slide to our south and east, although Thursday starts with a southeasterly low-level jet shifting northward. There is some diffluence to the flow aloft over our region, which will enhance the ascent. At the surface, low pressure is forecast to gradually move up the Ohio Valley and into the eastern Great Lakes. Meanwhile, a warm front lifts north across Delmarva while a new surface low develops at a triple point. It is this new low that tracks northeastward and strengthens while moving to near Cape Cod early Friday morning. This development typically keeps the warm front from clearing much of our area, therefore overunning showers with some elevated instability may allow for some thunder. Despite the trough taking on a negative tilt, enough stable air across the northern half of the area may result in less thunder. This though will depend on the location of the warm front and thus the warm sector, as some guidance has stronger instability in the afternoon across parts of Delmarva. A burst of warm air advection in the morning may result in a band of heavier showers/convection with some thunder near and north of the warm front. The most concentrated showers may be focused prior to midday Thursday. As of now, any severe thunderstorm threat appears to be just to our south given the placement of the true warm sector. Some showers should continue Thursday night as the closed low moves overhead, then improving conditions Friday as strengthening low pressure moves away. However, cyclonic flow combined with another short wave with even a surface trough reflection should result in some showers. A gusty west-northwest wind should develop for a time Friday as the pressure gradient tightens and cooling occurs aloft allowing for better mixing. For Saturday and Sunday...The flow turns more zonal for much of this timeframe with perhaps a weak but brief ridge in the east Saturday. The model guidance overall shows a short wave within this flow although the timing is a bit different and therefore it is not real clear what affect this may have on our weather. It may enhance a frontal zone to our southwest, which gradually lifts northward Sunday. Low pressure tied to another trough from the Midwest to the Great Lakes slides eastward, and this will push a cold front our way Sunday night. Given the idea of an incoming frontal zone from the southwest and nearby short wave energy, cannot rule out some convection mainly Sunday. For Monday and Tuesday...A closed low is forecast to expand across the Great Lakes and Midwest to the Ohio Valley during this time frame. This will drive surface low pressure eastward mainly north of the Great Lakes. The cold front is forecast to move across our area Monday and shift offshore early Tuesday. The second half of Monday may be convectively active, however this will depend on short wave timing and convective development to our west. As a result, kept chance PoPs for Monday then slight chance to chance PoPs on Tuesday. We are anticipating warmer temperatures ahead of the cold front. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Currently, 1 kft AGL CIGs are borderline MVFR-IFR at KMIV and KACY. Expect these CIGs to increase throughout the day but likely remain MVFR. A brief period of MVFR CIGs possible at other TAF sites thru about 17Z but predominately VFR expected. After 00Z, lower clouds will begin to expand further across the region, eventually followed by rain showers. Widespread MVFR ceilings and visibilities are expected no later than 06Z. Many locations may see IFR conditions after 06Z. NEly winds this morning will veer slightly out of the east this afternoon. Wind speeds are expected to be 5-9kt this morning and 9-13 kt this afternoon and tonight. There may be a brief period of gusts near 20 kt mainly this afternoon. Outlook... Thursday...MVFR to IFR conditions, with showers especially during the day. Some thunderstorms are possible mainly during the day. Some fog possible especially at night, however stratus may dominate. Friday...MVFR/IFR conditions should start the day, then improving to VFR. A few showers are expected. West-northwest winds may gust up to 25 knots, then diminishing at night. Saturday...Generally VFR, however some showers possible at night with lowering ceilings. Sunday...MVFR/IFR conditions possible with some showers and afternoon thunderstorms.
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&& .MARINE...
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Winds and seas are expected to build through the day time hours. Seas above 5 feet are expected beginning this afternoon and continuing at least through the overnight hours. Winds gusts near or above 25 kt are possible this evening and overnight. On the Delaware and southern New Jersey coastal waters, seas may subside late tonight. Outlook... Thursday...A southeasterly low-level jet should maintain gusts of 25- 30 knots for a time Thursday before weakening across the northern ocean zones. The seas will therefore be enhanced especially across the northern ocean zones. Therefore, the Small Craft Advisory for our two northern ocean zones has been extended through 22z Thursday. For the remaining ocean zones, seas may remain elevated enough however confidence is lower at this time. Friday...West-northwest winds may gust to 25 knots in the afternoon and seas may still be near 5 feet on the ocean zones. The conditions should diminish at night. Therefore, a Small Craft Advisory may be needed. Saturday and Sunday...The conditions are anticipated to be below advisory criteria, however seas may build some during Sunday. RIP CURRENTS... There is a moderate risk for the formation of dangerous rip currents along the New Jersey Shore through this evening.
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&& .TIDES/COASTAL FLOODING... Some of the highest astronomical tides of the year are occuring through the end of the week. As a result of that and onshore flow, minor coastal flooding is expected with the high tide this evening and Thursday evening. One source of guidance shows water levels along the northern NJ shore approaching moderate flooding thresholds with the Thursday evening high tide, but this seems unlikely at this time. Minor coastal flooding may again be possible with the Friday evening high tide. No changes were made to the coastal flood advisory. However, there is a chance that it may need to be expanded further into the extreme upper part of the Delaware Bay and the tidal Delaware River downstream from the Commodore Barry Bridge. For now though, have chosen to hold off until we know what the tidal departures are with the high tide this morning at Reedy Point. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory from 6 PM this evening to midnight EDT tonight for NJZ012>014-020>027. DE...Coastal Flood Advisory from 6 PM this evening to midnight EDT tonight for DEZ002>004. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Thursday for ANZ452>455. Small Craft Advisory until 6 PM EDT Thursday for ANZ450-451. && $$ Synopsis...Gorse Near Term...Klein Short Term...Johnson/Klein Long Term...Gorse Aviation...Gorse/Johnson/Klein Marine...Gorse/Johnson/Klein Tides/Coastal Flooding...Johnson

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