Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 271027 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 627 AM EDT Tue Sep 27 2016 .SYNOPSIS... A cold front will move through the region this morning before stalling offshore this afternoon through Wednesday. Low pressure systems over the Great Lakes and Southeast U.S. are expected to drift toward the region for the later half of the week before finally lifting into New England for the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 615 AM...Latest forecast update sent to NDFD prior to the next scheduled ESTF. This update focused on hourly PoPs and weather grids to reflect an earlier end time to the showers today. Just about all of the recent hires model runs continued to back away from the previous notion that showers would persist across the coastal plain for much of the day. The back edge of the precip has already made it to the I-95 corridor before sunrise. Once the previously mentioned shortwave trough moves off the Delmarva coast by mid morning, the bulk of the shower activity should then shift offshore. However, a few showers will linger into the first part of the afternoon in southern DE and near the southern NJ coast but before eventually tapering off late. Previous Discussion... 07Z surface analysis shows a 994 mb surface low located just north of Lake Superior. An occluded front extends southeastward from the low all the way down the eastern side of the Appalachian spine. The front is currently on our western doorstep and will move eastward through the area this morning. Looking aloft, the low near the Great Lakes is vertically-stacked so it is not going to make much progress downstream today. Warm conveyor belt around the eastern side of the closed low was associated with a band of showers that was precedes the fropa. The back edge of the precip has already pushed east of RDG-MPO as of 330 AM. Heavier showers were noted on the western side of the Chesapeake Bay. This enhanced activity was connected to a weak shortwave impulse (per water vapor satellite imagery) over the northern VA Piedmont. As the shortwave tracks northeastward, expect the heavier/more widespread showers to move into eastern MD and DE after 4 AM before shifting off the southern NJ/DE coast by about 8 or 9 AM. An isolated thunderstorm cannot be ruled out south and east of I- 95, where locally higher (elevated) instability situated near the theta-e ridge axis. Rainfall amounts of one-half to locally one inch are expected in these southern zones, while areas along and northwest of the I-95 corridor generally see less than one-quarter inch (and not much drought relief). The back edge of the showers should make it to the I-95 corridor around daybreak. The front is then expected to stall off the coast later this morning and afternoon, which would keep showers lingering along the coast through much of the day. The good news is the latest model guidance has shifted toward a more progressive scenario with the front making it farther offshore, and therefore precip may still end sooner than previously thought even along the coast. Tried to capture these near term trends in the PoP/weather grids but further adjustments may be needed over the next few hours. Similar to the precip shield, the back edge of the dense overcast cloud cover is expected to progress through the area from northwest to southeast over the course of the day. Anticipate some jet-induced high clouds to stream across the area today though the cirrus should be thin enough to not have a big impact on temps. The warmest temperatures this afternoon (highs in the mid 70s) will likely reside along the I-95 corridor and the warmer valleys farther to the north and west as these areas will clear out early enough in the day to receive strong daytime heating. Farther south and east toward the southern NJ coast and lower Delmarva, clouds will hold on longer into the afternoon, resulting in highs near 70F. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... The closed low will push slowly southward through the upper Midwest tonight. Meanwhile, a wave of low pressure is expected to develop along the stalled front to our south and east. We should be mainly dry tonight, even along the coast as the secondary low tracks far enough offshore. Good radiational cooling conditions may occur this evening across inland areas under clear skies and calm winds. Therefore, temps should drop quickly after sunset. Forecast lows are in the 40s for the valleys in northeast PA and northwest NJ. However, there is some uncertainty with the temperature forecast as high pressure builds southward. A light northeasterly wind may develop overnight, which could prevent temps from bottoming out. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Strong ensemble signal for a heavy rain event somewhere in the Mid- Atlantic region late this week with uncertainty on the location. Current indications are our CWA will be on the eastern edge of it. Wednesday through Friday: An area of low pressure (most noted in the upper levels) is expected to move southeastward toward our region from the Great Lakes and another area of low pressure will be forming in the Carolinas ( main surface low). The exact locations of these systems will determine where the highest risk for an excessive rainfall event will be. The GFS and Canadian operational models are more progressive with the upper low to our northwest which in turn forms the surface low just to our south. In turn a deep but narrow moisture axis ahead of these features tracks from the Eastern Gulf and Atlantic Oceans northwestward into DE and Eastern PA sending PW values from 1.5-2 inches, very high for this time of year. Sufficient lift and dynamics are present coupled with marginal instability to allow for heavy showers within this moisture axis. A few thunderstorms would be possibile as well with a tall skinny CAPE profile normally seen with more aggressive heavy rainfall events. A hindrance of the heavy rainfall threat looks to be fast moving convective elements in the axis though. This outcome would bring several inches of rainfall to areas south and west of Philadelphia. However, the NAM and the ECMWF model have all of these features further westward which would shift the moisture axis further inland to areas like Baltimore and State College. Waffling on the locations of the players and their respective locations is evident in the ensembles. Many members do suggest an axis of heavy rain sets up but are variable on location. The axis looks to be narrow, maybe only 100 miles wide. Overall, model trends have been to the west with the 00z cycle. So the forecast features heavy showers from Chester county PA south into Delmarva with just lighter showers from Philadelphia and points east. Showers will likely begin to develop from south to north across the region during the afternoon Wednesday. Precipitation is then likely to become more widespread along and near the main axis of moisture Wednesday night. This axis then could move little through Friday but gradually become cutoff from the deeper moisture. By Friday, the precipitation should become much lighter regionwide even with both areas of low pressure lingering over the Mid-Atlantic and Ohio Valley. Rainfall amounts from the individual ensembles are as impressive as the operational runs, so some totals over a few inches are likely just west of our region. It should be highlighted that areas outside of the heavy axis Wednesday night and Thursday may not see much more than lighter showers with rainfall under an inch total eventhough it rains off and on for a couple of days. Diurnal temperature swings should be very limited with clouds and rainfall with temperatures staying in the 60`s for most locations Thursday and Friday. Ahead of the rain, temperatures will be in the 70`s Wednesday. Winds will be out of the east from 10-15 mph on average Wednesday through Friday. Friday night through Monday: Lingering low pressure both at the surface and mid-levels could keep a few light showers around. Some breaks in the clouds are likely as well. The clearing process will be gradual. This will allow for high temperatures to get back into the 70`s for most of the region, with lows falling back into the 50`s. Ensemble guidance is likely a few degrees to cool by Sunday, not accounting for the more abundant breaks and warmer boundary layer temperatures. && .AVIATION /10Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. MVFR ceilings w/ showers continue across the area. Lower clouds below 1 kft are expected to move in from the west behind the showers between 08-12Z. Do not expect IFR conditions to persist more than 2 or 3 hours. Improvement to VFR as lower clouds clear out is forecast to occur from northwest to southeast this morning. Lower clouds may linger toward midday or early afternoon near the coast (ACY). SW winds currently will become W-NW after sunrise. Winds should back toward the SW during the afternoon. Wind speeds will be light...generally under 10 kt. VFR this evening. Patchy fog may develop overnight at the fog-prone terminals, resulting in MVFR or locally IFR visibility early Wednesday morning. OUTLOOK... Wednesday through Friday night: Mainly MVFR or IFR with periods of showers. easterly winds around 10 knots. Saturday: Improving to VFR with isolated showers. East winds 5-10 knots. && .MARINE... SCA continues for the waters thru early this morning. S-SW winds have mainly been below SCA criteria thru most of the night but there is a potential to see a brief (2 hour) surge of winds near 25 kt. Seas in our nearby offshore buoys are around 4-5 ft. Therefore, will let the SCA continue. Winds will become W or NW and diminish behind a cold front late this morning diminish. Winds and seas should remain below SCA criteria this afternoon and tonight. OUTLOOK... Seas building to five feet by Wednesday night with low pressure nearby. Seas will likely be very slow to drop as the low pressure systems linger. Winds will be easterly with SCA gusts by Wednesday night. Some gusts may reach gale force from ACY northward Wednesday night and Thursday. Winds should decrease by Saturday below SCA criteria. Rip Currents... The moderate risk for the formation of dangerous rip currents is forecast at the NJ/DE beaches today, especially this morning. Conditions may improve to yield a low risk by this afternoon and winds turn offshore and decrease. && .TIDES/COASTAL FLOODING... Spotty minor tidal flooding on the oceanfront is possibile with the Thursday and Friday high tidal cycles. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ430- 431-450>455. && $$ Synopsis...Gaines Near Term...Klein Short Term...Klein Long Term...Gaines Aviation...Gaines/Klein Marine...Gaines/Klein/Meola Tides/Coastal Flooding... is the U.S. government's official web portal to all federal, state and local government web resources and services.