Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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323 FXUS61 KPHI 240426 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 1226 AM EDT Mon Apr 24 2017 .SYNOPSIS... A low pressure system will organize over the Carolinas tonight and Monday then move up the east coast through the middle of next week. Another frontal boundary will approach our region from the north later in the week before meandering over the region this weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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1230 am update: Continued the trend set in the previous update by delaying the onset of precipitation. The low-level air remains fairly dry, especially just off the surface, in the Delmarva Peninsula, so any precipitation falling in Delaware and eastern Maryland is virga at the moment. High-resolution models are not overly aggressive in bringing steadier precipitation into the area overnight, with mainly sprinkles or light showers generally along and south of a Stevensville, MD, to Dover, DE, line. Reduced PoPs overnight, particularly north of the aforementioned line. Also made some adjustments to temperatures (generally too cool in eastern Pennsylvania and northern/central New Jersey) and dew points (generally too low everywhere). Slowed the onset of increased cloud cover in the northern CWA as well per latest satellite imagery. Previous discussion... High pressure that was over the area today will continue to shift offshore overnight. Meanwhile, a cutoff low aloft and surface low will drift out of the southern Appalachians and toward the southeast coast. Well north of these features, moisture will be streaming up the east coast overnight. Scattered showers may creep into the central/southern portions of the Delmarva and southern New Jersey by daybreak, but with dry air in the low- mid levels, it will take some time to moisten the column and the showers to advance north. The remainder of the area will remain dry through daybreak with just an increase and thickening of clouds. Few changes were needed with the evening update, I did slow the timing of the steady precip for Monday a little. Most of the op models are showing the starting time to be later than what the earlier models had.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/...
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1230 am update: Reduced PoPs for most areas north of the Mason- Dixon Line on Monday, as precipitation will be moving northward at a snail`s pace during the day. Also think it will take some time for the lower portions of the atmospheric column to moisten given substantial drying that occurred on Sunday. Most high- resolution guidance is keeping precipitation very light and fairly spotty during the morning with a stronger push of steadier/more widespread rain moving in from the south/southeast during the late afternoon. For this reason, lowered QPF during the day as well. Based on the latest trends in short-term model simulations, think the northern CWA might be a little warmer than initially forecast as well (with perhaps even some partial sunshine in eastern PA/northern NJ during the morning). Increased hourly/max temperatures in these areas, with considerable weighting given to non-statistical output, which appears to have a better handle on the slower storm-system motion. Previous discussion... The coastal storm continues to advance toward the southeast coast on Monday, but remains well to our south. However, there will continue to be plenty of moisture streaming up the east coast while several short wave/vorticity impulses move northward as well. The dry air will eventually be overtaken everywhere during the morning into the afternoon, and rainfall is expected to spread across most of the area by the late morning and early afternoon. With cloud cover and precipitation across the area, temperatures are expected to be several degrees below normal, especially the southern areas where the clouds and rain will begin sooner.
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&& .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Monday night through Tuesday night: A strong ensemble signal is present that low pressure over the Carolina coastline will move up the east coast. A westward shift has occurred with both the operational and ensemble modeling as they are still ironing out details with tropical moisture influx and convection along with how fast it closes off in the mid-levels. However, operational and ensemble modeling has keyed in on two areas of excessive rainfall the first being in the Carolinas and Virgina where the greatest amount of lift is located Monday night and the second over New England as the low moves up the coast Tuesday into Wednesday in association with the deeper tropical moisture. This leaves our region in between the two areas of heavy rainfall. Still periods of mainly light rain are expected and locations in Delmarva and along the coast should still see over an inch of rainfall with lower amounts further northwest. It should be noted the CMC and UKMET are still maintaining a wetter solution which was also incorporated into the QPF forecast along with the SREF. A dry airmass will continue to be very slow to erode north and west of Philadelphia as well, so while cloudy, it`s not hard to see an outcome where the Southern Poconos do not see a half inch of rain by Wednesday. Low clouds, rain and an easterly onshore flow will keep temperature ranges small and rather chilly with highs in most spots only in the 50`s. This forecast update is still a bit cooler than MET/MAV for highs and slightly warmer with lows. Winds should pick up as well from the east with some gusts around 20 mph at times based on bufkit analysis. It is not a favorable environment for mixing but with the low pressure nearby a tighter pressure gradient will be present leading to the wind gusts. Wednesday and Wednesday Night: Fairly good agreement is present to linger scattered showers into Wednesday, the westward jog in the track means a slower clearing trend from what we were looking at with previous model runs. With some breaks in the rain and maybe even the clouds we should be several degrees warmer than Tuesday. Thursday through Friday night: A busy late week and weekend in Philadelphia. A good piece of news is that high pressure will become more dominant near the region resulting more sunshine and a break from the gloom most of this week will bring. Current thinking is that a cold front may provide enough lift for a thunderstorm or two ( only slight chance and out of the point and click for now) as it moves into the region late Thursday and Friday. Confidence is higher today that we will see abundant southerly flow ahead of the front pushing highs into 80`s along with muggier conditions leading to warmer lows. Bufkit soundings indicate the potential for some 20 mph gusts Thursday afternoon as well. Next weekend: The modeling continues to show this front may evolve into a backdoor or warm front for the weekend somewhere near our region. A very wide temperature range was present both days with operational model and ensemble data. Overall the temperature spread has actually increased a standard deviation to 5 from 24 hours ago. An example is the high temperatures both days produced over 20 degrees of a difference between the 06z and 12z GFS runs. Needless to say the forecast stuck close to the previous forecast blending some of the the latest ensemble guidance. With the front nearby, enough lift still may be present for a shower or thunderstorm, slight chance and out of the point and click for now. && .AVIATION /04Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. VFR conditions will continue across the area through at least tonight and into the early part of Monday. Scattered showers are expected to make its way up the coast and begin moving into the central/southern Delmarva and southern New Jersey by daybreak Monday. Then rains will advance northward through the morning and into the afternoon, although it may have a hard time advancing for a while with dry air in the low-mid levels. Conditions are expected to become MVFR, possibly IFR, by late morning or early afternoon as the rain advances north. Light and variable winds early this afternoon will become southeasterly through the afternoon and into the evening. Winds may become light and variable again for many areas overnight, before increasing again out of the east-northeast and continuing through the day Monday. OUTLOOK... Monday night: MVFR and IFR conditions spreading from south to north with light rain moving in. East northeast winds 10-15 with gusts up to 20 knots. Tuesday through Wednesday: MVFR and IFR with several periods of rain. Perhaps some intervals of VFR for KABE and KRDG where more breaks in the rain are likely. Rain will end on Wednesday from west to east with a slow improvement to VFR. Easterly winds becoming northerly, gusts up to 25 knots on Tuesday from the east decreasing to under 10 knots from the north on Wednesday. --------. Wednesday night through Friday: Mainly VFR. Winds under 10 knots but southwesterly gusts up to 20 knots Thursday afternoon. && .MARINE... Conditions remain just below advisory levels most of tonight. However, winds and waves are expected to begin increasing across the southern waters tonight, then advance northward during the day Monday. The current timing of the Small Craft Advisory remains unchanged at this time. OUTLOOK... Seas will remain above five feet through Wednesday and potentially through Friday. Wind gusts will increase Tuesday from the east. Mixing continues to look poor but 925 mb winds are modeled around 40 knots for a short period Tuesday afternoon and evening moving from south to north. So gale gusts are still possible in this timeframe. Winds will gradually decrease and turn more northerly Tuesday night and Wednesday. Southwest wind gusts Thursday may approach SCA criteria as well. && .TIDES/COASTAL FLOODING... Positive tidal anomalies are expected with persistent onshore flow occuring through Tuesday night. Ensembles included in the Stevens Flood Advisory System indicate a high probability for minor coastal flooding at our tidal points along the oceanfront and DE Bay with the Tuesday afternoon-evening high tide. Coastal Flood Advisories may eventually be needed as tidal departures of around a foot above astronomical tide would be sufficient to produce minor coastal flooding with the new moon on Wednesday. A few of the outlier ensemble members in the Stevens tidal guidance predict moderate coastal flooding, particularly at Reedy Point and Lewes. However, there is a much higher likelihood that the magnitude of flooding would be limited to minor since onshore flow will not be particularly strong (predominately be below gale force) and the low pressure center will be relatively weak (MSLP above 1000 mb) at the time it moves in close proximity to our region. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 6 AM this morning to 6 PM EDT Tuesday for ANZ452>455. Small Craft Advisory from noon today to 6 PM EDT Tuesday for ANZ430-431-450-451.
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&& $$ Synopsis...Gaines Near Term...CMS/Robertson Short Term...CMS/Robertson Long Term...Gaines Aviation...Gaines/Robertson Marine...Gaines/Robertson Tides/Coastal Flooding...Staff

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