Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 191032 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 632 AM EDT Fri Apr 19 2019 .SYNOPSIS... A strong low pressure system will continue to develop along a cold front over the Ohio Valley with the potential for heavy rainfall and strong to severe thunderstorms this evening and through the overnight. This low will slowly move northeast, weakening as it does so through early Monday. Weak high pressure will build into the region Tuesday, before a backdoor cold front settles southward Tuesday night into Wednesday. Slightly cooler and unsettled weather look to affect the area for the latter half of the week as another low pressure system moves across the area. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... For the 630 PM update, made some adjustments to the hourly temperature, dew point and wind grids based on the latest observations and trends. Trapped cool air at the surface across parts of the far north is slowly eroding/mixing out. Some low clouds are developing mainly near and west of the I-95 corridor. This looks to continue, however with mixing these low cloud bases may rise at least some this afternoon. Delayed the mentionable PoPs some given a lack of shower activity thus far. Warm air continues to surge northward across the area ahead of a deep closed low. Low-level cool air though remains trapped across the far northern areas, however this is expected to mix out through the morning. The strong closed low centered across the Gulf Coast states today will slowly shift eastward. As this occurs, a deepening surface low will track up the Tennessee and Ohio Valley`s as a frontal zone remains to our north and west. Yesterday`s warm front continues to clear our far northern zones early this morning. The scope of the warm air across much of the area is evident by the ongoing southerly breeze and gustiness as strong flow above the surface with a mild boundary layer is maintaining mixing. This mixing will continue through today, however one of the forecast challenges is how much cloud cover will be around. This will have an impact on the temperatures as any period of sunshine will result in temperatures quickly rising. Much of the guidance continues to show stratus developing prior to daybreak, however this may be overdone. We still kept lots of clouds around (as there should be a decent amount of higher level clouds moving through) however did go warmer given the amount of mixing expected. It would not be surprising if more sunshine occurs especially for a time this morning into the afternoon (although perhaps some low clouds hug the coast as higher dew points advect northward over the cooler ocean waters). The amount of heating will have an impact on the how much instability is realized. If some thunderstorms do occur, steepening low-level lapse rates could result in locally strong wind gusts. Much of the hi-res guidance (and some CAMS) show not a lot of coverage today. A nearly 50-knot low-level jet shifts to mostly the coastal areas then offshore this morning, however this is forecast to expand inland toward evening. This combined with some surface convergence due to the southerly wind into the coast may result in some showers by later today for the coastal locales. Elsewhere, a few showers are anticipated within the moist southerly flow however a short wave rounding the upper-level ridge axis should organize showers and perhaps some thunder across eastern Pennsylvania later this afternoon. This may also be enhanced by southerly flow into the higher terrain thus much of this activity may remain west of the Fall Line, and also these areas may get within the right entrance region of a departing mid level jet streak. The guidance looks to continue with a slower trend, therefore PoPs were adjusted accordingly. The bulk of the forcing and thus more widespread showers look to occur tonight. The area of eastern Pennsylvania in particular may be the spot to watch this afternoon as showers/thunder may train for a time given the storm motion fairly aligned with the deep southerly flow. This setup within an increasingly high PW airmass can result in locally heavy rain (flash flooding potential). High temperatures are a blend of a model consensus and continuity then these were adjusted up some especially south of the I-80 corridor. Deeper mixing with boundary layer warming (steepening low- level lapse rates) should result in southerly wind gusts up to 35 mph especially this afternoon. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... The closed low is forecast to gradually turn more northward and up the Tennessee Valley overnight. This will maintain deep southerly flow across our region along with PW values surging to near 2 inches. Showers and some thunderstorms are expected at any time tonight, however the main focus for an organized band of heavy showers with embedded thunderstorms is late this evening into the wee hours of Saturday morning. It is during this time frame when enhanced lift moves through with an elongated short wave along with a ramp up of the low-level jet. This is also when an intense mid level jet starts to arrive from the south and southwest. The strongest part of the low-level jet is forecast to be along the coastal counties and offshore, however the flow aloft is still strong everywhere through the overnight. The amount of instability is less certain given it is at night, however strong southerly flow should maintain warmer temperatures and while we expect some thunderstorms, a well developed squall line with lightning all along it is less certain. Some of the hi-res/CAMS as well as the HREF indicate a better chance of stronger convective elements overnight near and south of Philadelphia. Given the amount of shear despite lower instability, some convection even if lower topped may pose a locally strong to damaging wind potential. This risk will be fine tuned later today. A more organized band of heavy showers will pose more of a flash flood threat, and given the latest guidance we opted to not expand the flash flood watch at this time. The closed low gets closer by late in the day Saturday, and after an unsettled start to the day for much of the region the showers/thunder should rapidly end and move offshore by late morning. An intense mid level jet arrives during the course of the day, and this should result in a dry slot moving across the region. While some additional showers cannot be ruled out for the rest of the day, the amount of drying aloft may lessen this potential. Some sunshine is expected to return and it will be cooler although a southerly breeze will continue. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Overview...deep 500 mb low will completely cut itself off from the main westerly flow, thus beginning to slowly fill and move off to the northeast through Monday morning. A weak ridge attempts to build across the mid-Atlantic Tuesday, but a quick-moving area of low pressure will skirt across the Northern Tier of the country, dragging a (backdoor) cold front through the area Tuesday night. Though some discrepancy exists between the long range guidance, cooler weather looks to dominate the latter half of the workweek as an upper-level trough digs southward across the Northeast. On a another note, it looks as if much warmer weather may make a return as we close out the month. By Sunday morning, the upper low looks to have reached its peak and is fully in the occluded phase, thus, it looks to begin weakening. Scattered showers will likely occur through at least Monday morning as the low continues to migrate ever so slowly eastward across the CWA. These showers will likely be more topographically favored over the Poconos and NW NJ and also diurnally driven, tending to tapper off Sunday night. Highs Sunday will be cooler in the mid 60s across the area. There is some disagreement with how long the upper low sits over the region, with the ECMWF tending to drag the progression the system out through the day Monday, while the GFS moves it to the east by Monday night. Tending to side with the latter, as the ECMWF sometimes stalls these systems a little too long. Thus, any showers should end, with drier night expected Monday night. Highs will be a little warmer Monday as the remnant upper low will have weakened and shifted eastward. Therefore, highs will range in the low to mid 70s. A little warmer Tuesday as a ridge of high pressure builds behind the upper low with highs sitting in the mid to upper 70s. This will be short-lived as a shortwave trough zips along the Northern Tier, driving a surface low and cold front into the region by Wednesday morning. There does not look to be a lot of precip with this system and dynamics are weak, however, a few showers look plausible Tuesday night into Wednesday. Highs Wednesday will be noticeably cooler, staying in the mid to upper 60s. A trough of low pressure looks to dig into the Northeast for the latter half of the workweek, keeping things fairly quiet, though cooler. Some showers are possible Thursday night and Friday. The GFS kicks the trough out a little faster than the ECMWF, thus am going with a more blended approach given the previous dialogue on this model tendency. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...Areas of MVFR ceilings this morning, then these may tend to improve to VFR for a time this afternoon before lowering again toward evening. This is of low confidence. Some showers around especially this afternoon and a few thunderstorms cannot be ruled out. Southerly surface winds 12-18 knots with gusts 20-30 knots. Low-level jet of 40-50 knots at 2000 feet will result in low-level wind shear mainly this morning at MIV and ACY. Tonight through Saturday...MVFR to IFR ceilings tonight and visibilities as low as IFR due to heavier showers and some thunderstorms, then conditions improve to VFR by early Saturday afternoon. The extent of the thunder tonight and early Saturday morning is less certain. Southerly winds 10-15 knots with gusts up to 25 knots. The gusts may drop off for a time tonight. Outlook... Saturday night...MVFR to IFR cigs in the morning. Southerly winds 10 to 15 knots. Sunday...VFR cigs may transition to MVFR at times. Showers may lower restrictions at times. South to southwest winds 10 to 15 knots, dropping to 5 to 10 knots at night. Monday...generally VFR, though a few widely scattered showers are possible in the morning. Light and variable winds with a prevailing westerly direction. Tuesday...For the most part, VFR. Showers arriving in the afternoon and into the overnight from northwest to southeast. Westerly winds turning northwest 5 to 10 knots. && .MARINE... Small Craft Advisory for Delaware Bay through early Saturday afternoon, and elsewhere through early Saturday evening. Strong southerly flow will be maintained, however the overall mixing is expected to be limited some due to much warmer air moving over the cooler waters. It is possible some low-end gale force gusts occur for a time late today and especially tonight, however confidence is low due to a strong low-level inversion expected over the waters. Seas will remain elevated and actually build today into tonight. Outlook... Sunday...Sub-SCA winds from the south at 10 to 15 knots. Seas remain on the higher side with wave heights lingering from 5 to 7 feet. They subside somewhat by Sunday night, however. Monday...Sub SCA winds. Seas 3 to 5 feet. Tuesday...southerly winds 10 to 15 knots. Seas 2 to 4 feet. && .HYDROLOGY... A Flash Flood Watch remains in effect for roughly along and north/ west of I-95. Areas of 1-2 inches of rain is expected with local amounts up to 3 inches. Hourly rainfall rates potentially nearing 2 inches later today and especially tonight will result in the locally higher amounts, and where this occurs is where the flash flooding potential is the greatest. River/Stream/Creek rises will begin late today and tonight and persist through the weekend. Mainstems won`t crest until at least Sunday. Smaller creeks and streams will crest on Saturday. Poor drainage and low-lying flooding will quickly occur during times of enhanced rainfall rates. Keep in mind that the hydrologic forecasts are based on predicted rainfall. Actual response will be different if the rainfall increases/decreases across a specific basin. Keep in touch with future forecasts into the weekend. && .TIDES/COASTAL FLOODING... Subsequent high tides also have potential for minor coastal flooding, especially if heavy rain materializes tonight. Additionally, with southerly flow continuing little evacuation of water will occur between high tides making it increasingly likely flooding thresholds will be reached, especially with the higher astronomical tide tonight and especially in the back bays. Another coastal flood advisory will be needed, however this will be re-evaluated later today. Response on the eastern shores of Chesapeake Bay will be slower, but prolonged southerly fetch is favorable for some minor tidal flooding. Models continue to suggest this possibility by Saturday morning`s high tide. We will continue to monitor this potential. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Flash Flood Watch from 2 PM EDT this afternoon through Saturday morning for PAZ054-055-060>062-070-071-101>106. NJ...Flash Flood Watch from late tonight through Saturday afternoon for NJZ001-008-010-012-015. Flash Flood Watch from 2 PM EDT this afternoon through Saturday morning for NJZ007-009. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EDT Saturday for ANZ450>455. Small Craft Advisory until 2 PM EDT Saturday for ANZ430-431. && $$ Synopsis...Davis Near Term...Gorse Short Term...Gorse Long Term...Davis Aviation...Davis/Gorse Marine...Davis/Gorse Hydrology... Tides/Coastal Flooding...

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