Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 230830 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 330 AM EST Tue Jan 23 2018 .SYNOPSIS... Strong low pressure will track northeastward from the eastern Great Lakes to just north of Maine tonight. It will whip a strong cold front across the mid Atlantic states early this afternoon. Strong high pressure will spread east during the balance of the week, before moving out to sea Saturday. A cold front and low pressure then arrives from the west on Sunday and proceeds offshore next Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... As a surface low moves through the Great Lakes region this morning, large-scale ascent will increase markedly downstream across much of the eastern U.S. today, aided by a combination of differential cyclonic vorticity advection downstream of a potent vort max in the Midwest, considerable low-level warm advection aided by a 50+ kt 925-mb jet, and left-front quadrant upper- level divergence as a cyclonic jet streak noses into the Northeast. Several areas of showers have developed this morning (with some embedded storms in the eastern Ohio Valley and adjacent areas), including from central PA south-southeastward into lower Delmarva. Hi-res model output suggests this precipitation will grow rapidly during the next few hours as it races northeastward into the northern Mid-Atlantic. Showers are already beginning to reach Berks County and far southern Sussex County (DE) at this time. Model QPF has trended upward overnight, with the Poconos and Lehigh Valley solidly in the 0.75-2.00 inch range from virtually all of the hi-res output. The 00Z GFS looks dry everywhere and generally discounted given its outlier ("outliar") look. QPF looks generally under an inch southeast of the Fall Line, though there is some indication of a second maximum in portions of Delmarva and south/east NJ that will need to be watched. For now, increased total QPF southeast of the Fall Line to generally a third to three-quarters of an inch. Given the showery nature of the precipitation, some spots will be relatively dry, with localized totals possibly higher than the area-averaged storm totals in the grids. With the relatively high totals in the Poconos, Lehigh Valley, and northern New Jersey, may see some isolated hydro issues (unlikely much more than nuisance in nature). Additionally, particularly strong cells may produce locally heavy rain and poor-drainage/urban flooding in isolated spots. The convective environment is at least somewhat favorable for isolated damaging wind gusts and even a brief/weak tornado. Shear is generally off the charts today, with SRH in the warm sector generally 300+ J/kg and deep-layer shear 60+ kts. Instability looks rather marginal, however, and it is not entirely clear that much (if any) surface-based instability can develop, particularly if widespread showers occur. Shear may be so strong that updrafts fail to acquire much vertical depth as well, with corresponding downdrafts unable to generate considerable downward momentum transport (further impeded by meager low-level lapse rates). Nevertheless, given the output of the 00Z NAM Nest and several recent HRRR simulations, it will not take much instability to generate reasonably strong convective cells, and with very strong winds off the surface, the potential is there for an isolated severe gust or two. Of note, several wind gusts 35-50+ kts reached the surface with a convective line to our west (west/central PA, WV, and eastern OH) overnight. That certainly suggests any convective cells require close monitoring today for strong-to-severe wind gust potential. Additionally and not unusual for these convective setups (given the time of year, for starters), convection may contain little or no lightning. Surface trough should move through most of the area by 21Z, with precipitation shunted eastward by this point. Skies will rapidly improve thereafter. Temperatures will be warm today in advance of the potent upper trough, with strong mixing and warm advection aiding in much above average temperatures today. However, there is some question regarding how high temperatures will go given the widespread precipitation expected. Kept things close to consensus for now, but the temp forecast is low confidence. Finally, with south/southwest winds increasing this afternoon in the warm sector and strong winds aloft, expect some gusts 25-35 mph generally along/southeast of I-95. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/... Conditions dry out tonight as the synoptic cold front moves through. Skies should be clearing out, and temperatures will be much colder than those observed early this morning. Kept forecast close to statistical MOS blend, with winds likely strong enough to mitigate more substantial radiational cooling. This leads to lows forecast to be in the middle 20s in the Poconos to the middle 30s in the urban corridor, Delmarva, and the NJ beaches. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... 500MB: Another short wave moves a cross the mid Atlantic coast Wednesday night followed by substantial ridging Friday and Saturday, then another trough develops to the east coast next Sunday and Monday. Temperature: Calendar day averages should range between 3 and 7 degrees above normal Wednesday, near normal Thursday and Friday, on Saturday near 10 degrees above normal, Sunday probably 12 to 18 degrees above normal, cooling a bit to 5 to 10 degrees above next Monday. Forecast basis: Wednesday-Thursday is based on a blended 00z/22 GFS/NAM MOS, Thursday night-Friday was based on the 00z/22 GFS MEX MOS and then D4-8 was based on the 0441z WPC guidance. Wednesday...Considerable cloudiness after early sunshine. A slightest chance for a few flurries in the Poconos. Brisk with a northwest wind gusty 20-30 MPH. Clearing at night. Max temps Wednesday generally 2 above normal with the Wednesday night mins similar. Confidence: Above average. Thursday...Mostly sunny and brisk. Northwest gusty 20-25 MPH. Max temps 2 to 7F below normal. The Thu night mins about 2F below normal. Confidence: Above average. Friday...Mostly sunny. Wind becoming light southwest. Max temps near normal. Confidence: Above average. Saturday...Increasing clouds and milder. Southwest gust 20 to 30 MPH. Chance of showers at night. Max temps 10 to 15F above normal. Sat night mins. probably 15 to 20 degrees above normal! Confidence: Above average. Sunday...Showers with a coldfront and developing low pressure on the front. This part of the fcst differs considerably than what I`d anticipated earlier and it is a GEFS combo with the GGEM and ECMWF both pretty confidence on a wet 12 to 24 hours much more so than the 00z/23 GFS op. So have followed WPC guidance POPS. Max temp 10 to 15 above normal. Gusty southerly flow with a wind shift to west probable at night. Confidence on overall scenario: Above average but below average on details including timing CFP. Monday...cooler but daytime max temps still probably 5F above normal. Showers should b e ending or gone. Confidence: average. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of tonight...Some patchy fog will occur at ABE/TTN with occasional low stratus at ACY through 10Z or so. Showers will move in from the southwest, with CIGs/VSBYs becoming sub-VFR promptly by sunrise. Moderate to heavy showers may occur at times and with little warning, causing quick/brief reductions to IFR/LIFR. Low-level wind shear will continue through the night (generally southwest 35-50+ kts at 2000 feet). Winds generally east or northeast at ABE/TTN and more southeast/south elsewhere, with speeds below 10 kts. Overall confidence is slightly above average. Tuesday...Generally sub-VFR through 20Z with rapid improvement west to east thereafter. Showers likely through the morning, and at least a slight chance of thunder. For now, no mention in the TAFs as coverage/timing are very uncertain. Winds will become southwest during the day, possibly becoming gusty during the afternoon. Showers are capable of producing strong/erratic gusts and localized convective turbulence. LLWS likely through early afternoon. Overall confidence is average. Tuesday night...VFR with west to northwest winds 5 to 15 kts. Confidence above average. Outlook... Wednesday...VFR sct-bkn aoa 3500 ft clearing at night. West northwest wind gust 25 to 30 kt. Thursday...VFR. West to northwest wind gust 20-25 kt. Friday...VFR. Light wind becoming southwest. Saturday...VFR gusty southwest wind 20-30 kt. Chance of MVFR conds later at night in showers. && .MARINE... Small craft advisory conditions are expected on Delaware Bay today and on the Atlantic waters today and tonight. There may be some marginal gale-force gusts on the Atlantic waters today as strong southerly flow increases this morning. However, the bigger threat will be isolated/sporadic gale-force gusts with showers/storms that move through the waters during the day. A few special marine warnings may be required, given that it will not take much for showers to transport strong winds aloft to the surface. Showers should move east of the nearshore waters tonight, and advisory conditions should cease on Delaware Bay. However, winds/seas will remain elevated (switching to west/northwest this evening) through the night on the Atlantic waters. Outlook... Wednesday...SCA extended Atlantic waters and will probably be needed for the De Bay waters. Wednesday night through Thursday...A west northwest wind could gust around 25 knots and the Small Craft Advisory may need to be extended in time. Thursday night through Friday...No marine headlines are anticipated. Saturday... A southwest wind small craft advisory may be needed. && .CLIMATE... Monthly departures should drop about 2 degrees from this mornings (Jan 23) CF6 values that are seen on our Mount Holly web site. The overall monthly average will probably end up somewhere around 1 to 2 degrees below normal for January. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 PM EST Wednesday for ANZ450>455. Small Craft Advisory until 6 PM EST this evening for ANZ430- 431. && $$ Synopsis...Drag 330 Near Term...CMS Short Term...CMS Long Term...Drag 330 Aviation...CMS/Drag 330 Marine...CMS/Drag 330 Climate...Drag 330

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