Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KPHI 232045 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 445 PM EDT Sun Oct 23 2016 .SYNOPSIS... A cold front will slide through our region early Monday morning. High pressure centered over central Canada will build to our north during mid week. A warm front, followed quickly by another cold frontal passage is expected through our region late Thursday or early Friday. High pressure follows to our south across the Carolinas next Saturday with another cold frontal passage likely Saturday night or Sunday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... An intense surface low will continue to roll northward across eastern Canada through tonight. As this occurs the upper-level troughing relaxes over our area for a time, however a potent short wave diving southeastward from the Midwest this afternoon will arrive in our area late tonight. The combination of thermal advection reversing, an inversion and lingering cyclonic flow has resulted in a decent amount of stratocumulus that expanded across much of our area this afternoon. This is expected to dissipate from south to north into this evening as some additional warming aloft occurs and the flow aloft starts to back. The pressure gradient will lessen as well and with the onset of less mixing, the gusty winds will diminish fairly quickly late this afternoon and early this evening. The short wave for the overnight time frame looks to be tracking a bit farther north this afternoon as seen on the water vapor imagery. Some of the guidance has shifted a bit farther northward with the west to east precipitation axis. While there is a weak surface low forecast to develop and slide across the northern half of our area toward morning, much of the forcing appears to be tied to warm air advection on the nose of a strong 850 MB jet. This will tighten the thermal gradient and where this sets up will be the placement of a ribbon of frontogenetic forcing. Our northern zones look to be on the southern end of the precipitation axis, however are expected to see at least some showers as this system zips east-southeast. As a result, we continued the idea of quickly increasing POPs after Midnight but also tightened the gradient southward. Looks like much of the measurable QPF is north of I-80 northward. Farther southward, just some increase in cloud cover is anticipated overnight. As a low-level jet arrives overnight along with accompanying warm air advection, temperatures are not expected to drop off as much and may even rise a bit toward morning. Low temperatures were mostly a blend of continuity and MOS. && .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... A quick moving short wave and weak surface low will exit to our northeast early in the morning. This feature will pull a cold front offshore to start the day and will also amplify the trough aloft across the region. As a result, any showers up north end very early. As cold air advection increases during the day combined with cyclonic flow and a tightening pressure gradient, there will be a gusty wind once again. It looks like gusts should peak at around 30 mph especially from late morning on. The increasing cold air advection combined with just enough moisture should result in some stratocumulus lingering or developing. This could be fairly extensive across the northern areas especially as another strong short wave is forecast to arrive during the afternoon. We therefore kept more clouds across the northern areas through the day. Since the cold air advection is forecast to be more notable in the afternoon, many areas should have temperatures get into the 60s before the cooling starts. We used mostly a MOS blend for the high temperatures. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... 500 MB: A cold trough in the northeast USA will weaken to the Maritimes late Wednesday as a short wave moves through the Great Lakes- Ohio Valley region. That short wave crosses the mid Atlantic coast Friday with a broader trough to move east across our area Sunday. Temperatures: October through the 22nd has averaged around 4 degrees above normal. The chill during midweek will make a significant dent in that above normal departure but still leave the month as a whole, averaging around 2 degrees above normal. Calendar day averages Tuesday should be around 3 TO 4 degrees below normal, 5 and 10 degrees below normal Wednesday and Thursday, thereafter within 3 degrees of normal Friday through Sunday. Forecast basis: Unless otherwise noted, a 50 50 blend of the 12z/23 GFS/NAM MOS guidance Monday night-Tuesday night, thereafter the 12z/23 GFS MEXMOS Wednesday and then the 15z/23 WPC guidance Wednesday night -Sunday, at times blended with the 12z/23 GFS MEXMOS guidance. The dailies... Monday night...Clear to partly cloudy except there could be a sprinkle or flurry in the Poconos toward dawn Tuesday. Seasonable. a gusty northwest wind to 20 mph high terrain and I-95 region to the coasts. Confidence: above average except below average any pcpn. Tuesday...scattered sprinkles/flurries possible during the morning, mainly Poconos, otherwise...considerable diurnal driven cloud cover, breezy and cool. Northwest wind gusts to 25 mph or 30 mph. Confidence: above average except below average any pcpn. Tuesday night...Freeze and or frost is an above average likelihood for e PA and NJ countryside (probably excluding Philadelphia center city) and adjacent sections of ne MD and N DE as northwest winds diminish. -4C at 850. Confidence: above average. The 12z/23 nam boundary layer winds tend to show a little bit of stirring (transfer) most of the night east of I-95. That may indicate a cool bias in the NAM temps near the shore. On the other hand, SREF modeled PWAT which is near .3" inches early Tuesday morning, lowers to less than .25" from ACY- PHL northward, indicating the columns cold and dryness. This very low pwat tends to be a prerequisite for below normal night time cold. Continues to be highlighted in the HWO. Hourly temps sampling <36 for areas of frost and <33 for widespread frost. Since there may be a slight stirring of the wind, the frost might be spottier than anticipated but the freezing temp fcst looks pretty solid along and north of I-78. Wednesday...Quite cool with highs 5 to 10 degrees below normal despite partly to mostly sunny skies. Northwest to north winds. Confidence: above average. Wednesday night...There will be another risk for freeze and or frost on Wednesday night. However, unlike Tuesday night, increasing high clouds after midnight may somewhat limit radiational cooling. If the clouds come in early enough, this could inhibit frost development. For now, continue a mention of patchy frost for areas with a forecast min of 35 degrees or less, since temps will plummet during the evening,despite bands of thin cirrus. Thicker cirrus should not arrive until after 2am when the damage may already be done. Confidence: average Thursday and Thursday night...Low pressure and its waa pattern will run into marginally cold airmass over ne Pa and nw NJ where a a little snow, sleet or freezing rain is possible during the morning, conditional on spotty precipitation arriving prior to 11 am. Will add the potential for a few slippery spots in the HWO that area. Used the partial thickness tool for ptype, manually adding snow. there is quite a bit of model discrepancy regarding whether the column will still be cold enough for snow to reach the ground but it is possible. No accums expected, other than a trace of sleet and/or snow in the hills and mountains. Small chance of an isolated thunderstorm during the evening on the Delmarva but not mentioned in the grids. Otherwise, max temps may not occur until sometime late in the day or evening. Am suspicious that the fcst max temps issued at 330 PM along and north of I-78 are 3F too warm. Please see WPC for qpf amounts (around half an inch give or take a quarter inch). I count this as somewhat beneficial rain for our area. Confidence: Average except below average on any mixed ptype vcnty I-80 north. Friday...Partly sunny and seasonable. northwest wind gusts 15 to 20 mph. Confidence: average. Saturday and Sunday...Fair and seasonable. A cold frontal passage is probable but with model timing differences. Confidence: below average, especially Sunday. && .AVIATION /21Z SUNDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Rest of This Afternoon...VFR scattered to broken cloud bases around 5000 feet. West-northwest winds 10-20 knots with gusts to around 25 knots, then diminishing toward evening. Tonight...VFR with an increase in clouds, especially north of KPHL. Lower VFR ceilings should be confined from about KABE to KTTN on northward. The main shower activity after 04z should stay north of KABE and KTTN. West to southwest winds diminishing to 10 knots or less, although some increase may occur toward daybreak Monday with a wind shift to the northwest. Monday...VFR ceilings around at or above 5000 feet. Northwesterly winds 10-15 knots with gusts up to 25 knots. Outlook... Monday night through Wednesday night...mostly VFR conditions expected. Gusty northwest winds to 25 kt expected Tuesday afternoon. Confidence: above average. Thursday...VFR ceilings Thursday should lower to MVFR conds in periods of rain (with possible localized IFR) Thursday night. East wind becoming southeast. Confidence: above average. Friday...VFR. northwest wind gust to 15 or 20 kt. Confidence: average. && .MARINE... Intense low pressure will continue to move well north of New England through tonight, while an upper-level disturbance arrives from the northwest toward morning. This will allow the winds to decrease through this evening from south to north, however a low-level jet is forecast to arrive toward morning before shifting offshore during Monday morning. The strongest low-level flow is forecast to occur during warm air advection, therefore it is questionable how much mixing can occur down to the surface. Cold air advection then increases during Monday especially in the afternoon. The combination of both of these is expected to contribute to advisory level gusts through Monday. Since the stronger winds aloft are forecast to move through during a less favorable mixing signature, gusts were kept below gale force. While there should be a lull for awhile this evening, especially on Delaware Bay, it should be short enough and therefore a Small Craft Advisory is in effect for all zones through Monday. Outlook... Monday night and Tuesday...SCA conditions are expected to continue intermittently as northwest winds occasionally gust above 25 kt (though seas may drop off below 5 ft. SCA headline as of the 330 PM forecast was extended through Monday evening with future shifts to rereview for extensions. Confidence: above average. Wednesday and Thursday...No marine headlines anticipated. Northwest to north winds gusting to 15 kt Wednesday becoming east to southeast Thursday. Confidence: average. Thursday night...Southeast to south winds could increase above SCA criteria on the Atlantic coastal waters late. Confidence: average Friday...No marine headlines anticipated at this time. Northwest wind gusts to 15 or 20 kt. Confidence: average && .CLIMATE... Our forecast indicates October should average at least 2 degrees above normal for PHl and ABE. Some of the recent forecasts have been colder and that may mean PHL and ABE slip to around 20th warmest October on record, which is only noteworthy in that the excessive monthly warmth of July, August, September eased in October. Will reevaluate tomorrow and Tuesday. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until midnight EDT Monday night for ANZ430-431-450>455. && $$ Synopsis...Drag Near Term...Gorse Short Term...Gorse Long Term...Drag 444 Aviation...Drag/Gorse Marine...Drag/Gorse 444 Climate...444 is the U.S. government's official web portal to all federal, state and local government web resources and services.