Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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525 FXUS61 KPHI 151040 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 640 AM EDT Sun Oct 15 2017 .SYNOPSIS... A cold front will progress to the east and move through our region later tonight. High pressure is then expected to settle over the eastern part of the country for the period from Monday through Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... For the 630 AM update, adjusted temperatures and dew points up a bit for most places to start. Changed the drizzle and fog mention to be more widespread. Changes are expected today as much warm air surges in ahead of a strong cold front which arrives tonight. First, we deal with the low clouds and fog, as low-level moisture is trapped beneath an inversion within a zone of light winds. This is also producing some drizzle early this morning, and the radar imagery indicates very weak returns across parts of the area. Some denser fog may occur, however this is not anticipated to be widespread. The forecast challenge is the timing of this dissipating. The forecast soundings continue to show increasing vertical mixing occuring later this morning and especially this afternoon from south to north. Given a southwest to west flow up through the atmosphere today along with a strengthening low-level wind field, the trapped low-level moisture should get mixed out. However some forecast soundings, especially the NAM/3km NAM, indicate a lower level inversion holding or evening strengthening some this afternoon within a zone of warm air advection. The issue is that these same soundings do show better mixing with time beneath the inversion. Given the chance of drier air entrainment from above the mixed layer, we believe there will be increasing sunshine through the afternoon. If not, then temperatures will need to be probably lowered. Assuming that we effectively mix this afternoon, a gusty southwest wind up to 30 mph can occur. This should be more likely in the coastal plain and also the higher elevations. Given the expected mixing and also the anticipated improving sky conditions, afternoon temperatures are expected to surge into the upper 70s to lower 80s for many places. As mentioned above though, the temperature forecast today is dependent on the extent/timing of the clearing. Overall, we did not make any significant changes to the high temperatures for today. In addition, dew points should favor the 60s for much of the area therefore it will feel a bit on the humid side. This will all be short-lived though as a strong cold front arrives later tonight. Showers associated with this front are expected to remain well to our west during the day. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... An upper-level trough moving across the Great Lakes and Ohio Valley will drive surface low pressure well to our north tonight. This will sweep a strong cold front across our area later tonight, with a sharp wind shift to the northwest with its passage. The guidance shows that the bulk of the large scale forcing shifts to our north, however enough convergence with the cold front along with some lingering instability should allow for a broken band of showers to arrive from the west later this evening. The question is how much does this weaken with an eastward extent. The organization of the showers should be on the decline with an eastward extent, at least initially, given the timing and little instability in place. As a result, held off on adding thunder and continued with mainly chance PoPs (scattered showers) from west to east. Much of the model guidance indicates that the showers occur behind the cold front/wind shift, and this characteristic could allow for some more organized showers to continue farther east. The showers may reorganize however toward morning across portions of Delmarva to southern New Jersey as the right entrance of a 500 mb jet moves across. Some increase in the winds could occur for a time right behind the cold frontal passage due to pressure rises and the onset of cold air advection. Low temperatures are mostly a continuity/MOS blend. && .LONG TERM /MONDAY THROUGH SATURDAY/... A cold front is expected to be well off the coast and on its way out to sea on Monday morning. Showers may linger near the coast through the mid-morning hours. Otherwise, cool dry air will build into our region on a northwest wind on Monday. Surface high pressure is forecast to build eastward, with its center settling over the Virginias on Tuesday before sliding off the Middle Atlantic coast on Wednesday. The center of the high is anticipated to move back to West Virginia and vicinity for Thursday through Saturday. Our region will remain under the influence of the high through the period. As a result, we are expecting dry weather conditions. A mid level short wave trough and an associated cold front may arrive from the northwest on Thursday night or Friday. There will be little moisture associated with these features and they should have limited impact on our weather. Temperatures are forecast to be near normal for mid-October on Monday and Tuesday. However, readings will trend above normal again during the period from Wednesday through Saturday. Temperatures are forecast to drop into the middle and upper 30s late on Monday night outside of urban areas and away from the immediate coast, Delaware Bay and Chesapeake Bay. We are anticipating patchy frost in the Poconos, the Lehigh Valley, upper Bucks County, northwestern New Jersey and the Pine Barrens. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...MVFR/IFR with areas of LIFR ceilings early this morning are expected to improve by late morning, then to mostly VFR this afternoon. MVFR/IFR visibility due to fog and even drizzle is expected to improve by mid morning. The timing of the improvements is still of lower confidence. Light winds, becoming southwest and increasing to 10-15 knots with local gusts to about 20 knots. While confidence is high regarding the increase in winds, especially this afternoon, there is lower confidence regarding the extent of the gusts. Tonight...VFR overall, although some showers move through later this evening and overnight (local MVFR possible) as a strong cold front arrives. Southwest winds around 10 knots, however gusts up to 25 knots are possible at times especially with a wind shift to the northwest behind the cold front. OUTLOOK... Monday through Thursday...Mainly VFR. && .MARINE... Low clouds, some fog and drizzle will give way to improving conditions later this morning and afternoon. Southwesterly flow increases mainly this afternoon and tonight ahead of a strong cold front. A low-level jet arriving mainly late this afternoon and tonight should result in wind gusts to around 25 knots. It is possible some gusts to 25 knots occur this afternoon as mixing develops, then a greater chance of gusty winds tonight just ahead of and behind the cold front. As a result, a Small Craft Advisory starts at 18z/2 PM today for all zones and this goes through at least tonight. Some showers are expected with the cold front late tonight. OUTLOOK... Monday and Monday night...A Small Craft Advisory is in effect for a northwest wind gusting around 25 to 30 knots. Tuesday through Thursday...No marine headlines are anticipated. && .FIRE WEATHER... A northwest wind 10 to 20 MPH with gusts around 25 MPH is expected for Monday. Relative humidity values are forecast to drop into the 30s in much of our region on Monday afternoon. Rainfall amounts associated with tonight`s cold frontal passage should be light. As a result, there is a heightened fire weather concern for Monday. && .CLIMATE... Much warmer air arrives during the day today, and depending on the extent of clearing and mixing some places could approach record high temperatures. Below are the record high temperatures for October 15th, for the official climate locations that could approach their record this afternoon. Again, this will depend on how much clearing actually occurs. Allentown, PA 82 in 1947 Reading, PA 83 in 1989 Trenton, NJ 84 in 1975 Georgetown, DE 84 in 1975 && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 2 PM this afternoon to 6 AM EDT Tuesday for ANZ430-431-450>455. && $$ Synopsis...Iovino Near Term...Gorse Short Term...Gorse Long Term...Iovino Aviation...Gorse/Iovino Marine...Gorse/Iovino Fire Weather...Iovino Climate...Gorse

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