Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 160127 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 927 PM 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 Sunday. There may be a weak cold frontal passage Thursday night. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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930 pm update: Cold front continues to blast eastward across the Northeast this evening. Objective 01Z surface analysis shows the front positioned from a 991-mb surface low near the Quebec/New Brunswick border to near Burlington, VT, to near Altoona, PA with very strong 3-hour pressure rises continuing upstream of the front/low. Front should reach the western CWA within 2 to 3 hours. Thin line of low- topped convection is slowly diminishing along the front, but there has been increasing lift and precipitation echoes (albeit quite light) upstream of the band recently. As synoptic- scale lift increases via upper-jet dynamics and differential cyclonic vorticity advection downstream of a potent vort max in the Great Lakes region, more light rain/showers should develop just behind the front. Latest HRRR and NAM Nest are quite consistent with this signal, especially southeast of the I-78 corridor, late tonight. Current PoP/Wx grids look OK for now, though they still might be timed a little on the fast side. Main changes to the grids were more edits to winds/sky cover given latest trends (finally some clearing in the area) and to increase dew points in the pre-frontal regime. Portions of previous discussion included below... Subjective 22Z surface analysis indicates a 992-mb surface low in far southeastern Quebec with a trailing cold front near Watertown, NY, to just west of Pittsburgh with substantial (7-9 mb) 3-hour surface MSLP rises on the upstream side of the front/low. Frontal lift is aiding in the generation of low- topped convection along the front, and with very strong winds aloft, some damaging wind gusts have occurred with the line this afternoon in western NY and northwest PA. This convection will wane rapidly this evening as what little instability is present will nocturnally diminish. However, synoptic-scale lift will get a boost from a strong vort max rapidly progressing eastward through the Ohio Valley and Great Lakes region tonight. Aided by upper-level divergence via the right entrance region of a 250-mb jet streak atop the Mid-Atlantic, post-frontal light precipitation should develop across the area late this evening through early tomorrow morning. The rapid progression of the system and unimpressive vertical profiles of moisture suggest accumulations should be light (generally near or below a tenth of an inch). Winds should turn northwest as the front moves through the area after midnight and may become gusty, especially in the vicinity of showers. The increased winds and continued mixing upstream of the front should hinder a rapid drop in temperatures after frontal passage. Instead, this should just prevent much of a diurnal rise in temperatures tomorrow.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... High pressure will begin to build over the area Monday. Clearing will continue to progress NW to SE over the area. Winds will remain gusty through the day as decent cold air advection continues. Temperatures will mostly remain level or continue a slow fall thru the day. Early highs in the low 60s north and low/mid 60s south/east. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Hazards...A frost advisory was issued for a small portion of ne PA and nw NJ early Tuesday and coverage may need to be expanded in future forecasts. Additionally... the GFS is flagging potential widespread freeze frost for eastern NJ early Wednesday. Fire weather: please see that section for a brief note on elevated fire danger potential midday and Monday afternoon. 500MB: A short wave crosses New England Monday evening. Another trough passes by, further north across Quebec Tuesday and yet another on Thursday, then very strong ridging (several standard deviations above normal) develops across the Great Lakes and northeast USA late in the week through at least next weekend. Temperatures... October has averaged 8 to 9 degrees above normal for the first 14 days of the month, excluding the ACY Marina where departures from normal are less due to the tempering influence of the nearby ocean (both warm and cold). Calendar day averages Tuesday should be a a couple of degrees below normal, warming Wednesday to nearly 5 degrees above normal, and eventually to daily averages 10 degrees above normal Saturday and Sunday. The daily diurnals will be larger than usual due to the dry airmass in place and light wind fields, nighttime lows probably a little colder than statistically modeled. Forecast basis...This forecast was a 50 50 blend of the 12z/15 GFS/NAM MOS Monday night-Tuesday night, the 12z/15 GFS MEXMOS Wednesday and thereafter the D4-8 WPC 15z/15 12 hourly max/min T and POP, and the 6 hourly td, wind and sky. The very warm GFS D4-8 was not used. The dailies... Monday night...Gusty northwest winds to 20 mph early, otherwise chilling with late night decoupling aiding radiational cooling and the likelihood of scattered to widespread frost near dawn Tuesday, north of I-78, especially near and north of I-80. A frost advisory was issued where temperatures appears to give us the best chance for frost or freezing occurrence. (Isolated frost and freezing temps have already occurred in this advisory area). The gusty north wind may continue all of Monday night along the coasts and there is uncertainty whether there will be enough decoupling s of I-78 to have issued any advisories there at this time. Tuesday-Sunday...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 Sunday. Our region will remain under the influence of the high through the period. As a result, we are expecting dry weather conditions. This flux of the high pressure centroid may also be associated with a slight temporary cooling Friday with the preceding possible weak cfp Thu night. && .AVIATION /01Z MONDAY THROUGH FRIDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Low-confidence forecast tonight. CIGs are beginning to scatter out at PHL/ILG/MIV/ACY, but fluctuations between SCT/BKN from 2000-3000 feet may continue this evening at all sites. CIGs should fill back in, though likely somewhat higher (3000-5000 feet) overnight as a cold front approaches. Scattered showers are likely 03Z to 12Z, with brief sub-VFR VSBYs and somewhat lower CIGs possible. Winds will veer to northwest in the 05Z to 10Z time window with potential for gusty/erratic winds near showers. Skies should rapidly improve after 12Z, but northwest winds will increase to 10 to 15 kts with gusts 20 to 25 kts possible by late morning through the afternoon. Given the low confidence in the CIGs, impacts of showers, and timing of cold front/wind shift...expect several amendments to the TAFs through the overnight hours. OUTLOOK... Monday night through Friday...VFR with mostly a light westerly wind...possibly gusty 15-20 kt Thursday and Friday. && .MARINE... 6:30 pm update: Winds have approached/exceeded advisory criteria at buoy 44009, and are slowly increasing elsewhere. In addition, seas are creeping toward 5 feet across the coastal waters. Current advisory looks good and made no changes to it. Did, however, slow the changeover to northwest winds slightly and slowed the increase in winds this evening to account for the latest observational trends. Previous discussion below... We will keep the SCA flag in place since winds will be increasing as the front nears, and gusty winds will continue after fropa. Scattered showers moving across the waters with the front after midnight and probably mostly done shortly after dawn. Winds mostly SW ahead of the front and turning NW behind it. Fair weather expected Monday. OUTLOOK... Monday night...A Small Craft Advisory is in effect for a northwest wind gusting around 25 to 30 knots. Tuesday..There may be a leftover northerly wind SCA lingering into the early daylight hours on Tuesday. Tuesday night through Friday...No marine headlines are anticipated with generally a westerly flow and gusts under 22 kt. && .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. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...Frost Advisory from 2 AM to 8 AM EDT Tuesday for PAZ054-055. NJ...Frost Advisory from 2 AM to 8 AM EDT Tuesday for NJZ001-007- 008. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT Tuesday for ANZ430-431- 450>455. && $$ Synopsis...Drag Near Term...CMS Short Term...O`Hara Long Term...Drag Aviation...CMS/Drag Marine...CMS/Drag Fire Weather...Iovino/O`Hara

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