Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 080737 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 237 AM EST Fri Dec 8 2017 .SYNOPSIS... A weak ridge of high pressure over the area tonight will weaken Friday. Low pressure will strengthen and move along a front across the offshore waters Friday night through Saturday. The low will then move up into New England Sunday while a ridge of high pressure builds back over the Middle Atlantic. A cold front will cross the area Monday night. Another system may affect the area later Tuesday and Wednesday. && .NEAR TERM /THROUGH TODAY/...
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**probable wxa for snow and WSA for snow late tonight into sat evening.. Decisions on the watch advy sitn in process now. UK has been steadfast heaviest on this event for the past 2 days of cycles. EC second and recently the GFS/NAM are on board. Strong rrq of the jet stream core up in se Canada is a big driver. SFC temps--melting-riming, especially during the day Saturday are problems. More from Mike in his section. For today...increasing and thickening clouds from south to north. light west wind. Rain develops s DE midday and spreads slowly ene this aftn with no accums due to warm sfc temps (melt as it falls). Otrw...all fcst elements are blended 00z/8 GFS/NAM MOS. The POPS were raised today in s DE per the HRRRX/UKMET.
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&& .SHORT TERM /TONIGHT/...
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Forecast basis: Guidance temps were far too cold tonight and so we stayed with the 330 PM Thursday forecast temps. POPS were adjusted upward tonight per the UKMET and ECMWF .10. Precipitation develops northward during the night. mostly snow from near Dov DE to ACY northward and either snow or rain to snow southward with accums of 1-3" possible in the 6z-12z time frame parts of s NJ through the DE. Wind tums light north except nne 10-15 mph along the snj and de coasts toward dawn. More later..
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/... 900 PM...Updated temperature, PoPs and weather grids for the Friday night-Saturday event. Will hold off on making changes the snowfall accumulations until the mid shift gets a complete look at the 00Z guidance that comes in later tonight. The biggest difference with the evening update was to increase PoPs late Friday night and Saturday (most notably from the I-95 corridor/Delaware Valley, south/east) with increasing NWP support for the occurrence of measurable precip across this part of the forecast area. The precipitation type should be snow for the bulk of the event. The exception will be coastal locales near the ocean/Delaware Bay where mixing will play a factor in limiting snowfall accumulations (evident in probability of frozen precipitation fields for the last several runs of NAM/NAM nest). Yes, models have trended westward/wetter over the since last night with 12Z/18Z model consensus painting one-half to one-inch of QPF for areas just to the east of I-95. However, a steadfast 10:1 snow ratio should not be valid in the coastal plain given a marginally warm surface and the potential for considerable riming to take place (more likely between 5:1 and 8:1 in the coastal plain for this event). Previous Discussion from Thursday Afternoon... The system that we have been watching for Friday night and Saturday is showing better organization today and also a track closer to the coast. All of the operational models are now showing a good chance for some accumulating snow across parts of Delmarva and southern / central NJ for the early part of the weekend. We have adjusted pops/qpf/snowfall fcsts accordingly. Much of the show will occur during the 4th period of the fcst, so we`ll hold for any winter weather headlines for now. Our fcst would align with advisory criteria snowfall, but with trends continuing to develop, we`ll see the next set of model runs before deciding which flags are needed and where. Our snowfall fcst may be conservative considering the QPF amts being forecasted by some of the models at 12Z. Temperatures will continue below normal through the weekend. Snow will taper off Saturday evening, but a few snow showers will occur up across the southern Poconos. Much of Sunday through Monday will be dry as a ridge of high pressure extends across the region. We have kept the chc for snow showers across the highest elevations of the southern Poconos. Temperatures will remain 5 to 10 degrees below normal thru this period. Later next week, the upper trough that is setting up across the east this weekend will be progressing across the Middle Atlantic and Northeast. More cyclogenesis is possible with this and its possible that more precip could be across the area Tuesday night and Wednesday. The models vary with regards to how cold it will be then, but its possible that we could have more snow across the area then. We`ll fcst temperatures back close to normal Tue, but then it appears that a cold front will bring back below normal readings for Wed and perhaps lasting into Thu. && .AVIATION /07Z FRIDAY THROUGH TUESDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR CIGS...mostly CI lower during the day from south to north. snow or rain to snow develops s DE midday and possibly reaches into s NJ late in the day. Confidence: moderate. Tonight...VFR cigs to start but IFR conds in snow are likely to develop most TAF sites toward 11z/9. Please see the upcoming 12z TAFS for details. Could be a little rain mixed with the snow at the start at KMIV and KACY. Light wind becomes north by morning. OUTLOOK... Saturday thru Sat night...Lower CIGS/VSBYS in snow. Conditions will be better further W (KRDG-KABE). Sunday thru Monday...Mostly VFR. Few snow showers far N/W. Monday night and Tue...Restrictions psbl with rain/snow. High uncertainty in this period attm.
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&& .MARINE...
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SCA up north will soon be expired. However we begin a new SCA for the southern waters at about 5am Saturday. Outlook... Saturday thru Saturday evening...Rain/snows with sub- SCA conditions. Winds/seas beginning to build late. Sat night thru Monday...SCA conditions expected much of the time. Winds and seas diminish on Delaware Bay Monday.
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&& .CLIMATE...
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We will add in the record daily snowfall for Saturday at about 5am after all our products post.
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&& .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EST early this morning for ANZ450-451.
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&& $$ Synopsis...O`Hara Near Term...Drag 236 Short Term...Drag 236 Long Term...O`Hara Aviation...Drag/O`Hara 236 Marine...Drag/O`Hara 236 Climate...236

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