Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 230508 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 108 AM EDT Wed Jun 23 2021 .SYNOPSIS... A cold front will continue moving offshore tonight. High pressure then builds in for Wednesday and Thursday before moving offshore. A slow moving frontal boundary will cross the Great Lakes region from this weekend into early next week. The mid-Atlantic will then be in between the front to the west and offshore high pressure for several days. && .NEAR TERM /THROUGH TODAY/...
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Cooler and drier air continues to filter southeastward into the region, with temperatures feeling chilly for this time of year. Made a few minor adjustments to temps and dew points for the early morning update. Some patchy fog is possible overnight, but it is not expected to become too widespread or dense with the drier air arriving. Lows will be chilly for late June with readings in the 40s in many areas with some low 50s across Delmarva and metro Philadelphia. Winds will be from the north or northwest settling to under 5 mph. Wednesday: High pressure will build across the area with plenty of great weather. Sunny skies and cool temperatures are expected. Highs will top out in the upper 60s to low 70s for the NW areas and mid 70s for Philadelphia and much of Delmarva. Winds will be from the northwest around 10 mph early then becoming light and variable late.
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&& .SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/... Mainly tranquil weather is expected this period. We remain in a post- frontal air mass by Wednesday night with surface high pressure overhead. Clear, calm, and rather cool conditions are expected with lows dropping well down into the 50s in most areas. Another mostly sunny and pleasant day is expected Thursday. High pressure will shift off to the north and turn our flow onshore. This will keep highs only in the upper 70s to near 80 despite some height rises aloft, and a little cooler at the coast with the easterly breeze. For Thursday night, most model guidance is indicating the development of a weak surface low pressure off the Outer Banks, likely along the remains of an offshore cold front, and moving in the general direction of the New Jersey coast. This could potentially bring some light showers to eastern and southeastern areas by later Thursday night, where slgt chc PoPs were added. But overall, should be a mostly dry and quiet night with lows a few degrees warmer than the prior night. && .LONG TERM /FRIDAY THROUGH TUESDAY/... Overview... A typical summer pattern develops for the long term as a Bermuda High becomes established offshore and a slow moving trough develops over the Great Lakes. This is a classic southwest flow regime which is likely to be in place for at least several days beginning the end of this week and heading into next week. The result will be a transition from the dry and cooler weather during the middle of this week to a sustained warm and humid pattern, especially by the time we get to the weekend. From Saturday onward, we will mostly be looking at highs in the mid 80s to near 90. This is nothing atypical for the season, but combined with the increasing humidity, we may flirt with Heat Advisory levels at times during the period. Rain chances in this pattern are quite uncertain at the moment. It will depend heavily on how fast a cold front to the west approaches as Great Lakes troughing slowly moves eastward and/or starts to lift out. Model guidance has a spread of several days on when this front will move through, or if it even will at all as opposed to just washing out. My general thinking is that rain chances will gradually increase from Saturday onward, especially to the west, but that overall there will be a lot of dry hours through the period. Dailies... Friday-Friday night... The weak wave of low pressure mentioned for Thursday night will likely still be in the vicinity on Friday. Have added some slgt chc PoPs to southeastern areas for the daytime Friday. With rising heights aloft and a lack of any dynamic support, this surface low should remain very weak, and any shower activity should be limited. Otherwise, as the flow turns more southerly combined with the height rises, we should warm up a bit on Friday, with highs rising into the low 80s in most areas. By overnight, dry weather is expected, but it will be a warmer and stuffier night as humidity continues to increase. Saturday-Sunday... We enter the southwest flow regime in earnest over the weekend. Highs of 85 to 90 combined with dew points near 70 will yield heat index values of 90 to 95 degrees on both days, with Sunday being a little warmer than Saturday. As alluded to above, rain chances will heavily depend on the progress of a cold front to our west. Am inclined to think the weekend will be mostly dry due to the lack of forcing and weak shear. PoPs range from slgt chc to chc, with the highest values to the west (closer to the front) and PoP values on Sunday higher than on Saturday since the front should be a bit closer. Coastal areas will see slightly cooler temperatures and should have the lowest shower/storm chances, so current indications are for a good weekend at the beaches. Monday-Tuesday... Continued warm and humid weather is expected with similar temperatures to over the weekend. The cold front should continue to crawl eastward, and as a result my inclination is that shower and storm chances should generally be higher for these days than over the weekend. But much remains to be determined owing to uncertainty over the timing of the front. && .AVIATION /05Z WEDNESDAY THROUGH SUNDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight... Mostly clear with some patchy fog at rural sites possible through sunrise. Otherwise VFR. Light N/NW winds. Moderate confidence. Wednesday... VFR with a few daytime Cu/Sc. Winds N to NW around 10 knots during the morning then light and variable by late afternoon. High confidence. Outlook... Thursday-Thursday night... VFR. Winds east or southeast at 5 to 10 kt, possibly becoming light and variable overnight. High confidence. Friday-Friday night... Mainly VFR. Southeast wind 5 to 10 kt, becoming more southerly overnight. Moderate confidence. Saturday-Sunday... Mainly VFR. Scattered afternoon showers and thunderstorms possible mainly west of PHL on both days, with a slightly higher chance on Sunday. Southwest wind 5 to 15 kt. Low to moderate confidence.
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&& .MARINE...
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Tonight: SCA conditions continue on the ocean waters with gusts around 25 knots expected and seas up to 5 ft. Wind gusts have dropped below advisory thresholds over the bay with seas from 2 to 4 ft, thus the SCA has been allowed to expire here. Tuesday: Sub-SCA conditions expected as high pressure crests over the waters by late afternoon. NW winds 10 to 15 knots during the morning then decreasing by afternoon. Outlook... Wednesday night-Sunday... No marine headlines are anticipated. A couple of wind shifts through the period but gusts mostly 20 kt or less. Mainly dry weather expected. Seas 2 to 4 ft. Rip Currents... An underlying long period swell of 13 to 15 seconds, while not the dominant wave period, is expected to result in a moderate risk for the development of rip currents for Wednesday and Thursday.
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&& .TIDES/COASTAL FLOODING... Astronomical tides will increase over the next couple of days as we approach Thursday`s full moon. Offshore flow will prevail through Wednesday which should allow the decreasing surge values to partially cancel out the increasing astro tides. However some sites on the Atlantic and DE Bay could approach minor flood stage (but likely remaining below advisory) with Wednesday evening`s high tide cycle. The flow will shift onshore for a time starting Thursday and consequently this period may need to be monitored for additional minor coastal flooding concerns. && .PHI WATCHES/WARNINGS/ADVISORIES...
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PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EDT early this morning for ANZ450>455.
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&& $$ Synopsis...O`Brien Near Term...Davis/Fitzsimmons/O`Hara Short Term...O`Brien Long Term...O`Brien Aviation...Davis/Fitzsimmons/O`Brien/O`Hara Marine...Davis/O`Brien/O`Hara Tides/Coastal Flooding...WFO PHI

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