Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 251837 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 137 PM EST Sat Feb 25 2017 .SYNOPSIS... A strong cold front will move across the region from late this afternoon through early this evening. High pressure then builds south of the region on Sunday before moving offshore on Monday. Low pressure approaching from the west will lift a warm front through the region on Tuesday. The aforementioned low will track from the Great Lakes into northern New York, and then into eastern Canada on Wednesday, and this will drag a cold front through the region. Several weak disturbances may impact the Northeast for the end of the work week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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130 pm update: Forecast updated to include severe thunderstorm watch 39 for portions of southeast PA, southwest NJ, the Eastern Shore of MD, and northern/central DE until 8 pm. Grids updated to include isolated damaging wind gusts. Environment continues to become more favorable for severe storms, with storms rapidly intensifying from Harrisburg, PA, southwestward to Charlottesville, VA. These storms are moving into an increasingly unstable air mass, with MLCAPE 500-1000 J/kg and effective bulk shear 40-60 kts. With continued heating in the low levels, near-surface lapse rates are continuing to increase, with 7-7.5 C/km readings reaching the I-76 corridor. Though thermodynamic profiles do not appear favorable for large hail, some subsevere hail is possible with the strongest updrafts. 1230 pm update: Environment is becoming more unstable this afternoon as diabatic heating has increased with the partial clearing observed in much of the area this morning. Model analyses indicate MLCAPE 500-1000 J/kg generally south of I-76 (and away from the coast) with effective bulk shear 40-60 kts. Though low-level lapse rates have been slow to improve this morning, some 7+ C/km readings are now present in southeast PA. As the cold front nears the region this afternoon, scattered storms should continue to intensify in the increasingly unstable air downstream. Already have showers developing in the southern Poconos, with a clear uptick in convection in central PA during the past hour. With fast unidirectional flow oriented parallel to the front, one or more lines of storms should quickly organize as it/they move east into the area along/ahead of the front. Downward momentum transfer of this fast flow remains possible with the stronger updraft cores, at least in isolated instances, especially if surface heating continues this afternoon (improving low-level lapse rates). Made substantial modifications to the forecast this afternoon. Increased temps across the area a bit more, changed PoPs to account for latest trends, and kept mention of fog in coastal NJ, where southeast winds will likely keep patches/areas of fog around before the storms move in. 930 am update: Boundary layer mixing has successfully eroded most of the lower clouds and fog in far southeast PA and much of inland NJ; however, patchy dense fog continues across coastal NJ. Coastal webcams showing Seaside Heights shrouded in fog, with at least patchy fog also visible near Atlantic City, Belmar, Tom`s River, and Cape May. Though improvement is expected through the morning hours, nudged temperatures downward in coastal areas just a touch through the day as the consistent southeast fetch will likely advect cooler air over the ocean waters inland and may keep low clouds in place through much of the day. Temperatures were running warmer than forecast in areas where clouds have eroded, but suspect the warming trend will be brief given the increasing clouds moving in from the west. Tweaked hourly temperature grids to account for the current trends, but only modified max temps a degree or two in most locations, as the expected slowing trend of the warming will likely preclude temperatures from exceeding guidance excessively, as has been observed several times in the past week. Regarding the convective potential this afternoon, not much has changed with the forecast thinking. High-resolution guidance has been quite consistent with the timing of the main line of convection generally from 20Z to 01Z from west to east. Instability continues to look marginal but sufficient for updraft maintenance as the cold front approaches the region this afternoon. Within an environment of favorable deep-layer shear and unidirectional wind profiles with directions mostly parallel to the front, a line of convection will likely progress through the area. The line may contain little lightning given the marginal/thin CAPE profile, but with steep low-level lapse rates underneath fast midlevel flow, downward momentum transfer of these strong winds should occur with relative ease during the afternoon hours. With favorable low-level helicity in vicinity of the front, bowing structures and even an embedded rotating storm/mesovortex cannot be ruled out. Previous discussion below... Another rather mild early morning across the area. Several items to discuss today as we have more warmth ahead of a strong cold front. Some varying amounts of low clouds with patchy fog this morning, with much of this (at least low clouds) across portions of eastern Pennsylvania and northern New Jersey due to more of an upslope component of the low-level flow. Some thicker fog/stratus has been noted along portions of New Jersey coast and near the far upper portion of Delaware Bay near Wilmington. Some guidance keeps low clouds across much of the area all day, however the thinking is that enough mixing and boundary layer heating occurs to erode this by early afternoon. Some fog was kept from northern Delaware eastward to the New Jersey coast. We now switch to the convective potential with an incoming strong cold front this afternoon. A strong upper-level trough will continue to move from the Midwest to across the Great Lakes region today. This trough is forecast to start taking on a negative tilt as it reaches the eastern Great Lakes later today. This will drive strong height falls eastward this afternoon along with a strengthening wind field. The forecast soundings indicate deep unidirectional flow with some veering below about 900 mb. There should be enough instability to produce some thunder this afternoon, however strong forcing along the leading edge of the height falls along with robust short wave energy should result in a convective line. This line will probably be more low-topped, however enough instability is expected inland to produce some lightning. Given the deep unidirectional flow contributing to strengthening shear, some bowing segments are possible within the line. Momentum transfer of stronger winds from aloft down to the surface can result in locally damaging gusts, especially with any bowing segments along with stronger cores due to water loading of the downdrafts. While some smaller hail cannot be ruled out, instability should be limited as soundings show more of a thin CAPE profile. We will continue to carry gusty winds in the forecast with thunder (may need to add damaging winds later), with the convective line generally moving from west to east across the area from about 20-00z. The intensity of the convection should lessen as it reaches the coast due to the cooler ocean influence. Southerly flow ahead of the cold front will result in another very warm day, with several areas already starting rather warm. Highs will be able to over achieve expectations if more clearing occurs ahead of the cold front during peak heating. High temperatures were a MOS/continuity blend but then raised some away from the coast given a warmer start.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... A negative tilt upper-level trough will be lifting across New England tonight, with low pressure tracking across eastern Canada. This will push a strong cold front offshore early in the evening with and showers and thunder quickly coming to an end. Strong cold air advection occurs immediately behind the cold front through about the first half of the night. This combined with stronger flow should create a gusty west to northwest for a time. As the colder air advects into the area, it will also be drying out however some forecast soundings indicate that as a subsidence inversion develops, there will be some lingering moisture trapped for a time. This may result in bands or areas of stratocumulus for awhile especially across the northern zones. It will be noticeably colder tonight with an added wind chill to the mix, reminding us that it is still winter. Low temperatures were mostly a MOS/continuity blend. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Surface high pressure settles over the Southeast U.S. on Sunday as an upper trough builds across the Northeast U.S. Conditions will be dry, and temperatures return to normal throughout the forecast area, with highs topping off in the 30s in the Poconos, and generally in the low to mid 40s for much of SE PA and NJ. Highs climb into the upper 40s for the Delmarva. Upper flow flattens out Sunday night as both the upper trough and the surface high move offshore. Return flow sets up on Monday, allowing for temperatures to climb back up to well above normal levels, generally topping off in the 50s to low 60s. From there, unsettled weather on tap for the rest of the new week. Low pressure develops over the central U.S., and a warm front will develop ahead of that low. That warm front will lift north through the region on Tuesday, and isentropic lift associated with that front will touch off some rain across the area. Low pressure over the central U.S. will move through the Great Lakes and Ohio Valley on Wednesday, and deep SW flow ahead of that low will usher an unseasonably warm and humid airmass into the region. Highs may be around 70 for most of SE PA, SW NJ, and the Delmarva. As that low tracks to the north and east, it will drag a cold front to the east, and that front will pass through the region on Thursday. The best chances for precip across the region will be on Wednesday afternoon as one area of low pressure passes well north and east of the area, and then again on Thursday as a second low passes north and east of the area, but this low will drag a cold front through the region. Temps cool off a bit on Thursday, with highs generally in the 40s and 50s, but then temps return to normal on Friday as much cooler air spreads throughout the Northeast. Will carry slight chance PoPs for Friday as several shortwaves pass through the region. Some snow/rain possible in the Poconos, but temps will be warm enough for all rain for the rest of the CWA. && .AVIATION /19Z SATURDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. For the 18Z TAFs...Mostly VFR before storms move in this afternoon. Timing looks to be 19-23Z for KRDG/KABE, 20-00Z for KPHL/KPNE/KTTN/KILG, and 22-02Z for KMIV and KACY. Sub-VFR VSBYs/CIGs expected as the precipitation moves through, with gusty/erratic winds 30-45 kts possible in proximity to the stronger storms. Winds will quickly veer to westerly or northwesterly after shower/storm passage late this afternoon/evening, with winds gusting 20-30 kts through tomorrow afternoon before diminishing. Outlook... Sunday...VFR. W-NW winds around 15 KT with 20-25 KT gusts. Sunday night...VFR. Winds become SW 5 KT or less. Monday...VFR. SW winds 5-10 KT. Monday night through Wednesday...Periods of rain and occasional showers with sub-VFR conditions. Best chances during the day Tuesday, and then again on Wednesday. S-SW winds, generally around 10 KT. Low confidence on forecast and timing. && .MARINE... 1230 pm update: Dense fog continues to plague the waters off the coast of New Jersey. Extended the dense fog advisory through 6 pm. Winds are slowly increasing over the waters as well, so current SCA looks good. Rest of forecast unchanged. 930 am update: Webcams continue to show dense fog over the coastal waters of New Jersey. Expanded the dense fog advisory to cover all marine zones adjacent to the coast through noon. Otherwise, forecast is in good shape. Previous discussion... A narrow area of stratus and/or fog continues right near the coast of New Jersey with this mostly confined from portions of Ocean County northward. Farther south, the low-level trajectory suggests less development however some fog has been reported. The dense fog advisory this morning remains in effect for just the northern two Atlantic coastal waters zones. Otherwise, a southerly flow will continue today and the increase may be slowed due to much warmer air over cooler water. This will increase especially later today as a strong cold front arrives, with colder air quickly arriving tonight in its wake. It is tonight when stronger winds are expected as mixing becomes much more efficient. There could be some low-end gale force gusts tonight, however it looks marginal and potentially short in duration therefore we will maintain the Small Craft Advisory. Seas will also respond to the increasing wind, however this was slowed a bit for this morning. A line of showers and embedded thunderstorms will move across the area late this afternoon and especially early this evening. These may weaken some as they encounter the marine influence, however some local gusts of 30-40 knots are possible along portions of the line. Outlook... Sunday...SCA conditions continue on the waters, subsiding to sub-SCA criteria late. Sunday night and Monday...Sub-SCA conditions likely. Above average confidence. Monday night through Wednesday...Generally sub-SCA expected, though winds and seas may approach criteria after potential warm frontal passage Tuesday night or Wednesday. Periods of rain/showers likely, especially during the day Tuesday and again Wednesday afternoon. Very low confidence. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Dense Fog Advisory until 6 PM EST this evening for ANZ450>453. Small Craft Advisory until 6 PM EST Sunday for ANZ450>455. Small Craft Advisory from 6 PM this evening to 6 PM EST Sunday for ANZ430-431. && $$ Synopsis...MPS Near Term...CMS/Gorse Short Term...Gorse Long Term...MPS Aviation...CMS/Gorse/MPS Marine...CMS/Gorse/MPS

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