Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KPHI 220108 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 908 PM EDT Mon May 21 2018 .SYNOPSIS... A weak area of low pressure will develop and move eastward across the Saint Lawrence Valley Tuesday through Wednesday bringing a weak cold front through the region late Tuesday into early Wednesday. High pressure builds in behind this exiting system through the end of the week. Uncertainty then becomes rather high for the next potential system to affect our area by Memorial Day weekend; however, it does appear that another frontal system will impact the region bringing unsettled weather. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
-- Changed Discussion --
High pressure remains just offshore with the mid-level ridge across the central Appalachians. As this ridge gradually breaks down overnight, cloudiness will increase across the area. With added low-level moisture within in a southeasterly flow regime, the approach of a mid-level shortwave, and more favorable jet dynamics, maintained a chance of showers in the forecast over the eastern shore of Maryland and northeast of the fall-line. Despite the convection to the south and west, the atmosphere remains stable north of the front, so do not expect thunder tonight. Updated hourly temperatures and dew points with LAMP and continuity, otherwise, remainder of forecast is on track. Previous Discussion... A very nice afternoon in progress across the region, however it is cooler toward the coast due to a breeze off the ocean and a sea breeze front making inland progress. An upper-level trough over the Midwest is forecast to gradually shift eastward tonight and be in the Great Lakes region by daybreak Tuesday. Meanwhile, a west-east ridge is across the southern states to the southwestern Atlantic ocean. This will allow surface high pressure to gradually shift eastward and become more centered off the southeastern New England coast by daybreak. As the upper-level trough shifts eastward, a weak area of low pressure will shift eastward as well and be located near the eastern Great Lakes as we approach daybreak. A frontal zone to our south will gradually shift northward as a warm front, however this feature looks to remain south of our area through tonight. The flow aloft is more zonal therefore short wave energy will easily roll eastward especially across our northern areas later tonight. As the flow increases some out of the southwest in the lower levels, warm air advection increases. The bulk of this through the night is forecast to be focused to our west, however the leading edge of it nears northeast Pennsylvania late tonight. There is also a surge in theta-e advection especially late tonight. In addition, parts of our region start to be within the right entrance region of an upper- level jet. This all leads to some increase in lift later tonight and clouds will increase, therefore some showers should arrive especially in parts of eastern Pennsylvania. We cannot rule out a few showers in Delmarva near the incoming surface front, however the greater concentration of showers toward morning should be near northeastern Pennsylvania. As a result, focused the highest PoPs there late tonight. The hourly temperature, dew point and wind grids were adjusted based on the latest observations, then the LAMP and some hi-res guidance was blended into this evening to assist with trends. Low temperatures are mostly a blend of continuity and MOS.
-- End Changed Discussion --
&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... An upper-level trough is forecast to amplify some across the Great Lakes region during Tuesday. This will drive weak surface low pressure northeastward, with a warm front lifting into our area and then a weak cold front approaching from the west late in the day. An area of warm air advection ahead of this system will also help in advecting more moisture across our region within a south-southwest flow. An initial area of showers in the morning especially north and west of the I-95 corridor should lift northeastward by early afternoon as the initial surge in warm air/theta-e advection occurs. A stronger short wave is forecast to move across the area in the afternoon, and this should result in renewed shower development from west to east. The surface flow looks to remain more backed from the southeast during the day, keeping the low-levels more stable. Some of the forecast soundings show the potential for decent elevated instability especially toward late afternoon, with the better chance of some surface-based instability across portions of Delmarva. The shear looks to be 30 knots or less, however wind profiles are such that convection in the afternoon may develop into some bands from the west. There is some drier air aloft, and therefore if convective cores are able to be strong enough then locally gusty winds are possible especially in the west/southwest zones which is further away from the cooler ocean influence. It appears that an abundance of clouds though will be around for much of the region through the day with a lack of a true warm sector surging northward. As a result, high temperatures will be on the cooler side. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Main challenges in the long term are the residual precipitation chances Tuesday night and very poor model continuity/agreement this weekend and beyond. Vort max will be moving through New England on Tuesday night with a predecessor perturbation moving off the Mid-Atlantic coast during the evening. Scattered convection should be ongoing in the central/southern Mid-Atlantic, possibly affecting central and especially southern portions of the CWA Tuesday evening. Convection should also exist along the main cold front to our northwest, but most of the high-resolution guidance indicates these storms will dissipate as nocturnal cooling develops and the stronger ascent progresses north/northeast of the region by Tuesday night. However, there remains some debate on when this will occur, with the WRF-NMM and RGEM suggesting more prolonged convection during the evening/night along this front, making it well into northern/central portions of the CWA. The 12Z NAM Nest is not as optimistic, but does suggest storms may persist into the Poconos and vicinity at least. Coarser guidance is more aggressive in producing QPF in much of our area during the evening, and with large-scale lift and considerable elevated instability remaining well into the evening hours, felt that keeping PoPs at least in the chance category was wise. Included some likely PoPs north of the Mason-Dixon Line, with the more aggressive coarse output along with modest support from mesoscale models enough to boost PoPs. The environment is not altogether unfavorable for a strong storm or two given the presence of 1000+ J/kg of MUCAPE in the evening, dry air aloft, and some potential cold-pool/linear organization. The shear remains weak through the evening, though I would not be too surprised to see small hail with the strongest cores. This threat should be short-lived, as a nocturnal lowering of instability should preclude a more substantive/longer-duration threat. The cold front is expected to move through the area by Wednesday morning. High pressure will build into the Northeast for the rest of the week, setting up a dry and fairly pleasant period. Midlevel ridging will develop to our west, but the Northeast will remain on the periphery of eastern Canada troughing, so temperatures will not get too out of hand during this period (though they should be a few degrees above seasonal averages, in general). Kept the temperature forecast close to a model consensus blend through this period. Model agreement deteriorates rapidly this weekend, as the 12Z GFS deepens a reinforcing trough over eastern Canada, with the ECMWF/CMC maintaining more progression. This has ramifications on the ejection of a western U.S. upper low and the evolution of the low trudging northward in the Gulf, not to mention the strength/speed of a cold front and upstream surface high pressure associated with the eastern Canada trough. Owing to the stronger/deeper trough, the GFS blasts a cold front through the area this weekend, bringing a cooler end to Memorial Day weekend. Meanwhile, the CMC/ECMWF are considerably slower with this front, leaving the Mid-Atlantic/Ohio Valley in long- duration/moist southerly fetch as the Gulf low approaches the Mid-South and Southeast. These models are much closer to each other than the GFS, so weighted the forecast much more strongly to these solutions Saturday onward. This would lead to increasing chances of showers/storms Saturday onward (especially Sunday and Monday) and temperatures remaining above seasonal averages, though I did trend them downward somewhat Sunday and Monday for at least a nod to the GFS (and to the increasing precipitation chances). This period bears watching, not only for the strong model disagreement but also for the potential for heavy rainfall, should the ECMWF/CMC solutions be in the more- correct camp. && .AVIATION /01Z TUESDAY THROUGH SATURDAY/...
-- Changed Discussion --
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR with an increase in clouds and gradual lowering. Light and variable winds at most terminals. Tuesday...VFR ceilings lowering to MVFR (locally IFR possible late in the day). Some showers and perhaps a few thunderstorms, especially in the afternoon. South or southeast winds less than 10 knots. Low confidence regarding the timing of the MVFR ceilings. Outlook... Tuesday night: Sub-VFR conditions possible with a chance for showers/storms, especially in the evening. South winds becoming west/northwest as a cold front moves through the area, with speeds 5 to 15 kts (possibly with some gusts near the frontal passage). Low confidence. Wednesday through Friday: VFR, though patchy fog may occur nightly in rural/valley locations. Winds generally northwest Wednesday and Thursday (10 to 15 kts during the day; below 10 kts at night) becoming south/southwest Thursday night and Friday (probably with some gusts 20+ kts on Friday). Moderate confidence. Friday night and Saturday: Predominantly VFR, though chances for showers/storms begin to increase on Saturday. Winds south/southeast 10 to 15 kts with higher gusts during the day Saturday. Low confidence.
-- End Changed Discussion --
&& .MARINE... The conditions are expected to remain below Small Craft Advisory criteria through Tuesday. An onshore wind this afternoon is generally below 10 knots, however some gusts up to 20 knots are occurring nearshore due to a sea breeze that has moved inland. A southeasterly wind is anticipated tonight through Tuesday with speeds around 10 knots. Some showers and possible thunderstorms on Tuesday, especially in the afternoon. Outlook... Tuesday night: Chance of showers/storms in the evening, with potential for some gusty winds and locally higher waves. Outside of storms, sub-advisory winds/seas expected. Wednesday through Friday: Sub-advisory winds and seas and fair weather expected. Friday night and Saturday: May see marginal advisory conditions develop as winds become southeasterly and strengthen. Predominantly fair weather expected, though the chance of storms will be increasing gradually during the weekend. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Staarmann Near Term...Gorse/LF Short Term...Gorse Long Term...CMS Aviation...CMS/Gorse/LF Marine...CMS/Gorse/LF is the U.S. government's official web portal to all federal, state and local government web resources and services.