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 300232 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 932 PM EST Tue Nov 29 2016 .SYNOPSIS... Strong low pressure is forecast to pass across the Great Lakes tonight and Wednesday. It is expected to move into southwestern Quebec on Wednesday night and it should pull a cold front through our region at that time. A secondary low is forecast to develop over southern New England on Wednesday night before reaching the Canadian Maritime Provinces on Thursday and Thursday night. High pressure is anticipated to build into our region from the west for Friday and the weekend. Low pressure may approach from the southwest and west early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
-- Changed Discussion --
The million dollar question remains. Patchy or widespread fog? Stat guidance is fog heavy while higher resolution forecast soundings are suggesting more stratus. Looking to our southwest, we can see what will happen is we get the clearing. Portions of LWX`s CWA is experiencing dense fog with vsbys down to zero. While bases have lifted along the coastal plain, cloud cover hasn`t budged since this afternoon...almost back to Harrisburg. For now, we`ll continue with mentioning patchy dense fog and not run with a Dense Fog Advisory for a more widespread event. Previous discussion... Low pressure will continue to weaken over the Upper Midwest through tonight. A cold front arcs southeastward from the low around the Great Lakes and then southward along the spine of the Appalachians. A warm front was located in the Mid-Atlantic region close to the I- 95 corridor. While a portion of warm front has progressed northward in the coastal plain this afternoon, the inland portion of the boundary has gotten hung up north and west of the fall line. Light winds and temperatures still in the mid 50s are observed in NE PA and NW NJ while stations in Delmarva and S NJ were seeing a gusty south winds with temperatures in the 60s. The steady rain that has affected the area for much of the day is quickly exiting the area during the mid afternoon as the southerly low-level jet and enhanced lift is shifting northeastward into New England and off the coast. Leftover light showers and sprinkles will hang around through early this evening, particularly over SE PA and the coastal plain of NJ and Delmarva. Low clouds will result in overcast skies through early this evening. However, clearing was noted on visible satellite to our west across W PA. Just about all of the model guidance shows the column drying out above the boundary layer later this evening as mid-level subsidence strengthens. The million dollar question for tonight is how much drying will occur in the lowest 1500 ft above the surface. Some of the guidance (e.g., HRRR/RAP) indicate enough drying in the boundary layer for breaks in the stratus to develop tonight. With light winds and a wet ground, any breaks in the cloud cover would promote the development of fog. The forecast area is at risk for dense fog overnight and early Wednesday morning if persistent clearing were to occur. Other models such as the NAM and NCEP WRF ARW/NMM favor low clouds to persist tonight, which would limit the extent of the fog. We have added the mention for the potential of dense fog in the HWO.
-- End Changed Discussion --
&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... Low pressure will finally progress eastward across the Great Lakes region on Wednesday. The attendant cold front will likewise move eastward, crossing the Appalachians during the afternoon. The southwesterly low-level jet ahead of the main trough will restrengthen during this time. The warm front over the area right now is expected to stall through Wednesday morning. The interaction between the approaching low-level jet streak and the front will enhance isentropic lift over the area. Accordingly, we expect rain to move back into the area from southwest to northeast during the morning as another upper shortwave disturbance approaches the region. The rain should taper off from south to north during the early afternoon as the boundary begins advances northward as a warm front. Models indicate another band of precipitation ahead of the cold front to move into western portions of the CWA late in the afternoon. There is a potential for isolated thunderstorms embedded in this line. A large north to south temperature gradient is forecast across the forecast area tomorrow with highs ranging from the lower 70s in southern DE to mid 50s from I-80 north. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... A mid level low is forecast to be located over the Great Lakes on Wednesday night before reaching southern Quebec and Atlantic Canada late in the week. Subtle ridging may occur over our region during the weekend with a short wave trough possibly passing overhead around Sunday night. More substantial ridging is anticipated for early in the new week. The ECMWF has joined the GFS in its later timing in ejecting the system out of northern Mexico at that time. It continues to appear as though the cold front will pass though our region around midnight on Wednesday night. Temperatures will remain in the 50s and 60s until the front arrives. The lift and abundant moisture ahead of the front will likely result in another round of moderate to heavy rainfall on Wednesday evening. However, the duration of the shot of moderate to heavy rain should be only a couple hours or so. As a result, additional rainfall amounts should be limited to about 0.25 to 0.50 inches in general. The enhanced lift associated with the approaching front and the low level jet ahead of it may result in localized thunder. We have maintained the mention of a chance of a thunderstorm at that time. Dry air and a return to near normal temperatures should follow for the late week period into the weekend. A west northwest flow may bring some cold advection stratocumulus at times, especially to our northwestern counties. We have mentioned a chance of rain showers and snow showers in eastern Pennsylvania and northern New Jersey for Sunday night associated with a passing mid level short wave. We have limited the precipitation type to rain showers in the south. It appears as though amounts will be light. High pressure is expected to regain its influence over our weather for Monday with dry conditions and near or slightly above normal temperatures expected. While the guidance is trending toward the later arrival of our next precipitation system, there remains some spread in the details. We will indicate a chance of rain for Tuesday. Based on expected temperature profiles, there could be a period of freezing rain in the north but it is still a low confidence forecast. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
-- Changed Discussion --
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. More MVFR conditions across the terminals this evening than IFR. The expectation is for more IFR as we go through the overnight. Terminals west of the I95 corridor are locked in with a thicker and lower cloud cover. Bases have risen the last few hours across the coastal plain. Mixed signals with the models this evening...a few suggesting more stratus, a few suggesting more fog...and dense fog. We`ll stick with the mention of a 1/2 mile toward dawn, but this will depend on how much clearing we get. For tomorrow morning after sunrise, I was more pessimistic than last forecast with a good model consensus. Previous discussion... The steady rain is exiting the area late this afternoon. However, low clouds will persist thru the first half of evening, resulting in IFR or low-end MVFR CIGs. Breaks in the clouds are expected to develop from west to east between 03-06Z. Expect fog to form quickly after breaks occur. The fog could become dense if/where clearing becomes more widespread. There is still high uncertainty where this occurs and how widespread it will be. Have added a 4-h tempo group for SM and FG for all TAF sites sometime between 06-12Z. If breaks in the low clouds don`t develop, IFR or LIFR CIGs will still impact the terminals tonight. Future TAF updates will hopefully be able to hone in on the extent of the fog and low clouds. Rain moves back into the area from SE to NW Wednesday morning. Expect VSBYs and CIGs to improve slightly once the rain arrives but low-end MVFR or IFR restrictions will persist thru at least 18Z. There is a brief window for improvement during the afternoon before showers arrive ahead of an approaching cold front late in the day. Conditions may support LLWS tomorrow morning but shear values are not as impressive as today. Light E winds under 10 kt at the surface and a 35-40 kt SE low-level jet about 1500 ft AGL arrives around 15Z Wednesday. Directional shear would increase LLWS values briefly between about 15-18Z as the low-level jet winds veer out of the S while surface winds remain backed out of the east and light. LLWS may need to be added in future TAF amendments. OUTLOOK... Wednesday night...MVFR and IFR conditions in moderate to heavy rain during the evening with a chance of thunder. A strong cold front should arrive around midnight followed by clearing and improvement to VFR. Low level wind shear is possible in the evening due the the expectation of a 40 to 50 knot low level southwesterly jet at that time, especially affecting airfields from KTTN, KPHL and KILG eastward. Thursday through Sunday...mainly VFR.
-- End Changed Discussion --
&& .MARINE...
-- Changed Discussion --
The ocean waters are back to a Small Craft Advisory. The gales have been dropped. No headlines for the DE Bay at this time. A long duration SCA is expected going forward for the ocean. Beach cams are showing good visibility at this time. Previous discussion... Upgraded the Small Craft Advisory to a Gale Warning for the coastal waters of DE and NJ through 8 PM this evening. Upstream buoys to our south (near the SE VA coast) have reported southerly winds gusting 35-40 kt. Expect intermittent gale force gusts to develop farther up the coast late this afternoon and early this evening as the low- level jet moves directly overhead. The Small Craft Advisory for the DE Bay was extended through 8 PM. Once the GLW expires, a SCA will need to be added for the coastal waters thru the remainder of tonight and for Wednesday. OUTLOOK... Wednesday night...Wind gusts of 25 to 30 knots are possible initially from the southwest then from the west following the passage of a cold front after midnight. There could be a brief surge of gale force gusts on either side of the frontal passage. Thursday through Saturday...West to northwest wind gusts of 25 to 30 knots are possible through the period. Saturday night and Sunday...No marine headlines are anticipated.
-- End Changed Discussion --
&& .HYDROLOGY...
-- Changed Discussion --
Much needed rain across the region through Wednesday night. The first shot of rain is winding down this evening, but not before producing 1.00-2.00 inches in parts of eastern PA and northern NJ and 0.25 to 1.00 inch elsewhere in our CWA. A PNS has been issued. It had been rather dry, therefore there was just some ponding of water on roads during heavier rainfall rates along with some poor drainage flooding. Some poor drainage flooding was locally enhanced where fallen leaves clog storm drains. Much of tonight will tend to feature a lull before the next batch of rain arrives Wednesday. For Wednesday and Wednesday night...Additional rainfall is expected to favor the 0.50 to 1.00 inch range, a reduction from the previous forecast based on the latest guidance. We are not expecting main stem flooding or even smaller basin flooding since these rainfall amounts will be spread out over a 48 hour period (with tonight`s lull in between) and the antecedent conditions were rather dry. Poor drainage flooding is always a possibility especially during heavier rainfall rates but impacts from this type of flooding are generally minor, except for travel during the morning and evening commutes.
-- End Changed Discussion --
&& .CLIMATE... PHL should average 50 degrees for the month or 2.4 above the average of 47.6. It is estimated that the large positive departures (warmth) of today and Wednesday will warm the monthly departure by an estimated 0.3 to 0.9 degrees from the values calculated through the 28th. Meanwhile... Daily record High temperatures for Wednesday 11/30 where they appear vulnerable to being within 2 degrees of record. GED 74 - 2006 Daily record rainfall values for Nov 29 and 30 (less confidence on approaching records today, whereas on Wednesday, several sites may establish a new record rainfall for the date.) NOV 29 NOV 30 ACY 1.93-1963 1.25 1923 PHL 1.68-1971 0.96-1972 ILG 2.25-1945 1.28-1934 ABE 1.70-1935 0.86-1923 TTN 1.40-1987 0.76-1898 GED 1.47-1982 1.71-1987 RDG 1.69-1984 1.52-1987 MPO 2.71-2005 3.00-1923 && .PHI WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 6 AM EST Thursday for ANZ450>455.
-- End Changed Discussion --
&& $$ Synopsis...Iovino Near Term...Klein/Kruzdlo Short Term...Klein Long Term...Iovino Aviation...Iovino/Klein/Kruzdlo Marine...Iovino/Klein/Kruzdlo Hydrology...Staff/Kruzdlo Climate...Drag

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.