797 FXUS61 KPHI 041054 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 554 AM EST Sun Mar 4 2018 .SYNOPSIS... The departing storm over the western North Atlantic will continue to drift out to sea today and Monday as high pressure builds into our region from the west. Weakening low pressure is forecast to move from Iowa to the Great Lakes and Ohio River Valley on Tuesday. Another low is expected to develop along the North Carolina coast on Tuesday night. The low is anticipated to strengthen rapidly as it moves northeastward, passing near Cape Cod late on Wednesday night before reaching Nova Scotia on Thursday. High pressure is expected to gradually build into our region from the west on Friday and Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The intense low in the western Atlantic is still influencing our sensible weather, with strong north to north-northwest winds and a stubborn low-cloud deck overnight across the area. Infrared satellite imagery indicates the low clouds are beginning to scatter somewhat just west of our CWA (with very evident topographical waves in the northerly near-surface flow), and think this will slowly spread eastward through the day. Nevertheless, models have trended slower with the gradual dissipation of the lower clouds today, so have made the sky grids a little more pessimistic early this morning. This cloud deck (along with the strong north winds) has hampered cooling tonight, so hourly temperatures were raised through daybreak then lowered a little after daybreak to exhibit a more muted fall and rise in temperatures during this period. Besides determining the onset/rate of clearing today, the other main challenge is the maximum temperature forecast. The rather stagnant atmospheric pattern suggests little change to max temperatures compared to yesterday. Regarding this, the MAV MOS was noticeably warm compared to reality yesterday, often with errors in excess of five degrees. Weighted the forecast heavily to ECS MOS as well as hi- res 2-m temperature input, as verification with ECS has been considerably better and the hi- res input will likely account for the muted diabatic heating/cooling early in the near-term period better than statistical guidance. As a result, forecast highs were lowered about 2-4 degrees, with projections in the low-mid 30s in the southern Poconos to the mid to maybe upper 40s in the urban corridor and Delmarva. With surface high pressure continuing to seep southward from Canada but very gradually given the blocking pattern in the Atlantic, the pressure gradient will ease only very slowly. As such, expect another breezy day, with winds 10 to 20 mph and gusts 25 to 30 mph at times. Cannot rule out some flurries in the southern Poconos today, but currently think the odds are too low for mention. Nevertheless, this is hinted at in NAM/GFS, so it is at least worthy of mentioning in this discussion. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... A midlevel vort max will be racing south-southeastward this evening near the Mid-Atlantic coast. This will bring a weak cold front through the area late in the day and this evening. Passage will be dry, but tonight's temperatures will likely be a little bit cooler given the source region of the upstream air. Additionally, skies will likely be clearer, so nocturnal radiational cooling will be somewhat enhanced. However, winds will still be elevated (and may gust a little bit near the passage of the weak front), so temperatures will likely keep from tanking (except possibly in the most protected valley locations). For overnight lows, the MET MOS has been much too cold recently, though its performance is better in clearer conditions. As such, I did incorporate its values into the forecast, though with less weighting than ECS/MAV MOS. Operational models do break out some precipitation in advance of the vort max, but well offshore. Thus, PoPs are unmentionable in the current forecast. Nevertheless, will need to keep an eye on trends with the vort max, as a westward displacement and/or more pronounced lift will likely lead to increased clouds at the very least, especially in the eastern CWA. && .LONG TERM /MONDAY THROUGH SATURDAY/... High pressure is forecast to build across our region from west to east for Monday into early Tuesday. We are anticipating dry weather and near seasonable temperatures at that time. The expected weather system for the middle of the new week has some of the same characteristics as the storm that affected our region on Friday. However, the system should not be as robust as its predecessor. Low pressure approaching from the Great Lakes and the Ohio River Valley on Tuesday is forecast to weaken. Energy is expected to shift to another low developing along the North Carolina coast on Tuesday night. The coastal low is anticipated to intensify rapidly as it moves northeastward. It should pass to our east on Wednesday and it is forecast to track near Cape Cod on Wednesday night. Sustained wind speeds on land are expected to be in the 10 to 20 mph range with gusts perhaps around 30 mph, not nearly as strong as our last storm. Precipitation totals may range from 1.0 to 1.5 inches with locally higher amounts. At this time, the guidance is focusing the heaviest precipitation in our region on the coastal plain. The most problematic aspect of the forecast should be the precipitation type. As the precipitation moves into our region late on Tuesday, the boundary layer should be warm enough to support rain at most locations. However, precipitation rates are expected to increase on Wednesday and the dynamic cooling wild card will come into play. Wherever the heavier bands of precipitation develop will likely dictate where the rain will change to accumulating snow. There remains a great deal of uncertainty at this point. The area of low pressure is anticipated to continue moving to our northeast late in the week and it should allow dry air to build in from the west at that time. Another area of low pressure approaching from the west may bring some precipitation for next weekend. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR with north-northwest winds 10 to 20 kts with gusts to 25 kts or so. A stubborn cloud deck around 5000 feet has been slow to erode, and forecasts have trended slower with this erosion. However, CIGs should remain VFR. High confidence in VFR/winds, but low confidence on when skies will scatter/clear. Tonight...VFR with north-northwest winds 10 to 15 kts with gusts to 25 kts, especially in the evening. Moderate confidence. OUTLOOK... Monday and Monday night...Mainly VFR. Tuesday...Mainly VFR. A chance of rain late in the day. Tuesday night and Wednesday...Conditions lowering to MVFR and IFR in rain and snow. Wednesday night...Conditions improving gradually to VFR with precipitation ending. Thursday...Mainly VFR. && .MARINE... Gales continue on Delaware Bay and (occasionally) on the far southern NJ and Delaware Atlantic coastal waters; however, winds are clearly sub-gale to the north. Have lowered the gale warning for the northern/central NJ waters, replacing it with a small craft advisory. Winds should diminish to advisory levels everywhere by late this morning (if not sooner). There is a chance of another round of gales this evening and tonight, especially in Delaware Bay and the southern New Jersey and Delaware Atlantic coastal waters. Confidence is too low to forecast this in the northern/central NJ coastal waters, but think this is probable to the south. Seas will remain quite elevated today, with 8-14 foot swells (with very long periods) continuing on the ocean. Continued beach erosion is likely. Waves of 3-7 feet are expected on Delaware Bay. OUTLOOK... Monday morning...North northwest wind gusts of 25 to 30 knots are expected. A Small Craft Advisory will be in effect for our ocean waters and for Delaware Bay. Monday afternoon and Monday night...Wind gusts should remain below the Small Craft Advisory criterion. However, wave heights on our ocean waters are expected to be in the 6 to 10 foot range. A Small Craft Advisory will be in effect for our ocean waters. Tuesday and Tuesday night...Wind gusts should stay below the Small Craft Advisory criterion. However, wave heights on our ocean waters are expected to be in the 4 to 8 foot range. Wednesday...Wind favoring the east to north quadrant with gusts of 25 to 30 knots expected. Gale force gusts are possible. Wednesday night...Northwest wind gusts of 25 to 30 knots. Gale force gusts are possible. Thursday...Northwest wind gusts of 25 to 30 knots. && .TIDES/COASTAL FLOODING... The coastal counties of New Jersey and Delaware are now under a Coastal Flood Advisory. It is in effect until 2:00 PM this afternoon. The prevailing wind flow has remained a little west of north and that has prevented surge values from getting much above 2 feet. As a result, the threat of moderate flooding has been on the decrease. We are anticipating widespread minor flooding with this morning's high tide. A few pockets of moderate flooding are possible, mainly in coastal areas of New Jersey from Long Beach Island up to Sandy Hook. The potential for minor flooding should decrease with subsequent high tide cycles as the strong storm over the western North Atlantic drifts away from our region. At this time, we are anticipating some residual minor flooding around tonight's high tide and one more advisory may be necessary. The need for an advisory for Monday seems unlikely at this point. The departing storm will continue to produce a pounding long period swell along the coasts of Delaware and New Jersey today. As a result, additional beach erosion is likely. No tidal flooding is expected for the lower Delaware River or for the upper eastern shore of Chesapeake Bay. && .EQUIPMENT... The Doylestown ASOS remains out of service. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...Coastal Flood Advisory until 2 PM EST this afternoon for NJZ012>014-020-022>027. DE...Coastal Flood Advisory until 2 PM EST this afternoon for DEZ002>004. MD...None. MARINE...Small Craft Advisory until 6 AM EST Tuesday for ANZ450>452. Gale Warning until 10 AM EST this morning for ANZ430-431- 453>455. && $$ Synopsis...Iovino Near Term...CMS Short Term...CMS Long Term...Iovino Aviation...CMS/Iovino Marine...CMS/Iovino Tides/Coastal Flooding...Iovino Equipment...Staff