785 FXUS61 KPHI 290630 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 230 AM EDT Wed Sep 29 2021 .SYNOPSIS... High pressure centered over the Great Lakes early this morning will gradually build south and east into the region through the end of the week into the weekend. Low pressure with an associated frontal boundary is likely to affect the area late in the weekend into early next week. && .NEAR TERM /THROUGH TONIGHT/... A closed upper low across southeastern Quebec will drift slowly southward toward New England into tonight. This is part of a broader scale Omega blocking pattern with ridging building well to our west. Resulting prevailing flow will be from the north to northwest. At the surface, high pressure will slowly build into the region from the north through the period. Temperatures are forecast to climb into the upper 60s to around 70 degrees this afternoon (a few degrees below normal). Dewpoints are expected to mix down into the low to mid 40s resulting in a very pleasant early autumn day. There will be a north to northwesterly breeze around 10-20 mph, making it feel perhaps a bit cool at times. Skies will initially be mostly clear/sunny, but some cumulus is expected to develop into the mid-late afternoon and persist into the overnight period. The breeze will diminish this evening as mixing dwindles and temperatures are forecast to fall into the mid 40s to around 50 degrees (again a few degrees below normal). && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... An upper level low will continue to sit and spin over eastern New England into the Canadian Maritimes during this period while high pressure gradually builds in to the region from the north and west. This will result in cool, dry, and tranquil weather over the area. For Thursday, as a spoke of upper level energy rotates around the upper low this will result in a fair amount of cover but not expecting any precip. Highs will be mostly in the 60s except around 70 over southern Delmarva and only in the upper 50s over the southern Poconos. As the surface high builds into central PA Thursday night this will mean diminishing winds with clearing skies. As a result, expecting a chilly night with temperatures getting down into the upper 30s over the southern Poconos. Elsewhere, it should be mostly in the 40s for lows. This will give way to a bright and sunny Friday as the high moves in right over the area with continuing seasonally cool temperatures. Tranquil weather continues Friday night under high pressure but it shouldn't be quite as cool compared to Thursday night. && .LONG TERM /SATURDAY THROUGH TUESDAY/... In the big picture, there will be rex block type pattern that will continue to persist over eastern Canada through the first part of the weekend as an upper level low sits over the Canadian Maritimes and upper level ridging persists to the north over Hudson Bay into northern Quebec. This will keep high pressure locked into the area through Saturday with continuing sunny skies and temperatures returning to more seasonable levels. The big question beyond this time is how quickly this pattern breaks down and how this breakdown in the pattern evolves. There continues to be differences in the different forecast models and from one run to the next so forecast confidence for the period Sunday through Tuesday is a little below average. It appears likely though that the ridging near Hudson Bay eventually gets undercut but another broad upper level low or trough as it digs in somewhere near the central CONUS. Depending on where this new upper level low sets up it could turn unsettled over the area for the Sunday through Tuesday period as a deep layer SW flow sets up over the east. This could bring the area periods of rain/showers and storms. If the ridging over Canada remains stronger though a lot of the unsettled weather could get surpressed just to our south. Long story short, given the uncertainty, we stayed close to the NBM guidance which has chance POPs in the forecast for most of the area for each period Sunday through Tuesday. Temperatures during this time should generally be around average for this time of year. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR. Winds initially northerly around 5-10 kts will increase to 10-15 kts with gusts up to 20 kts around 12Z. Winds will turn more northwesterly after 18Z as scattered cumulus develops. High confidence. Tonight...VFR with scattered cumulus. Northwesterly winds around 5 kts or less. High confidence. Outlook... Thursday through Saturday...Mostly VFR expected. Northwest winds 5-10 knots or less, becoming west during the day Saturday. High confidence. Sunday-Sunday night...Generally VFR conditions with a chance of showers which may lead to lower conditions. West to southwest winds 5-10 knots. Low confidence. && .MARINE... Sub-advisory conditions expected through tonight. Winds around 10-15 kts with gusts up to 20 kts from the north then turning northwesterly this evening. Seas 2-4 feet. Outlook... Thursday through Saturday...Winds and seas are expected to remain below advisory levels through the period, although winds could gust around 20 knots at times and seas could approach 4 feet at times. Sunday...Small Craft Advisory level conditions possible as wave heights over the ocean waters increase to around 5 feet. Rip Currents... With a low period offshore swell and offshore winds, the rip current risk is forecast to be LOW through Thursday. Swells from Hurricane Sam will begin arriving late Thursday or Friday, which may increase the risk of rip currents to close out the Surf Season. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Fitzsimmons/Robertson Near Term...Staarmann Short Term...Fitzsimmons Long Term...Fitzsimmons Aviation...Fitzsimmons/Staarmann Marine...Fitzsimmons/Staarmann