193 FXUS61 KPHI 180735 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 335 AM EDT Fri Jun 18 2021 .SYNOPSIS... High pressure will dominate the sensible weather through today as it continues shifting slowly offshore. A weak cold front will approach the region Saturday night. The remnants of what may become tropical cyclone Claudette is forecast to pass just south of the region Monday of next week, then another cold front is anticipated to cross the area on Tuesday. High pressure will return for the middle of next week. && .NEAR TERM /THROUGH TONIGHT/... Another quaint late spring day is in store as high pressure drifts south and east of our region. This sets up a stronger return flow from the southwest, which will help increase temperatures this afternoon some five degrees or so compared to yesterday into the low to mid 80s. Meanwhile, an area of low pressure will traverse Ontario into Quebec, dragging a cold front southeastward across the Great Lakes towards the East Coast. Given the strength of the aforementioned surface high across the Southeast and an additional shortwave trough trailing the heals of today's shortwave and surface low, this front will only sag into our region before stalling into tonight. As the additional shortwave and surface low push eastward across the Midwest tonight, this cold front will stall and begin to lift northward as a warm front once again. While widespread precipitation is not expected in this process, we will need to watch as several QLCSs and/or MCSs develop across the Ohio Valley this afternoon. Should these become strong enough and push eastward towards our area fast enough, we will have a better chance for seeing overnight showers and thunderstorms. These will have to battle the stabilizing boundary layer with the loss of diurnal warning this evening and into the overnight. CAMs this morning have not been overly confident in this idea, however. && .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/... Synoptic overview: The short term period begins Saturday morning with zonal mid-level flow. A mid-level jet maxima associated with a shortwave trough will pass just north of the region throughout the day. Multiple mid-level vorticity maximums are forecast to pass over the area into the overnight associated with upstream convection across the Midwest and Ohio Valley. Mid-level heights then build Sunday as troughing begins to dig into the Midwest Sunday through Sunday night. At the surface, a cold front will approach the region late Saturday and likely stall out somewhere near or north of the forecast area overnight. The remnants of what may soon become tropical cyclone Claudette are forecast to remain south of the region, but will be making its approach from the Southeast Sunday night. Details: The prolonged southerly return flow will result in further increasing temperatures and dewpoints on Saturday with highs in the mid to upper 80s and dewpoints in the mid to upper 60s. Sunday should see similar temperatures and dewpoints as Saturday. Lows will be mostly in the mid to upper 60s Saturday night and Sunday night, with lows in the low 70s expected across much of Delmarva. An upstream MCS and associated vorticity maxima should be passing across the region by early Saturday morning. As a result, some scattered showers or thunderstorms may be possible during this period, especially along and north/west of I-95, along with increasing cloudiness. The details of this will depend on the MCS evolution upstream, however it appears unlikely that this activity would yield any severe risk given an unfavorable environment (little to no instability during the morning hours). By late Saturday afternoon or evening, we should see the development of new convection and/or the arrival of more organized convection from nocturnal activity upstream across the Ohio Valley or Appalachian regions, potentially in multiple rounds. Instability, while not particularly high, will rise to near or over 1000 J/kg (MLCAPE) across some portions of the region, even persisting into the evening/overnight hours. Deep layer shear will be supportive of organized severe weather with values of 35-45 kts. Meanwhile, a cold front will be approaching the region from the north and we should see some convectively enhanced vorticity maxima pass across the region from upstream convection as well. Long story short, Saturday could be active with potentially multiple rounds of convection which could last into the early overnight hours, but the details and confidence in timing of any hazards remains low. At this juncture, I believe the greatest threat with any organized storms would be damaging winds given the setup (northwest flow), but again confidence on the details is low. This setup will largely depend on how the upstream convection plays out later on today and tonight. SPC continues to highlight our entire area in a Marginal Risk of severe thunderstorms, but this may need to be upgraded to a higher risk level at a later time if and when the details on timing and placement become clearer. Sunday should be mostly quiet with prevailing westerly flow and rising heights aloft to help suppress any new convection from developing. && .LONG TERM /MONDAY THROUGH THURSDAY/... The long term period appears to remain largely active, especially into early next week. The biggest forecast challenge and uncertainty will be the potential impacts from what may soon become tropical cyclone Claudette in the Gulf of Mexico. Guidance still varies a good bit with the solutions to this currently very weak and ill- defined system. The biggest question mark is how it interacts with the trough digging into the Midwest once over land Sunday and into Monday. The GFS, ECMWF, and even CMC all have the trough picking up the weak system and ejecting it quickly northeast south of the Mid- Atlantic region and racing offshore. This will continue to be monitored closely in the next couple days. I suspect the forecast will not become more clear on the potential impacts for the Mid- Atlantic region for another few forecast cycles. Depending on how close the system gets, we could see a period of tropical (efficient rain producing) showers and thunderstorms on Monday. If the system remains well south of the area, dissipates, or stays surpressed, Monday could end up being largely dry. As such, have suck with chance PoPs for the Monday period. On Tuesday, the main trough axis and jet maxima will pivot across the area sending a strong cold front through the region from the west. Guidance seems to be quite consistent on the synoptic aspects of this system at this range (beyond the potential remnant tropical system interaction on Monday) bringing the cold front through sometime Tuesday. Expect that there will be a round of showers and thunderstorms with the potential for severe thunderstorms given that bulk shear will increase to favorable levels for organized convection. The exact timing and details on the evolution of this remains quite unclear though, and will likely be dependent on how the synoptic system and front interact (if at all) with the remnant tropical system on Monday. Sunday through Tuesday will be very warm to hot with highs in the upper 80s to around 90 degrees. The dewpoints should stay in the low 60s Sunday with the front in the vicinity of the area, but they will rise considerably to around 70 degrees (or higher) ahead of the stronger cold front Monday and Tuesday. So Monday and Tuesday are likely to be very muggy and humid until the front passes. In the wake of the front Wednesday and Thursday, expect below normal temperatures around 80 degrees and much more comfortable dewpoints in the 50s as high pressure builds across the eastern CONUS. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Today...VFR with southwesterly winds from 5 to 10 knots and gusts to 15 knots. Can't rule out an isolated shower or thunderstorm late this afternoon and evening, especially across southeastern Pennsylvania. High confidence Tonight...VFR with high clouds streaming into the region. Winds turning southerly from 5 to 10 knots. High confidence. Outlook... Saturday through Saturday night...Prevailing VFR, but expect multiple rounds of showers and thunderstorms (especially 21Z Saturday through 06Z Sunday). The showers and thunderstorms may locally cause MVFR or IFR restrictions. Winds becoming westerly around 10-15 kts Saturday with gusts up to 20 kts possible. Winds decrease Saturday night to 5 kts or less. Moderate confidence overall, but low confidence on the details of convection. Sunday...VFR. Westerly winds around 10 kts. High confidence. Monday through Tuesday...Prevailing VFR, but showers or thunderstorms may locally cause MVFR or IFR restrictions, especially Tuesday. Winds southerly and increasing to around 10-15 kts with gusts of 20-25 kts possible. Winds will become westerly late Tuesday behind a cold front. Moderate confidence overall, but low confidence in the details of convection. && .MARINE... Sub-advisory conditions anticipated for this morning before southerly winds increase in the presence of a developing low level jet. An SCA will go into effect at 4pm for wind gusts increasing to near 25 knots after 4 pm and last through the overnight with seas increasing to near 5 feet at times. Outlook... Saturday...Seas will remain near 5 feet across the ocean waters through mid-morning Saturday, and a Small Craft Advisory has been issued through this period. Otherwise, expect southerly winds around 15-20 kts and seas 3-4 feet. Expect multiple rounds of showers and thunderstorms Saturday, especially during the evening and overnight hours. Sunday...Sub-advisory conditions expected. Southerly winds 10-15 kts and seas around 3 feet. Monday through Tuesday...Advisory conditions appear probable to develop during this period (especially by Tuesday), but confidence is low on the potential impacts from the remnants of what may soon become tropical cyclone Claudette. The remnants of this system appear most probable to race offshore south of the area during the Monday period, and winds and seas may be higher than currently forecast. Otherwise, expect showers and thunderstorms Monday and Tuesday with prevailing southerly winds of 15-20 kts with gusts over 25 kts possible. Seas possibly increasing to over 5 feet, especially Monday night and into Tuesday. Rip Currents... Rip current risk remains low through today due to seas 3-4 feet or less with swell of 7-8 seconds. Although winds should start offshore, by mid day a sea breeze will develop leading to breezy onshore winds. A low rip current risk looks to persist for Saturday as seas remain around 3-4 feet with a swell around 6 seconds and SW winds 10 to 15 knots. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 4 PM this afternoon to 10 AM EDT Saturday for ANZ450>455. && $$ Synopsis...Staarmann Near Term...Davis Short Term...Staarmann Long Term...Staarmann Aviation...Davis/Staarmann Marine...Davis/Staarmann