textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Lower confidence in fog coverage this morning.

Increasing chances for rainfall Friday.

KEY MESSAGES

1. Potential fog development this morning and again overnight tonight, but otherwise, generally tranquil weather prevails through the middle of the week.

2. The next chance for rain returns late this week and into the weekend.

DISCUSSION

KEY MESSAGE 1...Potential fog development this morning and again overnight tonight, but otherwise, generally tranquil weather prevails through the middle of the week.

High pressure will remain in place within the vicinity of our area through the middle of the week. Dry weather is expected to continue through Wednesday amid warming temperatures each day.

Scattered to broken high cloud cover will persist through the middle of the week despite surface high pressure. Temperatures are expected to increase back to above normal values, warming each day through Wednesday. High temperatures look to be in the upper 60s to low 70s Tuesday, and in the low to mid 70s Wednesday. Overnights will becoming increasingly milder (no frost/freeze potential). Dry weather is expected to persist through Wednesday.

There is potential for areas of fog development early this morning and again overnight tonight into early Wednesday morning. Dewpoints will also be on the rise each day. Given mostly clear skies, light winds, and long nights, we anticipate some fog development across the area during the overnight periods. The latest hi-res guidance continues to suggest the best chance for fog development will be along and southeast of the I-95 corridor and across the coastal plain. Confidence remains low on any dense fog. Where fog does develop, it will likely be slow to mix out after sunrise Tuesday and Wednesday mornings. Light southerly flow will promote dewpoint advection and the solar insolation is relatively weak until closer to midday, which will limit mixing earlier in the morning.

KEY MESSAGE 2...The next chance for rain returns late this week and into the weekend.

The ridging that has be in place across the area for the past several days will finally begin to break down by Wednesday night into Thursday as a potent upper trough over central CONUS cuts off and lifts northeastward into portions of the Great Lakes and southern Canada by the end of the week. As this occurs, broad troughing is expected to take shape across the eastern CONUS as a few lobes of upper level energy will pivot through the region Thursday and over the weekend. At the surface, low pressure over the Midwest is forecast to track northeast into Canada as it weakens into Thursday with a trailing cold front back to the southwest. Latest guidance shows this frontal boundary washing out and stalling across the region Thursday night. General QPF guidance shows around 0.25 to 0.35 inches through daybreak Friday. After this time, there is still considerable uncertainty as to how this system will evolve to impact our local area.

Heading into Friday, while uncertainty remains, the latest guidance shows a polar trough phasing with a lobe of upper level energy, resulting in a rather poignant troughing patter across the eastern US. Guidance varies on the exact evolution of this trough through Friday, but several models show it digging before ultimately becoming a cut-off low across the Mid Atlantic or Northeastern states by Friday night. In the process, a surface low may form along the previously stalled boundary, beginning across VA/NC and trekking along the front into coastal New England. PWAT values look to creep up into the 1.50-1.75" range, which would push into the climatological maximum for this time of year. QPF forecasts this far out are also pointing out the potential for a corridor of heavy rainfall of a few inches or more, though the exact placement of where the heaviest rain will occur remains highly uncertain at this time.

Heading into this weekend, surface high pressure attempts to build in from the west, but will struggle to gain full control with broad upper level troughing and a cut-off low pattern overhead. Any upper level pattern like this will be highly dependent on how these cut-off lows and their lobes of shortwave energy evolve, a set-up which always bring more uncertainty than usual to the forecast. The takeaway message here is that the weekend may not be fully dry and could remain somewhat unsettled.

AVIATION /08Z TUESDAY THROUGH SATURDAY/

The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas.

Rest of tonight (through 12Z)...Some patchy fog development may cause sub-VFR conditions at various terminals. Hi res guidance suggests the best chance for impactful fog will be along the I-95 terminals and points southeast, though confidence remains low on exactly where fog will decide to develop. Decided to keep TEMPO groups for lower conditions at most sites, but eased up on the worst expected impacts. Calm air most spots. Low confidence.

Tuesday...Restrictions in fog are expected to continue through around 14Z, with conditions improving to VFR thereafter. South to Southwest winds 5-10 kt. High confidence.

Tuesday night...Primarily VFR. Patchy to areas of fog possible which may cause sub-VFR conditions at times. Low confidence.

Outlook...

Wednesday through Wednesday night...VFR. No significant weather expected.

Thursday through Saturday...Sub-VFR conditions probable. Rain chances daily with a slight chance of thunderstorms on Thursday.

MARINE

No marine hazards through Tuesday night. South-southwest winds generally 5-10 kt through tonight, becoming more southerly at around 10 kt Tuesday afternoon and Tuesday night. Seas 1-3 feet.

Outlook...

Wednesday through Wednesday night...No marine headlines expected. South winds generally around 5-15 kt with seas around 2-3 feet. Fair weather.

Thursday through Saturday...SCA conditions possible. While winds and gusts are expected to remain below 25 kt, seas around 3-5 feet are possible. Daily showers possible.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...None.