textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes from previous discussion.

KEY MESSAGES

1. A few showers or maybe a thunderstorm are possible into the early evening. 2. A large area of high pressure will set up over the Midwest, resulting in high heat and humidity over the Northeast and Mid- Atlantic. High temperatures will be well in the upper 90s to as high as 105 degrees with maximum heat index values between 100 and 110 degrees from Wednesday through Saturday.

3. There is the potential for afternoon and evening thunderstorms starting on Friday and lasting into the holiday weekend.

DISCUSSION

KEY MESSAGE 1... A few showers or maybe a thunderstorm are possible into the early evening.

A stalled front well to the south and weak high pressure building in across NY/NE will keep a light onshore flow across the area tonight and Monday. This humid airmass may yield a few showers this evening and perhaps a isolated one on Monday as some upper shortwave energy moves across PA/NJ. The chances will remain low (around 20% this evening and 10% for Monday). A tstm is possible but we'll only keep a slight chance for that attm. Any tstm is capable of producing decent downpours however with the high PWAT airmass in place.

KEY MESSAGE 2...A large area of high pressure will set up over the Midwest, resulting in high heat and humidity over the Northeast and Mid-Atlantic. High temperatures will be well in the upper 90s to as high as 105 degrees with maximum heat index values between 100 and 110 degrees from Wednesday through Saturday.

Strong mid-level ridging develops over the Great Lakes and Ohio Valley early this week. The high then builds into the Northeast and Mid-Atlantic by Wednesday and continues through into the holiday weekend. In general, a west to southwest flow develops on Wednesday, then becomes more west to northwest by Friday. High temperatures will climb into the mid and upper 90s on Wednesday, and then from 100 to 105 on Thursday and Friday. Temperatures will abate somewhat on Saturday, returning to the mid and upper 90s. However, with dew points in the 60s and 70s, this will yield max heat index values from 105 to 110 during the two hottest days, which will be Thursday and Friday. Still, max heat index values will be 100 to 105 on Wednesday and Saturday.

The Extreme Heat Watch remains in effect for New Jersey, southeast Pennsylvania, the Delaware Valley including Philadelphia and northern Delaware, and the southern Poconos. Future shifts will likely expand the Watch to include the rest of Delmarva.

The mid-level ridge will break and retrograde back towards the south and west on Saturday, and a mid-level trough will try to slide down into the Northeast. This pattern change will begin to favor slightly cooler temperatures and a break in the extreme heat by Sunday or Monday, as well as the potential for showers and thunderstorms next weekend (see Key Message 3 below).

KEY MESSAGE 3...There is the potential for afternoon and evening thunderstorms starting on Friday and lasting into the holiday weekend.

The mid-level ridge looks to break down and retrograde back to the south and west starting on Friday, and this will allow mid- level troughiness to develop over the Northeast. With several shortwaves riding over the top of the ridge and into the local area, this may touch off afternoon and evening showers and thunderstorms starting on Friday, but the better chances for convection arrive on Saturday and Sunday. Currently, PoPs are around 20 percent (slight chance) on Friday and 30 to 50 percent (chance) on Saturday and Sunday. Given the high heat and humidity, cannot rule out at least a few stronger thunderstorms.

AVIATION /20Z SUNDAY THROUGH FRIDAY/

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

thru sunset... Scattered showers perhaps an isolated tstm possible for many sites with VCSH inserted in the latest TAFs. CIGS will mostly remain high-end MVFR or low-end VFR with some improvement possible thru early tonight. Light E/SE winds. Low/medium confid.

Tonight... Gradually diminishing conditions after sunset as low level moisture arrives as ST clouds slowly advect from the South/East. Fog will develop across the northern/western terminals where some cooling occurs early. Overall. most sites heading for low MVFR or high IFR conditions many areas between 03Z/07Z. Light E/SE winds. Low confid.

Monday... Slow improvement back to VFR (11Z-14Z) then probably VFR afterwards. Couple afternoon showers possible N/W. Low/medium confid.

Outlook...

Monday night...Potential for VSBY restrictions in BR.

Tuesday through Friday...VFR. NSW.

MARINE

Good conditions across the waters with sub-SCA continuing into the overnight. A couple scattered showers are possible across Delaware Bay or the southern NJ and Delaware waters, but the chances are rather low. Winds will remain light 5 to 10 knots from the E or SE. Similar conditions for Monday. Aside from the showers a little fog is possible overnight and into early Monday.

Outlook...

Monday night through Friday...Overall, sub-SCA conditions expected. Cannot rule out a brief period of southerly 25+ kt gusts or seas approaching 5 feet late Tuesday and/or Wednesday afternoon, mainly north of Barnegat Light.

Rip Currents...

For Monday, breaking waves in the surf zone will be around 2 feet or less with east to northeast winds around 10 mph and a southeast swell. As a result, there is a LOW risk of dangerous rip currents for both the Jersey Shore and Delaware Beaches.

For Tuesday, conditions remain similar, with east to southeast winds around 10-15 mph and a southeast swell. Breaking waves in the surf zone will be around 1-2 feet as a result, and therefore, the LOW risk of dangerous rip currents for both the Jersey Shore and Delaware Beaches looks to continue.

For specific beach forecasts, visit weather.gov/beach/phi

CLIMATE

Here is a list of the high temperature records for July 2 (Thursday) and July 3 (Friday):

Thursday July 2 Allentown...102/1968 Atlantic City Airport...100/1966 Atlantic City Marina...97/1968 Georgetown...99/1953 Mount Pocono...93/1911 Philadelphia...103/1901 Reading...102/1966 Trenton...100/1901 Wilmington...98/1966

Friday July 3 Allentown...105/1966 Atlantic City Airport...104/1966 Atlantic City Marina...99/1919 Georgetown...101/1954 Mount Pocono...103/1911 Philadelphia...104/1966 Reading...103/1966 Trenton...102/1966 Wilmington...102/1966

PHI WATCHES/WARNINGS/ADVISORIES

PA...Extreme Heat Watch from Wednesday afternoon through Saturday evening for PAZ054-055-060>062-070-071-101>106. NJ...Extreme Heat Watch from Wednesday afternoon through Saturday evening for NJZ001-007>010-012>027. DE...Extreme Heat Watch from Wednesday afternoon through Saturday evening for DEZ001. MD...None. MARINE...None.