textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated aviation section for 12z TAFs.

KEY MESSAGES

1. Gusty winds develop this afternoon with milder air followed by showers with a slight chance of some thunderstorms this evening.

2.Temperatures will remain mild this weekend before a strong cold front arrives Sunday night into Monday morning. The cold front will bring a chance of showers, before ushering in colder air and gusty winds to start next week.

DISCUSSION

KEY MESSAGE 1...Gusty winds develop this afternoon with milder air followed by showers with a slight chance of some thunderstorms this evening.

An upper-level trough is forecast to sharpen as it slides across the Great Lakes and into the Northeast today into tonight. This will drive a quick moving surface low that tracks to our north tonight into early Saturday morning. An associated warm front will lift north across our area this morning followed by a weak cold front later tonight. The pressure gradient will tighten quite a bit during the day today ahead of this surface low, and with the low to mid level flow increasing in response, southwesterly surface flow will increase across our area. The model forecast soundings show some increase of the mixing layer during the day today, especially in the afternoon, and this will result in an increasing breeze. The peak wind gusts look to be 25-35 mph before diminishing diminishing in the evening. It will be turning warmer (highs for most areas in the 60s), however a southerly wind will keep coastal areas much cooler. There will be enough moisture and forcing for some showers to develop upstream and these then slide southeastward starting later this afternoon (northwest areas) and then reaching the remainder of the forecast area this evening. Also worth noting, the latest progged soundings depict some elevated instability getting into parts of the area this evening so we now have a slight chance of thunderstorms mentioned in the forecast for western portions of the area...essentially from around Philly north to the Lehigh Valley and then extending west and southwestward. Expect showers to move out overnight as the cold front crosses the area. Total rainfall amounts look to generally range from around a quarter to half inch from around the I-95 corridor north and westward with one to two tenths of an inch near the coast. Locally, amounts could be a bit higher than this in any heavier showers or storms.

KEY MESSAGE 2...Temperatures will remain mild this weekend before a strong cold front arrives Sunday night into Monday morning. The cold front will bring a chance of showers, before ushering in colder air and gusty winds to start next week.

By Saturday morning the axis of an H5 trough will be located off the Northeast and Mid Atlantic coasts. As it continues to shift eastward, the region will remain embedded within modestly enhanced northwesterly flow aloft, though modest height rises will likely take place. Sunday night into Monday morning, a shortwave will pivot through the region, with larger scale troughing setting up into early next week. At the surface, weak high pressure will initially be in control in the wake of the cold front, before another area of low pressure associated with the passing shortwave passes to the north Sunday night into Monday morning.

Saturday and Sunday look to feature benign weather with mild temperatures, especially Sunday. Afternoon highs on Saturday look to range from the mid 50s to low 60s for most, with lows Saturday night only falling into the low 40s. Highs on Sunday look to be in the mid to upper 60s for most, though slightly cooler in the Poconos and for coastal locations. Across portions of the Delmarva, 70 may be breached.

Sunday afternoon, showers are expected to develop ahead of the approaching cold front to the north of the area, and spread southeastward through Sunday night. For now, PoPs are in the 50-70% range generally for the Philly metro northward, and 30-50% for the remainder of the area. Some modest elevated instability could lead to a few rumbles of thunder with this activity. All showers look to exit the region by Monday afternoon as the cold front clears the area.

In the wake of the cold front, the region will see cooler temperatures filtering in. Temperatures may hold steady or even fall during the day on Monday, with breezy conditions also expected. Northwest winds could gust as high as 30-40 mph. Temperatures Monday night are expected to fall into the 20s and 30s areawide, and only climb into the mid to upper 40s on Tuesday.

Temperatures will again begin to moderate slowly beyond Tuesday, with a few passing systems bringing a chance for some light precipitation (mainly rain) Wednesday into Thursday.

AVIATION /12Z FRIDAY THROUGH TUESDAY/

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

Today...VFR. Winds start out of the south around 5-10 kt, increasing to around 15 kt with gusts 20-25 kt out of the south/southwest. Showers approach late in the day with clouds increasing as the day goes on. LLWS begins around 22z at KACY. High confidence.

Tonight...Conditions lowering to MVFR around mid evening as showers move in and then potentially even to a brief period of IFR around roughly the 4-7z time frame. S/SW winds around 5-10 knots with LLWS expected for the I-95 terminals and points S/E. Showers move out from west to east through the late evening and overnight with conditions improving to VFR by around 09z. Moderate confidence.

Outlook...

Saturday through Sunday...VFR. No significant weather.

Sunday night through Monday...Sub-VFR conditions are possible in showers Sunday night into Monday morning. Conditions will improve on Monday, though northwest winds could gust 25-35 kt.

Monday night through Tuesday...VFR. No significant weather.

MARINE

Stronger flow develops during the day today with a low-level jet arriving. This stronger flow will usher in milder air over the colder ocean therefore mixing will not be as efficient. However, a period of wind gusts to around 25 knots still expected with seas getting to 5 feet. Therefore, a Small Craft Advisory remains in effect for beginning this afternoon. The advisory runs until midnight for the Delaware Bay and to 6 AM Saturday for the ocean waters.

Outlook...

Saturday through Sunday...No marine headlines currently anticipated with winds under 25 kt and seas under 5 feet.

Sunday night through Monday...SCA conditions probably with southwest winds gusting 20-25 kt Sunday night, becoming northwesterly on Monday. Seas 4-6 feet.

Monday night through Tuesday...No marine headlines currently anticipated with winds under 25 kt and seas under 5 feet.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 2 PM this afternoon to midnight EDT tonight for ANZ430-431. Small Craft Advisory from 2 PM this afternoon to 6 AM EDT Saturday for ANZ450>455.