textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

1. Fog added to the forecast for early this morning.

2. Some increase in the potential for severe weather late Sunday afternoon into the evening.

KEY MESSAGES

1. Showers depart early this morning with some areas of dense fog possible in their wake. Becoming mostly sunny for the day Saturday with slightly cooler temperatures compared to Friday.

2. Warmer to end the weekend before a strong cold front moves through late Sunday into Sunday night with some showers and potentially strong to severe thunderstorms. Turning colder with gusty winds to start the new week.

DISCUSSION

KEY MESSAGE 1...Showers depart early this morning with some areas of dense fog possible in their wake. Becoming mostly sunny for the day Saturday with slightly cooler temperatures compared to Friday.

An elongated area of low pressure extends from eastern PA north and eastward into New England early this morning with an associated area of showers moving out of our area from west to east early this morning. However in its wake the low levels will remain quite Saturated as the flow remains weak. We've already seen some fog developing over portions of eastern PA and this could become more expansive over the next few hours. However as we head into the day Saturday and the low pulls away, NW flow will result in better mixing as well as drier air moving in so any early morning fog should diminish pretty quickly. Otherwise, weak, transient high pressure builds in for the day bringing partly to mostly sunny skies and slightly cooler temperatures compared to Friday. Generally expect highs ranging from the 50s north to the low to mid 60s for most areas south. However it will be cooler near the coast, especially heading into the afternoon as the flow will be light enough for a sea breeze to develop.

KEY MESSAGE 2...Warmer to end the weekend before a strong cold front moves through late Sunday into Sunday night with some showers and potentially strong to severe thunderstorms. Turning colder with gusty winds to start the new week.

The southern part of an upper-level trough across eastern Canada is forecast to amplify some Sunday into Monday across the Northeast and northern Mid-Altantic. This will drive surface low pressure eastward from the Great Lakes which tracks just to our north later Sunday, however another low develops along the front and slides over our area Sunday night. A frontal zone will be to our north and this will keep our area within the warm sector with southwesterly winds during most of the day Sunday. However a strong cold front will eventually move southward across the area bringing much cooler conditions by Monday. There are some differences in the forecast guidance regarding the timing of the cold frontal passage as it will depend on where exactly the new low develops along the front and the track this low takes. Some forecast guidance wants to bring the front through as early as Sunday afternoon into the early evening while other guidance holds it off until later in the evening into the overnight. Ahead of the front though, highs Sunday should reach into the upper 60s to low/mid 70s for most of the region. A more southerly wind component though will keep it cooler along the coast due to the wind off the cold ocean.

A ribbon of stronger forcing for ascent ahead of and with the cold front should result in an area of showers arriving in our northwest zones as early as Sunday afternoon then spreading southeastward Sunday night into early Monday morning. Also worth noting, forecast models are showing some instability in the warm sector ahead of the front so at least some thunderstorms will be possible. In fact, the Storm Prediction Center has expanded their area of MARGINAL risk for severe weather to include most of our CWA. A limiting factor is that a lot of this instability may be more elevated which could limit the severe potential. That said, the low and mid level flow will be quite strong so there's certainly the potential there for some severe wind gusts to be brought down to the surface. The timing for any severe weather looks to generally be late afternoon into the evening but will again depend on the exact timing of the front. As cold air advection develops later Sunday night in the wake of the cold front, temperatures will fall and this should result in high temperatures Monday occurring during the morning to perhaps midday hours before leveling off and then falling some during the afternoon. A pressure gradient tightens in the wake of low pressure departing to our east and northeast Monday and high pressure building toward the Great Lakes. This combined with strong cold air advection will steepen the low-level lapse rates and result in deeper mixing. A gusty northwest wind results during Monday especially in the afternoon into the evening, with peak wind gusts potentially reaching the 30 to 40 mph range. There will be less wind on Tuesday, however even colder/below average temperatures are forecast.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

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

Through 12Z...VFR CIGs for most terminals with a brief period of MVFR possible. Showers have cleared all but MIV and ACY and will exit the area by 08Z. Some patchy fog has developed in the wake of the showers, and may continue. This is primarily expected to impact RDG and ABE, but could lead to IFR/LIFR visibilities at times. Winds generally light and variable with periods of calm possible. High confidence in overall pattern, low confidence in how extensive fog coverage may become.

Today...VFR. Winds becoming north-northwest around 10 kt with a few gusts 15-20 kt in the afternoon. High confidence.

Tonight...VFR. Winds light and variable in the evening, eventually increasing out of the southeast at around 5 kt. High confidence.

Outlook...

Sunday and Monday...Periods of sub-VFR conditions expected in showers and perhaps a few storms Sunday afternoon through Monday morning. Improving conditions are expected Monday afternoon, though northwest winds with gusts 25-35 kt expected.

Monday night...VFR. Lessening winds.

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

Wednesday...Sub-VFR conditions possible with a chance of showers.

MARINE

A Small Craft Advisory is in effect for the Atlantic Coastal waters from Cape May NJ to Fenwick Island DE until 6 AM Saturday. A Small Craft Advisory is in effect for the Atlantic Coastal waters from Sandy Hook NJ to Cape May NJ until 10 AM Saturday. The winds are generally below Small Craft Advisory levels at this time but seas remain elevated.

For Saturday, SW winds early will become NW around 10-15 kt in wake of frontal passage. Winds will then gradually shift out of the east in the afternoon around 10 kt. Seas diminish to around 3-5 feet. Fair weather outside of SCA conditions.

For Saturday night, the conditions are anticipated to be below Small Craft Advisory criteria.

Outlook...

Sunday and Monday...Small Craft Advisory conditions probable Sunday night and especially Monday. Winds diminish Monday night but seas remain around 4-5 feet.

Tuesday and Wednesday...The conditions are anticipated to be below Small Craft Advisory criteria.

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until 10 AM EDT this morning for ANZ450>453. Small Craft Advisory until 6 AM EDT early this morning for ANZ454-455.