textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

There is a drier trend for Thursday and Thursday night. A storm could bring some wintry precipitation later in the weekend.

KEY MESSAGES

1) A storm system may bring some rain to wintry precipitation Wednesday into Thursday.

2)...A storm could bring wintry precipitation later in the weekend.

DISCUSSION

KEY MESSAGE 1...A storm system may bring some rain to wintry precipitation Wednesday into Thursday.

A strong upper-level trough is forecast to pivot into the East late Wednesday through late Thursday before lifting out into Friday. The strength and orientation of this trough will be a key driver regarding surface low pressure strength and track. A positively tilted upper-level trough lasting longer will keep low pressure weaker and more out to sea, while a neutral to especially negatively tilted trough will result in a stronger and westward storm track. The model consensus at this time points more toward a less impactful system due to the trough closing off farther north and the trough axis remaining more positive to neutral longer. An initial system tracks to our north with its cold front arriving during Wednesday. As the trough continues to amplify eastward, a new surface low develops along or near an existing baroclinic zone mostly off the Carolina coast, however it looks to mainly miss our area. The overall synoptic lift with the initial system and cold front should result in the development of a precipitation shield Wednesday and Wednesday night. This could be more focused initially across eastern Pennsylvania before pivoting eastward Wednesday night into early Thursday morning as colder air works in. If enough moisture remains as the colder air deepens, then rain would change to some snow before ending for parts of the area. There will probably be another area of precipitation developing offshore associated with the secondary low. One of the forecast challenges has been whether this can consolidate into one main storm for our area and the overall trends are looking less conducive for this. As a result, it looks like lower chances for precipitation Thursday and Thursday night and the NBM guidance has gone in this direction. It will be mild ahead of this upper-level trough Wednesday which would tend to favor rain, then cooling takes place Wednesday night through Thursday night. Given the low confidence with the specifics, did not adjust from the NBM guidance. If a drier trend continues for Thursday with the parent low deepening over New England, then a windier day would tend to result across our region. Much colder air follows for Thursday through Friday.

KEY MESSAGE 2...A storm could bring wintry precipitation later in the weekend.

Following an upper-level trough in the previous key message, the upper-level trough may reload into the East for the weekend and early next week. The depth of the trough looks to be strong on much of the guidance, which would support surface low pressure in the East. It will also keep the cold air in place. This system, just like the previous one, has uncertainty with it due to what happens with the track of the surface low. At least some guidance including ensemble members indicate some wintry precipitation potential mostly later in the weekend. We will have to see how this all evolves over the next several days regarding the details including the timing. The current forecast is the NBM guidance, with has up to 30 percent PoPS on Sunday.

AVIATION /18Z MONDAY THROUGH SATURDAY/

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

Rest of Today...VFR with high clouds. Decreasing West to Southwest winds 10 to 15 kts early then around 5 knots by sunset. High confidence.

Tonight...VFR. Clear skies. Light West to Southwest winds. High confidence.

Tuesday...VFR. Mostly clear skies. Southwest around 10 to 12 knots developing by afternoon. High confidence.

Outlook...

Tuesday night...VFR.

Wednesday and Thursday...A period of sub-VFR conditions due to low ceilings and some rain and/or snow. Gusty west-northwest winds may develop during Thursday.

Friday and Saturday...VFR overall.

MARINE

Winds and seas continue to diminish as a ridge of high pressure builds across the waters. sub-SCA conditions will remain into tonight and much of Tuesday. Later Tuesday, as the ridge moves away and low pressure moves across southern Canada, an increasing pressure gradient will result in increasing winds. South to Southwest winds 10 to 15 kts with gusts to 20 knots are expected by sunset. Fair weather tonight and Tuesday.

Outlook...

Tuesday night and Wednesday...Southerly flow increases and a period of Small Craft Advisory conditions are possible, mainly Tuesday night.

Thursday...Small Craft Advisory conditions probable, and a period of gale force wind gusts possible.

Friday and Saturday...The conditions may be mostly below Small Craft Advisory criteria.

PHI WATCHES/WARNINGS/ADVISORIES

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