textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Updated first key message and associated discussion. Updated Aviation discussion for 0z TAFs.

KEY MESSAGES

1. A cold front moves through the region tonight bringing some rain.

2. A frontal boundary south of the region will keep rain chances around through mid-week, especially across southern portions of the area.

3. A closed mid-level low ejecting from the Ohio Valley is expected to lead to the development of a surface low off the Eastern Seaboard during the middle and latter parts of this week. Depending on the track of the low, it could bring increased rainfall chances to the region.

4. Multiple rounds of tidal flooding are expected later this week, with tidal flooding potentially beginning as early as Tuesday evening.

DISCUSSION

KEY MESSAGE 1...A cold front moves through the region tonight bringing some rain.

A cold front will move south and eastward through the area this evening into the overnight bringing some rain/showers with it. A few rumbles of thunder are not out of the question either. There's not much instability to work with but PWATs are quite high at around 2 inches. Also, there are some upstream reports towards central PA of a quick .50 to 1.0 inches of rain with these showers as they've moved through. Generally not expecting any issues but if the line of showers currently moving through were to slow down and train near the urban corridor that could result in some localized flooding issues this evening. Expect showers to wind down overnight as the front continues to push south.

KEY MESSAGE 2...A frontal boundary south of the region will keep rain chances around through mid-week, especially across southern portions of the area.

The front will be south of the region on Monday and remain there through mid week. Confidence is not the highest pertaining to all aspects of sensible weather, namely precipitation. How far south that front sags will be a big factor in how much rain we see through the week. The farther south it stalls, the less likely our region will see continued rain chances through the mid week period. Guidance this afternoon continues to indicate the front may try to lift briefly northward as a warm front on Tuesday, which looks to feature the highest rain chances through mid-week. PoPs currently range from around 50% across far northern NJ to around 80% across portions of southeastern PA, far southern NJ, and most of Delmarva. For Wednesday into Thursday, the front should again be on the move southward, lessening rain chances across the area.

Regardless of exact frontal placement or rain chances, the entire area will be on the cool side of the boundary. Therefore, confidence is high that temperatures will be below normal. In fact, temperatures will likely struggle to get out of the 60s areawide through most of this week.

KEY MESSAGE 3...A closed mid-level low ejecting from the Ohio Valley is expected to lead to the development of a surface low off the Eastern Seaboard during the middle and latter parts of this week. Depending on the track of the low, it could bring increased rainfall chances to the region.

Model guidance this afternoon continues to indicate that a mid- level low will track eastward out of the Ohio Valley region towards the Mid-Atlantic by Thursday. From there, it may become cutoff and meander off the Eastern Seaboard into late this week and potentially even into this weekend. As the mid-level low tracks eastward, surface cyclogensis will become favored off the coast of the Carolinas. This could begin as early as Tuesday night as synoptic- scale ascent increases. The surface low is then likely to track northeastward into late this week.

There is considerable uncertainty in how close the low will track to our area, and therefore, what impacts it may bring. Most guidance features a track well offshore, but the deterministic ECMWF and GFS continue to feature a westward deviation of the low late this week, which would lead to increased rain chances across the region. However, when looking at their ensemble members, the deterministic runs actually lie on the western edge of the overall envelope.

All this being said, the forecast for the latter half of this week into this weekend features considerable uncertainty. For now, have PoPs generally in the 20-40% range across the eastern two-thirds of the area Friday through Sunday. However, if a westward deviation of the low does occur these could certainly trend up, or if the low remains farther out to sea the region could be mostly dry for the latter part of the week.

KEY MESSAGE 4...Multiple rounds of tidal flooding are expected later this week, with tidal flooding potentially beginning as early as Tuesday evening.

Regardless of the exact track of the coastal low, it looks as if we will be in persistent onshore flow from at least Tuesday onward for the Atlantic Coast, Delaware Bay, and Raritan Bay. This long of an onshore flow component increases the risk for tidal flooding, especially for back bays. For the later half of the week, the track, strength of the low, and fetch will all play a role with tidal departures. In addition, the full moon is on Saturday, meaning astronomical high tides are increasing as we get towards the end of the week.

Minor tidal flooding could begin as early as Tuesday afternoon/evening's high tide with potential Moderate tidal flooding on Wednesday.

For the Chesapeake Bay, the expected wind direction tends to limit tidal flooding concerns, but it will need to be watched if persistent easterly flow at the mouth of the Chesapeake.

AVIATION /00Z MONDAY THROUGH FRIDAY/

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

Tonight...Mainly MVFR predominating however showers moving through this evening could produce brief IFR conditions. Expect that by around 09z conditions improve to VFR for TAF sites from the I-95 corridor and points north/west as drier air moves in. Winds mainly light and variable this evening becoming northerly around 5-8 knots overnight. Moderate confidence.

Monday...MVFR cigs may linger at ACY and MIV but VFR should predominate through the day north of these sites. Northeast winds generally around 5-10 knots. Moderate confidence.

Outlook...

Monday night through Tuesday night...Sub-VFR conditions likely in periods of rain and low clouds.

Wednesday through Friday...While uncertainty is high, sub-VFR conditions will remain possible at times in rain or low clouds. Gusty northeast winds are possible.

MARINE

A Small Craft Advisory is in effect from 11 AM Monday through 11 AM Tuesday for all Atlantic coastal waters.

Conditions are expected to remain below SCA criteria through tonight with winds mainly southerly this evening shift to northwest and then northerly overnight.

Winds will become northeast on Monday and increase to 15-20 kt with gusts 25-30 kt and seas 4-7 feet.

Outlook...

Monday night...Advisory conditions expected with northeast winds of 15-20 kt and gusts of 25-30 kt. Seas 4-7 feet.

Tuesday through Friday...Advisory conditions expected. Gales are possible for at least part of this timeframe, but uncertainty is high at this point in time.

Rip Currents...

Monday... We'll continue with the MODERATE risk for dangerous rip currents on Monday as winds will switch around to Northeast and increase to 15 to 20 mph during the day. Breaking waves in the surf zone will be around 2 to 4 feet.

Tuesday...Northeast winds strengthen to 20 to 30 mph with wave heights in the surf zone building to 3 to 5 feet. As a result, there is a HIGH risk of rip currents both for the New Jersey shore and the Delaware Beaches.

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

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from 11 AM Monday to 11 AM EDT Tuesday for ANZ450>455.