textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

Latest aviation discussion has been updated. No changes in discussion points attm.

KEY MESSAGES

1. Low pressure passes north of the area tonight bringing windy conditions which will last into Saturday.

2. A strong cold front sweeps across our area later Monday with the potential for strong winds and strong to severe thunderstorms, followed by a much colder air mass Tuesday into Wednesday.

DISCUSSION

KEY MESSAGE 1...Low pressure passes north of the area tonight bringing windy conditions which will last into Saturday.

Surface low pressure over the Great Lakes will track east tonight into eastern Canada. The base of an upper trough with strong shortwave energy associated with this low will dig down through the Northeast and pass through the region late tonight through Saturday morning. As this occurs, some rain/snow showers will move through our northern zones later this evening into the overnight (Berks County north through the Leigh Valley into the southern Poconos). Any snowfall will be limited mostly to the higher elevations and will generally be less than an inch. The other thing to watch will be a surge of strong winds in these aformentioned zones associated with these showers with west winds 20 to 30 gusting 40 to 50 mph. In fact we can't rule out a few gusts reaching the 50-60 mph range. The winds may briefly abate for a time overnight before picking up again for Saturday morning. A Wind Advisory is now in effect until 2 PM Saturday for Berks County north and east through Lehigh and Northampton Counties and then north through Carbon and Monroe Counties.

For Saturday, strong WNW winds generally gusting 30 to 40 mph (40 to 50 mph in northern zones discussed above) and abundant sunshine will promote efficient mixing up to around 850 mb, and this should result in dryer conditions. Will go ahead and undercut the NBM dew points by leaning on the NBM10 and blending with dew points mixed down from the mixing height. This gives dew points in the teens to around 20, and with surface temperatures in the 40s and low 50s, MinRH values will be around 25 percent to 30 percent. This could result in fire weather concerns. Limiting factors include recent rainfall and recent snowmelt.

KEY MESSAGE 2...A strong cold front sweeps across our area later Monday with the potential for strong winds and strong to severe thunderstorms, followed by a much colder air mass Tuesday into Wednesday.

A strong upper-level trough is forecast to evolve out of the Plains and Midwest and into the Great Lakes to Lower Mississippi Valley from Sunday through late Monday. This will drive deepening surface low pressure up across the Great Lakes and then into adjacent Canada. A strong cold front will sweep across our area mostly Monday evening.

Well ahead of the strengthening upper-level trough, low-level warm advection will increase across our region in conjunction with increasing flow and the arrival of a warm front. This southerly flow will also transport deeper moisture into our area ahead of the aforementioned strong cold front. The warm air advection plus a low- level jet will provide some lift and therefore some showers should tend to develop Sunday night into Monday before becoming widespread during Monday as forcing for ascent increases. Some elevated instability late Sunday night could result in a rumble of thunder. A much warmer air mass will overspread our area despite the showers breaking out on Monday, with high temperatures in the 60s across much of the area with even some lower 70s possible. It will also become more humid with dew points surging to around 60F ahead of the cold front Monday. The consenus of the model guidance shows some instability developing during Monday, and this will be within an environment with robust wind fields. As a result, strong shear will be present and model forecast soundings show veering winds in the lower levels. While thunderstorms will be possible, the overall instability may be limited due to cloud cover and ongoing showers. However, strong forcing for ascent arriving from the west can overcome the limited instability. Given the wind profiles that are forecast, a squall line could evolve ahead of or with the cold front or the storm mode could be some fast moving line segments. If the surface flow remains backed some, then storm rotation will be possible given the shear. As a result, there is a chance for some severe thunderstorms Monday afternoon and evening with locally damaging wind gusts the primary threat as of now. Areas of heavy rain will be probable given ample moisture (precipitable water values of 1+ inches) and plenty of forcing.

As the strong cold front shifts offshore during Monday night, strong cold air advection develops. It is possible this colder air catches up to the lingering precipitation with some changeover to snow before ending, however the forcing may have a sharp back edge to it with the showers cutting off quickly with the frontal passage. There will also be a gusty wind ahead of the front out of the south which then shifts to west and northwest in the wake of the front. Peak wind gusts at this time from the gradient wind looks to be in the 40-50 mph range, with wind advisories very possible.

A much colder air mass settles in for Tuesday with temperatures struggling to reach 40 degrees in some areas. A gusty westerly wind will also add a chill factor with wind chill values down into the 20s and 30s during Tuesday. Still chilly on Wednesday however less wind is forecast.

AVIATION /06Z SATURDAY THROUGH WEDNESDAY/

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

Overnight...VFR. Winds have been slow to develop overnight with most TAF sites decoupling instead. Light W to SW winds until dawn when speeds will quickly increase 10 to 20 kts with gusts 20 to 25 kts. After mid-morning, further increases expected with gusts 25 to 30 knots expected. Few clouds. Kept LLWS in until dawn with 40 to 45 kts around 2K ft. Medium/high confid.

Saturday...VFR. West to NorthWest winds 15 to 20 kt with 25 to 35 kt gusts. Winds begin to diminish after 18Z.

Outlook...

Saturday night through Sunday...VFR should prevail with lighter winds than on Saturday.

Sunday night...Conditions likely drop below VFR, possibly below MVFR, with rain enveloping the region.

Monday and Monday night...Conditions may improve to VFR at times as rain departs, but more showers and t-storms with potential sub-VFR conditions are likely at times especially later in the day before conditions improve later at night. Gusty southwest winds may approach 30-35 kts with gusts 35-40 kts possible at night behind fropa, not inclusive of stronger gust potential in storms.

Tuesday through Wednesday...VFR prevails with diminishing winds.

MARINE

Strong south winds continue into this evening, ranging from 15 to 20 kt with 25 to 30 kt gusts. Winds diminish a bit this evening and veer to the southwest. There may even be a lull in the winds prior to daybreak Saturday.

On Saturday, low pressure passes well north of the waters, and winds turn to the northwest and increase to 15 to 25 kt with gusts up to 35 kt on the northern New Jersey ocean waters, and up to 30 kt elsewhere.

Small Craft Advisories remain in effect from Sandy Hook to Great Egg Inlet until 6 am Saturday, then a Gale Warning is in effect from 6 am to 2 pm Saturday. A new SCA may be needed after.

Small Craft Advisories remain in effect from the Great Egg Inlet to Fenwick Island through 10 pm Saturday and for Delaware Bay through 6 pm Saturday.

Fair weather expected.

Outlook...

Conditions likely drop below SCA levels for a time Saturday night before ramping back up ahead of the next system later Sunday into Sunday night. Gales could develop at times on Monday, with gales more likely Monday night before ramping back down on Tuesday. Rain also likely Sunday night, with showers and possibly gusty thunderstorms expected Monday into Monday night.

Sub-SCA winds may return by late Tuesday night or Wednesday, but waves may remain elevated.

PHI WATCHES/WARNINGS/ADVISORIES

PA...Wind Advisory until 2 PM EDT this afternoon for PAZ054-055- 060>062. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory until noon EDT today for ANZ430-431. Small Craft Advisory until 6 AM EDT early this morning for ANZ450>452. Gale Warning from 6 AM this morning to 2 PM EDT this afternoon for ANZ450>452. Small Craft Advisory until 6 PM EDT this evening for ANZ453>455.