textproduct: Philadelphia/Mt Holly

WHAT HAS CHANGED

No significant changes at this time.

KEY MESSAGES

1. Gusty winds develop today as low pressure moves north of the region.

2. Breezy to windy Saturday along with dry conditions may create some fire weather concerns.

3. 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...Gusty winds develop today as low pressure moves north of the region.

Low pressure across the upper Great Lakes will move east today and be near Lake Ontario tonight. The low will begin to slow fill after this morning. Still, a decent pressure gradient accompanying the low will bring a notable increase in winds this afternoon and early tonight. Overall, winds will remain below Wind Advisory criteria, but a couple gusts in that range can't be ruled out. A few rain/snow showers across the far NW areas and north NJ will accompany the low/front but nothing significant is foreseen.

KEY MESSAGE 2...Breezy to windy Saturday along with dry conditions may create some fire weather concerns.

Low pressure will depart northern New England early Saturday as it tracks into the Canadian Maritimes, while high pressure begins to build in from the Great Lakes area. The pressure gradient between these systems will result in a strong west to northwest flow across the Mid-Atlantic region Saturday. The air that arrives will also be much drier by Saturday afternoon to produce minimum relative humidity values in the 20-30 percent range. The dew points will lower into the low to mid 20s, however a period of deeper mixing may result in the dew points dropping into the teens across much of the area during afternoon peak heating. If this occurs, the relative humidity would be lower during peak heating. Since the winds will be gusting between 30-45 mph through about early/mid afternoon, this could create fire weather concerns. The main potential limiting factor will be fuel moisture and how dry it will be and become following the very recent times of rain/snow.

KEY MESSAGE 3...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 30- 40 mph range.

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 FRIDAY THROUGH TUESDAY/

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

Overnight...VFR with clear skies. Some Ci/Cs far west towards dawn. NW to W winds 5 to 10 knots. High confidence.

Friday...VFR. Increasing high then mid clouds. No precipitation expected at the TAF sites, but light precip for the far N/W areas possible. South winds increasing 15 to 20 knots with gusts to 30 knots by afternoon. Medium/high confidence.

Outlook...

Saturday...VFR. West-northwest wind gusts 25-35 knots, then diminishing in the evening.

Sunday and Monday...Sub-VFR later Sunday night and Monday with a period of showers and potentially gusty thunderstorms. Gusty winds to 30-35 knots Monday outside of any thunderstorms.

Tuesday...VFR. Westerly wind gusts up to 30 knots.

MARINE

West to Northwest winds 10 to 20 knots on the waters presently (3AM) will diminish thru dawn. After that they will shift to South and steadily increase thru the afternoon. Tonight then will turn West- Southwest. Speeds will be in the SCA range after noon (or so) today and remain there overnight, The SCA flag will continue with gusts to 30 kts expected. Fair weather expected.

Outlook...

Saturday...Small Craft Advisory conditions diminishing.

Sunday and Monday...Small Craft Advisory conditions develop Sunday night, then Gale conditions probable on Monday. Some gusty thunderstorms possible Monday night. A strong wind field is forecast to be in place on Monday, however a warm air mass will be moving over the colder ocean waters and therefore how much mixes down to the surface is of lower confidence.

Tuesday...Small Craft Advisory conditions probable

PHI WATCHES/WARNINGS/ADVISORIES

PA...None. NJ...None. DE...None. MD...None. MARINE...Small Craft Advisory from noon today to noon EDT Saturday for ANZ430-431. Small Craft Advisory from noon today to 6 PM EDT Saturday for ANZ450>455.