textproduct: Gray - Portland

WHAT HAS CHANGED

No major changes were needed for this forecast package. The main challenges will be the chance for isolated strong to severe storms this afternoon/evening and the return of some smoke from distant wildfires.

KEY MESSAGES

1. Low pressure and an approaching cold front will bring a chance for rain and isolated strong to severe storms.

2. Potential for widespread downpours/thunderstorms Tuesday- Wednesday. Locally heavy rainfall possible.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The next shortwave and cold front approaches today, leading to warm air advection through the morning and afternoon as we transition back into southwest flow aloft. Most of the CAMs suggest that we will see some early day showers across much of the area, but the best chances appear to be across southern New Hampshire and coastal Maine. Severe weather chances tomorrow will greatly depend on how quickly this activity moves out and how much smoke returns from distant wildfires. As of now, forecast highs generally range from the upper 70s north, to the 80s south. The warmest temperatures should be over southern New Hampshire and southwest Maine where widespread highs in the mid to upper 80s will be possible.

Later in the afternoon, we will likely see some redevelopment of the early day showers and storms as well as new development approaching from the west and north. The magnitude of instability will be greatly dependent on cloud breaks. 00z HRRR soundings over southern New Hampshire by late afternoon suggest tall skinny CAPE profiles and plenty of shear. The HRRR also suggests that the next slug of smoke will move through early in the day with the highest concentrations likely over southern New Hampshire, spreading to coastal and central Maine by the afternoon hours. Thus, afternoon instability and temperatures may be limited by a combination of early day convection and smoke. Still, given the high shear setup, it won't take much instability for a few isolated storms to become strong to severe. Strong winds would be the most likely threat but there is a non-zero tornado threat over southern New Hampshire given 0-1 km shear around 30 knots and 0-1 km SRH in the 250 to 300 m2/s2 range. Additional storms will be approaching the north from Quebec along the cold front around and after sunset, but CAMs currently show this activity weakening as it moves into our forecast area which makes sense as we lose daytime heating. There may be a short window for a stronger storm or two in the evening north. As a final note, PWATS will be in the 90th percentile for much of the area with anamolous IVT so some heavy downpours are also expected.

Most of the showers and storms will move out of the area by the overnight hours. While we could see a few lingering showers in the mountains on Sunday, most will see a dry day with cooler temperatures behind the cold front. Highs are forecast to range from 60s and lower 70s over the north country, to the mid/upper 70s across central New Hampshire and the Interior of Maine, to the upper 70s/lower 80s over southern New Hampshire and coastal Maine. Northwest winds will also be fairly gusty behind the cold front on Sunday, potentially gusting up to 35 mph in the morning and afternoon hours.

Additionally we have a very long period but smaller swell affecting area beaches. This wave pattern will be complicated by increasing southwest winds and resulting seas, but forecasts are for high risk of rip currents this afternoon. Given that they have occurred in the past couple of days and the forecast is indicating it continues, I have issued a rip current statement for today.

KEY MESSAGE 2 DESCRIPTION...

A slow moving, but deep trough, digs into the Northeast. Deep southwest flow aloft will really transport in a juicy environment. Dynamics look decent with this system as a 995 mb surface low slowly spins through Quebec, with some models hinting at a secondary coastal low developing near New England that would enhance moisture locally.

EPS and GEPS ensembles both show PWATs surging to 1.8-2.0 inches, which is quite a strong signal for this range on ensemble guidance. In addition, some deterministic output is suggesting robust IVT transport. NBM mean guidance is still advertising a healthy 1.0-1.5" of QPF across the region, which some individual ensemble members that feature significantly higher totals.

Will have to watch closely how much of the precipitation is convective, given this is an environment primed for efficient rainfall, and possibly a long-duration event. The best chances for thunder will be Tuesday or Wednesday afternoon if we can destabilize with any sunshine, but cannot rule out some embedded convective elements overnight too given rather strong dynamics. Do see a weak signal on CSU/CIPS for severe, especially on Wednesday with models show up to 1000 J/Kg MUCAPE developing.

Near seasonable temperatures should continue for the first half of next week. This will really just be a period of above average lows, and below average highs, given mostly cloudy skies and high dew points in the 60s-low 70s. It remains to be seen if wildfire smoke will have any tangible effects on temperatures, but given the upstream fires over Canada will continue to burn, we will need to keep an eye on air quality into next week.

AVIATION /06Z SATURDAY THROUGH THURSDAY/

Through tonight...VFR prevails through the morning, and then restrictions are possible at all terminals during the afternoon and evening hours under showers and thunderstorms. Due to timing and coverage uncertainty, will utilize PROB30 groups for now. Some valley fog is possible tonight, but otherwise VFR returns once the showers and storms move offshore around midnight.

Outlook...

Sunday: Conditions expected to return to VFR in the morning, except lingering MVFR ceilings and SHRA possible at HIE.

Monday: VFR expected.

Tuesday-Wednesday: MVFR to IFR possible with increasing chances for SHRA and a few TSRA.

Thursday: Becoming VFR after some morning showers.

MARINE

Southerly winds increase through the day ahead of an approaching cold front. Gale force gusts are likely further offshore, with high end SCA conditions likely across the coastal waters and bays this afternoon into the nighttime. A few gusts to 35kt are possible. Seas then take until at least Sunday morning to lower below 5ft.

Sunday-Thursday...The front crosses early Sunday morning, shifting winds to W and NW. Seas could stay above 5 ft much of the day Sunday. High pressure builds in for Monday, keeping conditions under SCA levels, and then the next front brings another chance at SCA conditions toward the middle of next week (be Tues-Weds or Weds- Thurs).

GYX WATCHES/WARNINGS/ADVISORIES

ME...Air Quality Alert from 10 AM this morning to midnight EDT tonight for MEZ007>009-012>014-018>028-033. High Risk for Rip Currents from 8 AM EDT this morning through this evening for MEZ023>027. NH...High Risk for Rip Currents from 8 AM EDT this morning through this evening for NHZ014. Air Quality Alert until 11 PM EDT this evening for NHZ001>015. MARINE...Small Craft Advisory from 2 PM this afternoon to 8 AM EDT Sunday for ANZ150-152-154. Small Craft Advisory from 2 PM this afternoon to 2 AM EDT Sunday for ANZ151-153. Gale Warning from 4 PM this afternoon to 2 AM EDT Sunday for ANZ180-182.