textproduct: Gray - Portland

WHAT HAS CHANGED

The threat remains for isolated to scattered severe thunderstorms this afternoon and evening. An axis of heavy rainfall has also trended south towards the mountains for this evening and tonight, increasing the chance for flash flooding. A Coastal Flood Advisory has been issued for this evening's high tide for Portland through the NH Seacoast.

KEY MESSAGES

1. A cold front will cross the region today and bring a possibility for some strong to severe thunderstorms as the heat breaks.

2. Expect high astronomical tides into early next week.

3. Seasonable temperatures return for much of the week. An unsettled second half of the week is likely with multiple rounds of precipitation in the forecast.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Overall this has been little change to the severe weather threat for the afternoon and early evening hours.

Parts of the area remain under a marginal or slight risk for severe weather. Various available severe weather guidance, including the HREF, CSU machine learning, and NCAR ensemble, all support the need for those risk areas.

Most concerning will be any afternoon or evening convection, along the warm front that is forecast to be hanging near the Canadian border. Instability will be adequate, but the shear profile is impressive, especially in the lower levels. Bulk shear magnitude in the 0-3 km layer is forecast to be in excess of 35 kt. That kind of magnitude can lead to a lot of potential modes for severe weather. We will have to watch both for supercellular structure and lines/segments which may contain mesoscale circulations. Forecast hodographs continue to show decent curvature and are sufficiently elongated that I think the tornado threat remains non-zero. Otherwise wind remains the primary hazard.

There may also be several rounds of strong to severe storms. Various CAMs have early afternoon convection, followed by more in the evening into the overnight. This will also add to the rainfall totals. This is especially true where storm motion will be largely parallel to the warm front for much of the day. Right now CAMs are falling mainly in the 2 to 3 inch range, but previous runs of the HREF supported the potential for 4 to 5 inches in parts of western and central Maine. Given the set up that seems entirely within the realm of possibility. Across the northern zones 3 hr flash flood guidance is around 2 inches, so flash flooding cannot be ruled out either. We are expecting a busy evening across the area, even if it that means babysitting the radar to make sure convection is behaving itself.

it will also be the last of the hot days for a while, especially away from the coast as southerly flow helps temperatures to warm into the upper 80s to lower 90s. Along the coast, the onshore flow will keep highs into the 70s to lower 80s. However with warm weather and generally fair conditions near the coast until thunder arrives overnight, will issue another Beach Hazard Statement for high paddlecraft risk, especially in the bays.

KEY MESSAGE 2 DESCRIPTION...

High astronomical tides continue today and into Monday. The evening tides are of greatest concern for Minor flooding, with the best chance for these levels to be met for Portland and Hampton this evening and Monday evening.

This evening's tide will align with increasing southerly flow as a front approaches from the west. This will result in growing sea state, but wave heights of 2 to 4 ft remain more tame vs. winter systems. A Coastal Flood Advisory has been issued for Portland through the NH Seacoast for this evening's tide after 9pm.

Subsequent tides will feature lower wave heights, but high still water will still rise into action stage, and approach minor flood stage.

KEY MESSAGE 3 DESCRIPTION...

Good model consensus remains for seasonable temps to return for much of the week. Widespread 70s and 80s are anticipated during the day and overnight dropping back into the 50s to lower 60s. The humidity will start to creep back up towards the latter half of the work week with the approach of the next chance of rain.

That will come as an upper-level low moves eastward across Ontario and Quebec. This upper-level low will allow for cooler temperatures and more unsettled weather. The first of three disturbances arrives on Wednesday evening as a decaying low brings scattered showers and storms. A more robust system then looks to overspread the northeast on Thursday, with more widespread rain in the morning followed by thunderstorms in the evening. Some wraparound showers from the aforementioned low may persist, with somewhat unsettled weather likely continuing through the weekend. High temperatures are expected to be in the upper 70s and lower 80s during this timeframe. Unsettled weather may continue through the weekend.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Through 12z Monday...Largely VFR conditions expected today. Rounds of SHRA or TS for HIE, mostly staying north of LEB this afternoon, before pushing east and becoming more widespread tonight. MVFR or lower conditions will be possible in showers/thunderstorms across the entire area overnight into Mon. LLWS will also accompany this period tonight into early Mon morning. MVFR begins to improve west to east early Mon, with VFR expected.

Outlook:

Monday - Tuesday: VFR conditions expected during the day. Some local valley fog possible in the vicinity of LEB and HIE at night.

Wednesday: VFR likely, with a chance of MVFR due to showers.

Thursday: MVFR expected with rain and thunderstorms.

Friday: MVFR likely with wraparound showers.

MARINE

Areas of fog remain possible in the vicinity of Penobscot Bay this morning. Marginal SCA conditions still expected late tonight into early Mon morning with some gusts near 25 kt and seas building to 5 ft outside of the bays. Winds and seas generally fall below SCA criteria through the first part of the work week, but could return Thursday into Friday as a storm system approaches.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Beach Hazards Statement through this evening for MEZ023>028. Coastal Flood Advisory from 9 PM this evening to 1 AM EDT Monday for MEZ023-024. NH...Beach Hazards Statement through this evening for NHZ014. Coastal Flood Advisory from 9 PM this evening to 1 AM EDT Monday for NHZ014. MARINE...Small Craft Advisory from 4 PM this afternoon to 8 AM EDT Monday for ANZ150>154.