textproduct: Gray - Portland

WHAT HAS CHANGED

The threat for isolated severe thunderstorms on Sunday afternoon and evening has increased some.

KEY MESSAGES

1. A cold front will cross the region Sunday and bring a possibility for some strong to severe thunderstorms.

2. Expect high astronomical tides at the end of the weekend into early next week. Seasonable temperatures return for much of the work week with widespread precipitation back in the forecast Thursday into Friday.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Tonight will be cooler than previous nights as the combination of clear skies, light winds, and lower dew points allows for some radiational cooling. This will allow temperatures to fall into the 50s to lower 60s by Sunday morning. Some coastal fog is possible, especially towards Penobscot Bay.

Sunday will be another warm day, 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.

Latest mesoscale guidance continues to show a possible prefrontal trough setting up over the region on Sunday afternoon ahead of the main cold front, which is not expected to cross until Sunday night into Monday morning. The timing of this front is not ideal for severe weather but the prefrontal trough will provide a conditional severe threat should cells develop during the afternoon and evening. Guidance shows ample low-level instability and rather significant deep layer shear, which will help to support storm organization despite rather weak mid- level lapse rates. Given inverted-v soundings, wind will be the primary threat but curved hodographs and potentially relatively low LCL heights will introduce a non-zero threat for an isolated tornado (especially towards the Canadian Border). The latest SPC D2 outlook places interior western ME and northern and eastern NH into a MRGL Risk (level 1/5) with a SLGT risk in portions of south-southwestern NH (level 2/5). Storms will also likely bring downpours with PWATs ~1-1.5".

The severe threat will diminish Sunday night but rounds of showers and storms will persist with overnight lows into the 50s to near 60 degrees.

KEY MESSAGE 2 DESCRIPTION...

High astronomical tides combined with storm surge up to around a half of a foot may bring total water levels to near minor flood stage around the times of high tide this weekend into early next week.

Monday will see the start of significant cool down with temperatures returning closer to seasonable normals for this time of year through the work week. A few lingering showers will be possible on Monday behind the cold front moving out to sea as we remain under cyclonic flow aloft. While most will see dry weather through mid-week given high pressure in place, we could see some isolated showers in the mountains each afternoon. Highs on Monday will range from the mid to upper 60s north, to the mid 70s to lower 80s across central/southern New Hampshire, the Interior of Maine, and the coast. Highs will then be very similar on Tuesday, but we could see a little bit of a warmup on Wednesday as an upper level ridge axis pops up over Maine in advance of the next shortwave trough that will move in on Thursday. Global models/ensembles are in fairly good agreement that this trough will bring our next widespread chances of precipitation to New Hampshire and western Maine Thursday into Friday. The latest NBM probabilities suggest a 40 to 70 percent chance of at least a half inch of rain for much of the area during this time frame and a 20 to 50 percent chance of at least an inch, which is a pretty good signal for widespread wetting rains this far out. As of now, the central and northern areas are favored for the higher probabilities. Models are suggesting limited instability, but with increasing dewpoints and an energetic trough in place, a few thunderstorms will be possible.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

VFR conditions expected. The exception is at KRKD where marine FG/stratus may bring MVFR or worse restrictions tonight through early Sunday morning. Light and variable winds will become S at 10-20 kts on Sunday. SHRA and TSRA will then arrive late Sunday through Sunday night, bringing restrictions along with lower ceilings.

Outlook:

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

Thursday - Friday: MVFR to IFR restrictions possible with showers and thunderstorms.

MARINE

SCA conditions expected late Sunday through Sunday night as S wind gusts increase up to 25 kts and seas outside of the bays build to 3-6 ft.

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 until 8 PM EDT this evening for MEZ023>028. NH...Beach Hazards Statement until 8 PM EDT this evening for NHZ014. MARINE...Small Craft Advisory from 4 PM Sunday to 8 AM EDT Monday for ANZ150>154.