textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes since the previous forecast cycle.

KEY MESSAGES

1. A cold front will cross this afternoon and evening, bringing scattered showers and gusty thunderstorms. High pressure brings fair weather with lower humidity over the weekend.

2. A cold front crosses Sunday night followed by a couple of disturbances around mid week bringing multiple chances for rain.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Radar and GOES imagery this afternoon shows developing showers and thunderstorms across the region with storms becoming more widespread in response to the approaching cold front and diurnally driven instability. Temperatures have managed to warm into the lower 80s in many locations with dew points well into the 60s south of the mountains. The latest SPC mesoanalysis shows up to around 1000 J/KG of MLCAPE stretched across portions of interior western ME along with 700-500mb lapse rates of ~6 C/Km. Effective bulk shear is modest at around 25 kts, which may help to sustain some stronger updrafts through early this evening, mainly across interior western ME. Any storms will be capable of producing locally strong to damaging winds along with perhaps hail. Showers and storms will wane this evening behind the front.

A much drier airmass will move in behind the front tonight with low temperatures falling into the lower to middle 40s across some interior valleys with 50s elsewhere. High pressure will build across New England this weekend, allowing for tranquil weather conditions with warm afternoons and cool nights. Highs on Saturday will be into the upper 70s to near 80 south of the mountain with highs on Sunday around 5 degrees warmer. Lows each night will be into the 40s and 50s.

KEY MESSAGE 2 DESCRIPTION...

An upper trough swinging through the Canadian Maritimes will send a cold front through northern New England Sunday night. This front will bring chances for showers and thunderstorms, mainly from the mountains to central Maine and points northward late Sunday. Latest deterministic and ensemble guidance favor mostly dry conditions Monday as high pressure builds in from the north and the cold front settles over southern New England as a stationary front.

A compact upper low over Canadian Prairies Tuesday will slowly translate east through the second half of next week. This feature will attempt to shift the stationary front northward as a warm front late Tuesday. Models diverge on how far north rain chances get on Tuesday with some solutions keeping all but far southern NH mostly dry. The latest NBM expands chance PoPs (30-50%) northeast across the area through Tuesday afternoon which seems reasonable given the spread in solutions. Model spread also exists with respect to the eastward progression of the upper low and a potential leading short wave crossing Wednesday and Thursday. This has resulted in a broad brush of 50 to 60 percent PoPs Wednesday through Thursday while it is likely that it will not be raining continuously through this time. Overall, the pattern supports some beneficial rainfall from the middle to end of the work week.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

VFR conditions will prevail through this evening outside of some possible TEMPO restrictions due to SHRA/TSRA, mainly at KAUG. VFR conditions are likely tonight for most sites but FG is possible late tonight through early Saturday morning, especially at KLEB and KHIE. S-SW winds will become NW at 5-10 kts overnight. NW winds of 10-15 kts and VFR conditions are then expected on Saturday.

Outlook:

Sunday-Sunday night: Mainly VFR, except low cigs and SHRA bring potential restrictions at HIE.

Monday-Monday night: Mainly VFR.

Tuesday-Wednesday: Lowering cigs and SHRA likely bring periods of at least MVFR.

MARINE

Onshore flow will become offshore behind a cold front this evening but winds and seas are expected to remain below SCA criteria through the period.

SW flow will freshen Sunday ahead of a cold front with gusts to 20 kts Sunday afternoon. Winds briefly turn offshore behind the front Sunday night and become light and variable through Monday. A front south of the waters slowly moves north through mid week allowing for increasing SE winds.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.