textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes were made since the previous forecast cycle.

KEY MESSAGES

1. Pleasant weather continues through Sunday under high pressure.

2. Quiet weather comes to an end with unsettled weather returning for the first half of next week with the potential for areas of heavy rain.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

The combination of clear skies, light winds, and low dew points will allow for another night of radiational cooling tonight. Forecast lows are mostly based on MOS guidance, with lower to middle 40s expected across the northern valleys and lower to middle 50s elsewhere. Valley fog is once again likely, especially towards dawn as temperatures cool to near their dew point and a large temperature gradient between air and water sets up.

Surface high pressure will drift towards the Canadian Maritimes on Saturday, allowing for light southeasterly flow across the region. Other than some diurnally driven cu across the foothills and points north, mainly sunny skies will prevail with high temperatures into the 80s, although cooler temperatures will be common along the coast due to the onshore flow. Saturday night will be on the cool side for late July with lows into the 40s/50s.

Dry conditions will persist on Sunday with partly to mostly sunny skies and near average high temperatures into the lower 80s. Low temperatures on Sunday night will mainly be into the 50s with partly cloudy skies.

KEY MESSAGE 2 DESCRIPTION...

Early next week, an omega block pattern establishes across the region as the western ridge amplifies and Atlantic high builds northward, parking a longwave upper level trough over the East Coast through at least late week. An upper-level low over Hudson Bay will deepen the trough, driving a surface low and accompanying cold front across New England, with a focus in the Tue-Wed timeframe. Model guidance shows notable spread regarding the surface low. The GFS/GEFS favors a weaker, more progressive solution further south, whereas the ECMWF/EPS depicts a stronger (wetter), closed low further north.

The predominant forecast impact will be increasing shower activity and thunderstorms from late Monday through mid-week, potentially lingering into Thursday if the trough slows. Activity will be enhanced diurnally rather than a continuous washout. Southerly return flow will advect deeper moisture across the region, elevating PWATs into the 1.502.00" range alongside dew points in the 60s. This juicy moisture profile, combined with lowering heights aloft and steepening mid-level lapse rates, will support modest instability (mainly in the afternoons) capable of supporting localized brief downpours. While initial guidance kept the core jet dynamics displaced to our south, there will be shear and CAPE overlap ahead of the front supporting storms. Limiting factors include cloud cover, height falls, and falling 1000500 mb thickness values, with surface temperatures seasonably cool in the 70s before weak ridging attempts to push the trough offshore by next weekend.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Through 18Z Saturday: VFR conditions expected. The exception is at KLEB and KHIE where valley FG is once again likely, especially between 09Z-13Z Saturday. Light and variable flow will prevail through Saturday. No LLWS is anticipated through the period.

Outlook:

Saturday night-Sunday Night: VFR conditions expected. An afternoon sea breeze is likely on Sunday with light and variable winds becoming SSE, especially across coastal TAF sites.

Monday/Monday Night: Mainly VFR conditions expected. Isolated SHRA may cause brief localized restrictions.

Tuesday-Wednesday: MVFR or worse restrictions likely due to RA and possible TSRA.

MARINE

Winds and seas will remain below SCA criteria through Sunday night as surface high pressure remains over or east of the Gulf of Maine.

Outlook: The gradient will remain relaxed with winds and seas safely below SCA through the first half of the forecast. A low pressure system is projected to develop south of the area along the east coast, supporting multiple rounds of widespread rain and the occasional thunderstorm mainly Tue-Wed. Looking at increasing winds and seas with potential for SCA conditions. Quieter weather follows for the later half of the week.

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...None.