Area Forecast Discussion - Porto

Issued: Mon, Dec 29, 11:03 PM | Valid through: Mon, Jan 12

KEY POINTS

• High-impact rainfall event expected 2026-01-09 to 2026-01-11, with potential for strong gusts (up to 86 km/h) and localized flooding due to prolonged heavy rain.
• Marginal cold air intrusion on 2026-01-11 may lead to trace snow accumulations on elevated surfaces and grass, with melting on pavement.

SYNOPSIS

A zonal flow regime dominates the initial period, maintaining dry and stable conditions across Porto. A transition to a more meridional pattern is anticipated by the long term, driven by a deepening trough over the Atlantic, which will tap into a moist, unstable airmass. This will lead to a prolonged period of heavy rainfall and strong winds, with embedded shortwave troughs enhancing precipitation intensity.

NEAR TERM (0-24 hours)

2025-12-30 (00Z-24Z): The near term remains governed by a strong surface high-pressure system, leading to clear skies and light winds. Temperatures will exhibit a diurnal range of 4-13°C, with no precipitation expected. Model agreement is high, with recent runs (as of 2025-12-29T22:10Z) showing stable temperature trends. The anomalous 2025-12-29T22:10Z run, which suggested slightly cooler temperatures, has been discounted due to its outlier status and lack of support from subsequent guidance.

SHORT TERM (24-72 hours)

2025-12-31 to 2026-01-02: A gradual transition begins as the high-pressure system weakens, allowing for increasing cloudiness by 2026-01-01. Precipitation probabilities rise to 100% by 2026-01-02, with light rain expected. The synoptic setup involves a strengthening low-level jet and increasing moisture advection from the southwest. Uncertainty exists regarding the timing of the initial precipitation onset, dependent on the progression of a weak shortwave trough. Ensemble spread indicates a 3-6 hour window of variability for the onset of rain.

LONG TERM (beyond 72 hours)

2026-01-03 and beyond: The pattern evolves into a more active regime, with a deep, slow-moving low-pressure system dominating the Atlantic. This will lead to a prolonged period of heavy rainfall (2026-01-09 to 2026-01-11), with model consensus indicating totals exceeding 30 mm. Strong southwesterly winds will develop, with gusts potentially reaching 86 km/h. A marginal cold air intrusion on 2026-01-11 may introduce a brief window for snowfall, though surface temperatures are expected to remain near or above freezing, limiting accumulations to elevated surfaces and grass.

CONFIDENCE & KEY UNCERTAINTIES

• High confidence: Prolonged heavy rainfall and strong winds 2026-01-09 to 2026-01-11, with potential for localized flooding and gust-related impacts.
• Moderate uncertainty: Timing and intensity of the cold air intrusion on 2026-01-11, which will dictate the presence and magnitude of snow accumulations. Models suggest a narrow window for snow, but small-scale thermal variations (e.g., urban heat island effects) may preclude accumulation in some areas.
• Low confidence: Precise rainfall totals for 2026-01-09 to 2026-01-11, as convective enhancement and orographic effects may not be fully resolved by current model guidance.