Area Forecast Discussion - Porto

Issued: Wed, Jan 7, 9:48 PM | Valid through: Wed, Jan 21

KEY POINTS

• Prolonged rainfall event with potential for moderate to heavy accumulations, especially from 2026-01-12 onward.
• Notable wind gusts (up to 87 km/h) accompanying heavier rainfall periods, increasing risk of localized disruptions.
• Temperature fluctuations near seasonal averages, but no significant cold air intrusion to suggest frozen precipitation.

SYNOPSIS

A complex, evolving synoptic pattern is expected to influence the Porto region through the next 96 hours. Initially, a weak trough axis will maintain a moist, unstable airmass, leading to persistent light rain. As the trough deepens and interacts with a developing low-pressure system to the southwest, a more pronounced plume of moisture will advect northward, supported by a strengthening low-level jet. This setup favors increasing rainfall rates and gusty winds, particularly from the 12th onward. Model guidance indicates a high probability of precipitation throughout the period, with notable discrepancies in timing and intensity of heavier rainfall episodes.

NEAR TERM (0-24 hours)

For the immediate 24-hour period (2026-01-07T21:48Z - 2026-01-08T21:48Z), focus is on the ongoing light rain event. Hourly data indicates a brief lull in precipitation through the early morning hours of the 8th, with a resurgence in activity by midday as the trough axis reorients. Temperature profiles remain stable, with lows near 6-7°C and highs around 12-13°C, precluding any threat of frozen precipitation. Notable wind gusts (up to 57 km/h) are expected during peak rainfall periods, particularly after 2026-01-08T18:00Z. Model agreement on precipitation timing has improved, though the oscillating trend in recent runs introduces some uncertainty regarding the exact onset of heavier rates.

SHORT TERM (24-72 hours)

The 24-72 hour window (2026-01-08T21:48Z - 2026-01-11T21:48Z) is marked by a transition to a more active pattern. The developing low-pressure system to the southwest will drive a significant increase in moisture flux, with PWAT anomalies exceeding 2.5 standard deviations by the 10th. This will manifest as moderate to heavy rainfall, with model consensus indicating a prolonged period of precipitation from the 10th onward. However, the exact timing and magnitude of the heaviest rainfall remain uncertain due to discrepancies in the handling of the low's track and intensity among guidance. Ensemble spread suggests a 12-hour window of variability in the onset of the most intense rainfall, with potential for localized totals exceeding 40 mm by the 12th. Wind gusts will become more pronounced, with peak values nearing 70 km/h during the 12th, warranting attention for coastal and elevated locales.

LONG TERM (beyond 72 hours)

Beyond 72 hours (2026-01-11T21:48Z onward), the pattern evolves into a more zonal flow regime, maintaining a moist, unsettled airmass over the region. While specifics become increasingly uncertain, the large-scale setup favors continued precipitation, albeit with potential for intermittent dry intervals. Notable wind gusts (up to 86 km/h) and heavy rainfall episodes (e.g., 2026-01-20) are highlighted as high-impact, low-probability events, contingent on the phasing of successive troughs and the resultant low-pressure systems. Temperature anomalies are expected to remain near seasonal norms, mitigating the risk of frozen precipitation.

CONFIDENCE & KEY UNCERTAINTIES

• High Confidence: Prolonged rainfall event with moderate to heavy accumulations; gusty winds accompanying peak rainfall periods.
• Moderate Confidence: Timing and magnitude of heaviest rainfall episodes, particularly from the 12th onward; track and intensity of the developing low-pressure system.
• Low Confidence: Exact wind gust maxima and localized rainfall totals, given model resolution challenges with small-scale features (e.g., coastal convergence zones).
• Key Uncertainties: (1) Envelope of model solutions for the low's track, which significantly impacts rainfall distribution and wind intensity. (2) Potential for microclimate effects in coastal vs. inland zones, which may not be fully resolved by guidance. (3) Oscillating model trends in precipitation intensity, suggesting a non-linear response to synoptic forcing.