Area Forecast Discussion - Porto

Issued: Mon, Dec 29, 6:49 PM | Valid through: Mon, Jan 12

KEY POINTS

• High-impact rainfall event on 2026-01-09 with gusts to 100 km/h, potential for localized flooding and wind damage.
• Prolonged wet period from 2026-01-01 to 2026-01-12, with multiple rounds of precipitation.
• Near-freezing temperatures during overnight hours on 2026-01-05, though no frozen precipitation indicated.

SYNOPSIS

A zonal flow regime dominates the next 96 hours, with a series of Atlantic low-pressure systems embedded within a strong jet stream. These systems will tap into ample moisture, leading to a prolonged period of precipitation across northern Portugal. The strongest system, arriving on 2026-01-09, exhibits characteristics of a hybrid low, with both baroclinic and tropical influences, raising concerns for heavy rainfall and strong winds.

NEAR TERM (0-24 hours)

2025-12-29T18:00Z - 2025-12-30T18:00Z

• Timing/Intensity: Dry conditions persist through the near term, with clear skies and light northerly flow. Hourly data indicates temperatures will bottom out at 4°C by 2025-12-30T06:00Z, with daytime highs recovering to 13°C.
• Uncertainty: Model agreement is high, with recent runs showing stable temperature and precipitation trends. The only notable discrepancy is cloud cover at 2025-12-30T23:00Z (85% in latest run), though this is not expected to impact surface conditions.
• Microclimate Effects: Coastal areas may experience slightly cooler temperatures due to marine influence, though diurnal ranges remain consistent with inland zones.

SHORT TERM (24-72 hours)

2025-12-30T18:00Z - 2026-01-02T18:00Z

• Synoptic Setup: A weak frontal boundary approaches on 2026-01-01, introducing a period of light rain (6.1 mm). This is followed by a more organized low-pressure system on 2026-01-02, producing moderate rainfall (14.1 mm).
• Uncertainty: Ensemble spread increases for the 2026-01-02 system, with some members suggesting a slower progression. This could delay the onset of precipitation by 6-12 hours, though the overall rainfall totals remain consistent.
• Thermal Structure: Temperatures will remain mild, with lows near 5-10°C. No threat of frozen precipitation, even in inland zones, as the warm sector dominates.

LONG TERM (beyond 72 hours)

2026-01-02T18:00Z - 2026-01-12T00:00Z

• Pattern Overview: The zonal flow persists, with a parade of low-pressure systems maintaining unsettled conditions. The strongest system on 2026-01-09 stands out, with a deepening trough and strong jet streak supporting intense rainfall and gusty winds.
• Notable Features: The 2026-01-09 system's potential for gusts to 100 km/h warrants close monitoring, as this could lead to power outages and property damage. The overall pattern favors above-average precipitation, with some inter-model variability in the exact timing and intensity of individual systems.

CONFIDENCE & KEY UNCERTAINTIES

• High Confidence: Prolonged wet period, heavy rainfall on 2026-01-09, and gusty winds.
• Moderate Uncertainty: Timing of individual precipitation events, particularly the 2026-01-02 system, which may be delayed.
• Low Confidence: Exact wind gust magnitudes on 2026-01-09, as small-scale features may not be fully resolved by models.
• Key Risks: The 2026-01-09 event has the potential to be high-impact, with flooding and wind damage concerns. While models agree on the synoptic pattern, localized effects (e.g., orographic enhancement, coastal wind funnelling) may exacerbate impacts in specific areas.