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Experimental Forecast This is NOT an official forecast. For official weather information, visit IPMA (Portuguese Met Service) .

São João da Madeira

Updated: 25 Dec 2025, 05:21 UTC 

AFDPRT
EXPERIMENTAL AFD FOR SÃO JOÃO DA MADEIRA
EXPERIMENTAL FORECAST OFFICE - PORTO METRO
05:21 UTC THU DEC 25 2025

Area Forecast Discussion - São João da Madeira

Issued: 05:21 UTC | Valid through: 05:21 UTC MON DEC 29 2025

SYNOPSIS

A stable high-pressure ridge centered over the northeastern Atlantic and extending into western Iberia dominates the synoptic pattern through midweek. This feature suppresses deep moisture advection and limits cloud development, supporting dry and generally clear conditions across the Porto metro region, including São João da Madeira, through 31 December. Weak shortwave energy embedded in the mid-level westerly flow begins to approach from the northwest by 01 January, gradually increasing mid-level moisture and lowering cloud bases. A stronger, moist Pacific-origin system is forecast to approach from the west on 05–06 January, associated with a deepening mid-level trough and strong low-level jet, bringing prolonged light precipitation and increasing winds. Thermal profiles remain near or just above freezing through this period, with no indication of frozen precipitation at low elevations in the current guidance.

NEAR TERM (0–24 hours)

Through the remainder of 25 December, high pressure maintains a stable airmass over northern Portugal. Satellite and model-derived cloud trends indicate increasing mid-level cloud cover during the morning hours (06–12 UTC), likely tied to weak diffluence aloft ahead of an approaching shortwave trough over the Azores. However, subsidence beneath the ridge axis limits deep convective development and moisture accumulation in the boundary layer. Dewpoints remain near 3–4°C, with surface-based RH values between 70–80% overnight, increasing to 85–90% near sunrise on the 26th.

Temperatures will continue to fall through the morning hours, bottoming out near 2°C by 09–10 UTC, then gradually rising through the afternoon to a peak near 11°C. The 15% chance of precipitation during midday hours (14–17 UTC) reflects isolated, shallow convection in a weakly unstable boundary layer (MUCAPE < 50 J/kg), likely tied to localized convergence along minor terrain gradients. Any showers would be brief and light, with negligible accumulation (<0.1 mm). Model runs over the past hour show minimal spread in temperature and moisture profiles, with recent HRRR and AROME-EPS cycles converging on a dry solution through 00 UTC 26 December.

Boundary layer stability increases through the evening as high pressure reasserts control, leading to clearing skies and light winds (<10 km/h). Radiational cooling under clear skies will support another frost event overnight, with surface temperatures falling to 1°C by 06 UTC 26 December. Wet-bulb temperatures remain at or just above 0°C, increasing the potential for slick spots on grass and elevated surfaces, though pavement-level freezing is unlikely due to urban heat retention and dry conditions.

SHORT TERM (24–72 hours)

From 26–28 December, the ridge axis shifts slightly eastward, maintaining dry conditions across the region. Diurnal temperature ranges widen under persistent clear skies: daytime highs near 11–12°C, overnight lows between 1–4°C. The 25% chance of precipitation on the afternoon of the 26th reflects lingering uncertainty in low-level moisture pooling near the coast, possibly enhanced by weak sea-breeze convergence. However, mid-level drying and lack of synoptic lift make any convective development isolated and transient.

By 27–28 December, increasing mid-level zonal flow (700 hPa winds 20–25 kt) begins to advect slightly moister air from the Atlantic, but boundary layer decoupling and subsidence inhibit cloud-to-ground precipitation. Gusts to 35 km/h are expected both afternoons due to modest low-level lapse rate steepening under strong insolation, though no widespread wind impacts are anticipated.

The thermal regime remains stable, with minimum wet-bulb temperatures hovering near 1–3°C. Frost remains possible on clear, calm nights, particularly in low-lying areas of São João da Madeira where cold air drainage may enhance surface cooling. No model ensemble (ECMWF EPS, GEFS, or AROME) indicates a departure from this dry, stable pattern through 30 December. Low-level temperature inversions are expected each night, with model soundings showing 850–700 hPa temperatures 4–5°C warmer than surface values.

LONG TERM (beyond 72 hours)

The synoptic pattern undergoes a gradual transition beginning 31 December. An upper-level trough progresses eastward from the central Atlantic, increasing diffluence over Iberia by 01 January. This promotes ascent and increased mid- to low-level moisture advection, with TPW values rising from near 12 mm to 18–20 mm by 05 January. The result is a shift from clear to overcast conditions on 31 December, followed by the onset of light drizzle during the early morning hours of 01 January.

Precipitation remains light (1–3 mm/day) and stratiform through 03 January, with model soundings indicating a saturated boundary layer and weak conditional instability. Temperatures remain near 8°C during the day and 2–4°C at night, with wet-bulb values near or just above freezing. No frozen precipitation is indicated in any deterministic or ensemble member, even during overnight periods, due to mild 850 hPa temperatures (>4°C) and strong maritime influence.

From 05–07 January, a stronger Pacific cyclone approaches from the west, with a deepening 500 hPa vorticity maximum and strong low-level jet (>60 kt at 850 hPa) advecting deep tropical moisture. This increases rainfall coverage and duration, with total accumulations of 20–25 mm possible across the period. Wind gusts may reach 70–75 km/h, particularly on 06–07 January, due to enhanced low-level flow and boundary layer mixing. However, precipitation efficiency remains low due to weak instability and lack of dynamic forcing at the surface. The 20% probability of precipitation on 07 January reflects uncertainty in timing and duration rather than intensity.

No significant cold air advection is forecast during this period. Minimum temperatures remain above 0°C at all levels except possibly on grass surfaces during clear breaks on 04 January, when a brief frost cannot be ruled out under light winds and dry skies.

CONFIDENCE & KEY UNCERTAINTIES

  • High confidence in dry conditions through 30 December, supported by strong model consensus (ECMWF, GFS, UKMO) and stable boundary layer structure.
  • Medium confidence in timing and coverage of light drizzle 01–03 January; small shifts in low-level frontogenesis could delay onset or reduce coverage.
  • Low confidence in exact rainfall totals 05–07 January due to spread in GEFS and ECMWF EPS in phasing of the upper trough and strength of low-level jet. Some ensemble members delay heaviest rain until 08 January.
  • Low confidence in surface temperature minima on 04 January; clear-sky radiational cooling could allow brief pavement frost if winds remain below 5 km/h, though urban areas likely remain above freezing.
  • No model guidance supports snow or ice at sea level. Any frozen precipitation would require an unmodeled cold air damming event, which is not supported by current ensemble spread.

Small-scale variability in low-level convergence (e.g., sea-breeze, terrain channeling) may produce isolated showers not resolved by convection-permitting models. These would be brief and localized, with minimal impact.

Experimental discussion - not official guidance

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