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Vila Nova de Gaia

Updated: 25 Dec 2025, 05:21 UTC 

AFDPRT
EXPERIMENTAL AFD FOR VILA NOVA DE GAIA
EXPERIMENTAL FORECAST OFFICE - PORTO METRO
05:21 UTC THU DEC 25 2025

Area Forecast Discussion - Vila Nova de Gaia

Issued: 05:21 UTC | Valid through: 05:21 UTC MON JAN 6 2026

SYNOPSIS

A shallow mid-level ridge extends over Iberia through the end of December, supporting generally dry and stable conditions across coastal northern Portugal. A weak shortwave trough approaches from the northwest late on 26 December, introducing modest moisture advection and increasing low-level cloudiness, though deep-layer ascent remains limited. Surface high pressure remains anchored near the Azores, with weak onshore flow developing late in the period. A stronger Atlantic cyclone approaches from the west on 01–02 January, increasing moisture flux and forcing prolonged stratiform precipitation into the region by early 02 January. The system deepens rapidly south of Ireland, generating strong westerly flow across Portugal by 03 January. A secondary surface low may develop along an old frontal boundary near the Iberian coast on 05–06 January, enhancing low-level convergence and precipitation efficiency. The pattern shifts to zonal to slightly meridional by 07–08 January, with lingering moisture and cooler temperatures.

NEAR TERM (0–24 hours)

As of 05:21 UTC, skies are partly to mostly cloudy with increasing stratocumulus development overnight, consistent with weak low-level moisture advection under a stable marine layer. Temperatures have cooled to around 5°C at the surface, with wet-bulb values near 3°C, indicating no frozen precipitation risk. Winds have increased to 35–40 km/h from the southwest, with gusts peaking near 40 km/h between 04–06 UTC, likely tied to low-level jet pulsing under the ridge’s eastern flank.

Through the morning hours (06–12 UTC), solar insolation will be limited due to persistent cloud cover, with maximum cloud fraction near 100% between 07–09 UTC. This should suppress daytime heating, though temperatures are expected to rise slowly from 8°C to 11°C by midday, supported by weak warm advection aloft (700–850 hPa). The boundary layer remains capped, limiting convective mixing. Clouds should erode from west to east during the afternoon as subsidence strengthens, with clearing likely by 15–17 UTC. Precipitation chances remain below 15%, confined to isolated drizzle or virga from saturated boundary layer pockets—any surface accumulation would be negligible.

Nocturnal cooling tonight (25/26 Dec) will bring lows to around 3–4°C, with wet-bulb temperatures near 1–2°C. Frost cannot be ruled out on grassy or elevated surfaces, particularly in sheltered inland zones of the Gaia metro area. Low confidence exists in exact dew point depression trends, as small errors in boundary layer moisture could shift frost potential. Winds will diminish slightly after sunset but remain gusty (30–35 km/h) due to continued low-level flow.

SHORT TERM (24–72 hours)

From 26–28 December, the synoptic pattern features a weak shortwave trough moving across the northeastern Atlantic, with its associated vorticity maximum passing several hundred kilometers north of Portugal. While deep-layer forcing is weak, modest isentropic lift is expected along the coastal zone due to weak frontogenetical forcing in the low to mid-troposphere. This supports an increase in low-level cloudiness and a rise in precipitation probabilities to 38% on 26 December, primarily between 06–16 UTC.

Model guidance (ECMWF, GFS, AROME) shows low confidence in exact timing and intensity of any precipitation. The bulk of moisture remains offshore, with 850 hPa specific humidity values only rising to 6–7 g/kg. Any precipitation would be light drizzle or very light rain, with accumulations under 1 mm and duration less than 6 hours. Surface temperatures will remain above 5°C during the day and near 3°C at night, precluding any frozen precipitation.

Winds increase again on 27 December, with gusts to 48 km/h, likely due to tightening pressure gradients between a building surface high over the eastern Atlantic and a weak trough offshore. This may enhance coastal convergence, but boundary layer stability and lack of deep moisture limit shower development. The most likely outcome remains dry with periods of high cloudiness and breezy conditions.

By 28 December, a broader upper-level ridge begins to re-establish over the region, promoting warming aloft and surface temperatures reaching 14°C—the highest in the past 10 days. This warming is consistent with increasing southerly component in low-level flow and adiabatic subsidence. Nocturnal minimums rise to 8°C, reducing frost risk significantly.

LONG TERM (beyond 72 hours)

The pattern undergoes a notable shift beginning 01 January as a deep Atlantic cyclone develops southwest of Ireland. Ensemble guidance (GEFS, ECMWF EPS) shows strong consensus on the cyclone’s track and intensification, with MSLP dropping below 970 hPa by 03 January. This will drive a strong, moist westerly flow into northern Portugal, with 850 hPa winds exceeding 25 kt and integrated vapor transport (IVT) values approaching 250–300 g kg⁻¹ m s⁻¹—moderate AR criteria.

Precipitation is expected to begin as light drizzle on 01 January, with a 10% probability and 0.6 mm accumulation. However, confidence increases for more persistent rainfall starting 02 January, as the low-level jet strengthens and warm advection enhances ascent ahead of the surface low. Model solutions diverge on the exact placement of the surface cyclone and associated frontal structure, but all indicate prolonged stratiform rainfall with embedded weak convective elements due to conditional symmetric instability (CSI) in the warm conveyor belt.

Peak rainfall is forecast 05–07 January, with totals exceeding 20 mm in the latest runs. The 06 January forecast shows 22.8 mm over 24 hours, with gusts to 84 km/h—indicative of strong boundary layer mixing beneath the cyclone’s cold front. While total QPF is not extreme, the duration and antecedent ground saturation could lead to minor urban ponding or slow-response stream rises, particularly in poorly drained zones.

Temperatures remain cool during this period, with highs near 8–11°C and lows near 2–5°C. Wet-bulb temperatures stay above freezing, so no frozen precipitation is expected at sea level. However, brief sleet or snow flurries cannot be ruled out on elevated terrain above 400 m, though accumulations would be transient and limited to grassy surfaces.

By 08 January, the system exits into the western Mediterranean, with high pressure rebuilding from the west. This should bring drier conditions and lighter winds, though lingering mid-level moisture may support patchy low clouds.

CONFIDENCE & KEY UNCERTAINTIES

  • 0–24 hours: High confidence in dry conditions and temperature evolution. Low confidence in exact timing of cloud dissipation, which could delay afternoon warming by 1–2 hours.
  • 24–72 hours: Medium confidence in overall dry trend. Low confidence in precipitation occurrence on 26 December—models disagree on boundary layer saturation and low-level convergence timing. Frost potential on 26/27 Dec night is possible but highly dependent on local radiative conditions.
  • 72–96 hours: Medium confidence in warming trend on 28 Dec. Increasing confidence in moisture return starting 31 Dec, though exact onshore transition timing varies by 6–12 hours between models.
  • Beyond 96 hours: High confidence in cyclonic development over the Atlantic. Medium confidence in rainfall timing and intensity; low confidence in exact surface low track and frontal placement, which could shift precipitation onset by 12–24 hours. Ensemble spread on 06 January QPF ranges from 12–35 mm—moderate but not extreme.
  • Winter weather: No significant risk of snow or ice at sea level. Brief snow flurries on elevated terrain (e.g., Serra do Pilar) on 01–02 January cannot be ruled out but would not accumulate on pavement.
  • Model oscillation: Recent runs (05:04–05:16 UTC) show stable temperature forecasts (4°–5°C lows, 11°C highs) and declining QPF (1.1 mm → 0.8 mm), suggesting model solutions are converging on a drier near-term outcome.

Experimental discussion - not official guidance

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