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Arouca

Updated: 25 Dec 2025, 05:21 UTC 

AFDPRT
EXPERIMENTAL AFD FOR AROUCA
EXPERIMENTAL FORECAST OFFICE - PORTO METRO
05:21 UTC THU DEC 25 2025

Area Forecast Discussion - Arouca

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

SYNOPSIS

A shallow surface high centered over the eastern Atlantic is reinforcing a stable, dry airmass across northern Portugal through midweek. Weak ridging aloft persists under broad subsidence, suppressing deep convection. A slow amplification of a mid-level trough over the northeastern Atlantic will begin to erode the ridge from west to east by the weekend, allowing for increased mid-level moisture advection into the Iberian northwest by early next week. Low-level temperature advection remains weak through 72 hours, with diurnal radiative processes dominating thermal trends. A progressive increase in low-level zonal flow and moistening is forecast from January 5 onward, coinciding with a deepening shortwave trough approaching from the northwest.

NEAR TERM (0–24 hours)

Current conditions at 05Z show clear skies with temperatures near 5°C and light winds, consistent with ongoing nocturnal radiative cooling under minimal cloud cover. Despite increasing mid-level cloudiness through sunrise (7–9Z), surface observations and short-term model guidance (HRRR, AROME) indicate continued dry conditions through the remainder of the night. The boundary layer remains decoupled, with dew points 1–2°C below temperature, limiting fog development.

Daytime heating will be rapid under clearing skies post-09Z, with insolation driving temperatures to near 11°C by 13–14Z. Low-level moisture remains marginal (surface dew points 3–4°C), and convective inhibition is strong, precluding any meaningful shower development despite isolated increases in boundary layer instability.

Precipitation probabilities remain below 15% through 00Z DEC 26, confined to weak orographic lifting pulses along the Serra da Freita, but no accumulating precipitation is expected. The primary uncertainty lies in low-level moisture advection timing from the west; however, model consistency (ECMWF-HRES, GFS-13km) shows minimal boundary layer moistening before 12Z DEC 26.

Wet-bulb temperatures will fall to near 1°C overnight, with minimum surface temperatures reaching 2°C by 06–07Z. Given the lack of snow level depression and no indication of frozen precipitation in soundings, no winter weather impacts are expected.

SHORT TERM (24–72 hours)

From 00Z DEC 26 through 00Z DEC 28, the region remains under the influence of a weak but persistent surface ridge, with 850-hPa temperatures near 0°C. Despite dry conditions, minimum temperatures are forecast to dip to 1°C on DEC 26 and 2°C on DEC 27, with wet-bulb values reaching -1°C and 0°C respectively. These values approach the threshold for frost formation on elevated surfaces and grass, though pavement temperatures are expected to remain above freezing due to residual heat retention.

A weak shortwave impulse may brush the coast late on DEC 26 (18–21Z), increasing cloud cover and briefly raising precipitation chances to 23% in the afternoon. However, model soundings (GFS, IFS) show a capping inversion at 800 hPa and limited moisture below 700 hPa, making any precipitation brief and non-accumulating. Any drizzle would be light and short-lived, confined to higher terrain.

By DEC 27–28, ridge re-strengthening leads to clearing skies and daytime highs rising to 12–14°C. Diurnal range widens due to dry soil conditions and minimal cloud cover. Gusts to 32 km/h are possible on DEC 27 due to modest low-level lapse rate steepening in the afternoon, though no widespread wind impacts are expected.

Model guidance has shown high stability in temperature forecasts over the past 6 hours, with ensemble spread (GEFS, EPS) for 2-m temperatures under 1.5°C through 72 hours. Precipitation remains the primary source of uncertainty, though all ensembles agree on totals <1 mm through DEC 28.

LONG TERM (beyond 72 hours)

The synoptic pattern undergoes a notable shift beginning DEC 31, as a deepening mid- to upper-level trough over the northeastern Atlantic drives a strong westerly flow into the Iberian Peninsula. By JAN 1, mid-level cyclonic vorticity advection increases across northern Portugal, with 700-hPa relative humidity rising above 70%. This supports a transition to overcast conditions and the potential for light precipitation.

The first measurable rainfall event in over a week is forecast from JAN 1–2, with ECMWF and GFS in reasonable agreement on QPF of 2–4 mm. However, ensemble spread increases notably from JAN 3 onward, particularly in the placement and intensity of the surface low. The 00Z DEC 28 GFS develops a stronger coastal low than the IFS, leading to a 10–15 hPa spread in MSLP across northern Portugal by JAN 5.

From JAN 5–7, sustained low-level onshore flow (850-hPa winds 15–20 kt) supports persistent moisture advection, with TPW values increasing from 18 mm to 24 mm. This coincides with a gradual cooling trend aloft, lowering freezing levels to near 1000 m MSL by JAN 6. While surface temperatures remain above 6°C, the combination of prolonged drizzle, gusty winds (up to 70 km/h), and reduced visibility may lead to minor travel impacts.

No model solution supports frozen precipitation at sea level during this period. However, should the surface low deepen faster than anticipated (e.g., GFS solution), cold air damming could become a factor, particularly if precipitation onset precedes full mixing. This scenario remains low probability but cannot be entirely ruled out for elevated terrain above 400 m.

CONFIDENCE & KEY UNCERTAINTIES

  • High confidence in dry conditions through DEC 28, supported by consistent model agreement on subsidence and low moisture availability.
  • Medium confidence in timing and intensity of light precipitation from JAN 1–3; while all models agree on increased cloudiness and some drizzle, QPF spread exceeds 2 mm in the EPS and GEFS.
  • Low confidence in wind speeds and precipitation duration from JAN 5–7 due to divergent handling of the coastal low development. The IFS maintains a weaker, faster system; the GFS amplifies it significantly. This affects both rainfall totals and gust potential.
  • Low probability, high-impact note: Should the coastal low deepen rapidly and precipitation commence during nighttime hours with surface temperatures near 2°C, brief sleet or snow flurries cannot be ruled out over elevated terrain (e.g., Serra da Freita). Accumulations on grass would be trace, with no impact expected on roadways.
  • Models continue to under-resolve fine-scale coastal convergence effects; any localized enhancement in precipitation near the escarpment west of Arouca would be brief and not reflected in gridded guidance.

Recent model runs show stable temperature trends with <0.5°C variation in min/max forecasts over the past 4 cycles. Precipitation trends remain stable in timing but exhibit minor oscillations in QPF (0.8–1.1 mm) for DEC 25–26, likely due to boundary layer nudging differences. No significant bias correction is warranted at this time.

Experimental discussion - not official guidance

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