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Matosinhos

Updated: 25 Dec 2025, 05:21 UTC 

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

Area Forecast Discussion - Matosinhos

Issued: 05:21 UTC | Valid through: 05:21 UTC MON JAN 6 2025

SYNOPSIS

A stable high-pressure ridge extends across the northeastern Atlantic and into Iberia through midweek, maintaining dry and generally clear conditions across coastal northern Portugal. Weak shortwave troughs embedded in a broad mid-level westerly flow will begin to approach from the northwest by late weekend, increasing cloud cover and introducing marginal instability and moisture advection by early next week. Surface pressures remain elevated through 28 DEC, then gradually fall as a deepening extratropical cyclone progresses southeastward from the Azores toward the western Iberian coast by 05 JAN. Low-level warm advection becomes increasingly pronounced ahead of this system, with isentropic lift supporting prolonged stratiform precipitation by 06–07 JAN. Despite modest total precipitable water (TPW ~25–30 mm), persistent low-level saturation and weak instability may support light to moderate drizzle, particularly in enhanced low-level convergence zones near the coast. No significant frontal passages are expected through 03 JAN, but baroclinic development increases thereafter.

NEAR TERM (0–24 hours)

As of 05:00 UTC, Matosinhos is under the influence of a subsiding air mass associated with the western periphery of a surface high centered near 40N 10W. Skies are partly cloudy with increasing stratocumulus development near dawn, consistent with nocturnal boundary layer stabilization and weak low-level moisture advection from the northwest. Surface temperatures are near 11°C, with wet-bulb values around 8°C, indicating minimal evaporative cooling. Cloud fraction has fluctuated between 15–40% in the past 3 hours, but model soundings suggest limited boundary layer depth (<600 m AGL), inhibiting convective development.

Through mid-morning (09–12 UTC), increasing solar insolation will erode low clouds from the south, leading to mostly sunny conditions by late morning. Surface heating will push temperatures to around 12°C by 14:00 UTC, with dew points near 7°C. A weak shortwave trough passing several hundred kilometers to the north will induce modest mid-level cooling (700–500 hPa lapse rates ~6.2°C/km), but boundary layer capping and lack of low-level convergence preclude any meaningful convective development. Isolated cumulus may form inland, but no precipitation is expected.

Tonight (18:00 UTC onward), radiative cooling will promote reformation of shallow stratocumulus, particularly along the immediate coast where marine layer depth increases under light northerly flow. Low temperatures are expected to bottom out near 9°C by 03:00 UTC 26 DEC. Wind gusts remain below 25 km/h, with surface pressure slowly rising to near 1028 hPa by 00:00 UTC 26 DEC. The 15% probability of precipitation in the afternoon and evening is associated with a brief mesoscale cloud band in high-resolution models (AROME, HRRR), but ensemble spread is low and no coherent forcing exists to support measurable accumulation.

SHORT TERM (24–72 hours)

From 26–28 DEC, the ridge axis shifts eastward into the Iberian Peninsula, allowing weak shortwaves to track across the North Atlantic just north of 45N. These disturbances will induce modest diffluence in the mid-level westerlies, leading to increasing cloud cover over Matosinhos, particularly on 26 DEC afternoon into 27 DEC. GFS and ECMWF show a subtle increase in 700 hPa relative humidity (from 50% to 80%) over the region by 12:00 UTC 26 DEC, coinciding with a 38% probability of 0.1 mm precipitation between 14:00–17:00 UTC. This signal is consistent across recent model runs, though timing varies by 2–3 hours between cycles.

The precipitation signal appears tied to weak isentropic ascent along a shallow marine boundary layer, with 850–925 hPa moisture advection peaking at ~5 g kg⁻¹ m s⁻¹. However, convective inhibition (CIN > 50 J/kg) and lack of surface-based instability suggest any precipitation will be light and stratiform, likely manifesting as brief drizzle or virga. Surface temperatures remain near 12°C during the day and fall to 7–8°C at night, with wet-bulb temperatures near or just below 6°C—insufficient for frozen precipitation, even if supercooled drops were present.

Gusts increase to 36 km/h on 27 DEC due to tightening pressure gradient between the departing ridge and a weak low approaching from the northwest. Cloud cover remains variable, with partial clearing possible during midday hours if subsidence persists aloft. By 28 DEC, high pressure briefly reasserts, allowing temperatures to warm to 14°C under mostly sunny skies. No significant changes in low-level thermal structure are expected, and confidence in dry conditions remains high outside of isolated coastal fog or stratus patches at night.

LONG TERM (beyond 72 hours)

From 29 DEC onward, the synoptic pattern undergoes a gradual transition. The Azores High weakens as a series of upper-level troughs deepen across the eastern Atlantic. By 31 DEC, increasing cloudiness is expected as low-level warm advection strengthens ahead of a broad upper low developing near 35N 20W. Surface temperatures remain near seasonal normals (9–11°C), but gusts increase to 34 km/h due to enhanced low-level flow.

The most notable change occurs between 05–07 JAN, as a deep cyclone (projected central pressure <980 hPa) moves southeastward from near the Azores toward 38N 12W. This system will drive persistent southerly flow into coastal northern Portugal, increasing TPW to near 30 mm by 06 JAN—well above the 25 mm threshold for prolonged drizzle in stable marine layers. ECMWF and GEFS ensemble members show strong consensus on sustained low-level saturation (925–850 hPa RH >90%) and weak conditional instability, supporting light to moderate drizzle for 12–24 hours.

Precipitation totals of 8–16 mm are indicated in the ensemble mean, though coverage is expected to be widespread but light, with peak intensities <0.5 mm/hr. The 6–11% probability of precipitation in the hourly data reflects the persistent nature of the event rather than convective timing. Gusts increase to 75–76 km/h on 06–07 JAN due to strong pressure gradient between the cyclone and a reinforcing high over central Europe. Despite elevated low-level temperatures (wet-bulb 8–9°C), any brief cooling aloft could support supercooled drizzle, but surface freezing is not anticipated.

CONFIDENCE & KEY UNCERTAINTIES

  • 0–24 hours: High confidence in dry, cool conditions with partial cloudiness. Low confidence in exact timing of afternoon cloud breaks due to model discrepancies in PBL depth (AROME vs GFS).
  • 24–72 hours: Medium confidence in brief, light precipitation on 26 DEC. Signal is consistent in HRRR and AROME but absent in some GFS ensemble members. Precipitation type will be liquid; no frozen forms expected.
  • 72–96 hours: Medium to high confidence in warming trend and increasing winds. Low confidence in exact timing and intensity of drizzle onset 05 JAN, as model solutions vary in low-level frontogenesis placement.
  • Beyond 96 hours (06–08 JAN): Medium confidence in prolonged light precipitation and gusty conditions. Ensemble spread in cyclone track is moderate (±3° longitude), which could shift heaviest rainfall offshore or inland.
  • Thermal profile: No model solution brings surface temperatures below 5°C during precipitation periods. Wet-bulb minima remain above 4°C, ruling out snow or ice at sea level.
  • Model oscillation note: Recent high-resolution runs (past 30 min) show minor fluctuations in low-level moisture timing (0.8–1.1 mm QPF), but no trend toward stronger forcing. Solutions have stabilized over the last 4 cycles.

Experimental discussion - not official guidance

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