Updated: 25 Dec 2025, 05:21 UTC

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

Area Forecast Discussion - Espinho

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

SYNOPSIS

A broad mid-level ridge spans the eastern North Atlantic through the coming week, promoting generally stable conditions across coastal northern Portugal. A weak shortwave trough is expected to approach from the northwest late on 2026-01-01, initiating a gradual increase in cloud cover and moisture return by early 2026-01-02. This system will deepen slightly under modest diffluence aloft, leading to a period of light precipitation and increasing low-level winds through midweek. A trailing cold front associated with a northeastward-propagating cyclone near the British Isles will pass through the region on 2026-01-06, bringing the most substantial rainfall and wind of the period. High pressure rebuilds briefly by 2026-01-08. Surface high pressure remains anchored near the Azores, steering weak disturbances northeastward along a subtle polar front. Low-level cold advection persists behind each passage, maintaining minimum temperatures near or below freezing at times, particularly in shallow drainage zones and elevated surfaces.

NEAR TERM (0–24 hours)

As of 05:21 UTC, skies are partly cloudy with increasing stratocumulus cover during the predawn hours, consistent with weak low-level moisture advection from the northwest under a 925 hPa temperature of approximately 6°C and a moist layer extending to 850 hPa. Surface temperatures are near 7°C, with wet-bulb values around 5°C, indicating no current risk of frozen precipitation. Diurnal warming will proceed under diminishing cloud cover through late morning, with insolation overcoming residual marine stratification by 10–11 UTC. High cloud erosion will allow for peak heating near 12°C by 13:00–14:00 UTC.

Isolated convective development cannot be ruled out in the afternoon due to weak daytime heating over land and a shallow moist layer, though CAPE remains negligible (<50 J/kg) and deep-layer shear is minimal. Any showers would be brief, localized, and driven by mesoscale boundaries or coastal convergence—not synoptic forcing. The 20% probability of precipitation in the afternoon reflects marginal instability and low cloud-to-ground efficiency in model soundings (GFS, IFS).

Low temperatures tonight will fall to around 3°C by 06:00 UTC on 2025-12-26, supported by clear skies and light winds (surface winds <10 km/h after 21:00 UTC). Wet-bulb temperatures will dip to 2°C, but no freezing is expected due to residual soil heat and maritime influence. Frost remains possible on grass and elevated surfaces where radiative cooling is unimpeded. Confidence in dry conditions is high, though a stray drizzle strand from a passing marine layer cannot be entirely ruled out near dawn.

SHORT TERM (24–72 hours)

From 2025-12-26 to 2025-12-28, the ridge axis shifts slightly eastward, allowing weak short-period waves to track across the far North Atlantic west of Portugal. The first of these, a subtle 500 hPa shortwave, will pass several hundred kilometers to the north on 2025-12-27, inducing weak cyclonic vorticity advection into the western Iberian coast. This may enhance low-level convergence marginally, supporting a 10% chance of light precipitation, primarily as drizzle or very light showers during the late morning to early afternoon hours. However, boundary layer stability remains high, and precipitable water values stay below 15 mm—limiting any output.

Temperatures will remain near seasonal normals, with daytime highs of 11–14°C and overnight lows of 3–6°C. The 27th features gusts to 40 km/h from the northwest, likely tied to low-level ageostrophic adjustments along a weak trough axis. On the 28th, a subtle ridge rebuild leads to warmer conditions (high of 14°C) despite persistent 10% POP, as subsidence suppresses deep convection.

Model guidance (ECMWF, GFS, AROME) shows strong agreement on the synoptic pattern through this period, with ensemble spread minimal in geopotential height fields. However, POP values remain sensitive to microscale boundary layer processes not resolved in convection-permitting models. Any precipitation would be brief and non-accumulating, with no impact on travel or infrastructure expected.

LONG TERM (beyond 72 hours)

The pattern undergoes a notable shift beginning 2026-01-01, as a deepening mid-latitude cyclone moves northeastward from the central Atlantic toward the British Isles. This system enhances southwesterly flow ahead of its cold front, advecting increasingly moist air (PWATs rising to 25–30 mm by 01-05) into northern Portugal. The approach will bring overcast conditions on 2026-01-01, with a narrow window of 6h light precipitation (0.6 mm) and a 10% POP—indicative of low confidence in timing and coverage.

By 2026-01-02, the warm sector becomes established, with low clouds and light drizzle persisting for 11 hours. Despite only 3.7 mm of total accumulation, the duration supports reduced visibility and damp surfaces. Wet-bulb temperatures remain at 1°C, with a minimum air temperature of 2°C—raising a marginal frost-in-drizzle scenario on elevated surfaces if precipitation occurs during overnight hours. However, pavement temperatures are expected to remain above freezing due to latent heat release and urban heat retention.

The primary event unfolds on 2026-01-06, as the cold front passes with strong low-level jet support (850 hPa winds >25 kt). This will produce 23.1 mm of rain over 24 hours, with peak rainfall rates of 1.5–2.0 mm/h. Gusts to 79 km/h are possible in squall-line embedded bursts, though no organized convective mode is indicated in current soundings. Post-frontal cold advection will drive temperatures back down, with a low of 4°C on 2026-01-08 and a brief return of 10% POP—possibly as light snow flurries at elevation (>200 m), but no accumulation expected at sea level.

Ensemble guidance (GEFS, EPS) shows increasing spread beyond +120h, particularly in the phasing of the upstream trough and the intensity of the surface cyclone. Some members delay the frontal passage by 12–18 hours, while others amplify the low more aggressively. However, the consensus maintains a moderate-impact rainfall and wind event on 01-06, followed by drier, cooler conditions.

CONFIDENCE & KEY UNCERTAINTIES

0–48 hours: High confidence in dry conditions, temperature trends, and cloud evolution. POP values are low and reflect marginal instability, not organized forcing.
48–72 hours: Moderate to high confidence in pattern evolution; low confidence in precipitation occurrence due to reliance on boundary layer processes.
Beyond 72 hours: Moderate confidence in pattern, low confidence in timing and intensity of the 2026-01-06 system. Ensemble spread increases notably in track and intensity of the parent low.
Frost risk: Brief frost possible on grass and elevated surfaces on 2025-12-25, 2025-12-26, and 2026-01-04, where wet-bulb temperatures dip to 1–2°C and skies are clear. Pavement temperatures unlikely to reach 0°C due to maritime influence.
Winter precipitation: No accumulation expected at sea level. Any snow flurries on 2026-01-04 or 2026-01-08 would be brief, localized to elevated terrain (>200 m), and non-impacting.
Model oscillation: Recent model runs (last 20 minutes) show stable temperature forecasts (4°–5°C overnight, 11°C daytime) and decreasing precip totals (from 1.1 mm to 0.8 mm), suggesting convergence in low-level moisture handling. No significant trend indicates a shift in synoptic evolution.

Experimental discussion - not official guidance