Updated: 25 Dec 2025, 05:21 UTC

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

Area Forecast Discussion - Maia

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

SYNOPSIS

A shallow mid-level ridge extends over the eastern Atlantic and Iberia through mid-week, supporting generally dry and stable conditions across northern Portugal. Weak shortwave troughs will brush the region from the northwest late weekend into early next week, coinciding with a gradual cooling trend aloft. Low-level moisture increases modestly by early January as a broader upper trough develops over the eastern North Atlantic, increasing low-cloud and drizzle potential. Surface highs remain dominant through 27 December, then retrograde westward, allowing weak baroclinic influences to approach from the Atlantic by 01 January. No strong frontal systems are indicated in the medium range, but subtle low-level convergence and cold air advection aloft may support marginal wintry precipitation late 02 January.

NEAR TERM (0–24 hours)

Through 26 December 06:00 UTC, high pressure remains centered over the Iberian plateau, promoting subsidence and dry conditions. However, increasing mid-level cloudiness is observed in the 05:00 UTC hour, with cloud cover rising from 26% to 95% between 05:00–08:00 UTC. This is consistent with weak diffluence aloft ahead of a subtle shortwave trough moving eastward across the Azores. Despite increasing cloud cover, no precipitation is expected through 12:00 UTC due to dry boundary layers (WB temps 5–7°C) and capping inversion evident in model soundings.

Surface temperatures have reached 10–11°C by 05:00 UTC and will peak near 12°C by 15:00 UTC as daytime heating briefly erodes low clouds. Winds remain brisk from the northwest, gusting to 37 km/h between 04:00–07:00 UTC due to modest pressure gradient between the surface high and a weak low over the Bay of Biscay.

From 18:00 UTC onward, skies clear rapidly as the shortwave passes and subsidence strengthens. Overnight cooling will be efficient under clearing skies and light winds, with radiative cooling favored. Low temperatures are expected to bottom out near 4°C by 23:00 UTC, with wet-bulb temperatures near 2°C. Frost is possible on grassy surfaces, though no accumulation is expected on paved surfaces due to urban heat retention and above-freezing pavement temps.

Precipitation chances increase after 00:00 UTC 26 December, rising to 48% by 15:00 UTC. However, this signal is marginal—model soundings show weak ascent and low-level moisture advection, but boundary layer temperatures remain above 3°C, and precipitable water values are below 10 mm. Any precipitation would be brief and light, likely virga or spotty drizzle, with no accumulation expected. Confidence in measurable precipitation remains low.

SHORT TERM (24–72 hours)

From 26 December 12:00 UTC through 28 December 12:00 UTC, the ridge rebuilds briefly, reinforcing dry conditions. Cloud cover decreases to <20% by 12:00 UTC 27 December, supporting daytime highs near 12°C. Low-level flow turns slightly more onshore by 28 December, increasing moisture advection from the Atlantic. Surface dew points rise from 2°C to 5°C, and low clouds become more persistent overnight into 29 December.

Gusts increase to 44 km/h on 27 December due to tightening pressure gradients aloft, though surface stability limits mixing. No precipitation is expected through 28 December due to insufficient lift and warm low-level temperatures (minimum WB: 5°C).

By late 28 December, a weak shortwave trough approaches from the northwest, increasing cloud cover and lowering cloud bases. Model consensus (ECMWF, GFS, AROME) indicates weak frontogenetical forcing in the lower levels by 01 January, but the absence of strong synoptic ascent or deep moisture limits confidence in widespread precipitation.

The most notable feature emerges late 02 January: a narrow corridor of cold air advection aloft (700–850 hPa temperatures dropping to -2 to -4°C) coincides with a shallow moisture plume. This creates a brief window for mixed-phase precipitation. Model soundings indicate freezing levels descending to near 300–400 m MSL, with surface temperatures near 2°C and wet-bulb temps at 1°C.

This setup supports a low-probability, high-impact scenario: brief light snow or sleet showers between 16:00–18:00 UTC 02 January, primarily over elevated terrain and inland areas. Accumulations are expected to be minimal:

Grass surfaces: trace to light dusting possible (melting begins rapidly after deposition)
Pavement: no accumulation expected (melting on contact due to above-freezing surfaces)
Elevated surfaces (bridges, overpasses): trace accumulation possible, but rapid melting expected

Precipitation amounts are modest (8.8 mm liquid equivalent over 17 hours), but duration and timing favor evaporative cooling, potentially allowing brief wet-bulb depression to freezing. However, model ensemble spread (GEFS, ECMWF EPS) shows only 10–15% of members producing frozen precip at the surface, and most terminate accumulation within 1–2 hours.

03 January sees rapid warming aloft, with 850 hPa temps rebounding to -1°C and surface temps rising to 4°C. Any residual precipitation will be light drizzle, ending by midday. Gusts remain near 37 km/h due to continued weak pressure gradients.

LONG TERM (beyond 72 hours)

The pattern shifts toward a deeper mid-level trough over the eastern Atlantic by 04–08 January, increasing zonal flow and maritime influence. This supports a series of weak disturbances moving eastward, each with limited moisture and lift.

Temperatures trend downward through 04 January, with lows near 1°C and wet-bulb temps near 0°C—raising concern for frost and black ice on untreated surfaces, particularly in rural and elevated zones. However, no additional frozen precipitation is indicated beyond 03 January.

From 05–07 January, increasing low-level convergence and boundary layer saturation support prolonged light drizzle, especially during overnight and morning hours. Total accumulations are modest (10–17 mm), but duration may lead to localized slick spots and reduced visibility. Wind gusts increase to 85–86 km/h on 06–07 January due to stronger cyclonic flow aloft, though no gale-force surface winds are expected.

The return to clear conditions by 08 January coincides with weakening flow and rebuilding high pressure. Confidence decreases beyond 06 January due to divergent model solutions in the phasing of upper troughs, but the overall regime favors cool, damp conditions with limited diurnal temperature range.

CONFIDENCE & KEY UNCERTAINTIES

0–24 hours: High confidence in dry conditions and temperature evolution. Low confidence in isolated precipitation late 25–26 December; timing and intensity of moisture return remain sensitive to shortwave position.
24–72 hours: Medium confidence in thermal trend and wind gusts. Low confidence in precipitation type 02 January—outcomes are highly sensitive to boundary layer cooling, timing of moisture arrival, and exact freezing level depth. A delay of 2–3 hours could prevent any frozen precip.
Beyond 72 hours: Low to medium confidence. Models consistently depict a cooler, wetter regime but disagree on the number and strength of disturbances. Precipitation amounts and wind speeds have high ensemble spread after 05 January.

Notable model oscillation: recent high-resolution runs (AROME, HRRR) have toggled between 0.8–1.1 mm of precipitation for 26 December, with slight shifts in shortwave timing. However, no run produces measurable accumulation, and all maintain surface temps above freezing.

Critical threshold monitoring:

Wet-bulb temps near or below 0°C on 02 and 04 January may support frost or brief snow on elevated surfaces despite above-freezing dry-bulb temps.
Pavement temperatures will be decisive for any winter weather impacts—urban areas unlikely to see accumulation, but rural and elevated zones may experience brief slick conditions.

Experimental discussion - not official guidance