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Paredes

Updated: 25 Dec 2025, 05:21 UTC 

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

Area Forecast Discussion - Paredes

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

SYNOPSIS

A broad mid-level ridge dominates the eastern North Atlantic and Iberia through midweek, reinforcing subsident conditions across northern Portugal. Surface high pressure centered near the Azores extends a weak anticyclonic circulation over the region, promoting dry and generally stable air through 27 December. A gradual erosion of the ridge begins late 27 December as a progressive shortwave trough approaches from the northwest, increasing mid-level cyclonic vorticity advection into the western Iberian coast by 30 December. This sets the stage for a moistening trend aloft and increasing cloud cover into early January. By 05–06 January, a deeper mid-latitude cyclone is forecast to pass north of Iberia, steering a warm front toward the coast and introducing prolonged low-stratus, light precipitation, and enhanced low-level winds. The synoptic evolution suggests a transition from radiational cooling under clear skies to advection-dominated conditions with near-saturated profiles and marginal freezing levels.

NEAR TERM (0–24 hours)

Through the remainder of 25 December, the region remains under the influence of a shallow nocturnal inversion with weak surface pressure gradients. Despite clear skies early in the UTC cycle, increasing mid-level moisture advection from the west has led to a transient increase in mid/high cloud cover between 05–12 UTC, with cloud fraction peaking near 100% between 07–09 UTC. This is consistent with satellite water vapor imagery showing a moist plume wrapping into the western periphery of the Azores high. Clouds are expected to erode rapidly during late morning and early afternoon as subsidence strengthens and boundary layer mixing increases. Surface temperatures will rise to near 10°C by 13–15 UTC, aided by weak downslope flow in the Douro Valley influence zone.

Low-level moisture remains limited, with dew points near 4–5°C, keeping wet-bulb temperatures below freezing only at night. The 13% probability of precipitation in the late afternoon (16–18 UTC) reflects isolated convective development in a weakly unstable boundary layer (estimated 100–200 J/kg MLCAPE), though deep layer shear is negligible and any showers would be brief and non-accumulating. Winds will remain light, gusting to 34 km/h in the early afternoon due to turbulent mixing.

Overnight into 26 December, radiational cooling will be efficient under clearing skies after 20 UTC. Surface temperatures are expected to fall to near 0°C by 02–04 UTC, with wet-bulb temperatures reaching -1°C. Frost formation will be widespread on grass and elevated surfaces, though pavement temperatures may remain just above freezing due to urban heat retention in the Paredes periphery. Low confidence exists in the exact timing of the minimum temperature, as minor variations in residual cloud cover or wind speed could delay the coldest period by 1–2 hours. The 23% chance of precipitation late in the night reflects marginal model signals of weak low-level convergence from a passing shortwave, but QPF remains near zero and no accumulation is expected.

SHORT TERM (24–72 hours)

From 26 to 28 December, the ridge axis shifts eastward, allowing for a modest increase in low-level westerly flow. Temperatures will rebound slightly, with daytime highs near 10–11°C and nighttime lows rising from 0°C on the 26th to 5°C by the 28th. This warming trend is driven by increasing low-level moisture advection and cloud cover, reducing nocturnal radiational cooling efficiency. Wind gusts will increase to 48 km/h on the 27th due to tightening pressure gradients aloft, though surface impacts will be limited by terrain shielding.

The most notable feature is the absence of precipitation despite increasing humidity. Mid-level drying and strong subsidence prevent deep cloud development, with models showing a persistent temperature inversion near 850 hPa. Wet-bulb temperatures remain above freezing during the day and only dip to 0°C at night, eliminating any frozen precipitation threat. Any brief showers would be convective in nature, tied to daytime heating, and confined to higher terrain west of Paredes.

By 29–30 December, the upper pattern shows signs of downstream development over the British Isles, increasing zonal flow across the eastern Atlantic. This enhances mid-level cyclonic vorticity advection over Iberia by 31 December, coinciding with increasing cloud cover and a return to cooler daytime highs (near 8°C). Low confidence exists in the exact timing of this transition, as GFS and ECMWF differ on the amplitude of the shortwave. However, both agree on a moistening trend in the boundary layer, with surface dew points rising to 6–7°C by 01 January.

LONG TERM (beyond 72 hours)

The pattern shifts toward a more zonal to slightly cyclonic regime from 01–08 January. A deepening extratropical cyclone is forecast to pass north of 45°N between 05–07 January, directing a warm conveyor belt toward the Iberian northwest. This will result in prolonged stratiform cloud cover, light drizzle, and sustained low-level winds (gusts 70–75 km/h) across coastal northern Portugal. Precipitation amounts are modest (up to 28 mm over 24 hours on 06 January), but duration and timing suggest potential for minor hydrological impacts, especially if antecedent ground conditions are saturated.

Of greater concern is the thermal profile during this period. Freezing levels will fluctuate between 600–900 m MSL, but surface temperatures are expected to remain just above freezing (1–2°C) during precipitation events. However, brief periods of wet-bulb temperatures at or below 0°C (notably on 02 and 04 January) cannot be ruled out, particularly during light precipitation at night. This raises the potential for frost or black ice on elevated surfaces and untreated pavements, despite negligible snow accumulation. Any frozen precipitation would be brief and non-accumulating at sea level, though grass-level frost or trace glaze cannot be ruled out under clear, calm breaks.

The return to drier conditions on 08 January coincides with a rebuilding ridge, but temperatures will remain cool (highs near 8°C, lows near 2°C), maintaining a risk of frost under clearer skies.

CONFIDENCE & KEY UNCERTAINTIES

  • 0–24 hours: High confidence in temperature trend and frost development. Low confidence in isolated afternoon showers (timing and coverage).
  • 24–72 hours: High confidence in dry conditions and warming trend. Low confidence in exact wind gust timing on 27 December due to mesoscale variability.
  • 72–120 hours: Medium confidence in cloud and precipitation onset. Low confidence in exact precipitation efficiency and duration due to model spread in low-level moisture convergence (ECMWF wetter, GFS drier).
  • Freezing risk: Low probability, but high impact if wet-bulb temperatures dip below 0°C during precipitation. Most models show surface temps just above freezing, but shallow cold air advection could enhance surface cooling, particularly on 04 January when a weak high follows a frontal passage.

Model guidance has been stable over the last 6 hours, with recent runs (05:04–05:16 UTC) showing consistent temperature and QPF solutions (±0.5°C, ±0.3 mm). However, ensemble spread increases beyond 30 December, particularly in the timing of the first significant moisture return. The 11% probability of precipitation on 06 January reflects this uncertainty—while the synoptic setup favors rain, exact placement of the warm front remains sensitive to upstream phasing.

Experimental discussion - not official guidance

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