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Póvoa de Varzim

Updated: 25 Dec 2025, 05:21 UTC 

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

Area Forecast Discussion - Póvoa de Varzim

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

SYNOPSIS

A shallow mid-level ridge extends over the eastern North Atlantic through the end of December, maintaining stable, dry conditions across coastal northern Portugal. Weak shortwave energy embedded in a broad westerly flow will begin to approach from the northwest by late Dec 26, increasing cloud cover and introducing marginal instability and moisture advection. A stronger, deeper shortwave trough will amplify into the Iberian coast from the west on Jan 5–6, coinciding with a deepening surface low to the northwest. This will drive a prolonged period of enhanced onshore flow, increasing low-level moisture convergence and sustained moderate winds. The synoptic pattern shifts toward a zonal to slightly cyclonic regime by early January, supporting a transition from dry to wetter conditions with prolonged drizzle and elevated wind gusts. No significant cold air advection is forecast; minimum temperatures remain above freezing at sea level throughout the period.

NEAR TERM (0–24 hours)

Through 05:21 UTC Dec 26, conditions will remain dry with temperatures stabilizing near 8–9°C under increasing mid-level cloudiness. Observed cloud cover has increased from minimal overnight values to near 30% at 05Z, consistent with weak moisture advection ahead of an approaching shortwave trough over the mid-Atlantic. Surface observations and model profiles indicate a well-mixed marine boundary layer with dew points near 6–7°C, supporting cloud development but not saturation at the surface.

Temperatures will rise through the late morning and early afternoon, reaching a peak near 12°C by 14–15Z. Insolation remains limited due to increasing mid- and high-level cloud cover, but sufficient to erode residual nocturnal inversion. Winds will remain light, gusting to 30 km/h in the afternoon due to increased boundary layer mixing.

By late afternoon into evening (18–21Z), cloud cover will decrease temporarily to near 1–10%, allowing for partial clearing. However, increasing mid-level moisture and vorticity advection from the west will promote cloud redevelopment after 00Z Dec 26. No precipitation is expected before 02Z, when isolated showers may develop offshore. The 63% probability of precipitation noted in the 12Z Dec 26 forecast reflects increasing ensemble spread in convection timing, not intensity. Any precipitation will be light (≤0.5 mm/h), brief, and driven by weak mesoscale convergence along the coast.

Thermal structure remains stable with surface-based CAPE near zero and lifted indices > +5. No thunderstorms are anticipated. Low confidence exists in the exact timing of initial shower development, as high-resolution models (AROME, HRRR) differ on coastal convergence efficiency. Most guidance suggests initiation between 02–06Z Dec 26, with peak coverage near 08–12Z.

SHORT TERM (24–72 hours)

From 05:21 UTC Dec 26 to 05:21 UTC Dec 28, the weak shortwave trough will pass just north of Iberia, inducing broad cyclonic flow aloft. This will sustain onshore low-level winds (15–25 km/h) and maintain moisture advection, with 850–700 hPa specific humidity values increasing 0.5–1.0 g/kg above climatology.

Precipitation chances will peak early on Dec 26 (63% at 17Z), then decline rapidly as the trough axis moves east. By 12Z Dec 27, dry air entrainment and subsidence behind the trough will suppress convection. The 15% chance of precipitation on Dec 27 reflects lingering low-level moisture and weak coastal convergence, but no synoptic forcing.

Temperatures will remain steady: lows near 6–8°C, highs 12–14°C. Diurnal range is constrained by persistent cloud cover and maritime influence. Wet-bulb temperatures will stay near 4–6°C, well above freezing. No frozen precipitation is possible at any elevation.

Wind gusts will peak near 35 km/h on Dec 27 due to increased boundary layer mixing under stronger low-level flow, but no wind advisories are expected. Cloud cover will decrease from 100% early on Dec 26 to less than 20% by midday Dec 27, supporting a return to clear skies.

Model agreement is high on the broad synoptic evolution, though ECMWF and GFS diverge slightly on the strength of moisture return post-trough. The ensemble spread in 6-hourly QPF remains below 1.0 mm across all members, supporting low-impact rainfall expectations.

LONG TERM (beyond 72 hours)

From Dec 28 onward, the large-scale pattern transitions toward a more active regime. A deepening extratropical cyclone will develop west of Portugal by Jan 5, with a strong 500 hPa shortwave amplifying into the western Iberian coast. This will establish a persistent westerly to southwesterly flow, enhancing low-level moisture advection (precipitable water values rising to 20–22 mm, near 85th percentile).

Prolonged drizzle is expected from Jan 5–7, with total accumulations of 10–20 mm over 48 hours. Despite low probability of precipitation per hour (<15%), duration will lead to measurable totals. The 81 km/h gust forecast on Jan 6 reflects strong pressure gradient forces as the surface low deepens to near 990 hPa northwest of Iberia.

Thermal structure remains maritime: surface temperatures will stay between 8–12°C, with no freezing level concerns. Wet-bulb temperatures will rise to 8°C by Jan 6, reducing evaporative cooling and increasing drizzle persistence. No convective instability is expected; precipitation will be stratiform in nature, driven by large-scale ascent and boundary layer saturation.

By Jan 8, the system will exit northeastward, allowing high pressure to rebuild and clear conditions to return. The cold pool remains offshore; no Arctic or continental air mass intrusion is indicated in any ensemble member.

Pattern confidence is moderate to high beyond 72 hours, as the GFS, ECMWF, and UKMO agree on cyclone development and timing. However, exact placement of the low (northwest vs west of Portugal) introduces uncertainty in wind and precipitation intensity. Spread in 10m wind gusts exceeds 20 km/h in ensemble means on Jan 6, indicating low confidence in exact peak wind values.

CONFIDENCE & KEY UNCERTAINTIES

  • High confidence in dry conditions through Dec 27, given strong model agreement on ridge axis position and lack of forcing.
  • Medium confidence in timing and coverage of light showers on Dec 26 (02–12Z); uncertainty stems from mesoscale model differences in coastal convergence and boundary layer mixing efficiency.
  • Low confidence in exact precipitation duration and wind gusts Jan 5–7; while synoptic evolution is consistent, small shifts in cyclone track could alter onshore flow intensity.
  • Very low confidence in sub-hourly precipitation timing beyond 72 hours; models resolve prolonged drizzle but not convective cell timing in stratiform regimes.
  • No confidence in frozen precipitation at any point; all profiles indicate surface and wet-bulb temperatures well above 0°C.
  • Model oscillation in recent runs (0.8–1.1 mm QPF for Dec 26) reflects sensitivity to boundary layer moisture initialization, not structural change. Trends are stable.

Experimental discussion - not official guidance

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