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Oliveira de Azeméis

Updated: 25 Dec 2025, 05:21 UTC 

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

Area Forecast Discussion - Oliveira de Azeméis

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

SYNOPSIS

A broad mid-level ridge dominates the Iberian Peninsula through midweek, reinforcing subsident conditions across northern Portugal. Surface high pressure centered over the eastern Atlantic maintains light northerly to northeasterly flow at low levels, promoting dry and stable air mass characteristics through 27 December. A slow erosion of the ridge begins late 27 December as a progressive mid-latitude trough approaches from the northwest, increasing mid-level cyclonic vorticity advection into western Iberia by 31 December. This sets the stage for a moistening trend and eventual low-amplitude frontal passage during 01–03 January. No strong surface cyclones are indicated in current ensemble guidance, though GFS and ECMWF ensembles suggest modest deepening of a low near the Azores by early next week, increasing onshore flow and low-level moisture advection into the region. Boundary layer temperatures remain near or just above freezing through the period, with repeated overnight radiative cooling supporting frost formation.

NEAR TERM (0–24 hours)

Through 12:00 UTC, residual mid-level moisture continues to erode, with satellite trends and model soundings indicating drying in the 850–700 hPa layer. Surface observations and short-term model output (HRRR, AROME) show increasing insolation with solar angle recovery, supporting diurnal warming. Temperatures have already reached 6°C at 05:00 UTC and are expected to rise steadily to around 11°C by 12:00–14:00 UTC. Boundary layer dew points remain near 4–5°C, limiting convective potential despite modest daytime heating (MUCAPE < 50 J/kg).

Cloud cover decreases from ~100% at 09:00 UTC to less than 10% by 13:00 UTC, enhancing surface heating. Isolated cumulus development may occur over inland areas by midday due to weak low-level convergence along the Ave and Vouga river valleys, but no organized convection is expected.

Post-sunset, rapid radiative cooling under clearing skies will drive temperatures down from 8°C at 18:00 UTC to near 2°C by 23:00 UTC. Wet-bulb temperatures fall to 1°C, with dew point depressions narrowing to 1–2°C, increasing the potential for patchy frost formation on grass and elevated surfaces by 06:00 UTC 26 December. Pavement temperatures may remain just above freezing due to urban heat retention, but frost cannot be ruled out in rural and open areas.

Precipitation chances remain low (10–15%) through the afternoon and evening, tied to weak transient shortwaves in the 700 hPa flow. These features lack sufficient moisture and lift to generate measurable precipitation, and any showers would be brief and localized. Model runs over the past hour show stable temperature and precipitation solutions, with minimal spread in 2m temperature and 850 hPa height fields.

SHORT TERM (24–72 hours)

From 26 to 28 December, the ridge axis shifts eastward, maintaining dry conditions but allowing for slightly increased low-level moisture return from the Atlantic. Temperatures remain steady, with daytime highs near 11–12°C and overnight lows between 1°C and 3°C.

On 26 December, weak isentropic lift along a shallow 850 hPa trough may enhance low-level cloudiness during the morning, particularly in coastal zones. However, subsidence inversions limit vertical development, and precipitation remains unlikely despite a 25% POP in the afternoon—this is tied to isolated convective cells in the boundary layer, not synoptic forcing.

By 27–28 December, increasing low-level southerly flow (7–10 kt at 850 hPa) supports warm air advection, with surface temperatures peaking at 12–14°C. Gusts to 35 km/h are expected each afternoon due to enhanced boundary layer mixing under stronger insolation and weak pressure gradients. No precipitation is forecast, but cloud cover may increase slightly during the afternoon due to boundary layer turbulence.

The main uncertainty lies in the strength and timing of nocturnal inversions. With clear skies and light winds, radiative cooling remains efficient. However, slight increases in low-level moisture (dew points rising to 3–4°C) may dampen cooling efficiency, potentially keeping minimum temperatures 1–2°C higher than model guidance suggests. This could reduce frost coverage compared to previous nights.

LONG TERM (beyond 72 hours)

From 29 December onward, the upper pattern transitions from zonal to increasingly amplified as a shortwave trough progresses into the eastern North Atlantic. By 31 December, mid-level cyclonic vorticity advection increases over Portugal, with 500 hPa Q-vectors indicating weak but persistent lift. This supports a gradual increase in cloud cover, with overcast conditions expected by midday.

Surface temperatures decline to near 8°C amid reduced insolation, though no precipitation is forecast. The dry signal continues into 01 January, despite increasing low-level relative humidity and a rising wet-bulb temperature (to 1°C). The first measurable precipitation is not expected until late 01 January, with light drizzle developing under a moist, stable marine layer.

From 02–07 January, ensemble guidance (GEFS, ECMWF EPS) shows a trend toward stronger onshore flow as a low-pressure system develops southwest of Portugal. Low-level jet intensification (850 hPa winds to 25–30 kt) supports sustained moisture advection, with IWV values rising from 15 mm to 25+ mm by 06 January. This coincides with the highest precipitation totals in the period (up to 23 mm), though coverage remains widespread light drizzle rather than convective activity.

Notably, boundary layer temperatures remain near or just above freezing through 04 January, with a minimum of -1°C forecast. Wet-bulb temperatures fall to -2°C, raising the potential for brief freezing drizzle on elevated surfaces or untreated pavements, particularly in rural areas. However, no widespread ice accumulation is expected due to limited liquid equivalent and short duration. Confidence in frozen precipitation is low, but the risk cannot be ruled out during the overnight hours of 03–04 January.

Wind gusts increase to 70+ km/h by 06–07 January, tied to tightening pressure gradients between the developing low and a building high over central Europe. These winds may enhance wind chill but are not expected to cause structural damage.

CONFIDENCE & KEY UNCERTAINTIES

  • 0–48 hours: High confidence in dry conditions and temperature trends. Model consistency in 850 hPa heights and surface pressure fields supports stable forecast.
  • 48–72 hours: Medium confidence. Slight differences in low-level moisture advection between IFS and GFS could affect nocturnal cloud cover and minimum temperatures.
  • Beyond 72 hours: Low to medium confidence. While the large-scale pattern shift is consistent across ensembles, timing and intensity of moisture return and frontal passage vary. The 05–07 January period shows notable spread in precipitation timing and totals (ECMWF drier, GFS wetter).
  • Frozen precipitation: Low confidence, but high impact if realized. Any occurrence would be brief and limited to grass, elevated surfaces, or untreated roads during overnight hours of 03–04 January. Pavement accumulations unlikely due to marginal thermal profile and short duration.
  • Wind gusts: Increasing confidence in stronger winds 05–07 January, supported by consistent ensemble clustering in SLP gradients.

No significant changes from recent model runs; solutions have stabilized over the past 30 minutes with minimal drift in temperature or precipitation fields.

Experimental discussion - not official guidance

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