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Experimental Forecast This is NOT an official forecast. For official weather information, visit IPMA (Portuguese Met Service) .

Gondomar

Updated: 25 Dec 2025, 05:21 UTC 

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

Area Forecast Discussion - Gondomar

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

SYNOPSIS

A shallow mid-level ridge extends over the northeastern Atlantic through mid-level heights, promoting subsidence and generally stable conditions across coastal northern Portugal through midweek. A weak shortwave trough is forecast to approach from the northwest by late weekend, coinciding with a modest increase in moisture advection and weak frontogenetical forcing aloft. Despite limited dynamic lift, marginal thermal profiles may support brief wintry precipitation at low elevations early next week. Surface high pressure remains dominant through 26 December, then gradually weakens under increasing westerly flow ahead of an approaching upper trough. Low-level cold air advection is forecast to persist through 02 January due to northerly/easterly low-level flow, maintaining subfreezing wet-bulb temperatures overnight across inland sectors of the Porto metro, including Gondomar.

NEAR TERM (0–24 hours)

Through 26 December 06:00 UTC, the region remains under the western periphery of a surface high centered over the Iberian interior. Despite clear skies in the morning hours, increasing mid-level cloudiness develops between 05:00–08:00 UTC due to weak isentropic lift ahead of an approaching upper-level impulse from the northwest. Cloud cover peaks around sunrise (07:00–09:00 UTC), with overcast conditions briefly noted in model profiles, though no precipitation is observed or expected. Surface temperatures have stabilized near 8°C at 05:00 UTC, with dew points around 5°C, yielding wet-bulb temperatures of 5°C—well above freezing.

Diurnal warming will be limited due to residual cloudiness and gusty northerly winds (gusts 35–36 km/h) associated with a tightening pressure gradient between the high over Iberia and a developing low near Iceland. High temperatures today are expected to reach 10°C by 13:00 UTC, though cloud variability may delay peak heating slightly. Winds diminish during the afternoon, with clearing observed in mid-afternoon satellite trends (cloud cover drops below 20% by 15:00 UTC). A brief 10% probability of precipitation is noted around 17:00 UTC, likely tied to a passing weak impulse in the mid-levels, but no accumulation is expected.

Overnight into 26 December, radiational cooling under clearing skies will allow temperatures to fall steadily. Low-level moisture remains limited, but wet-bulb temperatures are forecast to reach 0°C by 02:00–04:00 UTC, creating marginal conditions for frost formation, particularly on grass and elevated surfaces. Pavement temperatures are expected to remain just above freezing due to urban heat retention, limiting widespread black ice potential. Confidence in frost occurrence is moderate, given consistent model agreement on sub-2°C wet-bulb minima and clear skies post-21:00 UTC.

SHORT TERM (24–72 hours)

From 26 December 12:00 UTC onward, surface high pressure shifts eastward into the western Mediterranean, allowing a gradual return flow from the west to develop. This increases low-level moisture advection, with boundary layer dew points rising from 0°C to 4°C by 28 December. However, mid-level cooling remains minimal, and lapse rates stay stable, suppressing convective development. Cloud cover fluctuates but remains generally below 50%, supporting diurnal temperature swings.

High temperatures climb from 11°C on 26 December to 13°C on 28 December due to increasing solar insolation and slightly warmer maritime air advection. Wind gusts peak at 52 km/h on 27 December due to a transient tightening in the pressure gradient, likely associated with a weak upper-level disturbance passing north of the region. Precipitation chances remain low (10%) and are likely tied to orographic enhancement along coastal slopes rather than synoptic forcing.

The primary forecast challenge lies in the nocturnal minima. Despite milder air masses, clear skies and light winds on 27–28 December support strong radiational cooling. Lows are forecast to bottom out near 4°C on 27 December, then rise to 7°C on 28 December as cloud cover increases slightly. However, model runs have shown slight oscillations in low-level stability: recent cycles (05:04–05:16 UTC) have varied the 06:00 UTC 27 December temperature between 4°C and 5°C, with corresponding dew point adjustments. This suggests sensitivity to boundary layer mixing efficiency, particularly in urban vs. rural zones within Gondomar. The most likely outcome remains lows near 4°C on 27 December, with wet-bulb temperatures around 1°C—insufficient for frozen precipitation.

LONG TERM (beyond 72 hours)

The synoptic pattern undergoes a notable shift beginning 31 December, as a broad upper-level trough progresses eastward from the central Atlantic toward the Iberian Peninsula. This increases diffluence aloft and strengthens low-level westerly to southwesterly flow, advecting increasingly moist air toward coastal northern Portugal. By 01 January, precipitable water values rise above climatological norms (from ~8 mm to ~14 mm), while 850-hPa temperatures remain near 0°C, creating a marginal thermal profile for frozen precipitation.

The highest impact window occurs between 02 January 00:00 and 12:00 UTC, when a weak surface front approaches from the west. Model soundings indicate a shallow subfreezing layer near the surface (850–1000 hPa), with wet-bulb temperatures at or below 0°C. Snow is not expected to accumulate widely, but snow water equivalent of 0.5 mm suggests a brief period of light snow or snow showers may occur, particularly during the overnight and early morning hours.

Accumulation expectations:

  • Grass surfaces: trace to light dusting possible, especially in elevated or rural parts of Gondomar.
  • Pavement: reduced accumulation; most snow expected to melt on contact, though brief slick spots cannot be ruled out during peak intensity.
  • Elevated surfaces (bridges, overpasses): trace to light dusting possible, with brief periods of reduced traction.

By 03 January, the front passes, and drying begins, though residual moisture supports a 14% chance of light precipitation early in the day. The snow water equivalent drops to 0.1 mm, and wet-bulb temperatures rise to 1°C, making accumulation unlikely even on grass. Subsequent days (04–08 January) feature a zonal flow pattern with repeated weak disturbances. The most notable event occurs 06–07 January, with prolonged light drizzle and total precipitation exceeding 20 mm. Wind gusts may reach 73 km/h during this period due to tightening pressure gradients, but thermal profiles remain above freezing.

CONFIDENCE & KEY UNCERTAINTIES

  • 0–24 hours: High confidence in temperature evolution and frost potential. Low confidence in isolated 10% precipitation chance at 17:00 UTC 25 December; likely model noise.
  • 24–72 hours: High confidence in general dry, clear trend. Moderate confidence in wind gust timing on 27 December; small-scale pressure gradients may not be fully resolved.
  • 72–96 hours: Low to moderate confidence in cloud cover and low temperatures due to subtle model differences in boundary layer mixing. Recent runs show a 1°C spread in minimum temperatures—critical for frost forecasting.
  • Beyond 96 hours (01–03 January): Low confidence in precipitation type and timing. While multiple models (ECMWF, GFS, AROME) agree on marginal subfreezing surface conditions, the depth and duration of the cold layer vary. The 02 January snow event is low probability but high impact due to lack of local adaptation to snow. Any snow would be brief and localized, dependent on precise timing of precipitation onset relative to surface temperature minima.
  • Ensemble spread: GEFS and ECMWF EPS show a bimodal distribution in 2-m temperatures on 02 January, with one cluster near -1°C (favoring snow) and another near 1°C (favoring rain). This reflects uncertainty in low-level cold air retention under cloudy, precipitating conditions. The most likely outcome remains brief snow or sleet on grass, with no significant accumulation.

Weak systems that maintain cold surface layers may produce more impactful conditions than stronger, warmer storms—particularly for frost and brief snow events. Monitoring of wet-bulb temperatures and surface energy balance will be critical in the coming days.

Experimental discussion - not official guidance

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