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

Trofa

Updated: 25 Dec 2025, 05:21 UTC 

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

Area Forecast Discussion - Trofa

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 early next week, steering weak shortwave troughs along its southern periphery toward the Iberian Peninsula. Surface high pressure remains anchored over the Bay of Biscay through 26 December, promoting subsidence and dry conditions across northern Portugal. By 01 January, a broad but weak cyclonic circulation develops over the central Atlantic, increasing low-level moisture advection into the region. A slow-moving, low-amplitude trough approaches from the west during the 03–06 January period, enhancing cloud cover and supporting light, prolonged precipitation. Despite modest synoptic forcing, cold air damming persists near the surface due to nocturnal radiational cooling and weak diurnal mixing, allowing wet-bulb temperatures to dip below freezing on multiple nights, particularly 01–02 and 03–04 January. The pattern suggests repeated marginal cold-weather impacts despite minimal storm intensity.

NEAR TERM (0–24 hours)

Through 25 December, high pressure dominates with clearing skies observed in the early morning hours. Cloud cover has diminished from earlier in the night, with satellite trends and model analyses indicating dissipation of a weak nocturnal stratus layer by 06–07 UTC. Surface observations and model soundings show a shallow moisture layer eroding under modest subsidence, consistent with GFS and IFS analyses showing 500 hPa heights near 558 dm over the region. Temperatures have stabilized near 6°C at 05 UTC, with dew points around 4°C, and will begin a diurnal rise under increasing insolation. Weak cold air drainage continues in rural sectors of Trofa, but urban heat retention limits frost development in built-up areas.

Daytime heating will peak around 11°C by 13–14 UTC, supported by 850 hPa temperatures near 10°C and clear skies. Boundary layer mixing is expected to deepen to ~1.2 km by midday, limiting convective development. Nocturnal cooling tonight will be efficient under clearing skies post-sunset, with surface temperatures falling to near 0°C by 06 UTC 26 December. Wet-bulb temperatures will dip to 0°C, raising potential for frost on grass and elevated surfaces. Pavement temperatures, moderated by residual heat, are expected to remain just above freezing in most areas. Low confidence exists in exact timing of the coldest period due to microscale variability in cloud cover and urban-rural contrasts.

Precipitation chances remain below 15% through 12 UTC, but increase to 43% by late afternoon on 26 December as a weak shortwave trough approaches from the Atlantic. This feature is evident in 700 hPa Q-vectors and weak positive vorticity advection in the 500–400 hPa layer. However, deep-layer moisture remains limited (PWATs ~18 mm), and instability is negligible. Any showers will be brief, isolated, and convective in nature—likely driven by diurnal heating rather than synoptic lift. Model guidance (ENS, AROME-EPS) shows low spatial consistency in timing and coverage, suggesting any precipitation will be scattered and non-cumulative.

SHORT TERM (24–72 hours)

From 26 to 28 December, the weak trough passes just south of the region, with the primary jet axis remaining offshore. 850 hPa temperatures rise slightly from 9°C to 11°C, supporting daytime highs of 11–12°C. Low-level flow shifts to southerly, promoting warm air advection, though boundary layer inversion strength limits vertical mixing. Gusts to 43 km/h on 27 December are tied to a shallow low-level jet (~800 hPa) associated with the trough’s exit. Cloud cover remains variable, with model consensus (IFS, GFS, ARPEGE) indicating partial clearing by late 27 December.

The period 29–31 December features gradual cooling aloft as the ridge weakens and a broader upper low develops over the central Atlantic. Despite clear skies through 29–30 December, nocturnal radiational cooling becomes more effective due to drier air masses and lighter winds. Lows of 3°C are expected, but wet-bulb temperatures will dip to 1–2°C, increasing frost potential on grass and shaded surfaces. On 31 December, increasing mid-level cloudiness develops ahead of an approaching upper trough, with 700 hPa relative humidity rising above 70% by 12 UTC. Temperatures fall to near 1°C overnight, with wet-bulb values at or just below freezing—raising the potential for black ice on untreated surfaces, though accumulation remains unlikely due to minimal moisture.

Precipitation chances remain low (10–15%) through 01 January, but model ensembles (GEFS, ECMWF EPS) show increasing consensus on weak frontogenesis along a stalled surface boundary near the western Iberian coast. Any precipitation would be light and intermittent, with QPFs below 2 mm in most members. The primary uncertainty lies in low-level thermal structure: if cold air remains trapped near the surface, a brief transition to freezing drizzle cannot be ruled out during overnight hours of 01–02 January, particularly in low-lying areas of Trofa. However, model soundings show a shallow sub-freezing layer (<300 m deep), making significant ice accumulation unlikely. Most model solutions maintain pavement temperatures above 0°C during precipitation periods.

LONG TERM (beyond 72 hours)

The pattern shifts toward a zonally negative phase in the North Atlantic Oscillation (NAO) by early January, with a deepening upper low near 30°W and a reinforcing shortwave train moving into the eastern Atlantic. This setup supports repeated weak trough passages across Iberia from 02–07 January. The most persistent system arrives 05–06 January, with a broad 1000–500 hPa thickness gradient supporting sustained low-level convergence. PWATs increase to 22–25 mm, the highest in the 96-hour window, supporting prolonged light precipitation.

Despite modest dynamics, the persistence of cold surface air (due to repeated overnight radiational cooling and limited mixing) results in wet-bulb temperatures near or below freezing on multiple nights. The 02 January event features the highest risk: surface highs only reach 7°C, with lows to -1°C and wet-bulb temperatures down to -2°C. If precipitation begins while surface temperatures are below freezing, brief periods of freezing drizzle are possible, particularly on grass and untreated elevated surfaces. Pavement accumulations would be trace at most, but black ice remains a concern during early morning hours.

From 05–07 January, sustained southerly flow increases low-level moisture, but 850 hPa temperatures remain near 8–9°C, limiting instability. Precipitation is expected to be light but persistent, with model QPFs ranging from 7–21 mm over 24 hours (ECMWF on the high end, GFS on the low). Gusts increase to 70–75 km/h due to tightening pressure gradients aloft, though surface impacts will be moderated by terrain. By 08 January, the system exits, with high pressure rebuilding and temperatures rebounding slightly.

CONFIDENCE & KEY UNCERTAINTIES

  • High confidence in dry conditions and frost potential through 26 December, supported by consistent model agreement on subsidence and clear skies.
  • Medium confidence in timing and coverage of isolated showers on 26 December; spatial variability due to convective nature limits precision.
  • Low to medium confidence in precipitation type 01–02 January. While most models keep surface temperatures above freezing during precipitation, cold air pooling in localized areas could support brief freezing drizzle. Urban areas likely to remain above freezing.
  • Low confidence in exact QPF totals 05–07 January due to spread in ensemble members (ECMWF vs. GFS moisture advection timing). However, consensus on prolonged light rain is increasing.
  • High confidence in repeated sub-freezing wet-bulb temperatures overnight, especially 01–04 January, due to consistent model representation of clear skies, light winds, and dry air masses.
  • Key sensitivity: Any increase in cloud cover or wind speed during overnight hours would mitigate frost and freezing precipitation potential. Conversely, earlier clearing or stronger cold air advection could enhance impacts.

Experimental discussion - not official guidance

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