Updated: 25 Dec 2025, 05:21 UTC

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

Area Forecast Discussion - Santo Tirso

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

SYNOPSIS

A shallow but persistent upper-level trough over the northeastern Atlantic is steering weak shortwave impulses toward the Iberian northwest coast, though the primary jet axis remains displaced well to the north. Surface high pressure is centered over the central Atlantic, promoting gentle northeasterly to easterly flow across coastal northern Portugal. Low-level stability is maintained by subsidence and cool maritime air trapped beneath a weak mid-level ridge. This pattern supports dry, clear conditions through the weekend, with minimal thermal advection. A gradual increase in mid-level moisture and cyclonic vorticity advection begins late Dec 30, ahead of a broader upper low approaching from the west early next week. The synoptic evolution suggests a transition to unsettled conditions by Jan 05, with increasing low-level saturation and prolonged light precipitation embedded within a slow-moving baroclinic zone.

NEAR TERM (0–24 hours)

Through 06Z FRI DEC 26, the region remains under the influence of a weak surface ridge with light easterly flow aloft. Satellite and model-derived cloud fields indicate increasing mid-level stratiform cloudiness between 05–12Z over Santo Tirso, consistent with weak isentropic lift along the easterly flow interacting with coastal orography. However, boundary layer moisture remains limited (dew points 1–3°C), and the 0–3 km layer is capped by a 2–3°C inversion near 850 hPa, suppressing deep convection.

Temperatures will rise from current values near 4°C to a daytime high near 10°C by 12–14Z, supported by weak insolation and modest downslope warming in the easterly regime. Cloud cover will diminish during midday hours (10–16Z), with model output indicating clearing to partly cloudy conditions by late morning. A brief increase in low-level moisture (700–850 hPa RH > 70%) returns late in the afternoon into early evening (18–22Z), but precipitable water remains low (8–10 mm).

A 15% chance of isolated drizzle or virga is maintained between 14–18Z, primarily over elevated terrain north and east of Santo Tirso, where weak orographic lift may briefly saturate the subcloud layer. Surface-based convection is not expected. Overnight cooling will resume after 22Z, with temperatures falling to near 1°C by 12Z FRI, aided by clear skies and light winds. Frost formation cannot be ruled out on grassy surfaces, particularly in sheltered valleys where cold air drainage may enhance low-level cooling. Wet-bulb temperatures will dip to 0°C, but no frozen precipitation is expected due to surface temps remaining at or just below freezing.

Model guidance has stabilized over the last 6 hours, with minimal spread in 2-m temperature and cloud cover fields. The latest HRRR and AROME-EPS cycles show consistent timing of diurnal clearing and overnight radiational cooling.

SHORT TERM (24–72 hours)

From 06Z FRI DEC 26 to 06Z SUN DEC 28, the upper pattern remains broadly zonal with weak shortwave troughs passing north of Iberia. A subtle increase in cyclonic vorticity advection at 500 hPa occurs over the far eastern Atlantic late on DEC 27, but the primary dynamic forcing remains offshore. Low-level temperatures will fluctuate between 1–5°C overnight and 10–11°C during the day, with diurnal cloud cycles likely—clearing by late morning and increasing mid-level cloudiness in the afternoon.

On DEC 26, a weak moisture plume (IVT < 100 kg/m/s) brushes the coast during the afternoon, increasing low-level relative humidity and raising the probability of isolated drizzle to 30% between 12–18Z. However, instability remains negligible (CAPE < 50 J/kg), and any precipitation will be light and transient. Gusts to 45 km/h are possible during peak heating on DEC 27 due to increased low-level lapse rates and boundary layer mixing, though no organized wind event is expected.

By DEC 28, high pressure rebuilds briefly, promoting drier air and gusts diminishing to 35 km/h. Temperatures rebound to near 14°C due to stronger insolation and a deeper mixed layer. The 0°C wet-bulb minimum on DEC 27 night (-1°C) suggests frost and possible black ice on elevated surfaces, though pavement temperatures may remain marginally above freezing due to urban heat retention. Confidence in exact timing of minimum temps is moderate, as low-level cloud cover could delay cooling.

Ensemble spread in the 00Z GFS and ECMWF cycles remains low for temperature and pressure fields, but GEFS and IFS show a 10–20% increase in low-level humidity on DEC 27, introducing low confidence in whether brief drizzle develops. Most likely outcome remains dry at the surface.

LONG TERM (beyond 72 hours)

From 06Z SUN DEC 29 to 06Z TUE JAN 06, a progressive but slow upper-level pattern unfolds as a deepening upper low moves eastward from the central Atlantic toward the western Mediterranean. By DEC 31, increasing mid-level cloudiness and a tightening pressure gradient signal the approach of the baroclinic zone. Temperatures trend downward, with highs falling from 14°C to near 8°C by Jan 01 due to reduced insolation and increased marine stratocumulus advection.

A prolonged period of light precipitation is forecast from Jan 02–07, embedded within a quasi-stationary front stalled over northern Portugal. Precipitation is expected to be stratiform and light, with peak intensity on Jan 06 (28.4 mm/24h), coinciding with a 75 km/h gust event likely tied to low-level jet intensification ahead of a surface trough. However, ensemble spread in track and intensity of the upper low remains high beyond 120 hours—ECMWF develops the system farther south, favoring lighter, more prolonged drizzle, while GFS amplifies the trough more aggressively, increasing rainfall totals and wind speeds.

Wet-bulb temperatures remain near or just above freezing through Jan 04, then rise to 5°C by Jan 06 as milder maritime air overrides the surface cold pool. No frozen precipitation is expected at low elevations, though brief sleet or snow flurries cannot be entirely ruled out on elevated terrain (above 300 m) during the Jan 01–02 cold snap, particularly if low-level saturation coincides with surface temps near 0°C. Accumulations, if any, would be trace on grass and negligible on pavement.

Given the duration of light rain and gusty conditions, potential for minor hydrological impacts (puddling, reduced visibility) and wind stress on vegetation exists by early January, though no widespread flooding or wind damage is anticipated under current guidance.

CONFIDENCE & KEY UNCERTAINTIES

High confidence in dry conditions through DEC 28 and diurnal temperature trends, supported by consistent model agreement in surface pressure and boundary layer structure.
Moderate confidence in timing and coverage of light drizzle on DEC 26 and 27; dependent on precise low-level moisture advection and boundary layer saturation, which models resolve marginally.
Low confidence in exact precipitation amounts and wind speeds from Jan 05 onward, due to divergent phasing of the upper low in ensemble members. Spread in GEFS 500 hPa height anomalies exceeds 120 m by day 7.
Low confidence in frozen precipitation potential on Jan 01–02; surface temps near 0°C and wet-bulb at -1°C create a narrow window for brief sleet or flurries, but boundary layer drying in most soundings limits risk.
Note: Weak synoptic forcing but persistent cold air near the surface could enhance low-level saturation efficiency, making light precipitation more persistent than model QPF suggests. This is particularly relevant for Jan 02–04, where observed drizzle may exceed model output despite low ensemble precipitation probabilities.

Experimental discussion - not official guidance