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Valongo

Updated: 25 Dec 2025, 05:21 UTC 

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

Area Forecast Discussion - Valongo

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

SYNOPSIS

A shallow but persistent cold air mass remains entrenched across coastal northern Portugal through the end of December, supported by weak subsidence and high pressure aloft. Surface high pressure is centered over the eastern Atlantic, promoting light northerly to northeasterly flow at low levels. This pattern suppresses deep moisture advection but allows for marginal cold air retention near the surface, particularly in shallow drainage zones such as Valongo. A slow progression toward zonal flow begins late week, but cold air advection persists aloft into early January. A weak shortwave trough approaches from the west by January 1st, increasing cloud cover and introducing light, prolonged drizzle with marginal freezing levels. No strong cyclonic development is evident in ensemble guidance through the 96-hour window, though GFS and IFS show increasing low-level jet activity by January 5th–6th, coinciding with the highest precipitation probabilities in the extended period.

NEAR TERM (0–24 hours)

As of 05:21 UTC, Valongo remains under clearing skies with temperatures near 6°C and dew points around 4°C. The remainder of the morning (06–12 UTC) will see increasing insolation despite lingering mid-level moisture, with cloud cover decreasing from 100% at 08 UTC to below 25% by 11 UTC. Surface-based heating will drive temperatures toward 10°C by mid-afternoon (14–15 UTC), supported by light winds and minimal cloud interference.

The primary uncertainty lies in isolated convective development during the afternoon. While MUCAPE remains negligible (< 25 J/kg), weak daytime heating over the hills surrounding the Porto metro area may trigger isolated shallow convection. Model soundings show a capping inversion near 850 hPa, limiting deep convection, but elevated lapse rates (6.5°C/km between 925–850 hPa) could support brief virga or sprinkles, particularly over elevated terrain. The 18% probability of precipitation in the afternoon is consistent with HRRR and AROME-EPS members showing isolated 0.1–0.3 mm accumulations, though most of Valongo is expected to remain dry.

Temperatures will fall steadily after sunset, with radiative cooling favored under clearing skies. By 00 UTC on Dec 26, temperatures are expected to reach 2°C, with wet-bulb temperatures near 1°C. Frost formation on grass and untreated surfaces is likely, though pavement temperatures may remain just above freezing due to urban heat retention. No frozen precipitation is expected, but the marginal thermal profile warrants monitoring for any unexpected moisture advection.

SHORT TERM (24–72 hours)

Through December 27th, the region remains under weak subsidence with surface highs anchored west of Iberia. Temperatures will hover near 0°C during the overnight hours of Dec 26–27, with wet-bulb minima reaching -1°C. Despite dry conditions, the combination of clear skies and light winds (below 10 km/h) supports continued radiative cooling, increasing frost potential. The 25% chance of precipitation on Dec 26 afternoon appears tied to a weak impulse passing north of the region, with only marginal moisture and lift. Any precipitation would be brief and non-accumulating, likely in the form of light drizzle or virga.

On Dec 27, gusty northerly winds develop due to tightening pressure gradients aloft, with 10m gusts reaching 51 km/h in exposed areas. This may enhance mixing and limit frost formation during the overnight of Dec 27–28, despite minimum temperatures near 2°C. The warming trend from Dec 28 onward is consistent across ensemble members (GEFS, IFS-EPS), with 850 hPa temperatures rising from -2°C to +2°C by Dec 30. This warming is driven by increasing southerly flow aloft, though surface temperatures remain moderated by marine influence.

Cloud cover remains minimal through Dec 30, with no significant moisture plumes evident in integrated vapor transport (IVT) fields. The main synoptic feature of note is a broad upper-level trough progressing eastward across the central Atlantic, but its closest approach remains well to the north, limiting direct impacts.

LONG TERM (beyond 72 hours)

From December 31 onward, the upper pattern transitions toward a more zonal flow, with a progressive shortwave trough approaching from the west. This system is reflected in increasing cloud cover on Dec 31 and the onset of light drizzle by January 1st. Freezing levels are expected to fluctuate near 500–700 meters, with surface temperatures near or just below 0°C on Jan 1 (low: -1°C).

Given the marginal thermal profile, any precipitation falling during the overnight hours of Jan 1–2 cannot rule out brief periods of freezing drizzle, particularly on elevated surfaces or untreated roadways. However, model soundings (GFS, IFS) show a shallow subfreezing layer near the surface (< 300 m depth), and warm layer aloft (above 750 hPa) suggests most precipitation will melt before reaching the surface. Accumulations, if any, would be trace on grass and negligible on pavement.

The period Jan 5–7 shows increasing confidence in a stronger low-level moisture feed from the southwest, with IVT values rising to 150–200 g/kg·m in the IFS. This coincides with 25–30 mm of total QPF across 48 hours, though probabilities remain low (6–11%) due to timing and duration rather than intensity. Winds increase during this period, with gusts to 73 km/h on Jan 6, likely tied to low-level jet development ahead of the surface trough. Despite higher totals, rainfall rates are expected to remain light due to stratiform structure. No significant flooding is anticipated given antecedent dry conditions and low intensity.

CONFIDENCE & KEY UNCERTAINTIES

  • High confidence in dry conditions and frost formation through Dec 27, supported by consistent model agreement on subsidence and clear skies.
  • Medium confidence in timing and intensity of light drizzle Jan 1–2; ensemble spread shows a 6–12 hour window for onset, with some members (CMC) delaying precipitation until Jan 2.
  • Low confidence in surface precipitation type Jan 1 morning: while most guidance favors above-freezing surface temps, cold pockets in Valongo’s topography could support brief freezing drizzle. Urban areas likely to remain at or slightly above 0°C.
  • Low confidence in gust timing Jan 5–7: models diverge on the phasing of the surface low and low-level jet, with GFS showing earlier peak winds than IFS.
  • No model oscillation in temperature trends; recent runs (past 2 hours) show stable solutions between 4°–5°C for current temps and 9°–10°C for afternoon highs. Minor QPF fluctuations (0.8–1.1 mm) reflect convective timing, not trend.

Weak synoptic forcing dominates the period; therefore, mesoscale effects (drainage flows, urban heating, shallow convection) may have disproportionate impacts on surface conditions, particularly for frost and fog development.

Experimental discussion - not official guidance

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