Executive summary
Late-year 2025 is evolving with a prevailing northeast monsoon over southern China through much of November, and dynamical seasonal guidance favours a weak La Niña signal through early NDJ with a possible transition toward neutral by late winter. Expect generally drier tendencies relative to some recent wet summers, periodic replenishments of cool, windy air from the north and northeast, occasional cloud bands and storm risks coming off the South China Sea during transitional windows, and enhanced cool spells when the north (Siberian High / Himalayan cooled air) strengthens. The risk of Strong Weather systems dininmishes rapidly in this year phase.
Key large-scale drivers (what to watch)
As the hemisphere moves into winter, the Siberian High strengthens and cold air outbreaks become possible — these supply the cold, dry northerly/northeasterly flow that the NE monsoon channels toward southern China and Hong Kong. When strong, expect cooler temperatures, increased windiness and lower humidity at times.
The NE monsoon typically establishes in late Oct–Nov. For NDJ 2025/26 model runs and monthly bulletins indicate the NE monsoon and its 'replenishments' will influence coastal Guangdong and Hong Kong during November and intermittently into December, giving more frequent cool, breezy and cloudier conditions than autumn.
Global seasonal guidance in Nov–Dec–Jan 2025/26 suggests a weak La Niña-type sea surface pattern or a near-La Niña tendency is favored early in the NDJ season. A weak La Niña often tilts rainfall probability patterns — for East/Southeast Asia this can favour relative drying in parts of the central equatorial Pacific and modulate the risk of tropical activity in the South China Sea. Expect subtle, not dramatic, ENSO-driven signals.
Even in a cooling/NE monsoon environment, the upper South China Sea can still generate cloud bands, frontal surges or a late-season tropical system that pushes north — these bring episodic heavy rain and strong southerly-southeast wind bursts before being overtaken by northerlies. Recent regional bulletins have highlighted such replenishment events.
A colder-than-average Himalayan / Tibetan Plateau region tends to strengthen katabatic drainage and can modulate the pool of cold air available to downstream East Asian winter outbreaks. Published studies and recent regional assessments show complex local cooling/drying feedbacks from the Himalayan region that can occasionally enhance cold air advection toward southern China. Expect stronger cold surges when the upstream Himalaya–Siberia region is anomalously cool.
South Tropical Jet / upper-level influences
The South Tropical Jet (STJ) and the orientation of the subtropical jet stream modulate upper-level divergence, storm track steering and convective bursts. For NDJ 2025/26, seasonal analyses indicate subtropical jet variability — linked to tropical SST patterns and the weak La Niña tendency — may cause occasional enhancement of upper-level support for storms moving from the South China Sea toward southern China. Practically this means: when the STJ is active or positioned favorably, short windows of stronger uplift and heavier rain are more likely; when it shifts poleward or weakens, Hong Kong tends toward drier, more stable conditions.
Month-by-month practical outlook (Nov, Dec, Jan)
November 2025
Primary signal: Establishment of the NE monsoon with periodic replenishments from the north and northeast. Expect more frequent cool, breezy days and greater cloudiness along coastal Guangdong and Hong Kong early in the month; interspersed with short milder intervals when high pressure builds. Rainfall: near to slightly below average overall, but episodic cloud bands or a passing tropical remnant could produce localized heavy showers. Temperatures: cooling from late-autumn values — daytime highs trending toward the low-to-mid 20s °C with cooler nights. Winds: northeasterly to northerly breezes, gusty on exposed coasts.
December 2025
Primary signal: Stronger boreal winter influence; episodes of colder air and clearer, drier spells when the Siberian High strengthens. Rainfall: typically lower than summer — expect mostly dry or scattered-showers conditions, though frontal surges from the South China Sea can still produce heavier rain on occasion. Temperatures: generally cooler; nocturnal minima fall further, especially during cold surges (teens °C). Cloudiness: more variable — cloud bands appear during monsoon replenishments. Winds: persistent northeasterly; stronger during surges. The weak La Niña-like pattern tends to slightly favour below-average rainfall for some parts of the region.
January 2026
Primary signal: Peak of boreal winter influence. Expect the coldest monthly conditions of the three, with periodic strong northerly surges. Rainfall: usually low, but brief heavy showers possible when the STJ / upper-level dynamics or a South China Sea disturbance combine with frontal forcing. Cloudiness: more frequently clear after surges, otherwise variable. Winds: northeasterly persistent, gusty during surges. Overall, seasonal guidance suggests NDJ will not be an anomalously wet season for Hong Kong, but local heavy rainfall events cannot be ruled out.
Practical impacts & preparedness
- Expect cooler, windier conditions — pack layered clothing and plan for gusty coastal winds during outings.
- Although the seasonal signal leans toward drier conditions, short heavy rainfall events from South China Sea disturbances can produce flash-flooding and urban runoff — keep an eye on local warnings during cloud-band events.
- Marine & ferry operations: northeasterly winds can increase sea state; mariners should monitor local warnings.
- Air quality: stagnant conditions between surges can trap pollutants; cooler clear spells usually improve dispersion.
- Infrastructure: continuous monitoring is sensible — the region remains exposed to occasional intense rain or late tropical activity even during the NE monsoon phase.
Short technical note on uncertainty
Seasonal forecasts give probabilistic guidance. A weak La Niña signal and variability in subtropical jet positioning mean NDJ 2025/26 will have shot-to-shot variability: most days will be typical NE monsoon cool/windy or dry, but occasional deviations (milder intervals or cloud-band heavy rain) are expected. Local forecasts (HKO local forecasts and watches/warnings) should be consulted for event timing and intensity.