2026 shown against confirmed surge years 2018 and 2022, plus 2024 (no surge) and 2025 (confirmed surge). Dashed lines mark the 218mm and 320mm risk markers.
Square 161 (SE England / Greater London area). Deciduous tree SMD* is the primary tracking metric for shrinkable clay subsidence risk.
* Soil moisture deficit (SMD) is a measure of how dry the soil is relative to its maximum water-holding capacity. A high SMD means the ground has lost significant moisture. In areas underlain by shrinkable clay, high SMD causes the clay to contract and crack — the principal mechanism behind subsidence damage to buildings and infrastructure.
† The 218mm and 320mm risk markers are Geobear-defined thresholds derived from analysis of historic subsidence case report data and previous surge years. They are not official Met Office or industry-standard figures. The 218mm level represents the SMD at which Geobear's historic data shows a material increase in subsidence activity on shrinkable clay soils; the 320mm level is associated with extreme and sustained surge conditions.
Most recent four weeks of Square 161 data. Week-on-week SMD change shows the direction of travel.
| Week ending | Sun (hrs) | Rain (mm) | SMD grass | SMD deciduous | Wk change |
|---|
Soil moisture is lost through evapotranspiration — the combined process of water evaporating from the soil surface and transpiring through plant leaves. In summer, this is the primary mechanism driving soil drying. Large deciduous trees such as oak, willow, poplar and elm extract water from deep within shrinkable clay layers via extensive root systems, even when the surface appears dry. Their evapotranspiration rates are substantially higher than grass, which is why deciduous SMD diverges from grass SMD as the season progresses — and why heatwaves, which dramatically accelerate transpiration rates, cause such rapid SMD jumps.
This is why this dashboard tracks deciduous tree SMD rather than grass SMD. It better reflects the deep moisture extraction in clay soils that causes shrinkage and ground movement beneath buildings. MORECS deciduous SMD is modelled using canopy resistance and available water capacity parameters calibrated specifically for tree-covered soils, based on the Hough & Jones (1997) methodology.
Sources: MORECS Square 161 week ending 1 Sep 2026; Met Office September 2026 outlook; NOAA CPC ENSO Diagnostic Discussion 13 Aug 2026; IRI mid-August ENSO Quick Look.
The September outlook confirms the pattern likely to continue. September is unlikely to settle into one dominant weather pattern, with Atlantic systems expected to keep parts of the UK wet while drier spells develop elsewhere. Northern and western areas are likely to see more frequent rain, while the south and east should have better opportunities for dry weather. The Met Office identifies the south and east — the clay belt — as likely to see the driest and sunniest weather in September. A warmer, relatively dry start to September, with drought conditions easing slowly and increasingly unsettled, wetter and windier spells from late October into November.
The following is an interpretive assessment based on historical MORECS patterns and current climate signals. It is speculative and should not be treated as a forecast.
SMD will remain at or near the model ceiling until sustained and repeated rainfall events drive meaningful recharge into the deep clay root zone. This is not triggered by a single shower — it requires the kind of persistent Atlantic low-pressure systems that typically establish themselves from late September onwards as the jet stream shifts southward for autumn. Based on the two most recent confirmed surge years, that process looks like this:
| Year | 320mm crossed | Recharge began | Weeks above 320mm | Peak claims activity |
|---|---|---|---|---|
| 2018 | Wk 29 (mid-Jul) | Late Oct (wk 43) | ~14 weeks | Aug–Oct 2018 |
| 2022 | Wk 26 (late Jun) | Early Oct (wk 40) | ~14 weeks | Jul–Oct 2022 |
| 2025 | Wk 30 (late Jul) | Mid-Oct (wk 42) | ~12 weeks | Aug–Oct 2025 |
| 2026 (projected) | Wk 28 (mid-Jul) | Late Sep–Oct (wk 39–43)? | ~11–15 weeks (speculative) | Aug–Oct–Nov 2026? |
Applying the same pattern to 2026: SMD crossed 320mm in week 28. If recharge follows the same late-September to mid-October timeline as 2022 and 2025, the surge would remain active for approximately 11–15 weeks — meaning SMD would remain above 320mm until around late September or mid-October 2026.
There is one factor that could extend this: a very strong El Niño. During strong El Niño winters, the UK tends to see wetter and stormier conditions — but this typically becomes the dominant pattern from November onwards, not September or October. The autumn transition period (September–October) tends to be less reliably wet in El Niño years than mid-winter. This means the surge may not benefit from early El Niño-driven autumn rainfall; the recharge signal is more likely to come from the return of normal Atlantic westerly flow in late September.
| Year | Profile | First above 218mm | First above 320mm | Peak SMD | Notes |
|---|
Sources: NOAA CPC El Niño Advisory, ENSO Recent Evolution and Status (10 Aug 2026), full ENSO Diagnostic Discussion due 13 Aug 2026.
| Layer | Current status | |
|---|---|---|
| ENSO / El Niño | Global context signal | 10 Aug: Niño 3.4 +1.8°C, Niño 1+2 +3.3°C. 81% chance very strong OND 2026. |
| North Atlantic Oscillation | Jet stream / blocking driver | Sept: N-W/SE split. South and east drier. Late heatwave possible. Recharge slow to arrive. |
| UK pressure pattern | Regional driver | SON 2026 outlook: above-average temps. Dry start to Sept in south/east. Wetter from Oct/Nov. |
| MORECS SMD | Ground condition evidence | 332.4mm — wk 6 at ceiling (28 Jul–1 Sep). 14.1mm rain wk to 1 Sep, SMD unchanged. Surge sustained. |
| Case reports | Impact validation | 2022 rose from 18 Jul. 2025 from 11 Aug. 2026 active monitoring. |
London used as a proxy for clay-rich South East England.
A surge-risk hypothesis strengthens only when multiple signals align and MORECS confirms ground drying.
Learn more: What is subsidence?
Last updated: · MORECS week ending:
Sources: MORECS Square 161 weekly data (Met Office Hadley Centre), Met Office UK weather summaries and ENSO commentary, NOAA Climate Prediction Center, WMO El Niño/La Niña Update (2 Jun 2026), Geobear internal subsidence case report trends.
This dashboard is an operational early-warning framework, not a property-level diagnosis. It should not be used as a basis for insurance or engineering decisions without professional assessment.
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