Subseasonal Outlook — days 1–35

What the anomalies are measured against. The 1991–2020 normal, the period everyone quotes. Reaching it takes a step, because the hindcast spans 2001–2020 and the model's 1991–2020 climatology therefore does not exist — it has to be estimated. It is estimated by shifting the model climatology by the observed change between the two periods, so what is computed is

anomaly(L) = [ forecast(L) − model climatology(L) ] − [ ERA5 1991–2020 − ERA5 2001–2020 ]

The first bracket removes GEPS's drift and its mean bias, because both terms are the same model at the same lead — that is what the hindcast archive is for. The second moves the reference period, and nothing else: it is two decades of climate, measured from observations, with no model in it.

The correction is small and physical, which is the point: 1991–2020 is 0.12 K cooler than 2001–2020 and its 500 hPa surface sits 2.7 m lower, so anomalies quoted here are correspondingly warmer and higher. Precipitation moves 0.03 mm/day, outgoing longwave 0.05 W/m², the winds under 0.06 m/s.

What this deliberately avoids. Differencing straight against ERA5 would also be “vs 1991–2020”, and is the obvious way to do it — but it hands the model's mean bias back into the anomaly, measured here at +0.55 mm/day on precipitation and +9.7 W/m² on outgoing longwave. Those are an order of magnitude larger than the base-period change they would be sitting alongside, and they are a property of the model, not of the weather.

The climatology. The GEPS8 reforecast (2001–2020, four starts a week, four members, 39 leads) was pulled from the IRI Data Library for eight variables — 1,920 monthly files, 116 GB — and reduced to a lead-dependent model climatology: for each variable, lead and day-of-year, the mean over hindcast starts within ±15 days of that date across all twenty years, roughly 340–560 starts per cell. Anomalies are taken against that, at matching lead, so what the maps show is departure from how GEPS itself normally behaves that far into a forecast.

Two corrections worth naming, because both were wrong first and both were visible. The climatology is indexed by the day-of-year of the start, so it must be read at the forecast's init day-of-year, the same for every lead; reading it at each valid date instead compared a day-35 forecast against runs launched 35 days later, which over an autumn continent made the reference about 8 K too cold and produced a fake warming that grew with lead. And the ensemble mean must be weighted by member count: the perturbed file holds 20 members and the control one, so averaging the two files equally gave the control half the weight instead of a twenty-first, leaking one member's small-scale noise into every map.

The anomaly is computed on the climatology's grid, not the forecast's. The hindcast is 1° and the live forecast 0.5°, exactly nested. Interpolating the climatology up to 0.5° to keep forecast detail seems free but is not: the forecast resolves terrain and coastlines the 1° reference cannot represent, so that structure does not cancel and comes out as grid-scale speckle along every mountain range and shoreline — mean state masquerading as anomaly, and worst in precipitation. The forecast is now band-limited to the coarse grid first and the difference taken there. An anomaly cannot carry more resolution than its reference.

Outgoing longwave carries a model-version offset, and it is removed. The live operational GEPS radiates about 7.6 W m⁻² differently from the GEPS8 reforecast the climatology was built on — flat across all 35 leads (spread 0.53), so it is not a forecast signal; a global-mean OLR anomaly that size is not physically possible. Left in, it tinted every map green and put spurious widespread negative anomalies over North America. The per-lead global mean is removed for OLR, as it is for height. It is irrelevant to the RMM, where a uniform offset moves the projection by less than 0.05.

Height is treated differently from the rest. Between the 2001–2020 hindcast epoch and today there is a real climate offset in geopotential height — a flat +16 m at every lead, measured — which would otherwise sit over every 500 hPa map as a uniform ridge. For z500 the per-lead global mean is removed so the circulation shows. For temperature and precipitation the raw anomaly is kept, because there the departure from the 2001–2020 climate is part of what you want to see. Each figure says which it is.

The OLR channel. ERA5's top-of-atmosphere longwave runs about 13 W m⁻² above the NOAA-based reference climatology the RMM EOFs were built on, so a per-longitude offset measured over three whole years is removed before anything is projected. The GMGSI proxy that covers the final few days is offset-corrected onto ERA5 over their 220-day overlap (r = 0.80 in the anomaly, spread within 3% of ERA5's) and screened day by day — the mosaic occasionally publishes a day with missing passes, and one such day at the end of the record would otherwise become the observed MJO's latest point.

The stratosphere uses ERA5, not the hindcast — because it has to. Everything else on this page is de-drifted against the GEPS8 reforecast, which is the better reference. That archive cannot serve the vortex: it carries ua/va at 100, 200 and 850 hPa only, zg at 200 and 500 only, and no pressure-level temperature at all. There is no 10 hPa anything in it. So the stratospheric anomalies are taken against ERA5 instead, and the model bias that the reforecast would have removed is measured rather than assumed: GEPS analysis against ERA5 on the same day runs −26 m at 10 hPa globally and +0.5 m over the polar cap, with temperature within 0.5 K — small enough to leave alone, and stated here so it is not mistaken for signal. (100 hPa zonal wind is the one stratospheric field the hindcast could de-drift, and is a candidate for later.)

ERA5 was also checked against MERRA-2, the reference the site’s existing vortex monitor uses, so the two are not silently disagreeing: over 2,156 matching days they differ by 0.13 m/s in u(60°N) at 10 hPa (r 0.999), 0.11 K in cap temperature (r 0.997) and 9 m in cap height from 2001 onward. An archive-wide sweep of both records found a single corrupt day in 46 years of MERRA-2 — 1991-06-30, where the polar-cap height collapses to 0.80 m — now screened out; removing it also moved MERRA-2’s own height trend from +18.7 to +18.1 m/decade, against +18.9 measured independently from ERA5.

Caveats. This is an ensemble mean, so it understates the range of outcomes everywhere and increasingly so with lead; nothing here is a probability. Twenty-one members is also not many for precipitation: a single member differs from the member mean by about the field's own magnitude, so roughly a fifth of single-member variance survives into the mean and shows up as coherent-looking blobs at long lead. Those are ensemble sampling noise, not forecast structure. Extended cycles run Monday and Thursday only. The hindcast is a fixed model version while the live forecast is the current operational one, so a slow model change is not captured by the 2001–2020 reference — the outgoing-longwave offset above is exactly that showing up.

Data: ECCC GEPS via the MSC Datamart (Environment and Climate Change Canada, Open Government Licence). GEPS8 reforecast via the IRI Data Library, Columbia University. ERA5 via the Analysis-Ready Cloud-Optimized store, Copernicus Climate Change Service. GMGSI via NOAA on AWS. RMM reference EOFs after Wheeler & Hendon (2004).