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These parameters refer to temperature anomalies at 850hPa. They are available from T+12h to T+360h at 12-hour stepintervals. For the computation of anomalies a climatology derived from ECMWF's re-forecasts is used consisting of the closest single re-forecast run (11 members, 20 years back) preceding the real-time forecast date. The same climatology replaced also replaces the outdated fixed climatology that had been used for the following (pre-existing) parameters (also for the 850hPa level):
- Probability of temperature anomaly less than -2 K
- Probability of temperature anomaly of at least greater than +2 K
- Probability of temperature anomaly less than -8 K
- Probability of temperature anomaly less than -4 K
- Probability of temperature anomaly greater than +4 K
- Probability of temperature anomaly greater than +8 K.
Please note that the The first two of the aforementioned aforementioned parameters, namely probability of temperature anomalies less than -2K and at least greater than +2K, are available only for the following forecast ranges only: T+120-240h, T+120-168h, T+168-240 and T+240-360h. The other 4 parameters are available from T+12h to T+360h at 12-hour stepintervals.
An example of temperature anomaly probabilities in a case of cold snap over North America at the end of January 2019.
On the plot below comparison of products is shown between the old and new climatology. There are some significant differences. Anomalies with respect to old fixed climatology look more pronounced partly due to the climate change. Warm anomalies in the Arctic for example have diminished significantly when applying new reforecast-based climatology which is updated on the fly using always data in the past 20 years.
Comparison of temperature anomaly probabilities between the new and old climatology.