Status: Ongoing analysis Material from: Linus, Rebecca, Chris


1. Impact

In end of June 2021 North-western U.S and Canada was hit by a record-breaking heatwave that smashed many previous records. In Lytton, British Columbia, the temperature reached 49.6C. Large parts of the village was later burned down by a wildfire. 

For more discussion on the case, see ECMWF User Forum: Western North America Extreme Heat

2. Description of the event

The plots below show analyses for z500 and T850 from 24 June to 1 July.

The plot below shows time-series of T850 inside the box 49-51N,122W-120W (see outline in the EFI plots below) from March to October (see Day of the year on x-axis) for 2021 - green, 2020 -blue and 2019red. The thin lines are daily values and bold lines are 7 day running mean.

3. Predictability


3.1 Data assimilation


3.2 HRES

The plots below show the pre-operational HRES "heat health hazard" forecasts, based on the Universal Thermal Climate Index, which represents the thermal stress of the human body. Based on air temperature, humidity, wind speed and mean radiant temperature (which represents how human beings experience radiant fluxes). Plots show the 3-day maximum UTCI over 28-30 June. 

Scale: >46 = extreme heat stress (you must cool your body down immediately, drink as much as possible and stop any physical activity in order to prevent heat stroke), 38-46 = very strong heat stress, 32-38 = strong heat stress, 26-32 = moderate heat stress, 9-26=no thermal stress, 0-9 = slight cold stress, 0- -13 = moderate cold stress, -13 - -27 = strong cold stress, -27 - 40 = very strong cold stress, < -40 = extreme cold stress. 

3.3 ENS

The plots below show forecasts (HRES - red, ENS CF - blue) and observations (black) for Lytton, Canada (50N, 121W) in forecast from 23 June (top) with 6-hourly output and from 28 June with 1-hourly output.

The plots below show 3-day EFI for 2-metre temperature 28-30 June.

The plot below shows the forecast evolution for 3-day (28-30 June) maximum temperature inside the black box outlined above (centred on Lytton). The plot includes HRES (red), ENS distribution (blue box-and-whisker) and model climate (red box-and-whisker). The maximum in the model climate (sample of 1200 forecasts, with time-window day 5-7), is 39.3C.

3.4 Monthly forecasts

The plot below show weekly anomalies of 2-metre temperature valid 28 June - 4 July from different initial dates.

The plot below show weekly anomalies of z500 valid 28 June - 4 July from different initial dates.

3.5 Comparison with other centres

4. Experience from general performance/other cases

5. Good and bad aspects of the forecasts for the event

6. Additional material