New method to improve accuracy of winter weather forecast

Forecasters may be able to accurately predict extreme winter weather up to a month in advance by taking account of unusual winds miles up in the stratosphere, according to a new study led by an Indian-origin scientist.
Meteorologists at the University of Reading in UK, European Centre for Medium-Range Weather Forecasts (ECMWF) and Environment Canada found that by taking account of changing winds in the stratosphere, forecasters could be twice as certain of their winter weather predictions for between a fortnight and a month in advance.
The findings mean that forecasters can be more certain about predicting extreme winter weather up to four weeks before it happens, giving governments, businesses, and individuals more certainty when planning for extreme events, such as floods or snow storms.

"Forecasting the weather a full month ahead is a tough ask, but that's what businesses and emergency services really need to prepare for extreme winter weather," said Om Tripathi, from the Department of Meteorology of the University of Reading, who led the research.
"Accurate advance notice of prolonged cold spells, such as the 'polar vortex' that hit North America over the past two winters, can save lives and help keep power and transport networks running," said Tripathi.
"Our latest findings should give forecasters more confidence when issuing some winter weather forecasts up to a month in advance," he said.

Winds in the polar night jet stream, which is at around 40 kilometres above the surface of the Earth, usually blow from the west and have speeds of around 112 km per hour.

The research team found that during conditions in which the polar night jet stream wind speeds exceed 144 km per hour, or reverse their direction to flow from the east, forecasts in both the stratosphere and troposphere are more skillful.
While it was previously known that a sudden weakening of these winds, and subsequent warming of the stratosphere, was a source of predictability, the researchers found that the opposite was also true when the polar night jet stream strengthened and the polar stratosphere was unusually cold.
Such stratospheric conditions occur up to 3-4 times a winter, meaning that around one in five winter 'sub-seasonal' forecasts (those looking 2-4 weeks ahead) might benefit from this effect.

The strength of stratospheric winds can influence the position of the jet stream in the troposphere, having a major influence on weather across the North Atlantic, and allow freezing polar air to travel further south than would normally be expected.
The researchers examined 30 years' worth of past forecast data to see how the state of the stratosphere affected the accuracy of the forecast.
The study was published in the journal Environmental Research Letters.