Subseasonal to Seasonal Prediction and Predictability of Extreme and Severe Weather
Subseasonal to Seasonal (S2S) predictions reside between typical weather forecast and climate projections and are of considerable socioeconomic value. Successful forecast on the S2S timescale provides valuable information to the energy sector, water resources, agriculture, etc. Further investigation into S2S prediction and predictability is desired to aide in mitigation of extreme weather events and severe weather outbreaks.
In Part I, I will briefly present the S2S prediction of extreme and severe weather, which resulted in several prediction products that may be implemented in real-time. First, a statistical model was developed for the prediction of the winter seasonal blocking frequency and extreme temperature frequency over Eurasia one month in advance using sea surface temperature (SST), 70-hPa geopotential height, and sea ice concentration as predictors, and the model captures more than 65% of the interannual variance of blocking frequency. Next, an investigation of the relationship between large-scale weather regimes and tornado occurrence over the U.S. in boreal spring is presented. A hybrid model based on the weather regime frequency predicted by a numerical model was developed to predict above/below normal weekly tornado activity and has skill better than climatology out to week 3. Lastly, a simple statistical model was developed to predict the weekly frequency of extreme warm days and 14-day standardized precipitation index (SPI) during boreal summer in the United States (US) using a leading principal component of US soil moisture and an index based on the North Pacific SST as predictors.
In Part II, the mechanisms for atmospheric blocking onset were explored over four sectors, and the link between the mechanisms and the predictability of blocking onset was investigated. Atlantic blocks are associated with strong low-frequency components of the flow and resembles the negative phase of the NAO. Europe blocks are influenced by a traveling wave across the Atlantic Ocean which is dominated by the high-frequency component of the flow and develops rapidly due to strong anticyclonic Rossby wave breaking. Asian blocks are fixated within a stationary wave that spans to the western Atlantic Ocean and contains strong low- and intermediate- frequency influences. The Pacific blocks contain a low-frequency component resembling the PNA pattern but are largely influenced by intermediate- and high- frequency components of the flow. The Europe blocking events contain the lowest predictability owing to the strong high-frequency influence, followed by the Atlantic, Pacific, and Asian sectors which all had strong low- and intermediate- frequency components of the flow.
