Subseasonal forecasting during HMT2007
Klaus Weickmann, ESRL/PSD
A forecast process that uses operational prediction models, linear statistical models and a global synoptic model was implemented during the Hydrometeorological Test Bed (HMT) West Coast Operations 2007. A real time attribution component is applied as part of the process using diagnosis of the zonal mean angular momentum budget. The goal is to improve already skillful forecasts of tercile precipitation and temperature probabilities in weeks 1-3. A particular emphasis is placed on rapid circulation transitions and extreme events where subjective input by the forecaster is still deemed useful. This is primarily due to model errors centered around organized convective forcing and erroneous tropical-extratropical interaction. A global synoptic dynamic model (GSDM) involving multiple time scales is used as a framework for monitoring and forecasting the circulation subjectively.
Several key synoptic developments occurred centered on the Pacific-North American region during the winter. A prominent ridge retrogression around 9 January 2007 led to a cold regime across North America, which was anticipated at least 14 days in advance. The El Nino signal that developed thereafter in mid-January was relatively weak with little tropical-extratropical coupling. Real time attribution suggested that easterly upper wind anomalies related to Indian Ocean convective forcing during November-December 2006 inhibited the coupling. Instead, convection flared up briefly near the equatorial date line but then shifted back west toward the equatorial west Pacific by the end of January 2007. This forced a second ridge retrogression in the extratropics during 25-31 January and renewed cold conditions over North America. This event was also anticipated well in advance by using a combination of the GSDM and the initial tropical sea surface temperatures. The mid-latitude westerlies finally broke through the west coast ridge around 9-10 February after which a new active phase of the MJO developed over Africa and the Indian Ocean and is currently moving east.
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