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Fig2.6-3:  The recurring cycle of the early cut-off 4D-var analysis (SCDA).  Consider the 12UTC Early Cut-off (SCDA):  The 00UTC LWDA provides an 06UTC analysis that becomes the basis of an 06UTC short forecast.  This provides the first guess (at T+03, 09UTC); background data during the rest of the 4D-Var analysis is provided by T+6hr to T+9hr forecasts (12UTC and 15UTC) from the 18UTC forecast.  The cut-off time is 16:25UTC (4h25m after the nominal time (12UTC) of the analysis using CoDA between 16:00 and 16:25) to ensure as many as possible observations with data times between 09UTC and 15UTC are captured.  The cycle is repeated for subsequent analysis cycles 12hr later.  The early cut-off 4D-Var (SCDA) 12UTC analysis is used by Ensemble Control Forecast (ex-HRES).  It is also used as first guess for the 12UTC  LWDA.  The EDA runs in parallel with the 4D-Var and a) constrains the 4D-Var analysis and b) provides information for evaluation of perturbations for the ensemble.

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