Regional Fed trackers and bank models update their estimate every time a relevant series prints. Because the underlying monthly data is public, a nowcast is usually close to the eventual advance gdp-vintages estimate well before it is published.
Nowcasts are mechanical, not judgemental, so they swing hard on volatile inputs like trade-balance and inventories and they are least reliable early in a quarter when few inputs have arrived. Read the change in the nowcast, and the component driving it, rather than the level.
Example: the tracker starts the quarter at 2.2%. A strong retail-sales-control-group lifts it to 2.9%, then a wide trade deficit drops it to 2.4%. The advance estimate eventually prints 2.5%.
Related: gdp-vintages, retail-sales-control-group, trade-balance, consensus-estimate, data-revision