1. 先说说 forecast 的一个相关用法

forecast后面可以接that引导的宾语从句。如：

Mr Smith forecast that the economy’ s downturn would be short-lived. 史密斯先生预测经济走低将是短期的。

如果将此句改为被动语态，则是：

That the economy’ s downturn would be short-lived is forecasted by Mr Smith.

主语从句作主语，显得头重脚轻，改用形式主语it：

It is forecasted by Mr Smith that the economy’ s downturn would be short-lived.

2. 再回到网友的句子：

There could be a storm tomorrow, as forecasted by the Weather Network.

其实前面提问的网友已基本回了这个问题：

——有人认为 as 引导的定语从句，as充当主语指代前面整个句子的内容，省略了助动词 is。我不太认可这个分析，因为定语从句中的动词be通常是不可以省略的。

——前面网友的疑惑是：能否把as看作从属连词，引导状语从句，看作省略了 it is，即as it is forecasted by the Weather Network。我基本认为这个分析，网友的句子补充一下应该是：

There could be a storm tomorrow, as it is forecasted by the Weather Network that there could be a storm tomorrow.

句子省略的是形式主语 it 和外置主语 that there could be a storm tomorrow，即省略后的句应该是（此时如果it后面接的是动词be，这个be可以省略也可以保留）：

There could be a storm tomorrow, as (is) forecasted by the Weather Network.

又如：

I’ll do the job as quickly as (is) humanly possible. 我将在力所能及的范围内尽快完成工作。（《牛津英语用法指南》）

I’m enclosing my cheque for 50 pounds, as (was) agreed. 根据商定的安排，附上我的一张50英镑的支票。（《牛津英语语法精讲》）