waa! kowai!

Darasu no tenki wa kowai deshita yo! (The weather was scary.)

Darasu no tenki wa kawai desu yo! (Right now the weather is scary.)

You are female so you can replace "yo" with "wa" for emphasis, but you don't really but a "!" next to a verb without a emphasis particle.

waa! kowai!

Dallas no tenki wa kowai desu! Waa!

Darasu no tenki wa kowakatta desu!  

Kowakatta means passed. 

waa! kowai!

Dallas(*1) no tenki wa kowai desu! Waa!(*2)

*1 Dallas can be fine if you use alphabets in Japanese sentence.
*2 Waa is fine but personally I think kyaa is better to express the scared feeling.

(These are my suggestions)
1. In present tense
Darasu no tenki wa kowai desu. Kyaa! (;>_<;)

ダラス の 天気 は 怖い です！ きゃぁ！
           tenki   kowa
 
I can fully understand what you mean by your writing, good work:-)