The most common and most likely answer to this crossword clue is the 3 plus 4 plus 1 plus 2 letter word ALL HAIL - DD
I've seen this crossword clue in The Times
A
L
L
H
A
I
L
-
D
D
Buddy explains!
The straight part
We know that "Welcome/thoroughly bad weather forecast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case welcome/thoroughly bad weather forecast = all hail - dd.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.