Questions & Answers about הלוואי שהתחזית תטעה ולא תהיה לחות כזאת גם מחר.
הלוואי means if only, I wish, or sometimes I hope, but it usually sounds stronger and more emotional than plain אני מקווה.
So הלוואי... often introduces a real wish, especially when the speaker really wants something to happen or not happen.
The ש־ means that.
So הלוואי ש... is literally like saying I wish that....
In this sentence:
הלוואי שהתחזית תטעה...
= I wish that the forecast is wrong...
This is a very common structure in Hebrew.
Because the speaker is talking about a wish concerning the future or an unknown result.
תטעה is the future form of טעה: to be mistaken / to be wrong / to make a mistake.
So here שהתחזית תטעה means that the speaker wishes the forecast will turn out to be wrong.
After הלוואי, Hebrew very often uses the future tense for wishes about the present or future.