(a) bolt from the blue

To refer to something as a bolt from the blue means that it happened totally unexpectedly.
“The chairman’s resignation came as a bolt from the blue.”

on cloud nine

A person who is on cloud nine is very happy because something wonderful has happened.
“When the boss announced my promotion, I was on cloud nine.”

(a) cloud on the horizon

A problem or difficulty that is predictable, or seems likely to arise in the future, is called a cloud on the horizon.
“They are happily married and for the moment there appear to be no clouds on the horizon.”

(keep) in the dark

If someone is kept or left in the dark about something, they are not informed about it.
“The personnel was kept in the dark about the merger until the last minute.”

(a) fair-weather friend

Someone who acts as a friend when times are good, and is not there when you are in trouble, is called a fair-weather friend.
“I thought I could count on Bill, but I’ve discovered he’s just a fair-weather friend.”

(the) heavens open

When the heavens open, it suddenly starts to rain heavily.
“As soon as the race started, the heavens opened and the runners were soaked.”

once in a blue moon

If something occurs once in a blue moon, it happens very rarely.
“Bill has very little contact with his brother. They see each other once in a blue moon.”

reach for the moon

If you reach for the moon, you are very ambitious and try to achieve something even if it is difficult.
“His parents were hardworking people who encouraged their children to reach for the moon.”

come rain or shine

If a person does something come rain or shine, they do it regularly, whatever the circumstances.
“He goes to the gym club every day, come rain or shine.”

it never rains but it pours

This expression is used to comment on the fact that when something bad happens, other bad things often happen too, and make the situation even worse.
“First he forgot his briefcase, then he lost his wallet, and when he reached the car park, his car had been stolen – it never rains but it pours!”

take a rain check (on something)

To say that you take a rain check on something means that you cannot accept an invitation or offer now, but you will be happy to accept it later.
“Do you mind if I take a rain check on that lunch invitation? I’m going to be away all week.”

chasing rainbows

Someone who is chasing rainbows is trying to get something they will never obtain.
“She’s trying to get into Oxford, but I think she’s chasing rainbows.”

waiting for a raindrop in the drought

When someone is waiting for a raindrop in the drought, they are waiting and hoping for something that has little chance of happening.
“For many people, finding a job these days is like waiting for a raindrop in the drought. 

raining cats and dogs

If it’s raining cats and dogs, it is raining very heavily.
“We’ll have to cancel the picnic I’m afraid – it’s raining cats and dogs.

(a) storm is brewing / there is a storm brewing

If you say that a storm is brewing, you mean that the atmosphere indicates that there is going to be trouble, probably with outbursts of anger or emotion.
“As soon as we saw Pete’s face, we knew there was a storm brewing.”

(a) storm in a teacup

To refer to something as a storm in a teacup means that people are making a lot of unnecessary fuss or getting excited about something unimportant.
“They were arguing about who should go to the supermarket, but it was just a storm in a teacup.”

in the eye of the storm

A person or organisation who is in the eye of the storm is deeply involved in a difficult situation which affects a lot of people.
“The Prime Minister was often in the eye of the storm during the debate on whether or not to intervene in Syria.”

(the) lull before the storm

A period of unnatural calm before a difficult time or excessive activity is called the lull before the storm.
“The sales start on January 1st. The quiet period before that is just the lull before the storm.”

weather the storm

If you weather the storm, you succeed in surviving a difficult period or situation.
“Given the current recession, the company is weathering the storm better than most.”

(a) stormy relationship

If you have a stormy relationship with someone, you have a lot of arguments and disagreements.
“After a very stormy relationship, they decided to separate.”

make hay while sun shines

The expression may hay while the sun shines is used as an encouragement to take advantage of a good situation which may not last.
“Successful athletes are advised to make hay while the sun shines.”

snowed under

Someone who is snowed under has so many things to do, usually work, that they unable to cope with it all.
“With the ‘flu epidemic, doctors and nurses are completely snowed under.”

(a) face like thunder

If someone has a face like thunder, they look very angry.
“When Dad is really angry, he has a face like thunder!”

(the) tide has turned

When a trend has changed from one thing to another, the tide has turned.
“Before, people wanted to live in residential suburbs; now the tide has turned and warehouses are being converted into fashionable loft apartments.”

under the weather

If you are under the weather, you are not feeling very well.
“You look a bit under the weather. What’s the matter?”

(the) chill wind of something

If you face or feel the chill wind of something, you are beginning to encounter the problems or trouble it causes.
“Many building companies are facing the chill wind of the recession.”

get wind of something

If you get wind of something, you hear about something you were unaware of, usually a private or secret matter.
“The chairman didn’t want the press to get wind of the takeover before the agreement was signed.”

know which way wind blows

This expression means that it is advisable to know how a situation is developing, or what the tendency is, in order to be prepared for any changes.
“Before we decide on anything, we need to know which way the wind is blowing.”