Fasten your seat belts

In the most famous line from the 1950 movie All About Eve, the character Margo Channing (played by Bette Davis) said

Fasten your seatbelts, it’s going to be a bump-y night!

"Night" is often replaced by the more sensible "ride".

As anyone who has been in a plane has experienced, a "Fasten Seat Belts" sign comes on when the pilot has to make a difficult maneuver, or if the plane goes through turbulence.

So most people will understand that some sort of turmoil is expected if you say "Fasten your seatbelts". But it's often used jocularly, as if the difficulty is trivial or survivable.

"A storm is coming," or "a storm is brewing" is common and well-understood.

Gird your loins. Vocabulary.com says:

"Gird your loins and prepare for battle!" Okay, no one says "gird your
loins" anymore (which basically means "tighten your pants"), but gird
is still used as a verb to mean "get ready for a dangerous situation."

To gird is to prepare for a military attack, but more loosely it
refers to readying oneself for any kind of confrontation. When you
gird for something, you are preparing for the worst-case scenario.
Gird can also mean "fasten something tightly with a belt or a band"
(as in "gird your loins"), or it can mean "to surround or encircle." A
field that is girded by trees is surrounded and encircled by trees.

The source is overly dismissive of gird your loins, although it is true that it is rarely said now except semi-humorously, as in:

One hour 'til the physics final. All we can do now is gird our loins
and pray there is no surprise.

The loins are the area of the sexual organs, and girding one's loins means to put protection around that area before going into battle.

Steel yourself.

This phrase has very much the same kind of feeling as brace yourself, albeit perhaps slightly more literary, and not with the same sense of immediacy that brace necessarily infers. Steel yourself is a warning to prepare your spirit for some kind of hardship to come. (It can, of course, also be used ironically when the difficulty is only slight.)

Collins gives the following definition:

verb

to prepare (oneself) for coping with something unpleasant

I was steeling myself to call round when Simon arrived.

---

Synonyms of steel yourself

brace yourself

He braced himself for the icy plunge into the black water.

grit your teeth

fortify yourself

harden yourself

Oxford Living Dictionaries defines steel as follows:

verb [with object]

Mentally prepare (oneself) to do or face something difficult.

‘his team were steeling themselves for disappointment’

with infinitive ‘she steeled herself to remain calm’

The sense is, of course, to harden yourself and make yourself like steel. The Online Dictionary of Etymology attests to the verb steel being used to mean make hard or strong like steel in the 1580s. (It could be used with a fair amount of thematic consistency along with the word mettle, if you're that way minded.)

Shakespeare used the verb steel in this sense on more than one occasion.

In Henry VI Part II, he gives the following lines to Richard Plantagenet, Duke of York, in a pep-talk soliloquy. Plantagenet (or York) is plucking up his courage for the task at hand - putting down a revolt in Ireland in the name of the King and, then, usurpation of the English throne for himself:

Now, York, or never, steel thy fearful thoughts,

And change misdoubt to resolution:

Be that thou hopest to be, or what thou art

Resign to death; it is not worth the enjoying:

Let pale-faced fear keep with the mean-born man,

And find no harbour in a royal heart.¹

The calm before the storm.

from The Free Dictionary:

A period of inactivity or tranquility before something chaotic begins.

You mentioned that something is about to happen, so if your scenario would do well to emphasize that the current circumstance is calm then this could work well.

Time to put on your brown trousers.

A fictional anecdote: two armies meet on either side of a revolutionary battle ground displaying colors and wielding muskets. Generals and assistants ride to the middle of the battleground on horseback to discuss terms of battle: agreement, surrender, or fight! After a long negotiation both generals in their disappointment have declare it's time to fight.

The first general looks at his assistant and says "captain, prepare for battle and bring my red blazer".

Overhearing this order, the second general asks "Sir, respectfully, why the red blazer".

In response the first general explains "very well my most worthy opponent, when one of our officers has been wounded in battle we prefer to hide the wound in order to preserve moral. The color of the blazer will help to lessen the visibility of blood."

In turn, the second general says "Understood general...", after which he turns to his captain and orders: "Captain, prepare for battle and bring my brown trousers."

Beware the ides of March

Shakespearean quote - it was a truthsayers warning to Julius Caesar that bad things were heading his way. It is a bit elitist to use, but when it fits, it fits.

Hold on to your britches

I'm not sure if you're interested in colloquialisms, however, "hold on to your britches" is typically a Southern United States expression used to indicate a sudden change of conditions. Typically it's used in driving (as in suddenly hitting the gas pedal), however, I have definitely heard it used at other times.

On a personal note, I tend to use "Hold on to your bobby-socks" which is a much much older variant (before even my time.)

If you use the allusion:

Winter is coming

...most will understand what you mean.

Update: tchrist♦ asked me to add an explanation into my answer. I understand I'm going to be acting as Captain Obvious right now, but he's got diamond in his nickname and I have to obey.

So, there is a well-known Saga "Game of Thrones". This is one of the most significant series in the last decade. The first episode of this series is called "Winter is coming", and throughout the series this thesis is repeated countless times. I won't give here any details so as not to create a spoiler for those who for any reasons haven't watch this series yet (I didn't meet such people, but there are rumors that they exist). So I'll just give a link to Wikipedia.

"ominous signs of tough times" comes to mind. Or, simply, "tough times ahead".

• 
  ominous -1. menacing, threatening: ominous black clouds; ominous rumblings of discontent. 2. Of or being an omen, especially an evil one.

From the web:

1 - "It would require a significant change in their strategy to prepare for the tough times ahead."

2 - The ominous signs of growing tensions were everywhere. In Asia, Emperor Hirohito's armies had invaded the Chinese mainland.

Politicians often say it in times of crisis: "Yet there are still tough times ahead, and tough decisions to be made. I have reluctantly concluded that a constitutional amendment, demanding we focus our efforts on balancing the budget, is the only sure way to make us accountable, and ..."

"a sense of impending doom".

It can also be used in an ironic or hyperbolic sense, where the 'doom' you see coming is actually a fairly minor struggle.

I felt a sense of impending doom as the first drops of rain landed
lightly on the bride's veil.

This is a fairly well known and well understood set phrase in English.

It is also the name of a medical symptom which, as you might expect, is a type of anxiety where one just feels for no apparent reason as if something terrible is about to happen.

If the context is that you're going to actively join a struggle, then "into the fray" is appropriate.

"...a prelude to..."

This borrows on figurative language, one common being very similar:

a prelude to war

Examples of use:

A Prelude to War (US Army article)

History Rediscovered: Prelude to War (book on Amazon)

Prelude to War (film by Frank Capra)

A Prelude to War (novella)

A "prelude" used this way doesn't need to be a "prelude to war" in particular. The connotation can stick if you use it with "prelude to rage", "prelude to divorce", "prelude to revolution", "prelude to failure", et cetera.

Having taken my own advice, I used this in my syndicated column, Prelude to Conflict, in which I began to anticipate the conflict between the US and China.

I could feel a sense of dread at the prospect, and though we laughed naively, the true weight of our actions would not be felt...perhaps too late.

Really, one word is nice, I.e foreboding, but consider the setting and use that to your advantage.

"I can't do this anymore, I want a divorce."

"Your test results came back positive."

"You have a right to remain silent."

"Your position has been eliminated as a result of the merger."

In general, relying on a common phrase to create suspense is not what you want to do. Simply describing the circumstances should be all you need to demonstrate that your characters are sitting precariously on the eve of battle and things are about to get hairy.