This is the spell you want to have with you when defending a gate or hallway against an incoming horde.

Helps bypass the warcaster feat, in terms of taking away advantage on the saving throw to maintain concentration. So if your DM ever concocts a deadly mage (or you are the DM to a well optimized warcaster), might be nice to keep in your back pocket to keep the other party on their toes. :)

Also, couldn't you use it to trap people? 40ft (at least) of difficult terrain means that a PC has use 3 normal turns (no dash) to leave, and if they dash on 'slick ice' then there is a much higher chance of slipping being offered up to the DM. I think the sheer value of of what is essentially a large blinding trap supersedes even the concentration effect.

Quote from 112358132134 >>

Also, couldn't you use it to trap people? 40ft (at least) of difficult terrain means that a PC has use 3 normal turns (no dash) to leave, and if they dash on 'slick ice' then there is a much higher chance of slipping being offered up to the DM. I think the sheer value of of what is essentially a large blinding trap supersedes even the concentration effect.

 It is even more powerful if you combine it with the thematically appropriate Ray of Frost to make one enemy in the area make even less progress at getting out.

Quote from gabrielrockman >>

Quote from 112358132134 >>

Also, couldn't you use it to trap people? 40ft (at least) of difficult terrain means that a PC has use 3 normal turns (no dash) to leave, and if they dash on 'slick ice' then there is a much higher chance of slipping being offered up to the DM. I think the sheer value of of what is essentially a large blinding trap supersedes even the concentration effect.

 It is even more powerful if you combine it with the thematically appropriate Ray of Frost to make one enemy in the area make even less progress at getting out.

 The area is heavily obscured. You cannot see your target under normal circumstances, so Ray of Frost is a tricky proposition.

Quote from Meadbeard2019 >>

Quote from gabrielrockman >>

Quote from 112358132134 >>

Also, couldn't you use it to trap people? 40ft (at least) of difficult terrain means that a PC has use 3 normal turns (no dash) to leave, and if they dash on 'slick ice' then there is a much higher chance of slipping being offered up to the DM. I think the sheer value of of what is essentially a large blinding trap supersedes even the concentration effect.

 It is even more powerful if you combine it with the thematically appropriate Ray of Frost to make one enemy in the area make even less progress at getting out.

 The area is heavily obscured. You cannot see your target under normal circumstances, so Ray of Frost is a tricky proposition.

 Ray of Frost does not require you to see your target (some spells specify that you must be able to see your target, but Ray of Frost does not).  And since your target cannot see you, your attack roll does not have disadvantage.

Indeed you are correct. Good catch.

May be a dumb question, but, would a flying creature inside the spell's area also fall prone on a failed Dex save?

This is a very interesting spell.

In addition to being an area-denial tool (and a 80 ft diameter is a pretty big area), it’s a sightblock.

Quote from Drenix >>

May be a dumb question, but, would a flying creature inside the spell's area also fall prone on a failed Dex save?

It certainly is ambiguously phrased.  But, I think that was the intention (because it’s hard to fly through a miniature hail-storm.)

Casting word is:

Delugium Torenticus

Yes, if they don't have a hover speed I believe. 

I have a slightly different answer than the others to your question.  

Since the spell's description doesn't explicitly state anything about flying creatures, I would say it's a DM's call.  Some may rule that flying creatures fall out of the sky on the failed save.  Others might rule that there's nothing forcing the flying creatures down.

It's a great question though.

if your target is prone at the time you cast Ray of Frost, then that attack would have disadvantage, correct?

Quote from steelballz >>

if your target is prone at the time you cast Ray of Frost, then that attack would have disadvantage, correct?

 No. You already have disadvantage because you can't see your target, and you already have advantage because your target can't see you. So whether or not your target is prone, it's still a normal attack because advantage and disadvantage cancel out.

Would it be possible to use the Shape Water spell in conjunction with Sleet Storm to effectively make a 5 foot square of the Sleet Storm have the effect of the Ice Storm spell, or something similar? Basically damaging hail in one part of the spell area?

Why would a flying creature slip on the icy ground?

The ground ice makes it difficult terrain. The spell is pretty unambiguous that it is 20 feet high and "When a creature enters the spell's area for the first time on a turn or starts its turn there, it must make a Dexterity saving throw. On a failed save, it falls prone." Unrelated to the ground ice.

I see no reason this wouldn't include things flying through the sleet, icing up their wings. If that wasn't intended, why make it 20 feet high?

How does it bypass the War Caster feat?

Warcaster only provides advantage on the concentration check when the caster takes damage. Sleet storm doesn't cause damage, so there is no advantage on it. Plus the dc is the sleet storm caster's spell save DC, not 10 or half damage taken (whichever is greater).