I've been bouncing around the idea of building UR Storm for Modern, and I've managed to come up with what I think is a decent list. I'd appreciate any and all feedback that can be offered!

Lands
4 Scalding Tarn
2 Misty Rainforest
4 Steam Vents
2 Sulfur Falls
4 Island
2 Mountain

Spells
4 Pyretic Ritual
4 Desperate Ritual
4 Pyromancer Ascension
4 Gitaxian Probe
4 Manamorphose
4 Past in Flames
4 Grapeshot
2 Empty the Warrens
4 Serum Visions
4 Sleight of Hand
4 Thought Scour

Sideboard
3 Echoing Truth
1 Polymorph
1 Emrakul, the Aeons Torn
3 Gigadrowse
2 Lightning Bolt
1 Dryad Arbor
2 Pyroclasm
2 Vandalblast

I think you probably have too many win cons in your main deck. Most people run 2-4 Grapshots and 0-1 Empty the Warrens because drawing into more than 1-2 doesnt really do anything for you.

Think about replacing 4 of your win cons with a play set of reach through mist or desperate ravings

Similarly Im not sure about your sideboard I think it could use some work.

The Polymorph + Emrakul is cute but you dont have consistent enough ways to get the tokens/dryad arbor you need to polymorph. Most importantly if you polymorph a warrens token without having arbor on the field you could end up hitting the arbor and that would not be good. Likewise Gigadrowse is neat but might be better off replaced with something else.

Consider stuff like for your sideboard.
defense grid
torpor orb
tormod's crypt
cyclonic rift
empty the warrens

I second TRZ's suggestions. I would also considering throwing in a Lightning Bolt or 2 as a means to get the final damage in if you cant get your storm count high enough or deal with a "Problem" creature...Looking at you THALIA...