The USGS won't say anything other than "90k will die". If the following takes place, we are looking at more than 50% of the flora and fauna of the earth being wiped out.

Yellowstone's effect on North America...

Yellowstone's ash coverage

Within the purple is the initial kill zone. If you are caught in this area you will most likely die from burns caused by the hot ash.

Within dark orange is the hot ash zone. The ash is still hot enough to burn trees, homes, people, and cause massive forest fires.

Within all coloured area is the raining ash zone. If the ash gets in you eyes you will go blind. If you inhale the ash, your lungs will turn into flesh bags of cement

Yellowstone's toxic gases

CO2. When CO2 is in a concentration of more than 15% in the air, it is lethal.
SO2. If SO2 is inhaled it will cause respiratory problems and cause irritation to skin and eyes
H2S. Is toxic in very low concentrations. This gives the classic "Rotten egg" smell to volcanos

Effect on whole world...

Volcanic winter & food shortage

The following years after a yellowstone eruption will have a devastating toll on the earth.
The combination of CO2 SO2 and H2S in the atmosphere will cause a reflective barrier blocking sunlight and will plunge the whole world into decades of volcanic winter. Some of the only people who will survive are people with an MRE stash or some other means of getting food that isn't poisoned. The eruption would cause most of the earths food chains to be filled with toxins and carcinogens. Most of the earth would starve to death.

Martial law?

Governments have been known to instate martial law and grab guns when crisis hits. This happened after Katrina in many places that received government aid. New orleans had a period of martial law during the first weeks after Katrina. If you live in a densely populated city, it is a good idea to leave before the government gets there to "help".

It isn't a question of IF - it's a question of WHEN

Yellowstone will erupt and when it does think of these facts and do anything you can to avoid all of them.

TIPS:
Keep a 4 month supply of MRE's or canned foods.
Keep several fine dust masks to avoid inhaling ash.
Don't drive during ash fall. Ash will clog the engine.