Researchers are still waiting for chemical analysis before they can say, for sure, what caused ‘milky rain’ to fall in Oregon and Washington this Saturday but some extremely plausible explanations are beginning to emerge.

When the rain began to fall, emergency officials were caught off guard as much as anyone.

“We have received reports of ‘white stuff’ on vehicles. The ash is more than likely from the Volcano Shiveluch,” Washington state’s Walla Walla County Emergency officials said in statement.

The volcano theory is still a possibility. According to Rice university, trace amounts of ash from the 1980 Mount St. Helens fell as far away as Oklahoma, Significant amounts of ash fell as far south as Colorado.

According to the University of Oregon, ash from the 1883 Krakatau eruption fell on Singapore 840 km to the north and on ships as far as 6076 km west-northwest of the volcano. Three months after the eruption ash and aerosol in the atmosphere caused vivid red sunsets in New York and New Haven, Connecticut. It was such a vivid red that fire engines were called out. Sporadic red sunsets continued for three years after the eruption.

The Shiveluch volcano, mentioned by emergency officials, is just one possible source of volcanic ash. That volcano, in northeast Russia created a 20,000 foot ash plume in during an eruption in late January. That eruption is known to have deposited ash in Oregon and Washington. The Shilveluch volcano experienced another violent eruption on Thursday, just a day before the mysterious rain began to fall.

Japan’s Sakurajima volcano has also been extremely active of late, experience up to three eruptions per day. In recent days it has become even more active.

“During the past few days, about 6-8 vulcanian explosions have occurred each day, often with ash plumes exceeding 10,000 ft (3 km) altitude. Milder, but near constant ash emissions occur during most of the intervals between the explosions,” according to Volcano Discovery.

Another very active volcano in Colima, Mexico is another possible source of the ash.

“The strong southerly flow from the jet stream could have brought it from an active volcano in southwest Colima, Mexico,” said CNN meteorologist Derek Van Dam on Saturday.

That volcano is 2,000 miles away which is half the distance to the Russian volcano. The Colima volcano has been particularly active recently, sending clouds of ash up to a mile into the atmosphere.

It is also possible, however, that it was not a volcano at all. A large dust storm was reported in Northwest Nevada on Thursday night and into Friday. The storm was driven by strong winds of up to 60 miles per hour, causing traffic accidents and poor visibility in the Reno area. A prolonged and powerful southerly wind could have pushed the dust into Oregon and Washington.

While none of these has been confirmed as the source of the ‘milky rain’ all of them are possible culprits. If it is from any of the three active volcanos, it is entirely possible that there could be more of the dirty rain. If it is from Nevada, it’s entirely possible that dirty rain could become a more regular event.

Nevada, like the rest of the US Southwest is in a multi-year drought. The longer the area remains dry, the easier it will be for weather systems to pick up dirt and dust.