On Friday and Saturday, southwestern Colorado experienced one of the largest fall rainstorms ever to hit the region. Rainfall totals from 2 to over 4 inches were reported at both lower and higher elevations, and the rain falling on the steep terrain resulted in major flooding.

One area that was hit particularly hard was Pagosa Springs. This video from Maria Kelly on Facebook shows the floodwaters rushing down the San Juan River through the area near the famous hot springs resort. (She has several other videos showing the flooding as well.)

There is a fantastic group of CoCoRaHS volunteer observers around Pagosa, and their measurements map out the rainfall totals from Friday and Saturday, with numerous observations of over 3″.

The stream gauge on the San Juan River at Pagosa Springs reached into the ‘major’ range with a crest at 12.66 feet on Saturday evening, before dropping down overnight.

Vallecito Creek, north of Durango and Bayfield, also experienced major flooding on Saturday with homes flooded and residents needing to be evacuated, as outlined in this Colorado Sun story.

The largest rainfall totals were at the higher-elevation SNOTEL stations, which are located upstream (and uphill) from the areas with the worst flooding. The Upper San Juan station (just west of Wolf Creek Pass) recorded 6.2″ of precipitation over the two days on Friday and Saturday. Several other SNOTEL stations recorded over 5″ in two days.

These are *huge* amounts of rain in this region, especially over such a widespread area. Below are tables showing the largest two-day precipitation totals at the Upper San Juan and Vallecito SNOTEL sites. At Upper San Juan, the 6.1″ from this storm is the 2nd highest on record, with data going back to 1978, and the 5.0″ at Vallecito is the highest on record with data since 1986. You might notice that most of the other storms in these rankings are in winter: it’s not too uncommon for the southern mountains to get a lot of moisture from a huge dump of snow, but it’s very rare to see 5-6 inches of rain.

Where did this storm come from?

The moisture source for this storm was the remnants of Hurricane Priscilla in the eastern Pacific Ocean. The map below on the left show precipitable water (the total amount of water vapor in the atmospheric column), and on the right is the standardized anomaly of PW — how unusual the amount of moisture is for the time of year, shown as standard deviations from the average. The stream of moisture ahead of Priscilla into the southwestern US is very apparent, and the standardized anomalies are basically “off the charts” for this time of year: 5 to 6 standard deviations above normal. The weather balloon from the National Weather Service in Grand Junction on Friday evening measured 1.05″ of precipitable water, the most ever recorded so late in the year. (Over an inch is somewhat common during the monsoon season in summer, but had never happened later than the first week of October.)

Speaking of Grand Junction, they also received a lot of rain: 2.01″ between Friday and Saturday. That is only the 5th time that the Grand Junction airport recorded over 2″ of rain in two days, and the first since September of 1941. Kudos also to the National Weather Service team in Grand Junction for issuing timely forecasts and warnings for this event (and continuing to release weather balloons), while they work without pay as the government is shut down. More heavy rain is expected on Monday in southwestern Colorado, as the moisture from yet another tropical cyclone (this time, Raymond) moves in. This may exacerbate the ongoing flooding, so be sure to pay attention to forecasts and warnings if you’re in the area.

In case you missed it, here’s the link to our September monthly climate summary, which includes some information about other flooding that occurred last month (including a remarkable, but very localized, rainstorm just south of Grand Junction). And if you’re interested, join us for the launch of the CSU Climate Hub at Spur on Tuesday afternoon in Denver.

We’ll crunch more numbers on this event as they come in, and we’re sending our best wishes to everyone affected by the flooding.