“Biblical” Flooding in Boulder. Nothing to See Here, Move along.. September 13, 2013





National Weather Service September 12:

AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DENVER/BOULDER CO 941 AM MDT THU SEP 12 2013 .UPDATE...MAJOR FLOODING/FLASH FLOODING EVENT UNDERWAY AT THIS TIME WITH BIBLICAL RAINFALL AMOUNTS REPORTED IN MANY AREAS IN/NEAR THE FOOTHILLS. PRECIPITABLE WATER VALUES IN EXCESS OF AN INCH AND A QUARTER ON GPS SENSORS CERTAINLY SUPPORT LOTS MORE RAIN TODAY AND TONIGHT. 13Z HRRR RUN HAS ANOTHER 1-3 INCHES OF RAIN PREDICTED IN THE FOOTHILLS THROUGH 04Z SO THINGS ARE NOT LOOKING GOOD. WILL EXTEND THE AREA OF THE FLASH FLOOD WATCH TO INCLUDE ZONE 43 AND ALSO EXTEND IT IN TIME THROUGH 12Z TOMORROW MORNING.

Weather.com

As of Friday morning, 12.30 inches of rain had fallen in Boulder, Colo. since Monday afternoon. According to the Western Regional Climate Center, the previous all-time record rainfall for any calendar day in Boulder was a 4.80 inch deluge on July 31, 1919., in records dating to 1893. Furthermore, the previous wettest September day was only 3.05 inches on Sep. 4, 1909. In fact, only two other September days featured over two inches of rain, none previously since 1938. The ten previous wettest calendar days in Boulder since 1893 all occurred between early April and early August, according to National Weather Service. Average September rainfall in Boulder is only 1.63 inches. So, Boulder picked up over seven times their average September monthly rainfall…in just over three days! The previous wettest single month in Boulder produced only 5.50 inches of rain. Early Thursday afternoon, a storm total of 11.5 inches came in from Aurora, Colo., an eastern Denver suburb.

Dr. Jeff Masters tells me in an email that these conditions are definitely “Not normal.” He goes on –

“Balloon soundings from Denver last night and this morning recorded the highest levels of September moisture on record for the station. The total precipitable water (PW), which is how much water would fall at the ground if the entire amount of water vapor through the depth of the atmosphere was condensed, was 1.33″ at 12Z (8 am EDT) on September 12, and 1.31″ at 00Z September 12. The previous September record was 1.23″, set on September 10, 1980. Balloon soundings began in 1948. The flow pattern resembles the one that brought Canada’s most expensive flood of all-time to Calgary earlier this summer, with an upper-level low trapped to the south of an unusually strong ridge of high pressure.

Probably as good a time as any to review my video with Jeff Masters and Jennifer Francis discussing the mechanism by which weather gets “stuck” – changes in the jetstream that appear to be related to climate change.

UPDATE:

Discover Magazine:

Some context: We went from breaking record high temperatures last week (including tying the record — twice — for the highest temperature ever recorded in September in the Denver area), to what appears to be unprecedented torrential rainfall that has caused devastating flash flooding and killed at least two people in this area, and another person further south. Some numbers to consider: Over the past 18 hours or so, Boulder has received 9 inches of rain. By comparison, average total precipitation for Boulder for the entire month of September is just 1.68 inches. And the record tally for the entire month was 4.67 inches, according to a weather forecaster on Channel 7 here. The proximate cause is tropical, monsoonal moisture streaming up from the south and a stalled front that is pinning it all right over us.