Thursday September 12th 2013 1:22pm
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The city of Boulder just set a record for the most rain in a 24 hour period and the most rain during one month. Flooding is on-going across much of the front range foothills and plains from Colorado Springs north to Denver, Boulder, and Fort Collins. Here are some details about the rain storm, my thoughts on why it happened, and the forecast for the next few days.
Boulder set a record with an official measurement of 7.21 inches of rain from 6pm Wednesday night through 9am Thursday morning. Over the last four days, Boulder reported 9.61 inches of rain, which is the most in any four-day period since records began. The next closest total was 7.37 inches in May, 1969. To put these numbers in perspective, the average precipitation in Boulder for a YEAR is 20 inches, and we just received half of that in the last few days. You can research more historical weather data here: http://xmacis.rcc-acis.org/
More rain has fallen throughout the day today, Thursday, and heavy rain remains likely through Thursday night before hopefully subsiding on Friday. I say "hopefully" because the weather models are not providing realiable short-term forecasts right now. More on this below.
Here is a map of rainfall totals from Wednesday morning through Thursday morning, courtesy of CoCoRahs (http://www.cocorahs.org/).
Rainfall totals were so high because the storms moved over the same areas throughout the evening. Here is the radar loop from Wednesday night between 1030pm and 1100pm. The rain is consistently moving from southeast to northwest into the foothills west of Denver and Boulder. Look closely and you'll see a small counter-clockwise spin in the radar echos just to the left of the "D" in "Denver". This area of spin helped to focus the rainfall over the city of Boulder.
As frequent readers know, there are two main ingredients to make weather. The first is Moisture, and the second is Rising Air. Over the last few days, we've had a lot of both.
The moisture is coming all the way from the tropics south of Mexico. Follow the ribbon of grey colors southward from Colorado to the bright colors just south of Mexico, and you'll see the source of most of our moisture. Another source is the Gulf of Mexico.
The air is being forced to rise because of a strong wind from the east. This "upslope flow" pushes the air from 4,000ft in elevation on the plains of Colorado to over 9,000ft in the foothills west of the Denver, Boulder, and Ft. Collins. The process of air hitting a mountain and being forced to rise is called "Orographic Lift" and is responsible for the majority of precipitation - rain and snow - in areas with big mountains. See the graphic below showing the wind from the east on a direct path into the mountains.
The big difference between this storm and other storms is its slow speed. Many storms bring an abundance of moisture, and many storms also create rising air through orographic lift. However, the moisture and lift is usually short lived, perhaps only sticking around for 6-12 hours. In this case, we'll see close to five days of moisture and lift. The slow speed is what turns a normal storm into a record-breaker.
This stream gauge on Boulder Creek shows a massive spike in water flow on Wednesday night and another rise on Thursday morning with additional rainfall. Other gauges in the area look similar to this one.
The amount of rain is too much for any river system to hold, and the heavy rainfall on the burn scars from recent wildfires also helped more water runoff into streams instead of being absorbed by the soil. High rivers have washed out many roads and sadly caused at least three deaths. Let's hope the bad news ends there. Here's a picture from north Boulder. The debris shows the path of a normally tranquel creek which took this opportunity to carve a new path.
Yes, if this were winter, the front range would see a snow storm of more than six feet, but for now we're left with days (weeks, months) of cleanup.
The weather models actually did a good job of predicting the possibility of flooding rains more than five days ago, but they are doing a bad job of predicting the short-term details over the next few hours and days. I'll write another post on how the models performed up until this point, but the question is what will happen over the next few days? After looking at many models, it appears to me that heavy rain for some areas is still likely Thursday afternoon and Thursday night, then somewhat drier air will move in on Friday and Saturday. But don't breathe easy just yet, as another surge of moisture and upslope flow is predicted for Sunday. Finally, the atmosphere dries out on Monday and this drier period should last for much of next week, though a few showers are still likely.
I'll provide more updates on a daily basis, or more often if things go bonkers, which they've tended to do this week.
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