Thursday, September 12, 2013

Epic rainfall and flash flooding in Colorado

Rainfall over the past several days including some extraordinary rainfall last night has led to a critical situation along the Colorado Front Range today.  Unfortunately the forecast for the next couple of days has more rain on the way.  Let's look at what has been going on.

There has been a highly amplified upper-air pattern over the continental US over the past few days. Here's yesterday morning's 500mb analysis from the HOOT site:
 You can see a large, cut-off low centered over the Great Basin region.  Notice where there are lots of height lines (the black contours) close together--off in the upper Midwest and the Canadian Prairies.  That's where the main jet stream is located--the strong, west-east flow that helps guide storms across the country.  The jet stream is well to the north of this low--in fact, the low is comfortably tucked under a very high-amplitude ridge that goes well up into Canada and is centered over the Pacific Northwest.  Here in Seattle this ridge has given us clear skies and record-breaking warmth of the past few days.  Unfortunately that same ridge is keeping the steering flow away from the low over Colorado, leaving it in place.

The green colors in that analysis image show relative humidity at the 500mb level.  There has been a steady stream of moisture being pulled up from the south around this low over the high plains and eastern Rockies.  This moisture can also be monitored in real-time using satellite-derived water-vapor imagery, like this image from last night:
The brighter whites, purples and blues indicate lots of moisture in the upper troposphere.  The CIMSS Satellite blog has an excellent loop of water vapor imagery for this event at their post here.  Water vapor imagery often isn't very good for showing us the low-level moisture; what you're seeing above is mostly moisture aloft.  Checking last night's sounding from Denver we can see that there was far more moisture in the lower levels of the atmosphere:
The dewpoint and temperature profiles are almost right on top of each other from the surface up to around 600mb--that's a very deep layer for the atmosphere to be saturated.  A lot of people have been commenting on how unusually moist this is for Colorado.  One way we estimate the amount of water vapor throughout the entire depth of the atmosphere is through a measure called Precipitable Water (PWAT).  This basically says that if all of the water vapor in the air above your head were to immediately condense into liquid water and fall down as rain, this is how much rain would fall.  On the sounding above, in the lower right corner you'll see the PWAT value calculated as 33.16 mm.  That's 1.3 inches of precipitable water.  For comparison, here is a climatology of the average precipitable water values in Denver throughout the year from the NWS Rapid City page:
You can see in September that the average precipitable water (the 50th percentile) is only about 0.55 in.  Where does 1.3 inches fall?  Above the maximum value (1.25 in.) for September! An extraordinary amount of moisture.

Another thing you'll note on the sounding above is that near-surface winds are out of the southeast.  This promotes upslope flow--easterly winds forced to rise when they meet the mountains.  This constant rising motion in very moist air produces continuous condensation, clouds and rain.  You can see how steady the rainfall was last night in looking at this timeseries of observations from the NCAR Foothills lab on the northeast side of Boulder.

The rain gauge reset itself to zero this morning (hence the big drop in precip down to zero again), but you can see that over just last evening over 6 in. of rain had fallen.  In one evening.  The rainfall was also quite steady over this entire period--no big spurts or jumps in the precipitation that you'd expect if this was due to strong convection.  In fact, not much lightning was reported with this rain (though there was some).  There have been several reports of particular locations in the foothills getting over 8" of rain so far.  This also shows an interesting feature of this event.  Remember our precipitable water above?  It was only 1.3 inches.  If that was the total precipitable water, how could we be getting 6-8 inches of rain?  Remember that there is that stream of moisture constantly being brought up on the south--we saw it on the water vapor imagery.  This keeps replenishing the water vapor in the atmosphere, bringing in more and more moisture as the current moisture rains out.  Still--too get the much rain with these precipitable water values, these clouds have been very efficient at their rain production.

The rainfall overnight consisted of broad areas of stratiform precipitation in some locations, but also (particularly near Boulder) a series of more intense precipitation cells, probably somewhat convective, that repeatedly formed east of town and then moved west over the foothills.  Here's an example radar image from last night:
So what are the consequences of so much water over such a short period of time?  Flash flooding.  Here's the stream gauge data for Boulder Creek at Broadway in downtown Boulder:
There were extreme rises in the stream level overnight.  You'll note that the typical river stage is only a little over 1 foot.  From 6 PM Wednesday through about 3 AM MDT, the creek rose rapidly to just over 7 feet--moderate flood stage.  There was a drop off early this morning as the rain eased a bit, but more rain is developing today and you can see the stream flow on the rise again.  Basically all of the canyons along the northern Front Range hit at least minor flood stage last night and into today.  Here's the gauge data for St. Vrain Creek near Lyons--approaching record levels.  There was also a dam failure last night upstream from Lyons, prompting extensive evacuations.
There continue to be problems with managing this massive amount of water.  Pretty much every road going into the mountains is closed due to flooding or mudslides at this time.  Many streets in Boulder itself are impassible due to high water.  There are evacuations underway in the Big Thompson River canyon (the location of an infamous flood in 1976--which had distinctly different meteorological origin, being due to a parked convective storm as opposed to widespread, more stratiform rain).

Unfortunately it doesn't look like it's time to take a breath just yet.  The rain died down a little overnight, but it's forecast to pick up again today.  We're already seeing that throughout the region.  Here's the latest High-Resolution Rapid Refresh model's forecast for total accumulated precipitation through tonight.
It's predicting another 1-2 inches over the northern Front Range.  It's also alarming to note the 3-5 inch accumulations suggested for the mountains behind Colorado Springs and Pueblo, suggesting an additional flash flooding threat further south.

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