Flash Flood Watch for the Mountains Today and Tonight

The National Weather Service
Building a Weather-Ready Nation

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An abnormally moist atmosphere will combine with day time heating and very slow moving thunderstorms to produce scattered showers and thunderstorms across the state. Most of this activity will be focused in the mountains as they provide a more significant source of lift. This combination of factors raises the flash flood and mudslide risk for those in the mountains and near the 2012 burn scar areas.   NOW is the time to figure out if you are in danger if the right storm hits one of these burn scars - Why are they so dangerous? Because it only takes less than one half inch of rain in less than an hour to cause flash flooding and debris flows in burn scars! The Waldo Canyon burn scar near Colorado Springs produced a flash flood and debris flow on Monday from similar storms.

Forecast PWAT- Click to enlarge
Click Image To Enlarge

The above graph shows the Climatologically "normal" precipitable water values at Riverton (red line). It also shows the record high (light green) a low (black)  values recorded by the balloon launch at Riverton/Lander through the years. Once the expected value nears 2 standard deviations above the mean, represented by the dark green dashed line, then we can expect some storms to cause flooding ...if any storms do form that day and they happen to be moving slow enough. In this case, we are expecting thunderstorms to form today AND they will be slow-moving if not stationary.

Models are forecasting anywhere from 0.85in to 1.25in of precipitable water this afternoon and evening, if the highest and lowest values are thrown out, then we end up with a range of values that falls within the 90th-99th percentile, which is around or above 2 standard deviations above the mean value (around 0.58in this time of year).

A high pressure ridge to our southwest will continue to break down today, that event alone can trigger thunderstorms - especially over the higher terrain. Today, we have the added support of a short-wave that will encourage the atmosphere to destabilize further. Also, we have an abnormally moist atmoshpere, both at the surface and aloft. Surface dew points are in the 60's at Cody and Buffalo, which is very, very high for this area; and the precipitable water values are over the 90th percentile, which is not seen too often. This moisture provides fuel for the thunderstorms that will work with the instability from the short wave and from the normal surface heating that goes on every afternoon. In addition, the storm motion will be quite slow if not stationary in some cases, which increases the chance for flash flooding. If you are camping in the mountains today, please move away from nearby streams or dry washes as they could flood today and tonight, especially since this short wave will not exit the region until after midnight and it could keep some of mother nature's fireworks going well into the night. Also, if you live near a burn scar please familiarize yourself with your level of risk. If one of these storms happens to form or move over one of these burn scars today, then flash flooding and debris flows are nearly a certainty.

 July 4 Pattern - Click to enlarge  Afternoon Instability and Storm Motion - Click to enlarge
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The upper level pattern shows a large ridge continuing to dominate the great basin and American southwest. Wyoming is on the periphery of this ridge with a short wave (white dashed lines across the western portion of the state) rippling along the periphery of the ridge today.
Click Image To Enlarge

Instablility will be present everywhere this afternoon (blue shades) with the greatest instability residing in the mountains (dark blue to red shades).

The blue arrows show the average storm motion over a given point this afternoon. The shorter the arrow, the slower the storms will move, and therefore the greater the flash flood threat with any given storm.
 Watch and Warning Map - Click to enlarge
Click Image To Enlarge

This image shows where watches and warnings have been issued. Please go to our homepage and click on your area on the map to see every watch, warning, statement, and the forecast for your area of interest.

In summary, there is a high likelihood of thunderstorms today, with the best chance of storms over the higher terrain. The main concern with these storms will be flash flooding and frequent lightning, but there will also be a chance of hail with these storms.

If you are one of the many visitors or residents enjoying the outdoors today, please keep an eye on the weather, do not camp near creeks, streams, or dry washes, and remember - if thunder roars, go indoors!

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