Riverton Weather Forecast Office has prepared a comprehensive discussion about this developing rain pattern.
An upper level low slowly shifting eastward will bring an unsettled pattern to the region for the next several days. The position of the low today and tomorrow will place much of the state in an easterly wind pattern, commonly referred to as an upslope pattern which is very favorable for precipitation - especially east of the Divide. In addition, an abnormally moist atmosphere will increase chances for heavy rain (see graph below) and some upper level dynamic support will increase the coverage of the rain and turn it into a more widespread event. Most of the heavy rain will be focused in the mountains as they provide a more significant source of lift, and in the foothills on the east and north side of mountain ranges like Lander, Jeffrey City, Casper, and Dubois.
Click Image To Enlarge
The above graph shows the Climatologically "normal" Precipitable Watervalues 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 value (dashed line), we can anticipate precipitation records at the surface to be threatened as well as some localized flooding.
This combination of factors raises the flash flood and mudslide risk for those in the mountains and near the recent burn scar areas in addition to the usual flash flood prone areas (small, steep, rocky basins and dry washes).
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 Sheepherder Hill burn scar has already produced a damaging flash flood this year and it could easily do so again. If you are unsure whether or not you are at risk from one of these scars after checking out the website, please give us a call at 800-211-1448 and we will do our best to help answer your questions.
An upper level low in the Great Basin will work northeast and bring plenty of moisture with it. Rain will be fairly steady across the southwestern half of the state through tomorrow with flooding possible. A Flash Flood Watch has been issued for the Central Mountains and Upper Wind River Basin from tonight through tomorrow to cover the more steady period of moderate to heavy rain. Areas of Utah and Colorado have received nearly 3 inches of rain from this system with multiple Flash Flood Warnings and road closures.
Click Loop To EnlargeThe above loop begins at 18Z Wednesday (Noon Wednesday) and ends at 18Z Monday, showing the amount of rain expected in 6-hourly time steps through the weekend.
Click Loop To EnlargeThe above loop shows the expected accumulatedprecipitation from noon today through noon on Monday.
As the loop shows, most of the rain will fall across the southwestern half of the state today and tomorrow...spreading across the rest of the state by Friday and continuing through the weekend. The heaviest rain is expected across the Absarokas, Wind River Range, and Upper Wind River Basin near Dubois.
Click Loop To EnlargeThis image shows the amount of moisture available for rain (called Precipitable Water or PWAT) from noon today (Wednesday) through Monday morning. The blue and green shades show where the most moisture is available. The 500MB pattern is represented by the white lines and the low level winds are represented by the orange arrows (the longer the tail, the stronger the winds).
As the chart at the top of the page shows, "normal" precipitable water for this time of year should be less than one half of an inch (gray shades in this image) anything in the green and light blue shades is well above normal with the dark blue shades representing near record amounts of PWAT.
|Keep an eye on our homepage for the latest watches and warnings - especially if you live in a flood prone area.|
In summary, there is a good chance of measurable rain and even some localized flooding over the next several days with more steady rain expected through Thursday, transitioning to a more showery regime for the weekend.
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!