August 13, 2022 Heavy Rain Event

Overview

Scattered thunderstorms brought heavy rain to portions of east Idaho. Between 2 pm and 4 pm, the southern Rockland and Arbon valleys received around an inch of rain. Subsequent flooding produced debris flows, washed out roads, and inundated basements. The late afternoon hours brought strong thunderstorms to the Upper Snake Highlands. There were reports from Marysville and Ashton of around 3” of rain. Widespread flash flooding followed with numerous roads washed out and over 1,000 acres of farmland flooded in Fremont County alone. There were numerous reports of flood damage to buildings. 15 people became stranded along the Henrys Fork around Island Park and needed to be rescued.

These storms developed in an environment associated with a northward push of moisture associated with the southwestern US monsoon. An area of high pressure was centered over the central Rockies. The flow across East Idaho was from the south which aided in pumping abnormally moist air into the region. An area of dew points in excess of 50 degrees and precipitable water values in excess of an inch extended through northern Utah and eastern Idaho; nosing into the Upper Snake Highlands. These precipitable water values were nearly 200% of normal. Scattered strong, but slow-moving thunderstorms initially developed across East Idaho where mixed-layer CAPE values were around 1000 J/kg while the storm motion was northwesterly at 15 to 20 kts. Meanwhile, a line of thunderstorms developed during the late morning hours across the Central Idaho mountains. This area was west of the better moisture but in a region of better upper level support and shear. The trigger mechanism for these storms was a subtle upper level trough. As the trough moved east, the storms blossomed upon entering the Arco Desert and the better moisture. Additionally, the entire Upper Snake Plain and Highlands was under an area of upper level divergence which provided additional lift to the thunderstorms. As the line of storms moved east, they combined with storms already in progress across eastern Idaho. The incredible rainfall amounts observed around Marysville and Ashton were likely a combination of strong thunderstorms interacting with an abnormally moist environment with a slow storm motion.

Flooding near Marysville, ID. Photo courtesy of Keith Richey.

Photos
Radar
Storm Reports
Environment

Pictures


Flooding near Ashton. Photo: Keith Richey.	Flooding in Ashton. Photo: Dave Garz.	Flooding near Ashton. Photo: Angela Neeley.

Flooding in Ashton. Photo: Dave Garz.	Flooding in Ashton. Photo: Dave Garz.	Flooding near Ashton. Photo: Angela Neeley.

Flooding near Ashton: Photo: Keith Richey.	Washouts near Marysville along the Ashton-Tetonia Trail.	Washouts near Marysville along the Ashton-Tetonia Trail.

North Fremont High School track flooding. Photo: Keith Richey.	Washouts near Marysville along the Ashton-Tetonia Trail.	Potato Crop near Squirrel damaged from hail and torrential rain. Photo: Dave Garz.

Radar


Composite radar loop showing evolution of heavy rain from thunderstorms on August 13, 2022.	Radar rainfall estimates. Areas in red are over 2", while areas in purple are over 3"

Storm Reports

Public Information Statement
National Weather Service Pocatello ID
413 PM MDT Mon Aug 15 2022

...CONVECTIVE HEAVY RAIN REPORTS FROM THE SATURDAY, AUGUST 13, 2022
FLASH FLOOD AND SEVERE THUNDERSTORM EVENT ACROSS SOUTHEAST IDAHO...

Location                     Amount    Time/Date       Provider             

...Teton Valley...
Ashton                       3.00 in   0700 PM 08/13   Emergency Mngr       
Marysville                   2.70 in   0620 PM 08/13   COCORAHS             
1 E Marysville               2.52 in   0749 PM 08/13   Public               
1 NE Squirrel                2.38 in   0839 PM 08/13   Public               
3 SSW Ashton                 1.97 in   0745 PM 08/13   MESOWEST             
5 NW Judkins                 0.93 in   0749 PM 08/13   Public               

...Raft River Region...
3 NNE Holbrook               0.99 in   0700 AM 08/14   COCORAHS             
4 NW Holbrook                0.59 in   0509 PM 08/13   Public               
11 SSE Rockland              0.55 in   0700 AM 08/14   COCORAHS             

...Marsh and Arbon Highlands...
11 NW Buist                  1.00 in   0300 PM 08/13   Trained Spotter      
6 E Roy                      0.83 in   0521 PM 08/13   Mesonet              

...Lost River Valleys...
2 SE Berenice                1.03 in   0330 PM 08/13   Public               
1 S Berenice                 1.00 in   0330 PM 08/13   Public               
Mackay                       0.67 in   1050 AM 08/14   AWS                  

...Franklin/Eastern Oneida Region...
7 WNW Buist                  1.72 in   0539 PM 08/13   Public               

...Centennial Mountains/Island Park...
2 NW Macks Inn               2.42 in   0755 PM 08/13   Public               
1 SSW Warm River             2.00 in   0630 PM 08/13   Public               
2 ESE Warm River             1.78 in   0804 PM 08/13   Public               
2 ESE Macks Inn              0.00 in   0605 PM 08/13   Law Enforcement      

...Big Hole Mountains...
1 W Victor                   1.05 in   1129 PM 08/13   Public               
2 SE Victor                  0.96 in   0929 PM 08/13   Public               
1 SSW Victor                 0.92 in   0824 PM 08/13   Public               
2 S Victor                   0.81 in   0809 PM 08/13   Public               

...Arco/Mud Lake Desert...
Lost River Rest Area         1.23 in   0645 PM 08/13   ARLFRD               
INL INTEC/Grid 3             1.09 in   0525 PM 08/13   ARLFRD               
INL Radioactive Waste Mgmt   1.02 in   0520 PM 08/13   ARLFRD               
INL Critical Infrastructure  1.00 in   0700 PM 08/13   ARLFRD               
INL Reactor Tech Complex     0.96 in   0650 PM 08/13   ARLFRD               
INL Central Facilities       0.95 in   0705 PM 08/13   ARLFRD               

Observations are collected from a variety of sources with varying
equipment and exposures. We thank all volunteer weather observers 
for their dedication. Not all data listed are considered official.

Environment


Figure 1: Water vapor satellite Image. Area of high pressure over Central Rockies with monsoonal flow moving north from the Southwestern U.S.	Figure 2: Surface dew points (green) showing relatively moist values in the 50s across East Idaho. Mid level lapse rates (red) with values over 7 degrees C per km.	Figure 3: Precipitable water shown. Values over 1 inch are relatively moist, nosing into the Upper Snake Highlands.

Figure 4: Mixed layer CAPE (red contours) values around 1000 J/kg indicating relatively unstable atmosphere.	Figure 5: Storm motion wind barbs (brown) at 15 to 20 kts indicating a relatively slow storm movement.	Figure 6: Upper level (300 mb) heights and winds (black) with divergence (purple). Note a bullseye of divergence over the Upper Snake Highlands which indicates a favorable area for strong lift and enhanced convection.

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Active Weather Pattern for the West into the Plains

August 13, 2022 Heavy Rain Event