Abundant moisture associated with a low pressure system and afternoon heating produced thunderstorms with heavy rain across parts of Southwest Idaho.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS BOI wrote it up.
Abundant moisture associated with a low pressure system and afternoon heating produced thunderstorms with heavy rain across parts of Southwest Idaho.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Boise, ID wrote its last area forecast discussion before the first event at May 23, 1:41 PM MST, 1 hour 24 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BOISE ID 241 PM MDT SAT MAY 23 2015 .SHORT TERM...TONIGHT THROUGH MONDAY...POTENT SHORT WAVE MOVING SOUTH INTO OUR IDAHO ZONES AT THIS TIME. THIS FEATURE APPEARS TO BE POORLY DEPICTED IN THE LATEST MODELS. A LINE OF CONVECTION HAS FORMED IN ADVANCE OF THE SHORT WAVE. THIS LINE STRETCHED FROM JUST NORTHEAST OF BOISE SOUTHEAST THROUGH THE NORTHERN EDGE OF THE UPPER TREASURE VALLEY AND ON INTO THE WESTERN MAGIC VALLEY AS OF 230 PM MDT. IT IS EXPECTED TO CONTINUE TO MOVE SOUTH THIS AFTERNOON AND EVENING...SPREADING SHOWERS AND STORMS INTO MOST OF SOUTHWEST IDAHO. ADDITIONAL CONVECTION IS DEVELOPING UNDER THE SHORT WAVE...AS DEPICTED ON WATER VAPOR IMAGERY...MEANING THE THERE IS STILL AMPLE INSTABILITY BEHIND THE WAVE AND IT WILL NOT SHUT OFF ALL CONVECTION IN ITS WAKE. ADDITIONAL STORMS WILL FORM IN A GENERALLY UNSTABLE AIRMASS OVER ERN OREGON EVEN THOUGH IT IS NOT IN THE PATH OF THE SHORTWAVE. HEAVY RAIN IS POSSIBLE IN THE STRONGER CELLS AS STORM MOTION IS ONLY 10-15 MPH. WE WILL SEE SIGNIFICANT DRYING OUT AFTER MIDNIGHT. TOMORROW...THE ACTIVITY WILL BE MAINLY IN THE IDAHO ZONES...WITH SOMEWHAT DRIER AIR MOVING INTO SE OREGON. WE ARE AGAIN EXPECTED TO BE FAIRLY DRY SUNDAY NIGHT...WITH ACTIVITY INCREASING MONDAY AFTERNOON AS A SHORT WAVE MOVES INTO WASHINGTON FROM THE NW. SHEAR WILL INCREASE MONDAY AS THIS WAVE APPROACHES... LEADING TO A BETTER CHANCE OF ORGANIZED CONVECTION. TEMPS WILL REMAIN NEAR NORMAL DURING THE DAY AND A LITTLE ABOVE NORMAL AT NIGHT OWING TO ABOVE-NORMAL AMOUNTS OF LOW- LEVEL MOISTURE. WINDS WILL GUST NEAR STORMS AS ALWAYS...BUT WILL OTHERWISE BE UNEVENTFUL. .LONG TERM...MONDAY NIGHT THROUGH SATURDAY...NO CHANGE IN THE PATTERN WITH UPPER LEVEL TROUGH OVER THE REGION THROUGH LATE NEXT WEEK. A SECOND SHORTWAVE TROUGH MOVES IN FROM THE NORTHWEST ON MONDAY AND WILL BE THE FOCUS FOR MORE SHOWERS AND THUNDERSTORMS MONDAY NIGHT THROUGH FRIDAY. TEMPERATURES WILL REMAIN NEAR NORMAL THROUGH FRIDAY. UPPER LEVEL RIDGE TRIES TO BUILD NORTHWARD ON SATURDAY BUT IS LARGELY SUPPRESSED BY WEAK SHORTWAVES EMBEDDED IN THE WEST- SOUTHWESTERLY FLOW. IT SHOULD BE DRIER BUT THERE IS STILL A THREAT FOR SHOWERS AND THUNDERSTORMS IN THE MOUNTAINS SATURDAY. TEMPERATURES WILL WARM INTO THE 80S IN THE LOWER VALLEYS FOR THE WEEKEND. && .AVIATION...WIDESPREAD MVFR SHOWERS AND SCATTERED THUNDERSTORMS THIS AFTERNOON AND EVENING. EXPECT HEAVY RAIN AND SMALL HAIL WITH THE STRONGEST STORMS. SHOWERS AND STORMS WILL BEGIN TO DISSIPATE AFTER SUNSET. SURFACE WINDS NORTHWESTERLY 10-20KT THIS AFTERNOON...GUSTS TO 30KT NEAR THUNDERSTORMS. WINDS ALOFT AT 10K FT MSL...NORTHEAST 15- 25KT. && .BOI WATCHES/WARNINGS/ADVISORIES... ID...NONE. OR...NONE. && WEATHER.GOV/BOISE SHORT TERM...SP LONG TERM....JS/WH AVIATION.....JS
The Boise radar (KCBX), 28 km away, was scanning. The volume nearest the first event began at 22:02 UTC.
1 flash flood. Local standard time, as filed.
A federal employee reported 0.88 inches of rain from a thunderstorm fell between 405 and 430 PM MDT and a mud slide or debris flow was reported by USACE employees. The mud slide affected a highly used campground in the area.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Idaho in 2015Every state in 2015This day in weather historyWarnings in Idaho, May 2015
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 580655.