An upper low tracked through Southwest Idaho and kicked off areas of strong thunderstorms in the West Central and Boise Mountains.
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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.
An upper low tracked through Southwest Idaho and kicked off areas of strong thunderstorms in the West Central and Boise Mountains.
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 Aug 30, 8:55 AM MST, 4 hours 10 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BOISE ID 955 AM MDT SUN AUG 30 2009 .DISCUSSION...VERY ACTIVE WEATHER DAY ANTICIPATED. HAVE ALREADY ISSUED A FLASH FLOOD WATCH FOR THIS AFTERNOON AND TONITE IN THE WEST CENTRAL AND BOISE MOUNTAINS. VIGOROUS UPPER LOW OVER SOUTHWEST IDAHO WITH PARAMETERS IN PLACE FOR AN EARLY START TO CONVECTION VS.THE LATE START YESTERDAY. ISOLATED SEVERE POTENTIAL OVER AREAS SOUTH OF THE SNAKE RIVER AS IMPRESSIVE JET STREAK WRAPS UNDERNEATH UPPER LOW. THUS...MODERATE SHEAR...LIFT AND MOISTURE OVER A VERY UNSTABLE AIR MASS. MAIN THREAT AREAS FOR STRONG TO SEVERE CELLS WILL BE EAST AND SOUTH OF BOISE...WHILE FLASH FLOOD THREAT IS FARTHER NORTH IN THE CENTRAL MOUNTAINS WHERE LOCAL QPF AMOUNTS BETWEEN ONE HALF AND ONE INCH HAVE ALREADY BEEN REALIZED. GRIDS AND FORECASTS TO BE UPDATED TO REFLECT THE CURRENT THINKING AND EARLIER HEADLINES. && .AVIATION...VFR WILL CONTINUE EXCEPT NEAR THUNDERSTORMS MAINLY AFTERNOON/EVENING KMYL-KBOI EASTWARD. SOME STORMS WILL HAVE SMALL HAIL...GUSTY WINDS...AND LOCAL BRIEF HEAVY RAIN...WITH LOCAL IFR CONDITIONS AND MOUNTAIN OBSCURATION. SCATTERED SHOWERS AND WIDELY SCATTERED THUNDERSTORMS WILL CONTINUE OVER THE MOUNTAINS EAST OF KMYL-KBOI THROUGH MONDAY. SURFACE WINDS WILL BE LIGHT NORTHWEST EXCEPT NEAR THUNDERSTORMS. WINDS ALOFT WILL BE LIGHT EXCEPT MODERATE SOUTHERLY OVER CENTRAL AND EASTERN IDAHO. && .PREV DISCUSSION...TODAY THROUGH MONDAY...UPPER LOW WILL TRACK THROUGH THE FORECAST AREA TODAY. BEST CHANCE OF SHOWERS AND THUNDERSTORMS WILL REMAIN OUT AHEAD OF THE LOW CENTER WHICH WILL BE OVER TREASURE VALLEY BY THIS EVENING. CONVECTIVE THREATS ARE THE SAME AS YESTERDAY...BEING HEAVY RAIN AND SMALL HAIL. SOME LOCATIONS IN THE WEST CENTRAL AND BOISE MTNS EAST OF MCCALL...CASCADE AND IDAHO CITY HAVE ALREADY RECEIVED OVER HALF AN INCH OF RAIN THROUGH 3 AM THIS MORNING. THESE LOCATIONS COULD SEE UP TO AN INCH OF RAIN TODAY. CONCERN WITH HEAVY RAIN WOULD BE POTENTIAL FOR SLIDES IN FAVORED AREAS...INCLUDING ALONG HIGHWAY 21 NORTH OF LOWMAN AND MIDDLE FORK RD TO ATLANTA. UPPER LOW SHIFTS EAST OF THE AREA TONIGHT CUTTING OFF PRECIP FOR MOST LOCATIONS. EXCEPTION WOULD BE FAR EASTERN SECTIONS OF VALLEY COUNTY AND THE EASTERN BOISE MTNS. FOR MONDAY...RIDGE IS QUICK TO BUILD IN BEHIND THE LOW AND TEMPS RESPOND...WARMING 4-8 DEGREES FROM TODAYS HIGHS. EASTERN MTN ZONES WILL KEEP SLIGHT CHANCE OF SHOWERS/THUNDERSTORMS WHILE REST OF FORECAST AREA REMAINS DRY. LONG TERM...MONDAY NIGHT THROUGH SATURDAY...VERY LITTLE CHANGE IN THE ECMWF FROM YDAY TO THIS MORNING...AND VERY LITTLE IN THE WAY OF CHANGES IN THE FORECAST PACKAGE IN THIS PERIOD. STILL LOOKING AT SW FLOW ALOFT AFTER THE CURRENT UPPER LOW MOVES OUT. A RIDGE WILL TRY TO BUILD IN WED...INCREASING TEMPERATURES TO AROUND 10 DEGREES ABOVE NORMAL. AS THE RIDGE DECREASES IN ADVANCE OF A LARGE INCOMING EASTERN PACIFIC TROUGH...TEMPS WILL MODERATE BUT STILL REMAIN ABOVE NORMAL THROUGH AT LEAST THURSDAY. TEMPS ARE CURRENTLY FORECAST TO DROP BELOW NORMAL FRIDAY AND SATURDAY...BUT THERE IS A SIGNIFICANT CHANCE THAT THE INCOMING TROUGH WILL SLOW DOWN AND WE WILL INCREASE THE FORECAST TEMPERATURES FOR THESE TWO DAYS TO BACK ABOVE NORMAL. THERE IS VERY LITTLE CHANCE OF PRECIPITATION THROUGH THE END OF THE WEEK. && .BOI WATCHES/WARNINGS/ADVISORIES... ID...FLASH FLOOD WATCH FROM 21Z THRU 06Z FOR IDZ011-IDZ013. OR...NONE. && WEATHER.GOV/BOISE DISCUSSION...GS PREV DISCUSSION...DG LONG TERM....SP AVIATION.....JT
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE BOISE ID 607 AM MDT SUN AUG 30 2009 IDZ011>016-028>030-ORZ061>064-311215- WEST CENTRAL MOUNTAINS-LOWER TREASURE VALLEY ID-BOISE MOUNTAINS- UPPER TREASURE VALLEY-SOUTHWEST HIGHLANDS-WESTERN MAGIC VALLEY- CAMAS PRAIRIE-OWYHEE MOUNTAINS-SOUTHERN TWIN FALLS COUNTY- HARNEY COUNTY-BAKER COUNTY-MALHEUR COUNTY- LOWER TREASURE VALLEY OR- 607 AM MDT SUN AUG 30 2009 /507 AM PDT SUN AUG 30 2009/ THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF SOUTH CENTRAL IDAHO...SOUTHWEST IDAHO...WEST CENTRAL IDAHO...NORTHEAST OREGON AND SOUTHEAST OREGON. .DAY ONE...TODAY AND TONIGHT NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION WILL NOT BE NEEDED.
The Boise radar (KCBX), 114 km away, was scanning. The volume nearest the first event began at 20:03 UTC.
1 hail. Local standard time, as filed.
A trained spotter near Cascade, Idaho reported three quarter inch hail and heavy rain.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Idaho in 2009Every state in 2009This day in weather historyWarnings in Idaho, August 2009
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: 193140.