259  
FGUS75 KBOI 041919  
ESFBOI  
IDC001-003-005-007-009-011-013-015-017-019-021-023-025-027-029-031-  
033-035-037-039-041-043-045-047-049-051-053-055-057-059-061-063-065-  
067-069-071-073-075-077-079-081-083-085-087-050600-  
 
WATER SUPPLY OUTLOOK  
NATIONAL WEATHER SERVICE BOISE ID  
118 PM MDT FRI MAY 4 2018  
   
..IDAHO SPRING FLOOD AND WATER RESOURCES OUTLOOK
 
 
THE POTENTIAL FOR SPRING FLOODING DUE TO SNOWMELT IS ELEVATED FOR  
PORTIONS OF EASTERN IDAHO AND NORTHERN IDAHO. THE SPRING FLOOD RISK  
FOR THE REST OF THE STATE IS AVERAGE OR BELOW AVERAGE.  
 
THERE REMAINS AN ELEVATED RISK OF SPRING FLOODING ACROSS PORTIONS OF  
THE UPPER SNAKE BASIN DUE TO ABOVE AVERAGE MOUNTAIN SNOWPACK. THIS  
INCLUDES THE MAINSTEM SNAKE RIVER ABOVE AMERICAN FALLS RESERVOIR AND  
THE HENRYS FORK NEAR REXBURG. AN ELEVATED FLOOD RISK ALSO EXISTS  
ACROSS THE PANHANDLE REGION WHERE SOME OF THE HIGHEST SNOWPACK  
PERCENTAGES IN THE STATE RESIDE. ELSEWHERE ACROSS IDAHO, EARLY MAY  
SNOWPACK CONDITIONS SUGGEST A LOW PROBABILITY OF SPRING FLOODING DUE  
TO SNOWMELT.  
 
THE PRIMARY FACTORS IN THE DEVELOPMENT OF SPRING FLOODING ARE THE  
OCCURRENCE OF PERSISTENT ABOVE NORMAL TEMPERATURES, AND RAIN ON SNOW  
PRECIPITATION EVENTS. EVEN FOR AREAS THAT HAVE LOW SNOWPACK, SPRING  
FLOODING IS POSSIBLE UNDER THE RIGHT CONDITIONS. ADDITIONALLY,  
WILDFIRE BURN SCARS CAN HAVE A SIGNIFICANT IMPACT ON LOCAL FLOOD  
POTENTIAL DURING SPRING SNOWMELT.  
 
WATER SUPPLY  
 
NATIONAL WEATHER SERVICE APRIL THROUGH SEPTEMBER WATER SUPPLY VOLUME  
FORECASTS FOR NORTHERN AND EASTERN IDAHO, AND THE MAINSTEM SNAKE  
RIVER ACROSS SOUTHERN IDAHO RANGE FROM 115 TO 150 PERCENT OF  
AVERAGE. ELSEWHERE, WATER SUPPLY FORECASTS ARE 85 TO 110 PERCENT OF  
AVERAGE FOR MOST OF THE CENTRAL MOUNTAINS AND ONLY 30 TO 70 PERCENT  
OF AVERAGE FOR SOUTH CENTRAL AND SOUTHWEST IDAHO. THE LOWEST ARE IN  
THE BRUNEAU AND OWYHEE RIVER BASINS AT LESS THAN 40 PERCENT OF  
AVERAGE.  
 
TEMPERATURE AND PRECIPITATION  
 
AS OF MAY 1, TEMPERATURES FOR THE 2018 WATER YEAR HAVE BEEN AVERAGE  
OR SLIGHTLY BELOW AVERAGE ACROSS THE NORTHERN HALF OF THE STATE  
WHILE MOST OF SOUTHERN IDAHO HAS EXPERIENCED A LITTLE ABOVE AVERAGE  
TEMPERATURES. WATER YEAR PRECIPITATION WAS ABOVE AVERAGE FOR THE  
PANHANDLE, SPOKANE, AND CLEARWATER BASINS. THE CLEARWATER BASIN HAD  
THE GREATEST ANOMALIES IN THE STATE AT 130 TO 150 PERCENT OF  
AVERAGE. THE SALMON BASIN AND UPPER SNAKE BASIN NEAR THE WYOMING  
BORDER WERE GENERALLY 100 TO 130 PERCENT OF AVERAGE. ELSEWHERE IN  
SOUTHERN AND SOUTHWEST IDAHO THE WATER YEAR PRECIPITATION WAS MOSTLY  
IN THE 70 TO 90 PERCENT OF AVERAGE RANGE WITH POCKETS OF AROUND 60  
PERCENT.  
 
SNOWPACK  
 
AS OF MAY 2, THE HIGHEST SNOWPACK PERCENTAGES IN THE STATE RANGED  
FROM 138 TO 145 PERCENT OF MEDIAN IN THE CLEARWATER, SPOKANE AND  
NORTHERN PANHANDLE REGION. NOT FAR BEHIND WERE BASINS IN EASTERN  
IDAHO SUCH AS THE LITTLE LOST AND BIRCH BASINS, HENRYS FORK, TETON,  
AND SNAKE BASIN ABOVE PALISADES AT 120 TO 130 PERCENT. THE PAYETTE,  
BOISE, SALMON, WOOD AND LOST BASINS RANGED FROM 74 TO 109 PERCENT OF  
MEDIAN. SOUTHSIDE SNAKE RIVER BASINS ALONG THE NEVADA BORDER WERE A  
MIXED BAG RANGING FROM A LOW OF 17 PERCENT OF MEDIAN IN THE OWYHEE  
BASIN TO A HIGH OF 86 PERCENT OF MEDIAN IN THE RAFT RIVER BASIN.  
NORTHERN IDAHO BASINS AND THOSE NEAR THE CONTINENTAL DIVIDE REACHED  
THEIR PEAK SNOWPACK IN MID TO LATE APRIL. THE LOW ELEVATION SNOW IS  
GONE BUT MELTING OF THE HIGH ELEVATIONS WILL INCREASE OVER THE NEXT  
FEW WEEKS AS TEMPERATURES WARM.  
 
RESERVOIRS  
 
RESERVOIR STORAGE ACROSS THE IDAHO IS IN GOOD SHAPE. AS OF MAY 1,  
STORAGE IN MAJOR RESERVOIR SYSTEMS THROUGHOUT IDAHO WAS 100 PERCENT  
OF AVERAGE OR GREATER, EXCEPT WHERE SYSTEMS WERE HEAVILY DRAFTED TO  
MAKE SPACE FOR ANTICIPATED SNOWMELT RUNOFF. WEATHER PATTERNS,  
IRRIGATION DEMAND, AND FLOOD CONTROL NEEDS WILL CONTINUE TO DRIVE  
OPERATIONS THROUGH LATE SPRING AS RESERVOIRS ARE TOPPED OFF. WET  
SPRING WEATHER OR EXTENDED PERIODS OF ABOVE NORMAL TEMPERATURES  
RESULTING IN RAPID SNOWMELT AND LARGE RESERVOIR INFLOWS COULD RESULT  
IN SIGNIFICANT FLUCTUATIONS IN RESERVOIR DISCHARGE AND DOWNSTREAM  
AND RIVER LEVELS.  
 
DROUGHT  
 
IDAHO IS CURRENTLY FREE FROM ANY OFFICIAL DROUGHT CLASSIFICATION  
ACCORDING TO THE U.S. DROUGHT MONITOR. HOWEVER, BELOW AVERAGE  
PRECIPITATION FOR THE WATER YEAR AND POOR SNOWPACK HAS PUT MUCH OF  
SOUTHERN IDAHO IN THE ABNORMALLY DRY CATEGORY. WEATHER AND  
PRECIPITATION FOR THE REMAINDER OF SPRING WILL DETERMINE WHETHER OR  
NOT CONDITIONS IMPROVE OR DETERIORATE FOR THESE AREAS EXPERIENCING  
DRYNESS. GOOD RESERVOIR STORAGE WILL EASE DROUGHT CONCERNS FOR THOSE  
SERVED BY MAJOR STORAGE PROJECTS.  
 
LONG RANGE AND SEASONAL OUTLOOK  
 
THE OUTLOOK FOR MAY FAVORS ABOVE NORMAL TEMPERATURES ACROSS THE  
STATE. THE MAY PRECIPITATION OUTLOOK FAVORS ABOVE NORMAL  
PRECIPITATION FOR SOUTHEAST IDAHO AND BELOW NORMAL PRECIPITATION FOR  
THE NORTHWEST HALF OF THE STATE. THE SEASONAL OUTLOOK FOR JUNE  
THROUGH AUGUST FAVORS ABOVE NORMAL TEMPERATURES AND BELOW NORMAL  
PRECIPITATION.  
 
REFER TO THE LINKS PROVIDED BELOW FOR THE LATEST INFORMATION ON  
WATER RESOURCES.  
 
WATER SUPPLY VOLUME FORECASTS NATIONAL WEATHER SERVICE-NORTHWEST  
RIVER FORECAST CENTER WWW.NWRFC.NOAA.GOV/WS/  
 
NATIONAL WEATHER SERVICE-COLORADO BASIN RIVER FORECAST CENTER  
WWW.CBRFC.NOAA.GOV/  
 
SNOWPACK INFORMATION NATIONAL WEATHER SERVICE-NORTHWEST RIVER  
FORECAST CENTER WWW.NWRFC.NOAA.GOV/SNOW/  
 
NATIONAL WEATHER SERVICE-NATIONAL OPERATIONAL HYDROLOGIC REMOTE  
SENSING CENTER WWW.NOHRSC.NOAA.GOV  
 
USDA-NATURAL RESOURCES CONSERVATION SERVICE  
WWW.NRCS.USDA.GOV/WPS/PORTAL/NRCS/MAIN/ID/SNOW/  
 
RESERVOIR STORAGE BUREAU OF RECLAMATION  
WWW.USBR.GOV/PN/HYDROMET/SELECT.HTML  
 
USDA-NATURAL RESOURCES CONSERVATION SERVICE  
WWW.WCC.NRCS.USDA.GOV/BASIN.HTML  
 
DROUGHT CONDITIONS U.S. DROUGHT PORTAL WWW.DROUGHT.GOV  
 
U.S. DROUGHT MONITOR WWW.DROUGHTMONITOR.UNL.EDU  
 
NATIONAL DROUGHT MITIGATION CENTER WWW.DROUGHT.UNL.EDU/  
 
PEAK FLOW FORECASTS NORTHWEST RIVER FORECAST CENTER  
WWW.NWRFC.NOAA.GOV/PEAK/  
 
COLORADO BASIN RIVER FORECAST CENTER  
WWW.CBRFC.NOAA.GOV/RMAP/PEAK/PEAKLIST.PHP  
 
TEMPERATURE AND PRECIPITATION OUTLOOK CLIMATE PREDICTION CENTER  
WWW.CPC.NCEP.NOAA.GOV/  
 

 
 
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