SNOW IS AN IMPORTANT COMPONENT OF THE GLOBAL HYDROLOGIC CYCLE WITH SIGNIFICANT IMPLICATIONS FOR WATER RESOURCES AND NATURAL HAZARDS. TRADITIONALLY SNOW WATER EQUIVALENT (SWE) HAS BEEN ESTIMATED FOR WATERSHEDS AND MOUNTAIN RANGES USING SPARSE IN SITU MEASUREMENTS (E.G. SNOTEL) AND/OR HIGHLY PARAMETERIZED MODELS CALIBRATED WITH LOW-RESOLUTION REMOTE SENSING PRODUCTS (E.G. MODIS FSCA). IN RECENT YEARS REPEAT AIRBORNE LIDAR OBSERVATIONS BY THE NASA AIRBORNE SNOW OBSERVATORY (ASO) OFFER DETAILED MAPS OF SNOW DEPTH WITH HIGH ACCURACY AND SPATIAL RESOLUTION BUT LOGISTICAL COSTS ARE HIGH AND COVERAGE IS LIMITED TO INDIVIDUAL WATERSHEDS. DESPITE THESE EXISTING CAPABILITIES IT REMAINS CHALLENGING TO ACCURATELY ASSESS SNOW DEPTH AND SWE ON A REGIONAL SCALE. TO ADDRESS THIS ISSUE WE WILL USE ~30-50 CM COMMERCIAL OPTICAL STEREO SATELLITE IMAGERY (DIGITALGLOBE WORLDVIEW/GEOEYE CONSTELLATION) TO ESTIMATE SNOW DEPTH AND SWE ON A REGIONAL SCALE. WE WILL GENERATE TIME SERIES OF CO-REGISTERED 2-M DIGITAL ELEVATION MODELS (DEMS) USING A MATURE AUTOMATED OPEN-SOURCE PROCESSING WORKFLOW (NASA AMES STEREO PIPELINE RUNNING ON THE NASA PLEIADES SUPERCOMPUTER). PREVIOUS WORK DEMONSTRATED THAT THIS APPROACH CAN BE USED TO MEASURE GLACIER AND ICE SHEET ELEVATION CHANGE WITH<30 CM VERTICAL ACCURACY. WE WILL DEVELOP NEW CORRECTIONS AND WORKFLOW REFINEMENTS TO FURTHER IMPROVE THIS VERTICAL ACCURACY TO<10-15 CM. EXISTING LICENSING AGREEMENTS PROVIDE NO-COST ACCESS TO ARCHIVED DIGITALGLOBE IMAGERY (2007-PRESENT) WITH NO RESTRICTIONS ON DISTRIBUTION OF DERIVED PRODUCTS (I.E. SNOW DEPTH MAPS). WE TASKED WORLDVIEW-3 TO ACQUIRE CLOUD-FREE STEREO IMAGERY OVER THE TWO SNOWEX'17 SITES IN SEPTEMBER 2016 (SNOW-OFF) AND THEN EVERY ~2 WEEKS FROM JANUARY TO MARCH 2017 (SNOW-ON). WE WILL USE THIS DATASET TO TUNE THE IMPROVED DEM PROCESSING WORKFLOW AND THEN INTEGRATE AVAILABLE AIRBORNE LIDAR AND ICESAT-1/2 LASER ALTIMETRY DATA FOR CO-REGISTRATION AND ACCURACY EVALUATION. WE WILL COMPUTE SNOW DEPTH MAPS RELATIVE TO SNOW-OFF DEMS. WE WILL THEN USE SEVERAL APPROACHES TO ESTIMATE THE SPATIAL AND TEMPORAL DISTRIBUTION OF BULK SNOW DENSITY VALIDATE USING SNOWEX'17 IN SITU OBSERVATIONS AND CREATE SWE MAPS FROM THE SNOW DEPTH TIME SERIES WITH UNCERTAINTY ESTIMATES. FINALLY WE WILL EVALUATE STEREO2SWE PRODUCTS FOR A RANGE OF DIFFERENT VEGETATION TYPES AND EXPLORE OPTIONS TO IMPROVE SWE ESTIMATES BENEATH DENSE FOREST CANOPY. WE WILL THEN EXPAND THE SCOPE OF OUR STUDY AND USE THE TUNED AUTOMATED STEREO2SWE WORKFLOW TO ESTIMATE SNOW DEPTH AND SWE FOR SITES ACROSS THE WESTERN US. IN LATE 2012 PI SHEAN INITIATED AN INTERAGENCY OBSERVATION PROGRAM TO COLLECT SEASONAL STEREO IMAGERY OVER HIGH-ELEVATION TARGETS (MT. RAINIER OLYMPIC MOUNTAINS ETC.) IN THE PACIFIC NORTHWEST. THIS OBSERVATION PROGRAM NOW INCLUDES ALL MAJOR SNOW- AND ICE-COVERED AREAS IN THE WESTERN US (~35 SITES) WITH MONTHLY OBSERVATIONS FOR SEVERAL SITES IN WASHINGTON STATE. THIS EXPANDED SCOPE WILL PROVIDE A THOROUGH EVALUATION OF PRESENT-DAY COMMERCIAL SATELLITE RESOURCES AND THEIR POTENTIAL TO PROVIDE HIGHRESOLUTION MAPS OF SWE ON A GLOBAL SCALE WHICH WILL BOTH COMPLEMENT AND INFORM PLANNING EFFORTS FOR A FUTURE NASA SWE MISSION.