September 11, 2018 (UT, WY, CO)
- Severe (D2) to exceptional (D4) drought conditions are now impacting about two-thirds of Utah and Colorado after a summer that brought little relief, with unusual warmth and below-normal rainfall continuing over most of the two states. During the June-August period, most gages in Colorado and Utah saw near-record-low streamflows.
- Streamflows in Utah and Colorado were well below normal in July and August, with the observed monthly flows for both months below the 10th percentile for the vast majority of gages. Many gages saw their lowest or 2nd-lowest monthly flows on record, even in parts of the South Platte basin where the snowpack was near average. Streamflows were well above normal in northern Wyoming, and normal to below normal in the southern half of the state.
- With the poor spring-summer inflows, water storage in Utah and western Colorado has sharply declined since late spring, with most reservoirs now well below the median storage volume for the end of August. Lake Powell held 11.2 MAF as of September 1, compared to 15.0 MAF one year ago . The 1.5 MAF decrease in Powell storage from July 1 to September 1 was more than triple the drop over the same period one year ago.
- July and August were on the dry side overall for the region. The southwestern monsoon brought above-average precipitation over those two months to Arizona into far southwestern Utah, but not elsewhere. And the seasonal flow of Gulf of Mexico moisture to the eastern portions of the region favored only northeastern Wyoming and southeastern Colorado. July and August continued the streak of above-average temperatures for Utah and western Colorado, but Wyoming and eastern Colorado were closer to normal .
- Drought conditions have improved in southwestern Utah and southeastern Colorado since early July, but worsened in northern Utah and central and northwestern Colorado. D3 conditions expanded significantly in western Colorado . The total area in the region affected by D2 or worse drought increased by about 20,000 square miles over the past two months. As of September 4, 73% of Utah is in D2 or worse, and the remainder in D0 or D1; in Colorado, 64% is in D2 or worse, and 16% in D0-D1; and in Wyoming, only 3% is in D2, and 23% in D0-D1.
- The CPC seasonal precipitation outlooks for the month of September and the September-November period show enhanced chances for above-normal precipitation for Utah and western Colorado, consistent with the elevated odds of El Nino development by winter. Those odds are now at about 70%, per the IRI/CPC Probabilistic ENSO Forecast .
