Happy Water Year!

Happy belated water year.

On October 3rd, a special session recapping climate of the previous water year and climate impacts within the state of Idaho was hosted during the annual meeting of the National Science Foundation Office of Experimental Program to Stimulate Competitive Research in Idaho. Four speakers, Ed Delgado (director of Predictive Service for the NIFC), Liz Cresto (Idaho Department of Water Resources), Troy Lindquist (NWS Boise, senior hydrologist) and myself presented different views on climate, hydrology and wildfire over the past 12 months. I provide a synopsis of the session along with a fuzzy outlook for this upcoming winter in this post.

Our water year ended with nominal precipitation recorded across the inland northwest during September. Although Jul-Sep is the nadir on the annual precipitation wave, a broad section of the NW had its driest Jul-Sep in 118 years this summer with some phenomenal dry spells. The primary culprit for this aridity: a large scale ridge. The upper-level ridge that had plagued the midwest in June and July retrograded west in the second half of summer bringing the big dry to the northwest and slight relief to the parched corn fields.

Temperatures in September were above normal across the region. Consistent with clear skies and little moisture, this warmth was primarily expressed through daytime highs temperatures 4-6 degrees F above normal and overnight lows close to or slightly below normal. This morning, Oct 5th, the Moscow-Pullman airport reported a low of 20F, a record for this date and only 10F warmer than the coldest temperature all last winter. It’s amazing what bone-dry air can do. That aside, temperatures in Jul-Sep across the region were at or near record highs for the 118 year period of record (1895-2012), consistent with large scale ridging and a lack of precipitation (full sunshine and dry surface conditions amplify surface warming).

Wildfire: The combination of warm and dry conditions sustained over the later half of summer, along with patches of higher elevation conifers weakened by beetle damage and a few rogue dry lightning busts, was the perfect recipe for wildfire according to Delgado. He reported that the amount of area burned in the Eastern Great Basin GACC through October was nearly four times above the 2000-2011 average, with most of that occurring in the state of Idaho. The mixed fire regimes in the inland northwest of rangeland and forest fires both succumbed to flames and contributed to one of the largest wildfire seasons in the last couple decades. Megafires were seen in both rangeland fires, including some extremely large cheatgrass-fueled fires across SE Oregon  (Long Draw fire > 550k acres) and parts of the northern Great Basin, as well as several 150k-plus acre forest fires in the Trinity, Halstead and Mustang Complex fires in Idaho.  Delgado noted that the current wildfires burning in the central mountains of Idaho and Cascades are doing so in regions that had above normal snowpack this spring, suggesting that a large spring snowpack does not imply an off-season for wildfire. The lack of substantial season-ending precipitation on the immediate horizon will allow fires to burn further into autumn.

Water Supply: In early April, signs were that 2012 would be another favorable water year across the managed watersheds in the Boise and Snake River basins. However, by the end of the water year, reservoir levels on the Snake River system had less than half of their storage compared to a year ago, according to Liz Cresto. And with irrigation season officially continuing through the end of October, the “insurance” of reservoir carryover is looking rather meager. Cresto also noted that 8 counties in Idaho had a drought declaration which allows for a temporary 1-year transfer of water rights without any financial implications. According to Cresto, the game changer this year was spring precipitation, which was 25-50% below normal across the upper and middle Snake Basins, while parts of eastern Washington through northwestern Montana had one of their wettest springs on record.  Better predictive seasonal outlooks of spring precipitation post-April 1 would dramatically increase the effectiveness of water management.

Flash Flooding: If it ever decides to rain again, the joy may be short-lived. Especially if you find yourself celebrating near a recently burned area in the inland northwest. Troy Lindquist noted that hydrologic warning techniques designed using precipitation intensity thresholds often fail in burned areas, putting such area at elevated risk of flooding and debris flow.  NWS Boise has put together a webpage for heavily used roads particularly at risk of debris flow.  Elevated flooding and debris flow are likely is such regions for upwards of two years post fire recovery, depending on fire severity and vegetation recovery.

All told, most of the western US had a lousy water year, with the northwest actually fairing best along the west side of the Cascades and from eastern Washington eastward through northwestern Montana.  Quite a difference from Water Year 2010-2011.

Outlook for 2012-13

The El Nino that had been developing rather nicely back in July and August has stuck his tail between his legs and crept back into his cage.  In the past month, both observations and dynamical forecast models have been more bearish on this winter’s El Nino.  As of October 1, my collective thoughts are that we will have a neutral, or La Nada winter.  It’s about time, as the last neutral winter was four years ago, and we have not seen a single year since 1950 where we’ve gone from a La Nina to El Nino in a single year (at least using the Multivariate El Nino Index).

In the absence of a strong signal, the skill of seasonal climate outlooks is weaker. Nonetheless, let’s take a look at the latest projections from both the Climate Prediction Center (CPC, top) and the Climate Forecast System model (CFS, bottom) for mid-winter (Dec-Feb), for precipitation (left) and temperature (right). First, let’s key in on precipitation as it is the current theme. The official CPC forecast has a bullseye “B” smack dab over Boise reflecting an El Nino-like signal across the rest of the US; however, the CFS model is pretty neutral and instead has abnormally dry conditions across the southern parts of the western US which is not consistent with an El Nino winter.  Both agree on elevated chances for above normal temperatures across much of the region, although the confidence and signal is rather weak.

From a purely statistical perspective, using projected magnitude of El Nino as our sole weapon in predictability of water year precipitation, I present the following plot, which shows the relationship between the Multivariate El Nino index (MEI) and cool season (Nov-May) precipitation averaged over the state of Idaho from 1950-2011.

Generally speaking the cloud of points shows above normal precipitation is more likely during La Nina conditions (MEI<-1) and below normal precipitation is more likely during El Nino conditions (MEI>1).  You can note here that the distribution for La Nina years is nearly all normal to above normal precipitation, whereas the distribution is less well behaved for El Nino years. With a MEI likely to be close to +0.5, we can see that we have had some dry years including the winter of 1976-77 when only about 50% of the normal cool season precipitation fell across Idaho and much of the Pacific Northwest. We’ll keep our eyes on the tropical Pacific to see if El Nino decides it’s time to strike, or to go on strike.

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