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September 2015

September Summary and Drought Update

Posted: September 28th, 2015


Correction: the image above is from 9/27. Originally posted in error was (for comparison) an image from 7/19.</em

Correction: the image above is from 9/27. Originally posted in error was (for comparison) an image from 7/19.


Seattle’s streak of warmer-than-normal months is about to come to an end. The last time we experienced a cooler-than-normal month was February 2014. Impressive as that 18-month warm streak may be, more notable is the fact that 6 out of the past 12 months broke all-time monthly average temperature records, and one month (July 2015) went down as our warmest month ever.


September’s break from warmth is by average temperature only, however. Its average minimum temperature will still finish above normal. With the exception of this past summer’s all-around extreme warmth, the soon-ending 18-month warm streak was largely achieved through above normal minimum temperatures.


The fact that it has not been cooling off at night like normal is due to anomalous warmth in the Pacific Ocean. The water off our coast is still relatively warm, so we should not be surprised if we see another handful of consecutive months—at least—of warmer-than-normal average temperatures again. As is explained below, the latest outlooks predict just that.


With respect to precipitation, September will finish below normal across the lowlands and near normal in the Cascades. Seatac will finish with 0.83 inches, which is 55 percent normal. The Central Cascades, home to SPU’s water supply, will end up with about 75 percent normal in the vicinity of the Tolt Reservoir, and closer to 100 percent normal around the Cedar River Watershed.


September’s decent precipitation numbers, which followed even better amounts in August, represent good news for those of us trying to manage water supplies during a drought. They do not, however, mean that we are by any means out of the woods. Last week, Washington’s Department of Ecology reminded us that we are “carrying a huge water deficit into this fall and winter.” Reservoir levels have stabilized somewhat but they’re still well below where they should be at this time of the year (see the image above captured on 9/27 by SPU’s Aaron Erman).


After moving to voluntary stages of water shortage response plans on August 11, Everett, Seattle, and Tacoma asked customers to reduce their water use by 10 percent to stretch water supplies, for people and fish, to the rainy season. SPU continues to make operational changes and activate supplementary water supplies—all in an effort to stretch our water supply as far as possible.


But the return of reliable fall rain, which normally happens around the middle of October, would be appreciated. Unfortunately, forecasts are not encouraging. A strong El Niño is now in full swing, and The Blob still has another season or two to go before it’s dead. So, while it’s difficult to imagine that this winter’s snowpack will be as bad as last year’s, it’s not likely to be substantial. Thus, we are hoping for rainy drought relief, but we are noting the Department of Ecology’s concern for a two-year drought.


Official Climate Prediction Center outlooks suggest that the next three months will be warmer and drier than normal. Those nicely smoothed outlooks are helpful, but it’s important to understand that they are the product of numerous models and analyses. One example is the North American Multi-Model Ensemble (NMME), which itself is comprised of seven individual members, each having their own biases and strengths.


Among the NMME’s constituents, forecast sea-surface temperatures are unanimously warm; therefore land temperatures are concomitantly warm through at least February. Forecast precipitation is more variable and a little less certain, but the majority of models suggest drier than normal conditions between now and March, with increasing agreement as the seasons go along.


Our warm streak may have come to an end, but our challenges have not. Let. It. Rain.