October Onslaught

It’s not supposed to be this wet this early

Climatologically, October is among chart-41the drier months in the northwestern US. A climograph generated from a tool we developed at the Applied Climate Science Lab depicts this for Spokane, Washington that from 1981-2010 averaged a measly 1.2″ for the month ~ around 7.5% of the annual total.  The average number of days with measurable precipitation for October in Spokane is 8.

The heavy usage of raincoats and windshield wipers for the past four weeks or so has provided a great boost to soil moisture and reservoir storage in many areas, and helped purge a bit more of the drought signal that has been lingering across the region from last years significant drought. Measurable precipitation was recorded for 22 of the 31 days in October in Spokane. Several spots west of the Cascades had 25-30 days with measurable precipitation. Curiously enough the moisture plumes last month seemed to have somewhat avoided southeastern Oregon and southwestern Idaho.

How much precipitation did we get during October 2016?

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Perhaps the most impressive record broken was the 6.23″ that fell at the Spokane airport during October 2016 making it the wettest month all time since records have been kept dating back to 1881 (although the station has moved). Have fun calculating a z-score for October 2016 precipitation.

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The water vapor onslaught of October also brought with it gloomy weather, namely an abundance of cloud cover that suppresses daytime high temperatures and keeps overnight temperatures from cooling off. This is perfectly represented in the maximum and minimum temperature anomalies maps from the NW Climate Toolbox shown below:

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While the NW and parts of northern California were hammered in October, our neighbors to the north were SOL.

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Juneau, Alaska recorded only 2.59″ for the month, making October 2016 the driest October on record (they average 8.5″).

Why?

Well, there is only so much moisture that is transported from the Pacific inland. The typical position of the jet stream in October is well north of the region, generally allowing for the hotspot of midlatitude cyclones to pass through SE Alaska and British Columbia before shifting southward later into the fall.

 

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Climatological 300hPa winds for October (1981-2010), note the maximum well north of the area.

A quasi-stationary trough off the Washington coast has coincided with the southward displacement of the storm track this past month and helped coax a bit of subtropical moisture in the form of atmospheric rivers that have contributed to the October onslaught.

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October 2016 300hPa winds.

 

Although this is an excellent start to the water year, snowpack contributions from the October precipitation have been lackluster due to high snow levels. When it comes to regional water supplies, snow is better than rain.