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Drought appears to be easing slightly over Northwest

January 2, 2019
By Eric Snodgrass
For Northwest Farm Credit Services

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The first 60 days of the wet season of the new water year was a bit sluggish outside of Montana, but the end of December 2018 featured multiple, large low-pressures systems that finally began to work on longer-term drought stress. Despite the rain/snow, the U.S. Drought Monitor classifies over 80 percent of Oregon in “severe drought,” but thankfully snow-depth reporting stations in the mountains made substantial improvements in the last two weeks of 2018.

December 2018 favored moderate temperatures for most of the Northwest, especially after mid-month.

Despite Montana’s relatively mild and wet/snowy December, the start of the New Year offered a brief dive into extremely cold temperatures.

This cold start will moderate quickly as the jet stream temporarily moves into a high-over-low pattern in the Gulf of Alaska. High-over-low patterns are very common during the Northwest’s driest Januaries on record and they are patterns we want to avoid.

Combatting this pattern in winter 2018-19 has been a strong central Pacific zonal jet stream that has resisted allowing the pattern to become blocked for long stretches of time. The end of the first week of January will feature multiple short-wave troughs that will push through the Gulf of Alaska, preventing the drier pattern from establishing. However, what the jet stream lacks this year is persistent onshore flow from the “Pineapple Express” or a deep, slow-moving coastal low-pressure system.

This is the main reason we have not made much progress on erasing long-term drought.

Polar Vortex
10mb winds showing the position of the polar vortex and stratospheric warming event (ridge) on December 31, 2018.
Confidence in long-range weather prediction is very low. Even the best forecast models are struggling with the interaction between several large-scale circulations. Here are a few of the details.

  1. On December 31, the polar vortex has been displaced by a large, sudden, stratospheric warming event near the North Pole. The polar vortex may split over Europe and the northeastern US, but models are struggling to capture the tropospheric response. Normally, this would signal a dry and warm January for the Northwest, but only about 45 percent of all sudden stratospheric warming events produce a large, widespread tropospheric response.

  2. The tropics have been dominated by the strength and phase of the Madden-Julian Oscillation (MJO) this fall and early winter. Its shift into phases 3-4-5 in December led to warming in Montana, the central Plains and the eastern U.S. as well as increased precipitation in Australia. The MJO is forecast to move slowly toward 6-7-8 through the first half of January, but do so at very low amplitude. This removes its presence as a dominant forecast feature in the near term.

3. El Nino has peaked in intensity in the central Pacific, but only as a weak event. At times the trade winds and Pacific Ocean pressure patterns have barely acted like an El Nino, which removes it from being a dominant part of our forecast.

The result is that we will have to watch the Pacific jet stream week by week as we progress through the wet season.

All long-range forecasts should be taken with a large grain of salt!

Eric Snodgrass is the Director of Undergraduate Studies for the Department of Atmospheric Sciences at the University of Illinois Urbana-Champaign. He serves on numerous committees and boards on campus, including the Illinois Teaching Advancement Board, Student Sustainability Committee and the Provost Task Force on Improving Large Enrollment Courses. Eric Snodgrass' research initiatives focus on K-12 science education as well as weather forecasting applications in financial markets.

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