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Unpredictability defines fall forecasting

September 4, 2018
By Dr. Eric Snodgrass

August 2018 was a roller coaster ride in terms of temperatures. Much of the Northwest saw a temperature pattern that at times featured near-record highs before a cold front would knock temperatures back for several days. Air quality was a major issue last month as smoke from fires all over western North America blanketed the region.

Thick smoke cover can reduce insolation (sunlight hitting the ground) by as much as 40 percent, which has a major impact on photosynthesis. Thankfully, the end of August brought strong westerly winds that cleared out the smoke, allowing air quality to improve. Summer has been dry, too, and drought conditions have developed over much of the Northwest except southeastern Montana, which has had a lot of thunderstorm activity.

September weather is crucial to finishing a growing season and planting winter wheat, and the greatest influence on September weather patterns is the position of the jet stream over Alaska.

The upstream flow in the atmosphere over Alaska dominates the precipitation and temperature forecast and the first half of September will feature a substantial ridge across much of Alaska and far eastern Russia. This will force a trough into the Northwest by the end of the first week of September, which will bring coastal rains and cooler weather.

Hot, dry weather along the east coast of the U.S. barring any land-falling tropical systems, which will help reinforce the trough over the Northwest too, but fall weather is notoriously unpredictable due to tropical cyclone activity in both the Atlantic and Pacific. One typhoon near Japan can cause a jet stream pattern flip in as little as five days.

Model forecasts favor near-average to cooler-than-average weather for much of September with no strong precipitation biases outside of coastal Washington and Oregon.

The start of the wet season is about a month away and one of the best pre-season indicators on winter weather conditions are Pacific Ocean Sea Surface Temperature (SST) anomalies. The most important features we see in forecasts are:
1. The development of a Modoki El Niño with warm water concentrated in the central Pacific Ocean that does not extend into the south Pacific Ocean;
2. Warmer than average SSTs off the west coast;
3. Potentially cooler water in the north central Pacific Ocean.

In the past, this configuration of ocean temperatures has led to above-average temperatures throughout winter. However, long-range predictability is low and in-season variability is very high with a Modoki El Niño. A potential analog for this winter forecast is December 2014 - February 2015.

Dr. 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. Dr. Snodgrass' research initiatives focus on K-12 science education as well as weather forecasting applications in financial markets.

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