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09/19/2018, 4:10 pm EDT

North America Season 1-3 Ahead Outlook

El Nino Modoki is favored, causes colder East U.S. winter Executive Summary: The Climate Impact Company season 1-3 ahead forecast is updated. The outlook indicates a variable autumn pattern with chilly weather forecast for October especially in the West while November reverses warm. A significant rainfall event takes place in Louisiana in October. El Nino Modoki is favored for winter supporting a colder than normal regime in the East. Climate discussion:  In an unusual twist, the polar vortex produced unprecedented presence through the 2018 warm season in the polar region (Fig. 1). For the first time in the 1950-2018 climatology a 6-month stretch of positive phase arctic oscillation (+AO) is observed (Fig. 2). The result of the +AO regime as late September arrives is (already) above normal snow cover in Canada (Fig. 3). Analogs of reasonably similar long-duration +AO regimes indicate the +AO regime is likely to continue during following winter. If so, the wintertime +AO regime will be somewhat unique due to the persistence of blocking high latitude pressure during recent winters somewhat related to the presence of a warm open ocean north of Europe caused by the climate change-inspired contraction of the polar ice cap. Interestingly, the Norwegian Sea has cooled somewhat dramatically during the past 30 days (Fig. 4) likely due to the influences of a stronger than normal polar vortex. So…Climate Impact Company expects +AO to continue during winter which at face value means very cold and snowy northern latitudes and mild ocean influence at middle latitudes with an eye northward for any cold outbreaks. Of course, El Nino southern oscillation (ENSO) has the most influence on the cold season climate. Currently, ENSO is in neutral phase but once again the subsurface equatorial Pacific Ocean is very warm and if the atmosphere can be engaged by the ocean warming by slowing or eliminating eastern equatorial Pacific trade winds then El Nino will come on rapidly. ENSO forecast models are predicting El Nino to develop lead by the Bureau of Meteorology/Australia outlook (Fig. 5).  However, the diagnostics producing an El Nino onset risk present now tried during summertime and failed as the atmosphere did not couple with ENSO warming. Will that happen again? Climate Impact Company expects El Nino to develop but not in a conventional way. The bulk of equatorial Pacific Ocean warming in 2018 has been near the Dateline both subsurface and surface. The warming in recent weeks centered on the Dateline indicates strengthening but reluctance to emerge in the far eastern equatorial Pacific Ocean (Fig. 6). Signs of El Nino Modoki are steadily emerging. Forecast confidence is low to below average and changes are possible but the forecast indicated in this report is based on presence of an El Nino Modoki and +AO regime. Finally, commentary on the Pacific decadal oscillation (PDO) and Atlantic multi-decadal oscillation (AMO) are required. The PDO has been in neutral phase for 14-15 months not seen before since 2006-07. The observational trend does not indicate positive or negative phase emerging anytime soon. +PDO is usually present with El Nino. Expectations of a continued neutral PDO does not support emergence of El Nino late this year. The northern Pacific Ocean and into the Gulf of Alaska is warming. If this trend continues a warm ridge pattern emerges over Alaska and could cause the stronger than polar vortex to buckle southward in Canada enabling greater risk of occasional cold into the U.S. during winter ahead. The AMO has been neutral during 2018 but with caveats. The tropics have generally been cool while record warm SSTA has occurred east of the Northeast U.S. Corridor. South of Greenland a strong cool pool related to presence of a stronger than normal polar vortex has occurred. Each of these very different regional SSTA regimes has affected downstream/upstream climate. The warmth east of the Northeast U.S. is likely to continue. The cool SSTA south of Greenland could expand and possibly add the Norwegian Sea. The cooler northern latitude SSTA suggests the polar ice cap may be a larger than recent years. The influence on the East U.S. is likely warm then stormy. Climate summary: The best fit analog years showing persistent +AO and an El Nino Modoki with both neutral PDO and AMO are favored to produce the Climate Impact Company constructed analog forecast for OCT/NOV 2018 to MAR/APR/MAY 2019 climate forecasts. Fig. 1-2: An unusually strong summertime polar vortex during summer 2018 which included a record 6 consecutive months of +AO index. Fig. 3-4: The Rutgers Snow Laboratory snow cover analysis for the northern hemisphere as of Sep. 19, 2018 (left) and the 30-day SSTA CHANGE analysis identifying cooling of the Norwegian Sea. Fig. 5: The Bureau of Meteorology/Australia ENSO phase forecast using Nino34 SSTA indicates El Nino ahead. Fig. 6: Upper ocean heat anomalies across the equatorial Pacific are warmest near the Dateline supporting development of El Nino Modoki. October 2018: The middle-of-autumn features a pattern change where the West and Central U.S. are mostly cooler than normal and the persistent national anomalous warm signature extending to last mid-spring ends. Above normal snowfall is likely in the central/northern Rockies and across the northern Great Plains. An excessive wet weather month is projected across Texas to the Mid-South. Fig. 7-8: The Climate Impact Company October 2018 temperature and precipitation anomaly forecast for North America. November 2018: Pattern reversal in November as weak El Nino influences appear. The northern latitudes flip warmer than normal. After a chilly October in the North-central U.S. a much warmer regime evolves in November thanks to Pacific influences. Early season snow cover retreats. The northern U.S. is drier than normal. The southern U.S. observes average rainfall and generally near normal temperature. In California a cooler than normal regime is expected. Excessive precipitation is forecast on the Alaska and western Canada Coast. Fig. 9-10: The Climate Impact Company November 2018 temperature and precipitation anomaly forecast for North America. DEC/JAN/FEB 2018-19: Increasing confidence in presence of an El Nino Modoki leads to a colder change in the East U.S. for meteorological winter 2018-19. The previous forecast was warm in the East. The anomalous warmth lifts to the north across eastern Canada. The western U.S. forecast is similarly warmer than normal. The outlook is also adjusted wetter for the southern half of the U.S. especially in the Southwest States including southern California and the eastern Tennessee Valley in the East. Above normal snowfall is likely Interior East to help justify the cold pattern in that region. The “ridge bridge” pattern across Alaska is in effect whereas an occasional cross-polar flow of arctic air into the U.S. can occur. The colder forecast is reliant on El Nino Modoki. If a conventional El Nino develops a somewhat milder national pattern is likely. Fig. 11-12: The Climate Impact Company DEC/JAN/FEB 2018-19 temperature and precipitation anomaly forecast for North America is indicated.  MAR/APR/MAY 2019: Next spring is largely temperate across the U.S. and warmer than normal for much of Canada. A wet scenario is forecast for the western states and the Gulf region. Fig. 13-14: The Climate Impact Company MAR/APR/MAY 2019 temperature and precipitation anomaly forecast for North America is indicated.