Theme: Changeable, warm-cool-warm flip SEP to NOV 2018. Executive summary: The Climate Impact Company month 1-3 forecast valid for September through November (meteorological autumn) is updated. The forecast is propelled by a combination of ENSO analog years favoring El Nino Modoki and one analog year which dissipates the El Nino risk. Forecast confidence is average for September but fades for mid-to-late autumn due to ENSO uncertainty. Indicated is a warm September especially in the North-Central to Southern California stretch. The warmth extends north and east of the immense western U.S. drought area. Theme of the 1-3 month forecast is changeable as October reverses chilly and could be cooler than indicated in the Central U.S. In November, the pattern reverses warmer (again). Fig. 1-2: The Climate Impact Company U.S. September/October 2018 temperature anomaly forecasts. Climate: The Climate Impact ENSO outlook consists of 3 El Nino Modoki years and 1 year when El Nino fails to develop. The clear signature of SSTA patterns in the equatorial East Pacific is warming of the central/east-central portion while eastern sections resist warming. The subsurface equatorial Pacific supports this trend. Described is an early sign of a potential El Nino Modoki ahead which occurred in 1994-95, 2002-03 and 2009-10. The SSTA pattern in the eastern equatorial Pacific is weak and subsurface warming remains robust but has faded in recent weeks. An outlier ENSO possibility for late 2018 is El Nino approaches and then retreats as observed in 2012. Each of the 4 analog years attendant climate patterns are averaged and contribute strongly to the 3-month forecast. Fig. 3: The Climate Impact Company ENSO analog forecast using Nino34 SSTA index. Cooling degree day discussion: The remainder of the MAY-SEP warm season indicates August is similar to the previous forecast at 314 CDD which is just beneath the 10-year normal (323). The (warm) 10-year normal has been exceeded every month of the warm season (so far). The September forecast is unchanged at 175 which is near the mid-point in-between the 10-year and 30-year climatology. MAY JUN JUL AUG SEP Forecast 141 265 376 314 175 Previous 141 265 376 313 175 Last Year 107 246 364 284 194 30-Year 97 213 321 290 155 10-Year 118 250 338 323 190 Table 1: The CIC warm season CDD forecast and various comparisons. Fig. 4: The CIC warm season CDD forecast versus 30-year normal. Fig. 5: The CIC warm season CDD forecast versus 10-year normal. September 2018: The September outlook trends wetter in the Gulf States/East-Central U.S. due to tropical influences. The Southwest (wet) monsoon is less aggressive. Anomalous heat continues to extend north and across the western drought area arcing eastward across the northern U.S. after entrainment into the prevailing westerly flow aloft. The eastern Canada polar vortex emerges and is capable of pushing occasional cooler air masses toward New England. A dry climate is forecast in September across the North-Central U.S. and into the Midwest. Fig. 6-7: The Climate Impact Company month ahead forecast for September 2018. The previous forecast is below. October 2018: The Canadian polar vortex is stronger than normal during mid-autumn supported by the presence of persistent cooler-than-normal SSTA south and west of Greenland. Cool air masses are generated and migrate into the Central U.S. Early season cold risk is expected in this region. The clash of the cooler air with lingering seasonal subtropical influences causes a widespread wet pattern from the North-Central U.S. to the Gulf region. The West and Southeast are warmer-than-normal. Fig. 8-9: The Climate Impact Company 2 months ahead forecast for October 2018. The previous forecast is below. November 2018: The cool October reverses much warmer and drier in November. The El Nino Modoki look is favored. However, the forecast is made with low confidence and susceptible to change. The typical wet southern U.S. pattern of an arriving El Nino in November is suppressed to just Florida to the Carolinas. Just west and north of this wet regime dryness is widespread from Texas and the Mid-South to the Great Lakes region. Broad mild westerly zonal flow stretches across central North America forced warmer just south of a strong polar vortex in north/northeast Canada. In the West a temperate climate is forecast with potentially wet weather across northern California. Fig. 14-15: The preliminary Climate Impact Company 3 months ahead forecast for November 2018. The previous forecast is below.