Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Mar 18 2021 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS La Nina conditions remain in place across the equatorial Pacific Ocean as indicated by both oceanic and atmospheric conditions. A transition to ENSO-neutral is favored during the late spring until autumn when La Nina once again becomes slightly more favored than ENSO-neutral at that time. In addition to the forecast ENSO evolution described above, current anomalous soil moisture and snow cover along with numerical and statistical model output inform the seasonal temperature and precipitation outlooks. The April-May-June (AMJ) 2021 temperature outlook favors above-normal seasonal mean temperatures for nearly all the contiguous U.S. and for portions of western and northern Alaska. Below-normal temperatures are most likely for a region from southeast Alaska to the extreme Pacific Northwest. Moreover, above-normal temperatures are most likely for the entire forecast domain as we move into and through the summer months. The AMJ 2021 precipitation outlook favors above-normal seasonal total precipitation amounts for portions of the Great Lakes, Ohio Valley, mid-Atlantic and Northeast as well as the west coast of Alaska. Below-normal precipitation is most likely for a region from the West coast across the Rockies to the southern Great Plains. The highlighted area of below-normal precipitation is forecast to slowly shift north and east during the summer months while above-normal precipitation is favored to expand southward along the eastern seaboard over the same period. Elevated odds for above-normal precipitation is introduced for parts of the Southwest during the monsoon season. Equal chances (EC) are indicated for areas where seasonal mean temperatures and seasonal total precipitation amounts are expected to be similar to climatological probabilities. BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS Note: For Graphical Displays of the Forecast Tools Discussed Below See: http://www.cpc.ncep.noaa.gov/products/predictions/90day/tools/briefing CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS La Nina conditions remain present in the equatorial Pacific Ocean as shown with both oceanic and atmospheric indicators. Equatorial sea surface temperatures (SSTs), however, have continued to become less negative with some spotty areas of small positive SST departures from climatology. The most recent weekly value of the Nino3.4 SST index is -0.3 degrees C. It is important to note, however, that the trend in less negative SSTs is likely being modulated by a recent and ongoing MJO event which has resulted in variations in low-level winds, cloudiness, solar radiation and precipitation. Below the surface, a shallow layer (to a depth of 100 meters) of negative ocean temperature anomalies is in place from 165 E to 100 W. A substantial area of positive subsurface oceanic heat content is located in the west-central Pacific Ocean ranging over a depth from 100 - 300 meters. Suppressed convection persists across the equatorial Pacific surrounding the Date Line, while enhanced convection continues across much of Indonesia and the Philippines. Even with a recent reduction in the Trade winds associated with the MJO, the recent monthly average of 850-hPa winds continue to show enhanced trade winds compared to climatology. Cyclonic circulations at 200-hPa symmetric about the equator to first order also are evident in the recent monthly mean. The assessment of these ocean and atmospheric indicators show the coupled ocean-atmospheric system remains consistent with La Nina conditions. PROGNOSTIC DISCUSSION OF SST FORECASTS Forecasts of the Nino3.4 SST index from the NMME show considerable spread of ensemble members across the modeling center forecasts and within each modeling center ensemble. Most guidance continues to favor a continued decrease of negative SST anomalies to ENSO-neutral territory during spring 2021. Some variation begins during the summer months with some guidance favoring once again a decrease in Nino3.4 SST hinting at a potential return to La Nina conditions by autumn 2021. Some statistical components of the CPC Nino3.4 SST consolidated forecast favor a different trajectory - toward ENSO-neutral or El Nino territory by late spring 2021 for the CA and CCA forecasts respectively. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Although equatorial Pacific SSTs continue to become less negative and are forecast to continue doing so through spring 2021, potential typical La Nina impacts are considered in preparing the AMJ and MJJ 2021 seasonal outlooks. ENSO composites, regressions anchored to Nino3.4 SST and forecast tools "bridged" utilizing Nino3.4 SST are used to inform the outlook in these seasons. Current anomalous soil moisture and snow cover are considered and contributed to the outlook in some locations during some seasons. Dynamical model forecasts from the NMME and Copernicus (C3S) multi-model ensemble systems and the CA based on soil moisture anomalies are utilized heavily as was the objective, historical skill weighted consolidation - guidance that objectively combines both dynamical and statistical forecast tools. Due to a number of back-to-back La Nina events in the historical record since the 1950s, a re-emergence of La Nina by the autumn is considered. At later leads, long term trends in temperature and precipitation are utilized heavily to inform the seasonal outlooks. PROGNOSTIC DISCUSSION OF OUTLOOKS - AMJ 2021 TO AMJ 2022 TEMPERATURE The AMJ 2021 temperature outlook favors above-normal seasonal mean temperatures for nearly all the contiguous U.S. and for portions of western and northern Alaska. The greatest odds for above-normal temperatures is located from the Four Corners region in the Southwest to southern Texas due to the confluence of factors including overwhelmingly consistent dynamical model guidance, low soil moisture conditions, long-term positive temperature trends and other statistical forecast tools. Modestly elevated odds for above-normal temperatures extend eastward across the Southeast as well - supported by model guidance and strong trends. More uncertainty and disagreement in forecast tools and lower historical forecast skill in forecast guidance decrease probabilities for above-normal temperatures in the north central CONUS. Consistent with lingering La Nina influence as indicated by ENSO composites, regressions and "bridging" tools there are elevated chances for below-normal temperatures from the extreme Pacific northwest to the south coast of Alaska. Negative trends in sea ice coverage and thickness and positive SST trends favor above-normal temperatures for parts of the west coast of Alaska and the north Slope. Consistent signatures across the range of model guidance and statistical forecast tools including those derived from land surface conditions favor continued elevated odds for above-normal temperatures for most of the forecast domain through the SON 2021 season with the greatest chances depicted for the western U.S. and along the eastern seaboard with slightly lower odds for the north central CONUS. Thereafter, beginning in OND 2021 and continuing through FMA 2022, there is a gradual increase in the EC designation as odds for La Nina re-development and so potential impacts increase for much of the northern tier of the CONUS from the Pacific Northwest to the western Great Lakes, and later southeast Alaska. The inclusion of areas of favored below-normal temperatures may need to be considered in these areas over the next couple of seasonal outlook release packages if odds of La Nina increase, but also, and perhaps just as important, if odds for El Nino remain very low during the late autumn and early winter 2021 as they are now. PRECIPITATION The AMJ 2021 precipitation outlook favors above-normal seasonal total precipitation amounts for portions of the Great Lakes, Ohio Valley, mid-Atlantic and Northeast primarily based on lingering La Nina influence, positive precipitation trends, other statistical forecast tools and to a lesser degree some model forecasts from the NMME and C3S ensemble systems. There is considerably stronger agreement, however, for a region of favored below-normal precipitation from the central West coast across the Rockies to the southern Plains. La Nina considerations, dynamical model guidance and the CAS forecast tool support this forecast. The highlighted area of below-normal precipitation is forecast to slowly shift north and east during the summer months - consistent with dynamical model guidance. Above-normal precipitation is favored to expand southward along the eastern seaboard through the summer months into the early autumn consistent with positive precipitation trends and a slight tilt favoring La Nina during the second half of the Atlantic hurricane season. Below-normal soil moisture and snowpack in the Southwest and southern Rockies may allow for more efficient heating of the land mass and so potentially an enhanced monsoon circulation earlier all factors being equal. Along these lines, elevated odds for above-normal precipitation is favored for parts of the southwest from JJA through ASO 2021. Some dynamical model guidance also support this forecast. La Nina conditions (first 2 leads) and dynamical model guidance support above-normal precipitation for the west coast of Alaska from AMJ - ASO 2021. The outlooks from OND 2021 through AMJ 2022 are primarily supported and based on long term precipitation trends. FORECASTER: Jon Gottschalck The Climatic normals are based on conditions between 1981 and 2010, following the World Meterological Organization convention of using the most recent 3 complete decades as the climatic reference period. The probability anomalies for temperature and precipitation based on these new normals better represent shorter term climatic anomalies than the forecasts based on older normals. For a description of of the standard forecast tools - their skill- and the forecast format please see our web page at http://www.cpc.ncep.noaa.gov/products/predictions/long_range/tools.html (Use Lower Case Letters) Information on the formation of skill of the CAS forecasts may be found at: http://www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/US/Outlook/outlook.shtm l (use lowercase letters) Notes - These climate outlooks are intended for use prior to the start of their valid period. Within any given valid period observations and short and medium range forecasts should be consulted. This set of outlooks will be superseded by the issuance of the new set next month on Apr 15 2021 1981-2010 base period means were implemented effective with the May 19, 2011 forecast release. $$