Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EST Thu Jan 21 2021 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS La Nina conditions are present across the equatorial Pacific Ocean, as indicated by current oceanic and atmospheric observations. La Nina is expected to continue through the Northern Hemisphere winter (~95% chance), with a potential transition to ENSO-neutral (~55% chance) during April-May-June 2021. The February-March-April (FMA) temperature outlook favors below normal seasonal mean temperatures for the southern half of mainland Alaska, the Alaska Panhandle, and from the Pacific Northwest eastward to the Northern Rockies. Above normal seasonal mean temperatures are favored across parts of northern and western Alaska, and for most of the CONUS, except portions of the Northern Plains, Northern Rockies, Southern Oregon, and Northern California. Maximum probabilities for above normal temperatures exceed 60% across parts of the Southwest and Southern Great Plains. The FMA precipitation outlook favors above normal precipitation for most of the northern tier of the CONUS, extending from the Pacific Northwest to the Great Lakes, then southward across eastern portions of the Middle Mississippi Valley, the Ohio Valley, and the Central and Northern Appalachians. Above normal precipitation is also favored for northern and western portions of Alaska. Maximum probabilities for above normal precipitation exceed 50% over the Ohio Valley. From central and southern California across the Great Basin to the Central and Southern Plains, below normal precipitation is favored. Below normal precipitation is the most favored outcome across portions of the Southeast, as well as along the southern coast of mainland Alaska and for the Alaska Panhandle. Equal Chances (EC) are forecast for areas where probabilities for each category of seasonal mean temperatures or seasonal total precipitation amounts are predicted 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 Tropical oceanic and atmospheric observations reflect ongoing La Nina conditions. During the past 30-days, an area of negative sea-surface temperature (SST) anomalies was located from about 160 deg E to the coast of South America. SST anomalies for the period ranged between 0.5 deg C to 1.5 deg C over most of this region, with the strongest anomalies from 170-150 deg W. Some warming has occurred recently, east of 135W. Subsurface temperatures were colder-than-average for the same region, down to a depth of about 175 meters. The Oceanic Nino 3.4 Index (ONI) for the latest observed season (OND 2020) is -1.3 deg C, which qualifies as a moderate La Nina at this time. Atmospheric observations reflect enhanced easterly trade winds at 850-hPa from the western to the east-central equatorial Pacific, westerly wind anomalies at 200-hPa over most of the Pacific, and suppressed tropical convection over the western and central Pacific. The atmospheric conditions reflect a canonical La Nina response. PROGNOSTIC DISCUSSION OF SST FORECASTS The CPC Nino3.4 SST consolidation depicts negative SST anomalies weakening (so warming) through the remainder of the winter and spring of 2021. The consolidation forecast predicts the ONI value will cross the -0.5 deg C threshold into ENSO-neutral territory during FMA 2021, and reach the zero anomaly line by AMJ 2021. This is slightly warmer than last month’s SST CON. The NMME ensemble mean SST plume for the Nino 3.4 region reaches the -0.5 deg C threshold by April 2021, and continues to warm slightly through summer 2021. The C3S (Copernicus) SST plume reaches the ENSO-neutral threshold slightly earlier, in Feb, then remains at or about -0.5 deg C until June. The CPC-IRI consensus predicts the likelihood of La Nina will drop to the same likelihood of ENSO-neutral by about AMJ 2021. The official ENSO forecast calls for approximately a 95% chance for La Nina to continue through Northern Hemisphere winter, with a potential transition during the spring 2021 (~55% chance of ENSO Neutral during AMJ 2021). PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Given a very high likelihood of the persistence of La Nina conditions, the seasonal outlooks utilized canonical impacts during past observed La Nina events as guidance for many areas of the forecast domain through at least MAM 2021. This guidance included regressions of temperature and precipitation relative to the CPC consolidation forecasts of the Nino3.4 region and via "bridging" techniques utilizing statistical relationships between dynamical model forecasts of the Nino 3.4 index and observed temperature and precipitation. Dynamical model guidance from the North American Multi-Model Ensemble (NMME) and the model suite from the Copernicus program are a significant component of guidance for the temperature and precipitation outlooks through JJA 2021. Beyond JJA 2021, the consolidation of various statistical tools, including decadal trends, was the primary basis for the outlooks, with little remaining influence from ENSO or other reliable large-scale signals of climate variability for these forecast leads. Predicted low soil moisture conditions as a result of predicted precipitation totals over already dry areas, influenced the temperature outlooks for the spring and early summer 2021 over the Central Plains and Southwest. At later leads, decadal trends in temperature and precipitation were the primary tool used in creating the seasonal outlooks. PROGNOSTIC DISCUSSION OF OUTLOOKS - FMA 2021 TO FMA 2022 TEMPERATURE The temperature outlook for FMA 2021 reflects current model guidance, blended with likely impacts of La Nina and trends. The favoring of below normal temperatures across portions of mainland Alaska, the Alaska Panhandle, and the Pacific Northwest reflect La Nina conditions. The outlook is slightly warmer than last month’s outlook but colder than the recent model guidance for this region. The FMA outlook for the Northern Great Plains is warmer than last month’s lead 2 outlook, reflecting the ongoing lack of snow cover and more recent model guidance, though probabilities are modest at best from North Dakota to Illinois. Odds for above normal temperatures were also increased across the Lower Mississippi, Gulf Coast, and Florida, reflecting recent model guidance and likely La Nina impacts. The outlooks for later spring and summer 2021 reflect recent model guidance and trends, with the lowest probabilities in the central portions of the CONUS where variances are highest, model skill lowest, and trends are small to negative. Outlooks for next autumn were unchanged, as the reasoning behind the outlooks has not changed for those periods, and trends are the most reliable tool. Some outlooks are calling for a second La Nina winter, which would not be unprecedented since 1950, and the outlooks for next winter are already aligned for that possibility. PRECIPITATION La Nina is the primary physical driver behind the subsequent FMA and MAM precipitation outlooks, so only minor adjustments were needed to the corresponding outlooks made last month. In MAM, the amplitude of La Nina and its associated precipitation impacts are expected to begin to decline. In AMJ 2021, below normal precipitation amounts are forecast to diminish in spatial coverage across the southern CONUS, and be centered over most of the southwestern quarter of the CONUS. After AMJ, with a predicted ENSO Neutral, trends are more highly factored, which favors below normal precipitation for portions of the Northern and Central Rockies during summer months, shifting southward during the winter. East of the Mississippi, summer and early autumn months have a wet trend. By next winter, Dec-Jan-Feb 2021-22, the only remaining signal is the trend for above normal precipitation over the Northern Plains. FORECASTER: Matthew Rosencrans 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 Feb 18 2021 1981-2010 base period means were implemented effective with the May 19, 2011 forecast release. $$