Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu May 20 2021 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS La Nina has ended and ENSO-neutral conditions are present across the equatorial Pacific Ocean, as represented in current oceanic and atmospheric observations. The official CPC ENSO forecast indicates that ENSO-neutral conditions are likely to persist through the summer. The June-July-August (JJA) 2021 temperature outlook favors above normal temperatures for a majority of the CONUS, with the largest probabilities (greater than 60 percent) forecast across the Southwest. The JJA 2021 precipitation outlook depicts enhanced probabilities of below normal precipitation amounts across the Pacific Northwest, Rockies, and High Plains. Above-normal precipitation is slightly more likely across the East, parts of the Gulf Coast, and much of Alaska. Equal chances (EC) are forecast for areas where probabilities for each category of seasonal mean temperatures and seasonal accumulated 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 The coupled oceanic and atmospheric observations reflect ENSO-neutral conditions. During the past four weeks (Apr 18 – May 14), sea surface temperatures (SSTs) were near to slightly below average with SST anomalies less than -0.5 degree C across the central and eastern equatorial Pacific. Positive subsurface temperature anomalies continue to persist at a depth of 50 to 150 meters throughout much of the equatorial Pacific. Integrated upper-ocean heat anomalies have remained positive since late March. Suppressed convection diminished during April across the equatorial central Pacific, due in part to a robust Madden-Julian Oscillation (MJO). Since early March the MJO has completed two circumnavigations of the global tropics. The enhanced phase of the MJO is currently over the Indian Ocean and is forecast to continue propagating eastward to the western Pacific and then Western Hemisphere entering June. PROGNOSTIC DISCUSSION OF SST FORECASTS The North American Multi-Model Ensemble (NMME) and the CPC SST consolidation for the Nino3.4 region depicts ENSO-neutral conditions persisting through summer 2021. Beyond this time, forecast tools diverge with the CFSv2 featuring a return of La Nina while other dynamical models such as the ECMWF and statistical tools maintaining ENSO-neutral conditions. The official CPC/IRI ENSO outlook indicates that ENSO-neutral conditions are the most likely outcome through August-September-October 2021 with probabilities for a return of La Nina increasing to 55 percent by October-November-December 2021 and continuing through the winter 2021-2022. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Dynamical model guidance such as the North American Multi-Model Ensemble (NMME) and the Calibration, Bridging, and Merging (CBaM) version of the NMME were used. The consolidation tool, which includes NMME input and various statistical tools, was used as well, especially at later leads. Also, anomalously wet (dry) soil moisture conditions were factors in the temperature outlook across eastern Oklahoma, eastern Texas, and the lower Mississippi Valley (northern Great Plains) during June-July-August 2021. At later leads, decadal trends in temperature and precipitation were the primary tool used in creating the seasonal outlooks. Slight adjustments to the seasonal outlooks, especially temperature, were needed due to the change in the 30-year base period from 1981-2010 to 1991-2020. PROGNOSTIC DISCUSSION OF OUTLOOKS - JJA 2021 TO JJA 2022 TEMPERATURE Above normal temperatures are favored throughout a majority of the forecast domain during JJA, except for parts of the Midwest where equal chances (EC) of below, near, or above normal temperatures are forecast. This EC area is due to a weaker signal among temperature tools coupled with poor skill historically from the NMME during the summer months. Positive decadal temperature trends and low soil moisture (below the 5th percentile) support the largest probabilities (above 60 percent) for above normal temperatures across the Southwest. Compared to the previous month, probabilities for above normal temperatures are decreased from eastern Texas to the lower Mississippi Valley where recent heavy to excessive rainfall resulted in soil moisture exceeding the 70th percentile. In addition, the June temperature outlook calls for EC for that region. Above normal temperatures are favored throughout Alaska during JJA, but the largest probabilities (above 50 percent) were shifted from northern Mainland Alaska to the Aleutians based on the latest model guidance and recent SST anomalies. Beginning in JAS through SON 2021, above normal temperatures are favored through the entire CONUS with the largest probabilities (above 50 percent) consistent with NMME dynamical model guidance and decadal trends. Although probabilities for above normal temperatures were reduced slightly across the North Slope of Alaska during SON and OND 2021 due to the new base period of 1991-2020, probabilities are larger compared to the remainder of Alaska. Seasonal temperature outlooks during the winter 2021-2022 and continuing through the summer 2022 are based on the CPC consolidation tool and decadal trends. PRECIPITATION Model guidance remains consistent from previous months depicting elevated probabilities of below normal precipitation for the Pacific Northwest, northern Rockies, and northern to central high Plains during JJA. Based on good agreement among the latest dynamical models, this favored area of below normal precipitation was expanded to include the southern high Plains and New Mexico during JJA and JAS. Due to a notable drying trend among the inputs to the NMME and C3S along with the consolidation tool, below normal precipitation is slightly favored for the desert Southwest through SON 2021. This favored dryness coincides with the monsoon, a wet time of year for the Southwest. The predicted MJO evolution is likely to provide a favorable large-scale environment for the development of an early season tropical cyclone in the Gulf of Mexico during June. This expectation along with it being a non-El Nino summer and fall support slightly elevated probabilities for above normal precipitation along parts of the Gulf Coast and Southeast. The favored area of above normal precipitation across the remainder of the eastern U.S. from JJA through ASO 2021 is related more to decadal trends. Based on the latest dynamical models, the coverage of elevated probabilities for above normal precipitation are increased to include more of mainland Alaska during JJA and JAS 2021. Although the NMME begins to favor below normal precipitation across the southern tier of the CONUS during OND 2021, the precipitation outlook maintains EC at this time range due to large uncertainty on the return of La Nina. This will be reassessed during subsequent seasonal outlooks as we draw closer to the fall and winter 2021-2022 when confidence in the state of ENSO is likely to improve. Seasonal precipitation outlooks during the winter 2021-2022 and continuing through the summer 2022 are based on the CPC consolidation tool and decadal trends. FORECASTER: Brad Pugh The Climatic normals are based on conditions between 1991 and 2020, 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 Jun 17 2021 1991-2020 base period means were implemented effective with the May 20, 2021 forecast release. $$