Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu May 21 2020 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS The June-July-August (JJA) 2020 temperature outlook favors above-normal temperatures across the West, Gulf Coast States, and along the East Coast. The largest probabilities (above 70 percent) of above normal temperatures are centered over the Four Corners region. Equal chances of below-, near-, or above-normal temperatures are forecast from the upper to middle Mississippi Valley along with parts of the northern to central Great Plains. The JJA precipitation outlook features increased chances of above normal precipitation for much of the eastern and central U.S., while below-normal precipitation is more likely across the Pacific Northwest, northern Great Basin, and much of the Rockies. Above-normal temperatures are favored throughout Alaska during JJA with increased chances of above-normal precipitation forecast for the Aleutians and most of mainland Alaska. During the late summer and early fall season, increased chances of above normal temperatures expand to include the entirety of the continental U.S. (CONUS) with the largest probabilities remaining over the Southwest. Meanwhile, chances of above-normal precipitation remain elevated across the Midwest, Southeast, and mid-Atlantic through August-September-October. By December-January-February 2020-2021, equal chances were introduced to the northern to central Great Plains and Midwest. Increased chances of above normal precipitation shift from the Southeast to the northern areas of the CONUS during October-November-December, while chances of below normal precipitation expand across the southern tier of the CONUS. Equal chances (EC) are forecast in areas where the likelihood of seasonal mean temperatures or 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 Oceanic and atmospheric observations across the equatorial Pacific indicate ENSO-neutral conditions persisted into May. The observed weekly SSTs, centered on May 13, feature average or slightly negative anomalies from 160-100 degrees W with positive anomalies west of the Date Line. Positive subsurface temperature anomalies (averaged across 180-100 degrees W) continued to decrease with an eastward expansion of below-average subsurface temperatures. Upper-ocean heat anomalies have declined rapidly since earlier this spring. From April 17 to May 16, easterly low-level wind anomalies were present east of the Date Line with westerly upper-level anomalies along and south of the equator. Outgoing longwave radiation (OLR) anomalies, during this same time period, featured suppressed convection across the equatorial central and eastern Pacific. PROGNOSTIC DISCUSSION OF SST FORECASTS The CPC SST Consolidation for the Nino 3.4 region indicates a steady decline into negative anomalies to -0.5 degrees C by JAS with negative anomalies peaking during the fall season. The SST constructed analog and Markov model indicate a persistence of ENSO-neutral conditions through the summer and fall. The CPC/IRI consensus forecast favors a continuation of ENSO-neutral conditions (with about a 65 percent chance) during the Northern Hemisphere summer with chances decreasing through the fall (to 45-50 percent). PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Tools used for the seasonal outlooks included dynamical model guidance such as the North American Multi-Model Ensemble (NMME) and the Calibration, Bridging, and Merging (CBaM) version of the NMME. Current soil moisture conditions played a role in the JJA temperature outlook, primarily across the Great Plains and Four Corners region. The consolidation tool, which includes NMME input and various statistical tools, was used especially at later leads. The likelihood that either ENSO-neutral or La Nina prevails later this summer through the fall season played a role in both temperature and precipitation outlooks. During the 2021 warm season decadal trends were relied upon. PROGNOSTIC DISCUSSION OF OUTLOOKS - JJA 2020 TO JJA 2021 TEMPERATURE Above-normal temperatures are favored for much of the CONUS and Alaska during JJA with the largest probabilities forecast across the West and Northeast, based on excellent agreement among tools including a strong decadal signal. Probabilities of above normal temperatures were increased across the Four Corners region from the previous outlook due to a notable trend in the latest model guidance, consistent with an increasing signal for below normal precipitation, and ongoing low soil moisture conditions. Conversely, high soil moisture was one of the factors for equal chances of below-, near-, or above-normal temperatures forecast across parts of the central U.S. this summer. Beginning in JAS, coverage with increased chances of above normal temperatures expanded as the effect of anomalous soil moisture wanes and decadal trends support a larger coverage of above normal temperatures. From ASO 2020 through NDJ 2020-2021, changes from the previous outlook were to slightly increase (decrease) probabilities of above normal temperatures across the Southwest (Pacific Northwest to northern Rockies). These minor changes were based on the latest consensus among dynamical and statistical tools. An expected lack of sea ice elevates chances for above normal temperatures across northern Alaska during the fall season. The temperature outlook at the longer lead times is based largely on trends. PRECIPITATION The precipitation outlooks are based primarily on the latest dynamical model guidance, through OND, along with long-term trends. Compared to the previous outlook for JJA, a slight tilt in the odds of below normal precipitation was expanded south to include the Four Corners region. Consistent with the NMME and consolidation tool, probabilities above 40 percent for above normal precipitation were shifted south to the Gulf Coast States and the Southeast. A large coverage of modest probabilities for above or below normal precipitation are forecast across the CONUS due to inherent uncertainty in convective rainfall during the summer. Elevated probabilities of above normal precipitation were slightly increased across mainland Alaska, in part due to the June outlook. Elevated chances of above normal precipitation across the Southeast and mid-Atlantic were maintained longer through the fall, compared to previous outlooks. The most notable trend in the latest dynamical model guidance, during the late fall and next winter was an expansion of a dry signal throughout the southern tier. Although no changes were made to the precipitation outlook beyond DJF 2020-2021, the future evolution of ENSO conditions will have to be closely monitored in subsequent outlooks. At the longer leads, MJJ through JAS 2021, the tilt in the odds towards above normal precipitation across parts of the central and eastern U.S. is consistent with long-term trends. Also, the favored area for below-median precipitation across parts of the Southwest during the spring 2021 is related to trends. FORECASTER: Brad Pugh 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 Jun 18 2020 1981-2010 base period means were implemented effective with the May 19, 2011 forecast release. $$