Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Aug 15 2019 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS The September-October-November (SON) 2019 temperature outlook favors above normal temperatures throughout the entire forecast domain, although probabilities vary. The highest probabilities for above normal temperatures are forecast across New England, parts of the Southwest, and northern Alaska. Probabilities for above normal temperatures are lowest across the northern Great Plains. The SON 2019 precipitation outlook indicates that above normal seasonal total precipitation is most likely across the Florida Peninsula, mid-Atlantic, and extending from parts of the Southwest north to the northern Great Plains and upper Mississippi Valley. Above normal precipitation is also favored for Alaska. Equal chances (EC; white areas) of below, near, and above are forecast in areas where the likelihood of seasonal mean temperatures or seasonal total precipitation amounts are expected to be similar to their 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 reflect a transition to ENSO-neutral conditions during July into early August. The observed weekly sea surface temperatures (SSTs), centered on August 7, are near average from 140 degrees W to the coast of South America with slightly positive anomalies limited to the central and western equatorial Pacific. The latest weekly Nino 3.4 SST anomaly is +0.4 degree C. Since mid-June, negative subsurface temperature anomalies have expanded in the eastern Pacific. Outgoing longwave radiation (OLR) anomalies indicated near average convection across the equatorial Pacific from July 12 to August 6. The Madden-Julian Oscillation (MJ0) has remained weak and incoherent for more than a month with other modes of subseasonal variability influencing convection across the global tropics. Enhanced trade winds have become more prevalent across the equatorial central Pacific during early August. Sea surface temperature anomalies of +2.5 to +3.5 degree C, or higher, persist throughout areas surrounding Alaska. These anomalies and likelihood for another fall season with low sea ice coverage were major factors in the temperature outlook for Alaska this fall. PROGNOSTIC DISCUSSION OF SST FORECASTS A large spread in the dynamical and statistical model forecasts of SSTs across the equatorial central and eastern Pacific during the fall and winter continues. The statistical model forecasts, including the Canonical Correlation Analysis, Constructed Analog, and Markov indicate a SST anomaly in Nino 3.4 at or slightly above +0.5 degree C through winter 2019-2020. However, the North American Multi-Model Ensemble (NMME) indicates near average SSTs during the late fall and winter. The CPC/IRI consensus forecast indicates that ENSO-neutral conditions are most likely through the winter with La Nina the least likely to develop. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS The temperature and precipitation outlooks were based on calibrated NMME forecasts, a statistical-dynamical hybrid model combining calibrated NMME forecasts with statistically bridged impacts from NMME Nino 3.4 forecasts, and an ENSO-OCN tool. Due to the expectation that ENSO is unlikely to provide a major influence on the mid-latitude circulation pattern at least through earlier lead times, decadal climate trends were weighed more heavily during the fall and winter 2019-2020. Decadal climate trends became the major tool for temperature and precipitation outlooks at the longer leads. PROGNOSTIC DISCUSSION OF OUTLOOKS - SON 2019 TO SON 2020 TEMPERATURE Above-normal temperatures are favored throughout the forecast domain during SON, based on excellent agreement among the dynamical models and statistical tools. Given the strongest signal in decadal trends, probabilities are largest across New England, the Southwest, and Alaska. Also, large positive SST anomalies and the high likelihood for a lack of sea ice this fall support more than an 80 percent chance of above normal temperatures across northern Alaska. Probabilities for above normal temperatures are tempered across the northern Great Plains where more variability is likely to occur during the three month period and the signal is weaker among dynamical models. Also, the potential for recurving tropical cyclones in the northwest Pacific and its effect on the longwave pattern downstream over North America limits confidence in the temperature outlook across the north-central CONUS. Probabilities for above normal temperatures were decreased slightly across the Pacific Northwest during OND and NDJ, based on the most recent NMME output and expectation of ENSO-neutral conditions. During the late fall and continuing through the winter 2019-2020, coverage and probabilities for increased chances of above normal temperatures were decreased across the central and eastern CONUS, compared to previous outlooks. This change is related to higher uncertainty on the evolution of ENSO this cold season. If ENSO-neutral conditions persist through the upcoming winter, subseasonal variability such as the Arctic Oscillation could play a greater role in temperatures. Temperature outlooks during the spring and summer 2020 were not changed from the previous month, since multi-decadal trends remain the primary tool. During these seasons, above normal temperatures are most likely across Alaska and a majority of the CONUS except for the north-central CONUS. By fall 2020, increased chances of above normal expand to include all of the forecast domain which follows long-term trends. PRECIPITATION Compared to the previous outlook for SON, an area favoring above normal precipitation was added to include the Florida Peninsula, coastal Southeast, and mid-Atlantic. The addition to the Florida Peninsula is supported by the latest NMME output, while long-term trends support increased chances for above normal precipitation across the mid-Atlantic during the fall. Enhanced odds for above normal precipitation during SON extends from the southwestern CONUS northeast to the northern Great Plains and upper Mississippi Valley, based largely on dynamical model guidance. A slight tilt in the odds for above normal precipitation, covering the desert Southwest and southeast California, during SON is also related to potential surges of moisture with east Pacific tropical cyclones (TC). SST anomalies are currently running +1.0 to +1.5 degree C in the primary TC genesis region of the east Pacific. The increased chance for above normal precipitation continues along the East Coast through OND, based on long-term trends. Later in the fall, the increased chance for below normal precipitation forecast for parts of the West Coast is maintained due to precipitation tools that combine statistical and dynamical output. During DJF 2019-2020 and JFM 2020, modifications to the previous outlooks were necessary due to a decreasing chance of El Nino this winter. Therefore, the area that favored above normal precipitation across the southern tier of the CONUS was changed to equal chances for below, near, or above normal precipitation. Increased chances of above normal precipitation for the north-central CONUS during the winter is consistent with the latest NMME output and long-term trends. A slight tilt in the odds for below normal precipitation across parts of California and the Southwest during JFM 2020 is supported by the calibrated NMME forecast. Enhanced odds for above normal precipitation forecast for parts of Alaska throughout each lead time is supported in part by multi-decadal trends. At the longer leads, AMJ 2020 through SON 2020, the tilt in the odds towards above normal precipitation across parts of the eastern U.S. is consistent with long-term trends. Also, the favored area for below-median precipitation, beginning with JJA 2020, across parts of the western and north-central CONUS is also 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 Sep 19 2019 1981-2010 base period means were implemented effective with the May 19, 2011 forecast release. $$