Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Sep 19 2019 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS The October-November-December (OND) 2019 temperature outlook favors above normal temperatures throughout the entire forecast domain, although probabilities vary. The highest regional probabilities for above normal temperatures are forecast across parts of the Southwest, the Northeast, and the North Slope of Alaska. Probabilities for above normal temperatures are lowest from the northern Great Plains and Upper Mississippi Valley to interior portions of the east-central Gulf Coast region, and for interior parts of southern Alaska. The OND 2019 precipitation outlook indicates that above normal seasonal total precipitation is favored near the Atlantic Coast from Florida to New Jersey, much of the west-central and central CONUS, and Alaska (excluding the Panhandle). Below normal precipitation is favored over the north-central portion of the West Coast. Equal chances (EC; white areas) of below, near, and above normal 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 reflected ENSO-neutral conditions during August and early September. The 7-day average sea surface temperature (SST) observations centered on September 11th are below average (-0.5C to -1.5C) over the central and eastern Pacific, and above average (+0.5C to +1.5C) over the west-central Pacific. The latest weekly Nino 3.4 SST anomaly is -0.3 deg C, down from +0.0C one week ago. Subsurface temperatures range from -0.5C to greater than -3.0C below average over much of the central and eastern equatorial Pacific, to a depth of about 175 meters. Outgoing longwave radiation (OLR) anomalies depicted near average convection across the equatorial Pacific from August 16th to September 10th, and suppressed tropical convection over Indonesia. Trade winds during the past month averaged close to normal over the equatorial Pacific. The Madden-Julian Oscillation (MJO) remained weak for at least the past 40 days, with other modes of subseasonal variability (such as Kelvin waves, equatorial Rossby waves and Tropical Cyclones (TC)) influencing convection across the global tropics. Sea surface temperature anomalies of +2.5 to +3.5 degrees C, or higher, persist throughout areas surrounding Alaska. These anomalies and the likelihood for another autumn season with significantly delayed sea ice onset were major factors in the temperature outlook for Alaska this fall. PROGNOSTIC DISCUSSION OF SST FORECASTS A substantial spread in the dynamical and statistical model forecasts of SSTs across the equatorial central and eastern Pacific during the fall and winter continues. Statistical model forecasts such as the Canonical Correlation Analysis and the Constructed Analog predict a Nino 3.4 SST anomaly at or slightly above +0.5 degree C through most of winter 2019-2020, with a very slow decline within the upper half of the Neutral category (that is, +0.0C to +0.49C) throughout spring. The CPC SST Consolidation and the NMME (North American Multi-Model Ensemble) predict only slightly reduced anomalies (relative to the tools noted above) throughout the upcoming winter and spring, though the forecast anomalies still range within the upper half of Neutral. 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 (CBaM), and a new Consolidation (CON) tool which uses the CCA (Canonical Correlation Analysis), the CA (Constructed Analog), the NMME, OCN (Optimal Climate Normals) and ENSO phase. The CA-SST (Constructed Analog which uses tropical Pacific SSTs as input) was also utilized. 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 - OND 2019 TO OND 2020 TEMPERATURE Above normal temperatures are favored throughout the forecast domain during OND, based on good agreement among the dynamical models and statistical tools. In Alaska, there is not only good agreement among the models and tools, but SSTs range from +1.5C to +3.5C above average surrounding most of the state, which contributes toward relative warmth in coastal regions. The greatest chances for above normal temperatures (in excess of 70%) are across the North Slope, where there is a strong historical trend during the past two decades for a significant delay in the formation of sea ice over the adjacent waters of the Arctic Ocean and Chukchi Sea. Over the Lower 48 states, probabilities for above normal mean temperatures exceed 60% over the Southwest, supported by the calibrated NMME, CBaM, historical temperature trends, and the CON tool. Probabilities for above normal (upper-tercile) temperatures exceed 50% over parts of the Northeast based on the CA-SST tool, trends, the calibrated NMME, the CON, and the CBaM. Probabilities for above normal temperatures are tempered across the northern Great Plains where more variability is expected during the three month period, and where the signal is weaker among dynamical models. In addition, the potential for recurving tropical cyclones in the northwest Pacific and their effect on the longwave pattern downstream over North America limits confidence in the temperature outlook across the north-central CONUS. These tempered probabilities for upper-tercile temperatures extend southeastward towards the central and eastern Gulf Coast region, as indicated by CCA, the calibrated NMME, CBaM, and the CON tool. If ENSO-neutral conditions persist through the upcoming winter as expected, subseasonal variability such as the Arctic Oscillation (AO) could play a greater role in determining the seasonal mean temperature pattern. During the next few leads (NDJ 2019-20 through FMA 2020), probabilities for above normal temperatures are reduced somewhat across much of the domain, with an area of EC (Equal Chances) being favored initially over the north-central CONUS. This area of EC expands across the Great Lakes and Ohio Valley, reaching a maximum spatial extent in JFM 2020 before decreasing in coverage. This area of increased uncertainty is highlighted in the calibrated NMME and CBaM tools, and to a lesser degree the OCN and new CON. As noted earlier, with ENSO-neutral favored to persist through the upcoming winter, the odds of other subseasonal factors such as the AO playing a larger role in the temperature pattern may result in episodic cold air outbreaks across the north-central states in particular. Temperature outlooks during the spring and summer 2020 were not changed from the previous set of outlooks issued last 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 The OND 2019 precipitation outlook favors above normal precipitation over Alaska (except the Panhandle), much of the west-central and central CONUS, and the Middle and Southern Atlantic Coast states. To various degrees, these areas are based on the CON, CBaM, trends (especially in the Carolinas and Mid-Atlantic), the CA-SST, and to a lesser extent the calibrated NMME. The anticipated delay in sea ice formation just north of Alaska, in addition to anomalously warm SSTs surrounding the state, also contribute to the relatively wet forecast. The Southern Atlantic region is also vulnerable to tropical cyclones, especially during October, as the main formation area returns to the Caribbean and Gulf of Mexico. Parts of the Southwest CONUS are also susceptible to tropical cyclones in October, and resultant Gulf surges with east Pacific tropical storms and hurricanes. Below normal amounts of precipitation are favored over north-central portions of the West Coast. This is supported by the CON tool, the CA-SST, and the CA. From NDJ 2019-20 to MAM 2020, odds favor above normal precipitation across much of Alaska. For the CONUS, the enhanced odds of above normal precipitation across the Middle Atlantic and Southern Atlantic Coast states gives way to EC, suggestive of a relatively wet month of October. The broad area of favored wetness over the west-central CONUS transitions to the northern CONUS, from about central Montana to the Great Lakes region. Increased odds of below normal precipitation shift southward with time from Oregon and northern California to most of California and Nevada by JFM 2020. Precipitation signals become very weak over the CONUS in FMA and MAM 2020, with EC favored. At the longer leads, AMJ 2020 through OND 2020, the tilt in the odds towards above normal precipitation across parts of the eastern CONUS is consistent with long-term trends. Also, the favored area for below normal precipitation, beginning with JJA 2020 across parts of the western and north-central CONUS is also related to trends. FORECASTER: Anthony Artusa 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 Oct 17 2019 1981-2010 base period means were implemented effective with the May 19, 2011 forecast release. $$