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HOME> Outlook Maps>Seasonal Forecast Discussion |
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Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Sep 21 2023 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS El Niño conditions are present, as represented in current oceanic and atmospheric observations. A continuation of El Niño is extremely likely this Fall and Winter, with over a 95 percent chance through January-February-March (JFM) 2024. Additionally, a strong El Niño is likely by late Fall and early Winter, with approximately a 70 percent chance of occurrence during the November-December-January (NDJ) 2023-24 season. The October-November-December (OND) 2023 temperature outlook favors above normal temperatures for Alaska, the western third of the Contiguous United States (CONUS), the Northeast, Great Lakes, parts of the Southern Plains, Florida, and the immediate Gulf and East Coasts. The largest probabilities (greater than 60 percent) of above normal temperatures are forecast across the North Slope of Alaska. Above normal temperatures are also likely (probabilities greater than 50 percent) for parts of the Pacific Northwest and northern New England. The OND 2023 precipitation outlook depicts enhanced probabilities of above normal precipitation amounts across the Southeast, Southern Plains, and parts of the mid-Atlantic states as well as northern and western Alaska. The greatest chances of above normal precipitation (above 50 percent probability) are indicated for parts of northern Florida and southern Georgia. 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 El Niño conditions. In August, sea surface temperatures (SSTs) were above average across the equatorial Pacific Ocean with strengthening positive anomalies noted in the central and east-central Pacific. In the last week, the Niño index values ranged from +1.1°C to +2.6°C. Area-averaged subsurface temperature anomalies increased compared to July in association with anomalous warmth in the central and eastern equatorial Pacific Ocean. Tropical atmospheric anomalies were also consistent with El Niño. Over the east-central Pacific, low-level winds were anomalously westerly, while upper-level winds were anomalously easterly. Convection was slightly enhanced around the International Date Line, stretching into the eastern Pacific, just north of the equator. Convection was mostly suppressed around Indonesia. The equatorial Southern Oscillation Index (SOI) and the traditional station-based SOI were both significantly negative. Collectively, the coupled ocean-atmosphere system reflected El Niño. The Real-time Multivariate Madden Julian Oscillation (RMM) index currently indicates incoherent intraseasonal activity, with the Madden Julian Oscillation (MJO) signal retreating westward to the Indian Ocean at a low amplitude during the past week. Dynamical model RMM forecasts generally favor the resumption of an eastward propagating MJO signal over the Maritime Continent, but remain divided in regards to its eventual amplitude with large ensemble spread. PROGNOSTIC DISCUSSION OF SST FORECASTS The most recent International Research Institute for Climate and Society (IRI) plume indicates El Niño will persist through the Northern Hemisphere winter 2023-24. Despite nearly the same ensemble mean amplitude as last month, the shorter forecast horizon means that the odds of a "strong" El Niño (≥1.5°C for the November-January seasonal average in Niño-3.4) have increased to 71%. In summary, El Niño is anticipated to continue through the Northern Hemisphere winter (with a greater than 95% chance through JFM 2024). PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Dynamical model forecasts from the North American Multi-Model Ensemble (NMME), the Coupled Forecast System model Version 2 (CFSv2), and the Copernicus (C3S) multi-model ensemble system were used extensively for the first six leads when they are available, as was the objective, historical skill-weighted consolidation and Calibration, Bridging, and Merging (CBaM) guidance, that combines both dynamical and statistical forecast information. Additionally, the official ENSO forecast depicts probabilities of El Niño that are exceptionally high through the upcoming winter, with a strong event likely. This anticipated El Niño signal played a key role in the construction of these outlooks. At later leads, decadal trends in temperature and precipitation were increasingly relied upon in creating the seasonal outlooks. PROGNOSTIC DISCUSSION OF OUTLOOKS - OND 2023 TO OND 2024 TEMPERATURE One of the major forecast challenges for the upcoming fall and winter is managing the competing influences between El Niño and recent trends . Across the southern CONUS in particular, El Niño composites tend to favor cooler than normal conditions while recent trends lean warm. Due to these competing influences, a large swath of EC is indicated across much of the interior South for OND. A slight tilt toward above normal temperatures is indicated along the Gulf and East coasts, where warm trends and above normal SSTs have a greater chance to counteract potential El Niño influences. Above normal temperatures are favored with greater confidence across the Northeast and parts of the Great Lakes, where recent trends and model guidance from the C3S are warm. Probabilities of above normal temperatures exceed 50 percent across parts of northern New England where recent trends are the strongest and SSTs in the adjacent Atlantic Ocean are warmer than normal. Recent trends and dynamical models guidance also favor warmth across the western third of the CONUS. The greatest probabilities of above normal temperatures (above 50 percent chance) are indicated for the Pacific Northwest, where dynamical and statistical inputs are in generally good agreement and adjacent SSTs are above normal. Relatively weaker probabilities of above normal temperatures (less than 40 percent chance) are indicated over the southwestern CONUS, where El Niño influences may counteract recent warm trends . Farther to the north and east, EC is indicated for the north-central CONUS where guidance from the NMME and recent trends yield weak signals . Above normal temperatures are favored throughout Alaska as El Niño, trend, and dynamical model guidance all favor milder than normal conditions across much of the state. The strongest probabilities of above normal temperatures (greater than 60 percent chance) are indicated across the North Slope as sea ice extent is currently much below normal across the adjacent Arctic Ocean. From November-December-January (NDJ) through March-April-May (MAM), impacts from the ongoing El Niño strengthen as above normal temperatures are favored across the northern CONUS and persist across Alaska. EC is indicated across the southern CONUS as El Niño favors cooler than normal conditions for most of the region while recent trends favor warm. A small area of enhanced below normal temperature probabilities are indicated for parts of the Southwest and southern High Plains during JFM and FMA where El Niño composites yield a cold signal and recent trends are relatively weak. After MAM, the forecast pattern increasingly reflects trends with above normal temperatures generally favored across the Northeast during Spring and then expanding across the entire eastern third of the CONUS by the early Summer. Similarly, an area of elevated above normal temperature probabilities across the Northwest during the Spring expands across most of the remainder of the western third of the CONUS during the summer months. Alaska is generally favored to experience milder than normal conditions throughout the spring and summer months. During the late summer and early fall, trends favor warmth across all of the forecast domain so above normal temperature probabilities are elevated across the entire nation from JAS through SON. Trends weaken later in the Fall across southeastern Alaska and the northwestern CONUS where EC is indicated during the OND 2024 season. PRECIPITATION Model guidance and El Niño composites are in good agreement in favoring above normal precipitation across the southeastern CONUS and along much of the East Coast during the OND 2023 season. Thus, above normal precipitation is favored for these regions. Probabilities of above normal precipitation exceed 50 percent across parts of northern Florida and southern Georgia, where ensemble guidance from the NMME and C3S yield the strongest wet signals . Elevated probabilities of above normal precipitation also expand west to the Southern Plains, consistent with combined El Niño/Optimal Climate Normals (OCN) composites. Conversely, below normal precipitation is more likely across the northwestern CONUS and the Great Lakes region, based primarily on a combination of El Niño composites and dynamical model guidance. Above normal precipitation is favored across northern and western Alaska due to anticipated El Niño influences, recent trends , and dynamical model guidance. Below normal sea ice extent in the adjacent Arctic Ocean may also contribute to increased precipitation chances across the North Slope. During the winter months, impacts from El Nino are expected to increase, and above normal precipitation is favored for most of the Southern Tier of the CONUS from NDJ to February-March-April (FMA). Additionally, above normal precipitation is favored along much of the Eastern Seaboard during the core of the winter as an East Coast storm track is generally favored during El Niño winters. Conversely, below normal precipitation is more likely across much of the Northern Tier, particularly for the northwestern CONUS and the Great Lakes region. Uncertainty is high across the West Coast as El Niño tends to favor wet along much of the California coast while trends favor dry. Given the increased possibility of a strong El Niño event, a tilt toward above normal precipitation is indicated for Southern California. A tilt toward above normal precipitation is also indicated across the North Slope of Alaska through FMA, consistent with recent trends . Confidence in potential ENSO impacts decreases substantially as we head to next Spring and Summer, resulting in increased uncertainty and coverage of EC across much of the nation. However, during late Winter and early Spring (FMA), guidance from the CFSv2 and other models yields a wet signal across the Southern and Central Plains. The wet signal generally progresses eastward across the Southeast by early Summer (April-May-June (AMJ)) and then to the Mid-Atlantic and Northeast by the middle of the summer (June-July-August (JJA)), consistent with trends . As this wet signal progresses eastward and northward, the statistical consolidation and trends become increasingly dry over parts of the Central and Southern Plains by late Spring and early Summer. As a result, below normal precipitation is slightly favored across these regions during May-June-July (MJJ) and JJA. A slight tilt toward below normal precipitation is also indicated for parts of the northern High Plains and Northern Rockies through August-September-October (ASO), consistent with trends. Farther to the north, uncertainty is high for most of Alaska, and EC is indicated for most of the state from late Spring through the Summer. As we enter the late summer and early fall, a wet trend signal is reflected across parts of the Tennessee Valley during July-August-September (JAS), translating farther to the south and west across the western Gulf Coast in ASO and Rio Grande Valley deeper into next Fall in September-October-November (SON). Above normal precipitation is favored for the North Slope of Alaska next Fall due to recent trends and the statistical consolidation also favors above normal precipitation for southwestern parts of the state during the same time period. Later next Fall and early next Winter (OND), recent trends depict a tilt toward dry for parts of Southeast Alaska and the eastern Great Lakes, and a wet signal for parts of the Upper and Middle Mississippi Valley and the Tennessee Valley. FORECASTER: Scott Handel 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 Cas e 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 19 2023 1991-2020 base period means were implemented effective with the May 20, 2021 forecast release. $$
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