371 FXUS05 KWBC 181252 PMD90D Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Apr 18 2024 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS During mid-April 2024, an El Nio Advisory remains in effect but the oceanic temperature anomalies associated with El Nio are rapidly fading. There is an 85% chance that ENSO neutral will be in place by the end of the April-May-June (AMJ) season. There is a 60% chance of additional transitioning of the ENSO phase from ENSO Neutral to La Nia by summer (June-July-August, JJA), with La Nia conditions favored to strengthen and continue through boreal autumn and winter. Therefore, a La Nia Watch has also been issued. The May-June-July (MJJ) 2024 temperature outlook favors above-normal seasonal mean temperatures for central and eastern Alaska, the far West excluding southwestern California, and most of the remainder of the Contiguous U.S. (CONUS) with the exception of the north-central states. Maximum probabilities (>50%) favoring above-normal temperatures are indicated over eastern Alaska, parts of the Northwest, the eastern Great Lakes and Northeast, and over New Mexico, southwestern Texas, and southern Florida. For southwestern California, near-normal seasonal mean temperatures are favored. For remaining areas of the CONUS and Alaska, Equal Chances (EC) of below, near, and above-normal seasonal mean temperatures are favored. No areas of favored below-normal temperatures are forecast for the MJJ season. The MJJ 2024 precipitation outlook favors above-normal seasonal total precipitation amounts for approximately the western half of Alaska, and from the southeastern quarter of the CONUS northeastward across the Mid-Atlantic region, southeastern New York state, and southern New England. Below-normal precipitation is favored for parts of the northwestern CONUS, and from eastern sections of Utah and Arizona eastward across Colorado, New Mexico, and much of West Texas. For the remaining areas of the CONUS and Alaska, where seasonal total precipitation amounts are favored to be similar to climatological probabilities, EC is forecast. 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 El Nio is fading rapidly as equatorial Pacific sea surface temperatures (SSTs) continue to cool, associated with upwelling from an oceanic Kelvin wave. The most recent weekly value of the Nio3.4 SST index is a surprisingly high +0.9 degrees C, which is a bit misleading given the very shallow layer of subsurface warmth underlying this area. Relatively cold water (departures ranging from -0.5 degrees C to at least -6 degrees C) is in place from the surface between 120W and 100W longitude, extending westward and deepening across most of the equatorial Pacific, reaching a depth of 150 to 300 meters at 150E longitude (the approximate longitude of the Solomon Islands and Coral Sea). Tropical convection is now slightly suppressed across the vicinity of the Date Line and the equator, and close to average over Indonesia. These anomalies, in addition to the return of low-level easterly wind anomalies to the west-central Pacific Ocean are indicative of the underlying transition from El Nio to ENSO-neutral. As of mid-April 2024, above-normal soil moisture is present in many areas of the CONUS including many areas west of the Continental Divide, the north-central Plains, from the southern Great Lakes eastward across the Northeast and Mid-Atlantic region, and from the vicinity of the Texas-Oklahoma Panhandles east-southeastward across the Arklatex region and Gulf Coast states. Drier than normal soils are evident over northern North Dakota, northwestern Minnesota, in the vicinity of the Washington Cascades rain shadow and Northern Rockies, and across much of New Mexico and southwestern Texas. Drier than normal soils are also indicated in a wishbone-shaped pattern that extends from the Upper Great Lakes region southwestward across Iowa, southeastern Nebraska, and Kansas, which then largely reverses direction and continues eastward across the Middle Mississippi, southern Ohio, and Tennessee Valleys. Soil moisture anomalies are an important consideration for the Seasonal Outlooks through the warm half of the year. PROGNOSTIC DISCUSSION OF SST FORECASTS Forecasts of the Nio 3.4 SST index from the North American Multi-Model Ensemble (NMME) are generally in good agreement for a nominal La Nia (Oceanic Nio Index or ONI <=-0.5 degrees C) by June. Most participant model predictions are slightly cooler than the NMME average, with the exceptions of the GFDL SPEAR and Canadian CanCM4i models. The former does not reach the La Nia threshold until mid-August. The International Multi-Model Ensemble (IMME, C3S, or Copernicus) average reaches the La Nia threshold by August. Three of the participant models (German DWD, ECMWF, and METEO-France) stay within the confines of ENSO-neutral limits at least into September, while the UKMO and CMCC are considerably cooler and reach marginal La Nia conditions by late May into June. The CFS is the coldest of solutions, exceeding the minimal La Nia threshold even before MJJ, and predicts a strong La Nia (ONI>=-1.5 degrees C) by July-August-September (JAS) 2024. CPCs SST Consolidation for the Nio 3.4 region passes into La Nia territory by JJA, peaks during the overlapping seasons of September-October-November (SON) and October-November-December (OND) at -1.4 degrees C (just short of a strong cold event), and then retreats into Neutral territory by March-April-May (MAM) 2025. As noted earlier, CPCs ENSO Team predicts the transition from El Nio to ENSO-neutral should be completed within the AMJ season (85% chance), with the odds of La Nia developing by JJA (60% chance). Historically, La Nia tends to follow strong El Nio events, adding confidence to the forecast. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Though some remaining El Nio atmospheric response could extend into the very early stages of the MJJ Outlook, it is unlikely to make a significant difference for the season as a whole. Dynamical model forecasts from the NMME and C3S multi-model ensemble systems are utilized, as is the Calibration, Bridging and Merging (CBaM) tool anchored to the NMME forecasts and bridged with the Nio 3.4 index primarily for temperature outlooks. The Constructed Analog (CA) statistical tools based on SST and soil moisture, along with the ENSO-OCN forecast tool that targets impacts from ENSO as predicted by the CPC consolidation Nio 3.4 SST forecast and long-term trends, played a large role in many of the outlooks. La Nia impacts are considered in the outlooks from JJA 2024 through February-March-April (FMA) 2025. PROGNOSTIC DISCUSSION OF OUTLOOKS - MJJ 2024 TO MJJ 2025 TEMPERATURE The MJJ 2024 seasonal temperature outlook favors above-normal seasonal mean temperatures for central and eastern Alaska, the far west excluding southwestern California, and most of the remainder of the CONUS with the exception of the north-central states. Maximum probabilities (>50%) favoring above-normal temperatures are indicated over far eastern Alaska, parts of the Northwest, the eastern Great Lakes and Northeast, and over New Mexico, southwestern Texas, and southern Florida. For southwestern California, near-normal seasonal mean temperatures are favored. The reduced coverage (from last months Lead 2 MJJ outlook) of 50%+ probability favoring above-normal temperatures over the Northwest is related to anticipated diminished influences from lagged El Nio impacts, slightly cooler effects of wet soils (through evaporation) over southern Oregon and southern Idaho, and trends. In southwestern California, favored near-normal temperatures is thought to be the best compromise given NMME and C3S SST regional forecasts of near to below-normal in this region extending southward along Baja, during the MJJ season. Near the coast of Southern California, this may be associated with prolonged or more frequent marine layer intrusions, a well-known climatological feature in this region. Across the central portion of the CONUS, anomalous warmth is favored to replace the southern part of the EC region (from last months MJJ Outlook), and is favored by statistical and dynamical model guidance. The recent very heavy rainfall (8-10+ inches) and severe weather over the Ark-La-Tex region and Lower Mississippi Valley is the reason behind the slightly lower odds favoring above-normal temperatures in that area. Model guidance and trends (as shown by the OCN tool) favor relative maxima in probabilities for above-normal temperatures across New Mexico and southwestern Texas, and from the eastern Great Lakes region to the Northeast and Mid-Atlantic coasts. Most tools support above-normal temperatures across the Southeast. To the north, the outlook for Alaska is based on a subjective consensus of model guidance, temperature trends, and anticipated residual influences from sea ice. For JJA and JAS 2024, the favored above-normal temperatures in central and eastern Alaska from the preceding MJJ outlook are expected to decrease in coverage and gradually become concentrated over the far northern part of the state. In JJA, favored below-normal temperatures are depicted over the southwest portion of the Mainland and the adjacent Alaska Peninsula, spreading eastward across the southern coast with time. This is partly due to the anticipated emerging effects of La Nia, and supported by the ENSO-OCN tool and the Statistical Consolidation (Stat-CON). For the CONUS, probabilities favoring above-normal temperatures exceed 50% for most of the interior West and south-central states, warranted by the ENSO-OCN, Stat-CON, Dynamical Consolidation (NMME-CON), and a skill-weighted mean of the two CON tools (the Final CON). This is also consistent with most other tools, such as the CBaM tool, the IMME, and longer-term trends. The favored area of EC in the vicinity of the Dakotas in JJA gradually gives way to favored above-normal temperatures in JAS 2024, consistent with emerging influences of La Nia. The three CON tools and trends favor elevated probabilities for above-normal temperatures in the vicinity of the Northeast CONUS. In August-September-October (ASO), the probabilities favoring above-normal temperatures begin to decline over the Pacific Northwest, in preparation for the typical La Nia impacts that set up during boreal autumn and winter. During SON through November-December-January (NDJ) 2024, the temperature outlooks favor a canonical La Nia response, with above-normal temperatures in the Northwest and north-central CONUS giving way to EC, and to below-normal temperatures across the Pacific Northwest by NDJ. Above-normal temperatures are depicted across much of the southern and eastern CONUS, also consistent with a cold event. During this period, SON through NDJ 2024, ENSO composites gradually begin to favor a replacement of the below-normal temperatures across southern Alaska to EC, and the expansion of favored anomalous warmth across northern and western Alaska. SON is the most likely season for high probabilities (>60%) for favored above-normal temperatures across the northwestern coast of Alaska due to the seasonally delayed onset of sea ice formation. From December-January-February (DJF) 2024-25 through MAM 2025, La Nia composites are primarily used in conjunction with model guidance. For the final two leads, AMJ and MJJ 2025, temperature trends are heavily utilized. For remaining areas of the CONUS and Alaska, EC favored. PRECIPITATION The MJJ 2024 seasonal precipitation outlook favors above-normal seasonal total precipitation amounts for approximately the western half of Alaska, and from the southeastern quarter of the CONUS northeastward across the Mid-Atlantic region, southeastern New York state, and southern New England. These anomalies are generally consistent with NMME & C3S multi-model ensemble guidance, CFS, and trends. Below-normal precipitation is favored for parts of the northwestern CONUS, mostly from ENSO-OCN and the last vestiges of lagged El Nio influences. Anomalous dryness is favored from much of the Four Corners region to West Texas. This is consistent with the three CON tools, CFS, C3S, NMME, and to some extent, the SST Constructed Analog (SST-CA) statistical tool. There is also a significant negative correlation between this strong dry signal over the eastern Monsoon region and abundant snowpack over the Central Rockies (Colorado and Utah). Historically, this correlation favors a slow start to, and potential underperformance of, the Southwest Monsoon. For the remaining areas of the CONUS and Alaska, where seasonal total precipitation amounts are favored to be similar to climatological probabilities, EC is forecast. From JJA and JAS 2024, widespread above-normal precipitation is favored in Alaska, consistent with NMME guidance. Afterwards, there is a slight tilt in the odds towards below-normal precipitation for Southeast Alaska for SON through NDJ 2024. From SON 2024 to MAM 2025, the main climate driver/forcing is the anticipated La Nia, which favors an enhanced storm track and above-normal precipitation amounts across the northwestern states, and also (starting with DJF 2024-25) over the Great Lakes and Ohio/Tennessee Valleys. Enhanced dryness and below-normal precipitation amounts are favored across much of the southern CONUS and Eastern Seaboard. For the last two leads, April-May-June (AMJ) and MJJ 2025, the seasonal precipitation outlooks are largely based on trends. FORECASTER: Anthony Artusa The Climatic normals are based on conditions between 1991 and 2020, following the World Meteorological 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 May 16, 2024 1991-2020 base period means were implemented effective with the May 20, 2021 forecast release. $$