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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 Mar 16 2023 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS In early March 2023, the final La Niña advisory was issued and La Niña has ended. ENSO-neutral is now present in the equatorial Pacific ocean and favored to continue through the spring and early summer months. Thereafter, El Niño is the most likely phase of ENSO from late summer into the autumn months. The April-May-June (AMJ) 2023 temperature outlook favors above-normal seasonal mean temperatures for the north slope of Alaska, parts of the Southwest, southern Plains and Southeast and northward to include the Ohio Valley, Mid-Atlantic and Northeast. Below-normal temperatures are most likely for portions of the Northern Plains and central interior West. The AMJ 2023 precipitation outlook favors above-normal seasonal total precipitation amounts for portions of the Great Lakes, Ohio Valley, Mid-Atlantic and Northeast. Below-normal precipitation is most likely for parts of the far Pacific Northwest and Southwest. Equal chances (EC) are indicated for areas where seasonal mean temperatures and seasonal total precipitation amounts are favored 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 La Niña has ended and ENSO-neutral is now present in the equatorial Pacific Ocean. Equatorial Pacific Ocean sea surface temperatures (SSTs) continued to warm over the past month with negative anomalies in SST only remaining in an area near and just east of the Date Line. The most recent weekly value of the Nino3.4 SST index is -0.1 degrees C. Further east toward the South American coast, positive anomalies in SST have developed with a region of greater than +0.5 degrees C departures in the eastern Pacific with the most recent value of the Niño1+2 SST index of +1.5 degrees C. Below the surface, the 0 - 300 meter depth equatorial temperature anomalies (i.e., integrated heat content) became positive in February 2023 and there exists a reservoir of warmer than normal water spanning a depth from approximately 75 m to 250 m from 130E to 150W with the maximum anomaly ranging from + 4-6 degrees C. Although the oceanic indicators for La Niña have decreased in the Pacific Ocean, there does remain some atmospheric signature and it will likely take some time for this response to completely disappear. For example, in the past 30 day average, suppressed convection remained present near the Date Line along the equator and upper-level cyclonic circulations, symmetric about the equator in both hemispheres, are evident. Trade winds have decreased in magnitude, but this is in part due to a strong MJO event and associated westerly wind anomalies crossing the Pacific within the last month. PROGNOSTIC DISCUSSION OF SST FORECASTS Forecasts of the Nino3.4 SST index from the NMME are in general agreement for a transition from small negative anomalies to positive departures from normal of varying magnitudes ranging from +0.3 to +1.2 degrees C (for the GFDL SPEAR model) by late summer 2023. The NMME ensemble mean forecast enters El Nino territory (greater than +0.5 degrees C) approximately during July 2023. The CPC Niño3.4 SST consolidation forecast follows a similar trajectory as the NMME models at the start, but hovers near or just below the +0.5 degrees C threshold during the late summer and early autumn months. Two statistical components of the CPC Nino3.4 SST consolidated forecast, the CA and CCA forecasts, predict a much more rapid transition towards El Niño with values of the Niño3.4 index near or surpassing +1.0 degrees C by autumn 2023. The official CPC ENSO outlook favors ENSO-neutral through the JJA season with El Niño favored thereafter at an approximate 60% probability through OND 2023. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS Given some likely remaining La Niña atmospheric response at times during April 2023, typical La Niña impacts are considered to a minor degree very early in this set of outlooks (i.e., AMJ 2023). Current anomalous soil moisture and snow cover/depth are considered and contributed strongly to the outlook in some locations during the AMJ and MJJ seasons. Dynamical model forecasts from the NMME and C3S multi-model ensemble systems are utilized heavily, although precipitation signals are particularly weak from this month's guidance as compared to normal. Moreover, the Calibration, Bridging and Merging (CBaM) tool anchored to the NMME forecasts and "bridged" to the Niño3.4 index is also significantly utilized. The ENSO-OCN forecast tool that targets impacts from ENSO, as predicted by the CPC consolidation SST forecast, and long term trends played a large role in many of the outlooks, especially at longer leads. Consideration and some slight adjustments consistent with potential development of El Niño are done in some areas beginning in late summer and autumn 2023, but any changes are minor at this stage given the uncertainty in the ENSO forecast initialized during this time of the seasonal cycle (i.e., spring predictability barrier). PROGNOSTIC DISCUSSION OF OUTLOOKS - AMJ 2023 TO AMJ 2024 TEMPERATURE The AMJ 2023 temperature outlook favors above-normal seasonal mean temperatures from the Southwest eastward to the southern Plains and Southeast as well as northward to include the Ohio Valley, mid-Atlantic and Northeast. The greatest odds of above-normal temperatures is for Texas and areas along the Gulf coast and Eastern Seaboard. There is strong support for warmer than normal temperatures in this region from nearly all dynamical model guidance and statistical forecast tools as well as long term positive temperature trends . Very dry soil moisture conditions and well as above-normal local coastal SSTs, further enhance these odds for the southern High Plains and Gulf / Atlantic coasts respectively. Elevated odds of below-normal temperatures are forecast for the Northern Plains where abnormally deep snowpack is in place and long term negative temperature trends exist. Dynamical model forecast guidance from some participant models of the NMME and C3S suite as well as the CBaM hybrid forecast tool support this forecast. A small region of favored below-normal temperatures is highlighted for an area in the interior western CONUS. Odds tilting toward below-normal temperatures in this area, however, are very modest. The AMJ and MJJ 2023 temperature outlook for the western U.S. is challenging due to considerably above normal snowpack and soil moisture as a result of recent storminess in February and March, offset by forecasts of warmer than normal temperatures predicted by some dynamical and statistical forecast guidance. Forecasts from some of the statistical forecast tools and dynamical models indicate high odds for above-normal temperatures in the western CONUS from lead 1 onward - perhaps linked to a more rapid transition to warmer than average equatorial Pacific SSTs and potential consequent atmospheric response. The higher odds for above-normal temperatures from these tools seems overdone at the start of this month's set of outlooks given the highly anomalous snowpack and soil moisture conditions at the start of AMJ 2023. Although the impact of enhanced surface wetness (either soil moisture to start or melting snowpack and consequent elevated soil moisture) is likely to diminish relatively quickly as the month of May progresses and in June, the season as a whole may still tilt slightly toward below-normal temperatures. The monthly temperature outlook for April 2023 favors below-normal temperatures for considerable areas in the western U.S. Equal-chances (EC) is forecast for much of the remaining area in the western CONUS due to these competing influences. There is also support for areas of favored below-normal temperatures in the western CONUS from the ECMWF model guidance, the CA forecast tool based on soil moisture, and the CBaM hybrid tool. Negative trends in sea ice coverage and thickness and so more open water earlier than normal for ocean areas along the northwest and northern coast of Alaska favors above-normal temperatures for these areas during AMJ 2023. Elevated odds for below-normal temperatures remains for portions of the northern Plains during MJJ 2023, while favored above-normal temperatures increase in coverage and probabilities across the western CONUS from MJJ through ASO 2023. Weaker and conflicting signals in the forecast tools increase uncertainty for the north-central U.S. and so the area forecast as EC increases from the JJA through OND 2023 seasons. Primarily beginning in SON 2023, the remaining outlooks are largely based on the ENSO-OCN forecast tool which is heavily influenced by long-term trends at these leads with some slight adjustments, at this stage, for the potential influence of El Niño. For Alaska, increased forecast coverage of favored above-normal temperatures is depicted from MJJ through SON 2023. PRECIPITATION The AMJ 2023 precipitation outlook favors above-normal seasonal total precipitation amounts for portions of the Great Lakes, Ohio Valley, Mid-Atlantic and Northeast based on long term positive precipitation trends as defined by OCN, dynamical model guidance from some participant models from the NMME and C3S ensemble systems, the CA forecast tool based on anomalous soil moisture and potential residual La Niña impacts. Below-normal precipitation is favored for parts of the far Pacific Northwest and the Southwest U.S. where some dynamical model forecast guidance, the CBaM hybrid tool, the ENSO-OCN forecast tool and long term negative precipitation trends supported the outlook. It is important to note that signals from the dynamical model suite for precipitation are especially weak for basically all forecast leads for which forecast guidance is available, even when considering shoulder seasons within the seasonal cycle as well as the transition to ENSO-neutral. Favored above-normal precipitation in the eastern CONUS is forecast to shift eastward and southward with time from MJJ through JJA and then transition westward by ASO 2023. This evolution is based on long term positive precipitation trends and to a lesser degree signals from the NMME and C3S model guidance. Favored below-normal precipitation forecast for the Pacific Northwest shifts to include parts of the northern Rockies from MJJ through JAS 2023 - a result of the ENSO-OCN forecast tool, long term negative precipitation trends and some dynamical model forecast support. For the Southwest, below-normal precipitation is most likely to continue during MJJ 2023, albeit with smaller forecast coverage. Unlike 2022, there are less climate signals to utilize for prediction of the first half of the Southwest summer monsoon. Enhanced wetness across the western CONUS from an extremely wet winter may lead to a potentially delayed start and less robust monsoon circulation due to perhaps less efficient heating of the land and so a weaker thermodynamic induced circulation. The CBaM hybrid forecast tool, does favor below-normal precipitation in the Southwest during the monsoon season, but there remains considerable uncertainty given the evolution of ENSO and how that may or may not influence precipitation in the region this summer. Climate signals for the state of Alaska for AMJ 2023 are quite weak and conflicting amongst the tools when present, so EC is forecast for the entire state. However, above-normal precipitation is most likely for parts of northern and central Alaska beginning in MJJ 2023 and continuing through the remainder of 2023. Model guidance and more open water earlier in the seasonal cycle are the basis for these highlighted areas. Primarily long term precipitation trends and slight tilts in deference to potential EL Niño development are used in making the remainder of the precipitation outlooks through AMJ 2024. FORECASTER: Jon Gottschalck 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 Apr 20 2023 1991-2020 base period means were implemented effective with the May 20, 2021 forecast release. $$
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