Prognostic Discussion for Long-Lead Seasonal Outlooks
NWS Climate Prediction Center College Park MD
830 AM EDT Thu May 16 2019

SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS

The June-July_August (JJA) 2019 temperature outlook indicates that above normal seasonal mean temperatures are most likely for the eastern and western thirds of the US, including Alaska. Below normal seasonal mean temperatures are more likely for parts of the Central Plains. The JJA 2019 precipitation outlook indicates that above normal seasonal total precipitation is most likely for much of the US, including the interior West across much of the Great Plains into the Central Mississippi and Ohio Valleys, stretching to the Mid-Atlantic coast. The greatest probabilities for above normal seasonal total precipitation are for areas of the Central Rockies. Below normal precipitation is more likely for a small area of the Pacific Northwest near the coast as well as southern Arizona. Above normal precipitation is more likely for most of Alaska. The earliest leads generally reflect the combined impacts of soil moisture, trends, and ENSO. Subsequent leads emphasize the evolution of long-term trend and ENSO impacts, the latter of which affect the forecast only through JFM 2020.

Equal Chances (EC; white areas) of below, near, and above normal seasonal mean temperatures or seasonal total precipitation amounts are where the likelihoods for these three categories are similar to their climatological probabilities. Review of subsequent seasonal outlooks and the scientific forecast basis for all outlooks are given below.

BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS

Note: For Graphical Displays of the Forecast Tools Discussed Below See:
https://www.cpc.ncep.noaa.gov/products/predictions/90day/tools/briefing

CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS

El Niño conditions continue in the Pacific Ocean during May 2019. Sea surface temperatures (SSTs) across much of the equatorial Pacific Ocean remain above climatological averages, though anomalies in the recent week are generally less than +1°C except near the Date Line. The latest weekly Niño 3.4 SST anomaly has fallen to +0.5°C. Oceanic heat content, determined from ocean temperature anomalies from the surface to a depth of 300 meters along the equatorial Pacific, has continued a rapid decline that began last month. Atmospheric conditions, however, continue to respond to the anomalous SST forcing with enhanced convection consistently along the equator near and just west of the Date Line during the last few months. Trade winds strengthened during April 2019, due in part to emerging MJO activity. Recently, however, a westerly wind burst is evident over the western Pacific as the MJO propagates across the Pacific. Collectively, these oceanic and atmospheric conditions represent a continuation of weak El Niño conditions as of May 2019. Other boundary conditions relevant to the seasonal outlook are soil moisture anomalies and, in some areas, near-coastal SSTs. Widespread and persistent above normal precipitation for most of the CONUS during recent months has resulted in positive soil moisture anomalies for many areas of the central and eastern CONUS, extending into parts of the Great Basin, with many of these areas exceeding the 99th percentile of climatological soil moisture. Sea ice coverage along much of the western Alaska coast is below normal, with SSTs above normal.

PROGNOSTIC DISCUSSION OF SST FORECASTS

The CPC SST Consolidation Niño 3.4 forecast indicates the continuation of an El Niño through 2019. Relative to last month the various input tools are in better agreement, with the the CFS and three statistical models all predicting at least a weak El Niño from autumn 2019 into winter 2019-2020. The CFS dynamical model forecast ensemble mean predicts a slow decline in positive Niño 3.4 anomalies, with the most recent runs indicating a transition to ENSO neutral conditions later this year. The NMME suite of dynamical model shows moderate spread this month with some ensemble members predicting a strong El Niño event by the beginning of winter, some maintaining a weak or moderate event through autumn, and some members predicting a return to ENSO neutral conditions by autumn. Little possibility of a La Niña event next winter is indicated by statistical and dynamical model forecasts. While La Niña events often emerge in the year following an El Niño in the climate record, at other times El Niño events extend beyond one year, as predicted by a significant number of the forecast tools. The probability of a continuation of the current El Niño into next winter is set at about 55-60% by the IRI/CPC consensus forecast.

PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS

The temperature and precipitation outlooks were based primarily on dynamical model guidance from the NMME, along with statistical model guidance, including a statistical forecast of the linear impacts of ENSO combined with decadal climate trend based on the CPC SST Consolidation forecast for Niño 3.4. The potential impact from a possible continuation of El Niño conditions were considered for the seasonal outlook period through next winter 2019-2020. Soil moisture conditions were strongly considered for the earliest leads along with near-coastal SST anomalies. Several statistical and dynamical models, as well as hybrid statistical-dynamical models, along with an objective consolidation of several forecast tools, were also examined in preparation of the seasonal outlooks. Decadal timescale trend were considered for all leads but take a primary role in later outlooks, as uncertainty in interannual climate signal increases.

PROGNOSTIC DISCUSSION OF OUTLOOKS - JJA 2019 TO JJA 2020

TEMPERATURE

The JJA 2019 through DJF 2019-2020 seasonal mean temperature outlooks utilize the predictability of several climate phenomena, including the ongoing El Niño event and its potential impacts, anomalous land surface and sea surface temperature states, and decadal timescale climate variability or trends. A statistical-dynamical hybrid model combining calibrated NMME temperature forecasts with statistically bridged impacts from NMME Niño 3.4 forecasts (known as calibration, bridging and merging or CBaM) suggest that below-normal temperatures are most likely during JJA over much of the central CONUS. Statistical guidance that emphasizes ENSO and long-term trend indicate that near normal temperatures are most likely for much of the same area. Subjective interpretation of objective tools that utilize the lagged relationship between soil moisture and temperature leads us to favor the colder CBaM guidance over much of the central CONUS. Long-term trend are generally weakest across the north-central CONUS, while ENSO and near-record soil moisture values favor below normal temperatures across much of the central CONUS. Likewise some consideration was given to historical patterns of covariance between seasonal temperature and precipitation anomalies. Over parts of the West, probabilities of above normal temperature were tempered to account for enhanced odds of above normal precipitation. The various forecast tools remain in good agreement on enhanced odds of above normal summer temperatures over much of the western and eastern thirds of the CONUS, respectively. Near-coastal SSTs and below normal sea ice concentration leads to increased odds of above normal temperatures over much of southern and western Alaska compared to last month's outlook. The various forecast tools suggest that above normal temperatures are likely to continue for Alaska and much of the western and eastern CONUS through all outlooks. For these regions interannual climate signal and other shorter timescale climate forcing are either in phase with decadal warming trend or positive temperature trend are greater than shorter timescale climate signals. Greater probabilities exceeding 50% for above normal temperatures expand across the West through late summer 2019, while probabilities of above normal temperatures decrease below 40% across southern tier states beginning in the Southwest region and continuing across the southern CONUS into the Southeast by early winter, due to the potential impact if El Niño persists through these seasons. Areas with enhanced probabilities of above normal temperatures expand into the Plains heading toward autumn 2019, as the signal due to positive soil moisture anomalies decreases. Temperature outlooks for NDJ 2019 and later are largely based on the consolidation, deriving signal from statistical model forecasts, based largely on decadal trend and the possible states of ENSO. Above normal temperatures are most likely for all regions in the NDJ and DJF winter 2019-2020 outlooks, with an area of uncertainty and decreased probabilities of above normal appearing in the north-central CONUS in the JFM through JJA 2020 outlooks.

PRECIPITATION

Beginning with the JJA 2019 precipitation outlook, above normal precipitation is forecast for a large region of the CONUS from the interior West across the much of the central CONUS and extending to the Mid-Atlantic coast, consistent with dynamical model guidance and a blend of long-term trend and summertime ENSO impacts. This general precipitation forecast pattern persists with some reduction in probabilities for the JAS 2019 season, and gradually shifts southward due to the seasonal evolution of long-term trend and expected ENSO impacts. For JJA 2019, below normal precipitation is most likely for parts of the Pacific Northwest, as well as parts of southern Arizona. In the case of the latter, there is good model support for delayed and shifted monsoon activity during 2019 due to the expectation of reduced temperature gradients that give rise to the monsoonal circulation. This is also is consistent with historical El Niño impacts during the warm season. The area of enhanced probabilities of above normal precipitation expands into the Southwest during ASO 2019, due to forecasts of continued above normal SSTs over the eastern Pacific Ocean, related to the potential persistence of El Niño, along with potential moisture surges into the Southwest monsoon region related to enhanced tropical storm activity. Elevated odds of above normal precipitation persist over the Southwest through NDJ 2019 due to the likelihood of continued El Niño conditions. During OND and NDJ 2019, a very slight tilt toward below normal precipitation is introduced over parts of the Northwest centered on Oregon. This is where the CBaM and consolidation are good agreement, albeit with very modest probabilities. Probabilities of above normal precipitation are enhanced for much of Alaska outside the Alaska Panhandle in JJA 2019, and for various regions of Alaska from JAS through all seasonal outlooks, driven by signal related to decadal climate trend and interannual climate signals, such as a persistence of El Niño. Decadal precipitation trend lead to enhanced probabilities of above normal precipitation for parts of the Northeast CONUS in AMJ through JJA 2020.

FORECASTER: Stephen Baxter

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 https://www.cpc.ncep.noaa.gov/products/predictions/long_range/tools.html. Information on the formation of skill of the CAS forecasts may be found at: https://www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/US/Outlook/outlook.shtml. 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 Jun 20 2019

1981-2010 base period means were implemented effective with the May 19, 2011 forecast release.

$$