Prognostic Discussion for Long-Lead Seasonal Outlooks NWS Climate Prediction Center College Park MD 830 AM EDT Thu Apr 16 2020 SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS ENSO-neutral conditions remain in place across the Pacific Ocean and ENSO-neutral conditions are favored to continue through Autumn 2020. However, there is some tendency toward La Nina conditions developing during autumn and early winter when the odds of La Nina (30-40%) exceed both that of El Nino and the climatological odds of La Nina. The ENSO forecast is quite uncertain from summer through the upcoming winter. The May-June-July (MJJ) temperature outlook favors seasonal mean temperatures to be above normal for nearly all of the United States with the greatest odds forecast for southwestern Alaska, much of the interior West, along the Gulf Coast and for the eastern one-third of the country. Lower probabilities for above-normal temperatures are forecast for areas of the central lower 48. A similar pattern is forecast through the summer and early autumn months moving forward. By late autumn through winter, above-normal temperatures are favored across the southern tier of the contiguous U.S. with weaker temperature signals forecast across the northern tier. This pattern is due to a combination of long-term trends and a forecast shift toward colder SSTs over the central and eastern tropical Pacific. For precipitation, there are elevated odds for above-normal seasonal total precipitation amounts for northwest Alaska and for most of the central and eastern U.S. Below-normal precipitation is most likely for the Alaska Panhandle and points southward and eastward to include the Pacific Northwest, northern California, and the northern Great Basin. Progressing into and through summer 2020, the aforementioned regions of above- and below-normal precipitation are largely maintained, but the east-west gradient take on a north-south structure during autumn through early winter. 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 ENSO-neutral conditions are in place across the Pacific Ocean at the current time. Sea surface temperature (SST) over the equatorial Pacific are almost entirely above normal, with a large region of anomalies exceeding +1.0 C centered on the Date Line. The Nino3.4 regional SST anomaly continues to hover around +0.5 C. At depth, positive ocean temperature anomalies dominate between 0-100 meters, except over the far eastern Pacific. Negative temperature anomalies are now observed between 100-200 meters depth from 150E to about 120W. Total 0-300m upper ocean heat content anomalies have turned negative for the first time since September 2019. Atmospheric conditions now show suppressed convection over the past month near and east of the Date Line. This convective anomaly is consistent with symmetric off-equatorial 200-hPa cyclonic circulations. Near equatorial zonal wind anomalies show some strengthening of the Walker Circulation over the central and eastern Pacific. Anomalously high soil moisture content is present from the northern Plains to the western Great Lakes southward to include most of the Great Plains, Mississippi Valley, Ohio Valley, Tennessee Valley and Southeast. Anomalously dry soils are currently observed over the Florida Peninsula and entire Gulf Coast including South Texas, as well as over much of the Pacific Northwest. Coastal SST anomalies are small in magnitude this year for waters in proximity to the Alaska south and west coasts and sea ice coverage in the Bering Sea and Bering Strait are for the first time in a while generally near the long term average - although ice is primarily first year ice, is thin and likely to melt out relatively quickly. Coastal SST anomalies near the West Coast of the CONUS are likewise low amplitude. By contrast, the Gulf of Mexico is much warmer than normal, as are waters off most of the Eastern Seaboard. PROGNOSTIC DISCUSSION OF SST FORECASTS The official CPC Nino3.4 consolidation forecasts a shift toward weak La Nina conditions by late summer and fall, due largely to the CFSv2 which continues to forecast developing La Nina conditions this summer. The SST constructed analog forecast largely follows the consensus forecast and nicely smooths over the jump between OND and NDJ where the CFSv2 times out of the forecast consolidation. The other two statistical tools (Markov and CCA) continue with warm neutral or weak El Nino during the entire forecast period. The NMME forecast overall follows the consolidation through SON, though it is interesting to note that three constituent modeling systems clearly favor La Nina (including the CFSv2), three favor neutral, and one is in between. The CPC-IRI consensus forecast is close to the objective consolidation, but slightly more conservative. It nonetheless favors a clear transition from warm-neutral to cold-neutral ENSO conditions over the next several months. It is interesting to note that the latest experimental tool we have is an artificial neural network model that quite strongly favors the La Nina outcome in this uncertain situation. PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS The temperature and precipitation outlooks for MJJ 2020 were based on a few factors, primarily bias-corrected and calibrated dynamical model guidance, statistical forecast tools utilizing various methodologies (including "bridging techniques") and recent trends. Objective, skill-based combinations of the above information was weighted quite heavily in the current set of outlooks. ENSO-neutral conditions are still generally assumed, but given the transition toward negative SST anomalies over the equatorial Pacific that is forecast, there have been subtle changes to reflect the fact that El Nino is not likely from summer through winter (only about a 20% chance). Recent climate trends are considered for all leads but were relied on more heavily for the middle and later forecast leads. Simultaneous and lagged relationships between soil moisture anomalies and subsequent temperature and precipitation anomalies are likewise considered. For the first several leads, diagnostic tools based on the NMME are utilized to better understand the source of forecast climate anomalies. These tools use the historical skill of predicting the large-scale 200-hPa circulation patterns observed during the various seasons to generate a type of post-processed forecast the is reconstructed based on these leading patterns of variability. This analysis indicates that large-scale circulation patterns more consistent with cold ENSO conditions are forecast through SON, the last full season in the analysis. PROGNOSTIC DISCUSSION OF OUTLOOKS - MJJ 2020 TO MJJ 2021 TEMPERATURE The MJJ 2020 temperature outlook favors seasonal mean temperatures to be above-normal for nearly all of the United States. A region in the north-central CONUS is designated as "Equal Chances" (EC) where forecast probabilities for either above-, near- or below-normal temperatures are no different than the 33.3% climatological odds for a 3-class system. The forecast is consistent with long-term trends, but is actually cooler than the trend over parts of the central and southeastern CONUS. This is due in part to the expected effect of anomalously wet soils on the local energy balance and is not supported by the forecast upper-level circulation. The greatest odds for above-normal temperatures is for southwestern Alaska, the interior West, along the Gulf Coast and for the eastern one-third of the country. Overwhelming agreement between numerous seasonal forecast dynamical models across a number of operational forecast centers and general consistency from previous months' forecasts strongly support this outlook. Also, nearly all statistical guidance is consistent with the dynamical model guidance, with the SST constructed analog depicting the coolest solution over the central CONUS. Strong long term positive trends (especially in the eastern CONUS, parts of the West and Alaska) contributed to the elevated odds of above-normal temperatures. A similar forecast pattern persists through JAS, with the weakness over the central CONUS shifting southeastward based on the latest guidance from both dynamical and statistical tools. By ASO and SON, long-term trends along with some weak tilt toward La Nina (and tilt away from El Nino), favor increased probabilities of above-normal temperatures across much of the interior West extending eastward across the central CONUS. Relative to the outlooks issued last month, there is very little change through the winter into next spring except to increase probabilities favoring above-normal temperatures over parts of the southern tier of the CONUS. Likewise probabilities favoring above-normal temperatures were decreased across parts of the Northwest and southern Alaska. Again, this is a modest reflection of the forecast tilt toward negative SST anomalies over the Nino3.4 region. PRECIPITATION The MJJ precipitation outlook depicts elevated odds for above-normal seasonal total precipitation amounts for northwest Alaska and for most of the central and eastern CONUS. The primary driver for the outlook across the central and eastern U.S. is the objective consolidation of the statistical tools and NMME, and is consistent with the long-term trend. The NMME forecast is a bit wetter over the northern and central Rockies and looks very similar to the lagged response to El Nino over that region. Analysis of the NMME circulation fields and associated reconstruction favors a much drier solution over the western and central CONUS. This increased confidence in using the consolidation forecast solution. Below-normal precipitation is most likely for the Alaska Panhandle southward and eastward to include the Pacific Northwest and northern California. Similar information as noted above is utilized as the basis for the outlook in these regions. Below-normal soil moisture content to a small degree increased the odds for below-normal precipitation for parts of California and Oregon due to potentially less local recycling of boundary layer moisture. Progressing through the summer and autumn seasons, the precipitation forecast takes on a more north-south orientation with above-normal precipitation favored over the northern tier of the U.S. with below-normal precipitation favored farther south. Unlike the MJJ and JJA forecast, by SON the NMME precipitation forecasts are largely consistent with the forecast reconstructed by the forecast circulation patterns. This is consistent with the extratropical circulation taking on a more La Nina flavor at times and leads to increased coverage for the autumn and early winter months relative to previous outlooks. Long-term trends favor modest probabilities of below-normal precipitation over parts of Southwest during late winter through spring. Forecasts for FMA through MJJ 2021 reflect the consolidated guidance which is largely informed by long-term trends. 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 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 21 2020 1981-2010 base period means were implemented effective with the May 19, 2011 forecast release. $$