NOAA Updates their Seasonal Outlook – El Nino coming but not here yet – May 27, 2023

CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS

Equatorial SSTs are near-to-above average across most of the Pacific Ocean and atmospheric conditions reflect ENSO-neutral conditions. Since early April 2023, near-to-above average SSTs have persisted across most of the tropical Pacific Ocean, and eastern Pacific positive SSTs have expanded westward to the east-central Pacific ocean. Weekly SST departures remain largest in the NINO1+2 region reaching 2.4 degrees Celsius above normal, while the NINO3.4 region has reached 0.5 degrees Celsius above normal. Subsurface temperature anomalies became positive in February and have increased through mid-April, and positive subsurface temperature anomalies now dominate the equatorial Pacific Ocean. During the last 30 days, weak suppressed convection was observed near the equator over Indonesia, and low-level winds were near average across most of the equatorial Pacific Ocean, reflecting ENSO-neutral conditions.

The Madden Julian Oscillation (MJO) has remained active, and the enhanced phase propagated eastward from the Maritime Continent into the western Pacific during the past week. The Realtime Multivariate MJO (RMM) index for early April 2023 through mid-May 2023 indicates that the MJO signal completed another global circumnavigation, however the MJO recently decreased in amplitude over the western Pacific due to destructively interfering Kelvin and Rossby Wave activity. RMM forecasts show the MJO regaining amplitude over the western Pacific at a slow phase speed in the upcoming week and propagating slowly across the western Pacific through late May. In RMM forecasts, some ensemble members from the Global Ensemble Forecast System version 12 (GEFSv12) and the European Centre for Medium-Range Weather Forecast (ECMWF) model forecast continuation of the MJO into the end of May, but the ensemble mean ECMWF indicates that MJO activity may wane.

PROGNOSTIC DISCUSSION OF SST FORECASTS

The latest Probabilistic ENSO Outlook issued in May 2023 favors a transition from ENSO-neutral to El Niño during May-July (MJJ) 2023, with chances of El Niño increasing to greater than 90% into winter 2023-24, and an El Niño Watch is in effect. The most recent plume from the International Research Institute (IRI) also indicates likely formation of El Niño in the May to July seasons and persistence into the winter. At least a weak El Niño is likely given high levels of above-average oceanic heat content, but the range of possibilities include an 80 percent chance of at least a moderate El Niño and a 55 percent chance of a strong El Niño by the end of the year. Dynamical model forecasts from the North American Multi Model Ensemble (NMME) and the Climate Forecast System model version 2 (CFSv2) favor a moderate to strong El Niño through November-January (NDJ) 2024 while they are available, and forecasts become more uncertain in spring and summer 2024 based on statistical models.

30-DAY OUTLOOK DISCUSSION FOR JUNE 2023

The June 2023 temperature and precipitation outlooks are based on the Climate Forecast System Model version 2 (CFSv2), North American Multi-Model Ensemble (NMME) and Copernicus model (C3S) suites for June, the week 3-4 model solutions, statistical tools, and soil moisture conditions. The Madden-Julian Oscillation (MJO) remains active with its enhanced phase over the west-central Pacific. The MJO influence on the mid-latitude circulation pattern and associated temperatures diminishes by late spring. However, it can modulate tropical cyclone (TC) development across the East Pacific and Atlantic basins. If the MJO continues to propagate eastward over the Western Hemisphere through late May into early June, that would provide a favorable large-scale environment for the genesis of an early season TC in the western Caribbean Sea. This was a factor in the June precipitation outlook for the Southeast. Since a transition from El Niño Southern Oscillation (ENSO)-neutral to El Niño is likely during the next couple of months, El Niño temperature and precipitation composites during May-June-July were also considered.

Temperature

A strong omega block has persisted over or in the vicinity of North America since late April. Model solutions depict a weakening of this blocking pattern during late May which lowers forecast confidence heading into June. During late May and early June, the Global Ensemble Forecast System (GEFS) and European Centre for Medium Range Weather Forecast (ECMWF) ensemble means maintain positive 500-hPa height anomalies over Alaska and the northern two-thirds of the contiguous U.S. (CONUS) with near to below normal heights across the southern tier. The prominent area of positive 500-hPa height anomalies at the middle to high latitudes of North America would favor a warm start to June throughout the northern CONUS. This likely warm start to June along with monthly dynamical and statistical tools support increased probabilities for above-normal temperatures from the Pacific Northwest east to the Northern Great Plains. This favored area of above-normal temperatures extends southward to the Central Great Plains and Middle Mississippi Valley where low soil moisture conditions are likely to warm surface temperatures. The week 3-4 GEFS and ECMWF models, C3S model, and recent daily CFSv2 model runs for June support to increased probabilities for above-normal temperatures across parts of the Northeast. Based on the NMME, C3S, and decadal trends , above-normal temperatures are favored for parts of the Southwest, Rio Grande Valley, Gulf Coast, and Florida. Increased probabilities for above-normal temperatures are forecast for nearly all of Alaska, consistent with dynamical and statistical tools. The one exception is the western coast of Alaska due to below-normal SST anomalies across the eastern Bering Sea.

Precipitation

The week 3-4 GEFS and ECMWF, valid during early to mid-June, are in good agreement for elevated probabilities of above-normal precipitation across portions of the High Plains, Central Rockies, and Great Basin. This wet signal is also present in the monthly NMME and consistent among daily CFSv2 model runs for June. Therefore, above-normal precipitation is favored for those areas during June. The NMME along with an increased potential for an early season TC to emerge from the western Caribbean Sea favors above-normal precipitation for parts of the Southeast. Based on the NMME and El Niño precipitation composites, elevated probabilities for below-normal precipitation are forecast for southeastern Alaska and parts of the Pacific Northwest. A large area of EC was necessary in the June precipitation outlook due to weak or conflicting signals among the guidance and lower predictability associated with convective rainfall during the late spring and early summer. This large coverage of EC will be reevaluated with the updated June outlook, released on May 31.

SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS (Focus or June through August)

El Niño Southern Oscillation (ENSO) neutral conditions are currently observed, and equatorial sea surface temperatures are near-to-above average across most of the Pacific Ocean. An El Niño watch is in effect as a transition from ENSO-neutral to El Niño is favored during May-July 2023, with chances of El Niño increasing to greater than 90% in winter 2023-2024.

Temperature

The June-July-August (JJA) 2023 temperature outlook favors above normal temperatures over the western contiguous United States (CONUS), the southwest, southeast, and along the eastern seaboard to New England. A tilt toward warmer than normal temperatures is also favored over most of Alaska. The largest probabilities of above normal temperatures (60 to 70 percent) are forecast over the Southwest, and probabilities reach 50 to 60 percent along the coast from Texas to New England.

Precipitation

The JJA precipitation outlook favors below normal precipitation probabilities over the Pacific Northwest, Southwest, and south coast of Alaska. Above normal precipitation probabilities are more likely for the eastern CONUS, including the Southeast, Mid-Atlantic, Lower and Middle Mississippi Valley. The North Slope of Alaska also tilts weakly above normal. 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

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

Dynamical model forecasts from the NMME, CFSv2, and Copernicus (C3S) multi-model ensemble systems are used for the first six leads when they are available as was the objective, historical skill weighted consolidation, that combines both dynamical and statistical forecast tools. The Calibration, Bridging and Merging (CBaM) tool anchored to the NMME forecasts and “bridged” to the Niño3.4 index is also significantly utilized. El Niño impacts were considered given the elevated probability of transition to El Niño during MJJ 2023, and El Niño composites and correlation/regression of temperature and precipitation with the NINO3.4 region were used to determine typical El Niño impacts. Statistical guidance such as the global SST based Constructed Analog (CA) and long term temperature and precipitation trends  played a role in many of the outlook seasons, particularly beyond lead 6 when dynamical model guidance is unavailable. Antecedent soil moisture conditions and coastal SSTs are considered at early leads.

PROGNOSTIC DISCUSSION OF OUTLOOKS – JJA 2023 TO JJA 2024

TEMPERATURE

The JJA 2023 temperature outlook favors above normal temperatures over the western CONUS, the Southwest, the Gulf States, along the eastern seaboard, and for much of the state of Alaska. The highest probabilities of above normal temperatures (60 to 70 percent) are over parts of the Southwest, Southern Rockies, and western Southern Plains where NMME and C3S show consistent strong probabilities of above normal temperatures, soil moisture is anomalously dry, decadal temperature trend is above normal, and where precipitation is forecast to tilt toward below normal for this season. 50 to 60 percent probabilities are indicated from the Southwest along the coast to New England where there was strong model agreement as well as support from above normal decadal temperature trends and above normal coastal SSTs. A weak tilt toward above normal temperatures (33 to 40 percent) is forecast over California owing to weaker temperature signals  in NMME and C3S, likely related to recent coastal SST anomalies that leaned below normal and recently transitioned to neutral. A weak tilt toward above normal temperatures is also indicated over western Oregon and Washington given a slight weakness in dynamical models  as well as mixed month to month temperature teleconnection to El Niño as it evolves through JJA. EC of above, near, and below normal temperatures are depicted over parts of the Northern Plains to the Great Lakes and parts of the Ohio River Valley were tools where weak or had inconsistent signals , and given currently neutral Great Lakes temperatures. Over Alaska, NMME, C3S, CFSv2, and CBaM all favor a tilt toward above normal temperatures over the southern half of the state, consistent with expected ENSO teleconnection in JJA, while statistical tools indicate EC over northern parts of the state and a spatially small, very weak tilt toward below normal decadal trend. Anomalously cold SSTs along the southwestern coast of Alaska led to a moderation of probabilities over the southwest part of the state.

For the remainder of the summer and into early fall (July-September, JAS through August-October, ASO), 50 to 60 percent probabilities of above normal temperatures are maintained over the western CONUS and stretching along the eastern seaboard where agreement remains among dynamical and statistical tools, and due to strong probabilities in the objective skill-based consolidation.  Probabilities over Alaska also remain above normal with the largest probabilities over the southern half of the state where there is dynamical and statistical model agreement. Compared to last month’s JAS and ASO forecasts, probabilities are slightly increased over the Ohio and Tennessee Valley regions and along the coast given strong probabilities in CBaM, NMME, and CFSv2 and where the objective skill based consolidation had higher probabilities. EC is maintained over the central CONUS where signals  were weak, though the eastern Great Lakes tilt above normal consistent with dynamical models .

Confidence decreases in September-November (SON) 2023 and following seasons as lead time increases and dynamical models  become more uncertain, however, El Niño is expected to become more dominant in the forecast(s), and the pattern begins to reflect El Niño conditions during fall and winter seasons. Above normal temperatures are favored over the western and eastern CONUS, with highest probabilities reaching 50 to 60 percent over New England where decadal trends are strong. EC is indicated over parts of the central CONUS in SON and October-December (OND) given lack of consistent signal in tools. By fall and winter 2023-2024 (November through January, NDJ and December through February, DJF) we expect to begin seeing a canonical El Niño pattern of increased temperatures over the Northern Tier and a reduction of probabilities over the southern CONUS. Warmer than normal temperatures are generally favored over Alaska in the fall and winter months. Forecasts for spring and summer 2024 are more reliant on decadal trends  given the comparatively uncertain forecast of El Niño and lack of dynamical model data at longer leads, though probabilities are higher where decadal trends  and constructed analogues based on SSTs agree. February through April (FMA) 2024 has elevated probabilities of above normal temperatures over Alaska and the east coast, with probabilities over CONUS expanding westward in April through June (AMJ) and JJA 2024.

PRECIPITATION

The JJA 2023 precipitation outlook depicts below normal precipitation probabilities over the Pacific Northwest and parts of the Northern Rockies, the Southwest, and south coast of Alaska. The tilt toward below normal precipitation over the Pacific Northwest and south coast of Alaska is consistent with expected El Niño teleconnections as well as below normal decadal trends . NMME and C3S are in good agreement on below normal precipitation over the southwest and wet soil moisture over the Four Corners region may provide a sluggish start to the Monsoon this summer. Above normal precipitation probabilities are indicated over parts of the Middle and Lower Mississippi Valleys, the Ohio and Tennessee Valleys, the Southeast, and mid-Atlantic. Parts of the Central Plains tend to be anti-correlated with the monsoon region, and as such a weak tilt toward above normal precipitation is indicated. The remainder of the above normal region is due to enhanced probabilities in C3S and above normal decadal trends with the highest probabilities aligned with where decadal precipitation trends  are strongest as well as recent high rainfall amounts. Though above normal precipitation probability is favored over the northern part of Alaska owing to weak decadal trends and above normal probabilities in CFSv2, probabilities are weak and EC is indicated for the central parts of Alaska given inconsistent signals and significant changes from last month’s forecast to this month’s. The dry signal over the Southwest is maintained through JAS 2023 as dynamical models agree on below normal probabilities. The above normal precipitation over the eastern CONUS shifts westward in JAS 2023 owing to a reduction of probabilities over the southeast in the forecast consolidation. Above normal precipitation is maintained over Northern Alaska through JAS and into winter 2023 as dynamical models generally forecast above normal precipitation. Over the Pacific Northwest, a weak tilt toward below normal precipitation is favored through FMA2024 consistent with decadal trends  and constructed analogues based on SSTs. Above normal precipitation is favored for the Southeast and Gulf States owing to canonical El Niño teleconnection during the winter months. Below normal precipitation is depicted over the north Central Plains and Great Lakes during the winter, also consistent with El Niño. Decadal trends  become the dominant player in spring and summer 2024, with below normal trends  over the Pacific Northwest and above normal trends  over the Mid-Atlantic. Confidence decreases over Alaska in spring and summer 2024, with only a weak tilt toward above normal rainfall in March through May (MAM) 2024 over the northwest consistent with above normal precipitation trend, and EC indicated for the remaining leads given lack of clear signal.