Climate Prediction Center - Seasonal Outlook (2024)

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 NiÃ±o Advisory remains in effect but the oceanic
temperature anomalies associated with El NiÃ±o 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 NiÃ±a by summer (June-July-August, JJA), with La
NiÃ±a conditions favored to strengthen and continue through boreal autumn and
winter. Therefore, a La NiÃ±a 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 NiÃ±o 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 NiÃ±o3.4 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 NiÃ±o to ENSO-neutral.

As of mid-April 2024, above-normal soil moisture is present in many areas of
the CONUS â€“ 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 NiÃ±o 3.4 SST index from the North American Multi-Model
Ensemble (NMME) are generally in good agreement for a nominal La NiÃ±a (Oceanic
NiÃ±o 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 NiÃ±a
threshold until mid-August. The International Multi-Model Ensemble (IMME, C3S,
or Copernicus) average reaches the La NiÃ±a 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 NiÃ±a conditions by late May
into June. The CFS is the coldest of solutions, exceeding the minimal La NiÃ±a
threshold even before MJJ, and predicts a strong La NiÃ±a (ONI>=-1.5 degrees C)
by July-August-September (JAS) 2024. CPCâ€™s SST Consolidation for the NiÃ±o 3.4
region passes into La NiÃ±a 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, CPCâ€™s ENSO
Team predicts the transition from El NiÃ±o to ENSO-neutral should be completed
within the AMJ season (85% chance), with the odds of La NiÃ±a developing by JJA
(60% chance). Historically, La NiÃ±a tends to follow strong El NiÃ±o events,
adding confidence to the forecast.

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

Though some remaining El NiÃ±o 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 â€œbridgedâ€