NOAA Updates its Four Season Outlook on August 17, 2023

And here are large versions of the three-month JAS 2023 Outlook

First temperature followed by precipitation.

Drought Outlook

NOAA also issues a Seasonal Drought outlook which is tied to the precipitation and temperature outlooks plus the initial conditions. That means among other things that current soil conditions need to be considered when doing a drought forecast.

The yellow is the bad news. There is a larger area where drought development is likely than the area where drought removal is likely plus there is a large area where drought is expected to persist.

Short CPC Drought Discussion

Latest Seasonal Assessment – There was a distinctive pattern to the changes in the Drought Monitor over the past four weeks. Broadscale improvement was noted across affected areas in the Northeast, the Mid-Atlantic Region, the Middle Mississippi Valley, the central Plains, and most of the Great Lakes. Dramatic Improvements by up to 3 categories were noted in some areas near the Mississippi/Ohio Rivers’ confluence and near the Great Lakes Region. Meanwhile, similar broadscale deterioration occurred from the Upper Mississippi Valley westward into the Pacific Northwest, and along the southern tier of states from the Desert Southwest eastward through the Lower Mississippi Valley. A few areas in northwestern New Mexico and the central and southern portions of Texas saw severe intensification by up to 3 Drought Monitor categories. Excessive heat made a significant contribution to the current drought conditions along the southern tier of states, and to a lesser extent the Pacific Northwest. The southern Plains and Lower Mississippi Valley felt the worst of the heat, and Excessive Heat Warnings were frequent during the past month, especially along the central and western Gulf Coast Region. Lakefront Airport in New Orleans reported heat indexes topping 115F every day during August 5-15, topping out at 124F late afternoon on the 15th. For 69 hours, from 7:15 CDT August 7 through 3:45 CDT August 10, the heat index remained at or above 100F. Drought intensification was also observed in Hawaii, contributing to fatal wildfires that scorched parts of Maui.A surge of tropical moisture – partially fueled by Tropical Storm Hilary – is expected to push into the Southwest, from the southeastern California deserts to eastern Arizona. This should bring some relief from dryness that has developed during the hot and drier-than-normal Southwest Monsoon. The dwindling patches of drought from the Appalachians to the Atlantic Coast are also expected to ease, along with the areas of drought currently affecting the Virgin Islands, Puerto Rico, and Alaska. For the rest of the country, not much relief is expected outside the most severely affected areas in extreme [D3] drought across the Upper Midwest. Heat and a lackluster Monsoon should keep drought intact along the southern tier of the country from the Southern Rockies through the central Gulf Coast Region, with some development expected in parts of the Four Corners Region, across central and northeastern Texas, and in northern Louisiana. Drought is also expected to persist and locally worsen from the Upper Mississippi Valley westward through the Pacific Northwest, with some expansion in the latter region, where the wet season beginning in late October is expected to get off to a slow start. Also, drought is expected to persist and develop across Hawaii, with all but central and eastern Kauai expected to be in moderate drought [D1 or worse] by the end of autumn.

Looking out Four Seasons.

Twelve Temperature Maps. These are overlapping three-month maps (larger versions of these and other maps can be accessed HERE)

The 12 temperature maps issued last month.

Maps tell a story but to really understand what is going you need to read the discussion. I combine the 30-day discussion with the long-term discussion and rearrange it a bit and add a few additional titles (where they are not all caps the titles are my additions). Readers may also wish to take a look at the article we published last week on the NOAA ENSO forecast. That can be accessed here.

I will use bold type to highlight some things that are especially important. My comments, if any, are enclosed in brackets [ ].

CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS

Oceanic and atmospheric observations across the equatorial Pacific reflect continuing El Niño conditions. The latest observed weekly SSTs indicate positive anomalies across the equatorial Pacific with anomalies exceeding +1 degree C near and east of the Date Line and exceeding +2 degrees C in the East Pacific. Equatorial upper-ocean heat anomalies (averaged between 180 to 100 °W and 0-300m depth) remain strongly positive but have shown some recent weakening at depths of more than 100 meters in the east-central Pacific. Outgoing longwave radiation (OLR) anomalies are negative near the Date Line, indicating enhanced convective precipitation, consistent with El Niño conditions. Low-level wind anomalies at the 850-hPa level were westerly over the western equatorial Pacific and easterly over the eastern Pacific, while upper-level wind anomalies at the 200-hPa level were westerly over the eastern tropical Pacific.

Strongly positive SST anomalies are present near the northern coast of Alaska. SST anomalies remain strongly positive along the Gulf Coast and the Southeast Atlantic coast as well, while positive SST anomalies near the Northeast Atlantic coast have decreased in the last two weeks.

PROGNOSTIC DISCUSSION OF SST FORECASTS

The CPC SST Consolidation for the Niño 3.4 region indicates an increase in the magnitude of positive anomalies through autumn into early winter, with a peak median anomaly exceeding +1 degree C through the DJF winter season. The median North American Multi-Model Ensemble (NMME) consolidation forecast exceeds +1.5 degrees C at its peak in late autumn and early winter before decreasing at longer leads with greater uncertainty. The CPC consensus ENSO outlook indicates greater than a 90 percent chance that El Niño will continue through winter, with greater than a 60 percent chance that the strength of the Niño 3.4 anomaly will exceed +1.5 degrees C at its peak.

30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2023

The September 2023 outlooks are issued against the backdrop of an ongoing El Niño signal. The latest Niño 3.4 weekly sea surface temperature (SST) departures stood at +1.2 degrees Celsius and positive subsurface temperature anomalies dominate the equatorial Pacific Ocean. Meanwhile, the magnitude of the Real-time Multivariate (RMM) based Madden Julian Oscillation (MJO) index has strengthened during early August, with the amplitude of the signal rising above the RMM unit circle. However, there is little evidence of eastward propagation at this time which is reflective of disorganized intraseasonal activity. The MJO is generally favored to weaken during the next several weeks with a possible re-emergence later in August over the western Pacific or western Hemisphere. Farther to the north, across the extratropics of the Northern Hemisphere, both the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) have generally been negative during early August while the Pacific North America (PNA) index has exhibited a positive signal in recent days. Combined natural analog composites derived from the recent evolution of equatorial Pacific, MJO, and extratropical indices depict a 500-hPa flow pattern dominated by below normal heights across the Bering Sea and western Alaska as well as over the northeastern Pacific off the coasts of Southeast Alaska and the Pacific Northwest. Anomalous ridging is generally favored across the western and north-central Contiguous United States (CONUS). Farther to the east, natural analog composites depict near normal heights across the Southern Plains, Southeast, and northward to much of the Great Lakes and Northeast. Meanwhile, trends  during the last 15 years favor above normal heights across most of the western and northern two-thirds of the CONUS as well as the Aleutians and parts of eastern mainland Alaska during the month of September, with the strongest positive anomalies over the north-central CONUS. Natural analog composites, trends , and dynamical guidance from the Copernicus Climate Change Service (C3S), North American Multi-Model Ensemble (NMME), the Couple Forecast System Version 2 (CFSv2), and the Forecast Consolidation Tool (CON) (which includes both dynamical model input and statistical guidance) form the basis of the September outlooks. The latest CPC Weeks 3-4 Outlooks and associated ensemble guidance from the European Centre for Medium Range Weather Forecasts (ECMWF), Global Ensemble Forecast System (GEFS), and the CFS also contributed. Antecedent conditions such as extratropical SSTs, sea ice extent, and soil moisture conditions were also considered where appropriate.

Temperature

Above normal temperatures are favored for most of the western CONUS, consistent with anomalous ridging. The strongest probabilities (greater than 50 percent) of above normal temperatures are indicated near the Four Corners region, where dynamical and statistical guidance show the strongest agreement. However, probabilities of warmer than normal conditions are tempered across much of the northwestern CONUS, due to cold signals  depicted by the ECMWF and CFS ensembles for the weeks 3-4 period, which includes most of the first half of September. Farther to the south and east, enhanced above normal temperature probabilities are indicated across the Southern Plains, Lower Mississippi Valley, and parts of the Southeast, due to good agreement among dynamical model guidance. Probabilities of above normal temperatures exceed 50 percent for parts of the Gulf Coast region, where SSTs in adjacent waters are much warmer than normal. Equal chances (EC) of above, near, and below normal temperatures are indicated for the remainder of the eastern CONUS due to weak signals  exhibited by NMME and CFSv2 output as well as due to conflicting statistical guidance. Above normal temperatures are favored for southern, eastern, and northern Alaska, consistent with the consensus of dynamical model guidance. Above normal SSTs also contributed to increased above normal temperature probabilities along the South Coast and North Slope of Alaska. However, EC is indicated for much of western mainland Alaska due to conflicting signals  between generally warmer dynamical model guidance and colder natural analogs  as well as the presence of below normal SSTs adjacent to southwestern mainland Alaska.

Precipitation

Above normal precipitation is favored across the Southeast northward to the Mid-Atlantic and westward to parts of the Tennessee and Ohio Valleys due to good agreement among the CON, El Niño Southern Oscillation (ENSO)-based natural analogs, and the latest CPC Weeks 3-4 Outlooks. Conversely, below normal precipitation is favored farther to the north and west for the western Great Lakes and Upper Mississippi Valley as the CON, CFSv2, and natural analog composites tilt dry across these regions. Latest weeks 3-4 guidance from the ECMWF and GEFS depict a wet signal across the Baja Peninsula and the Gulf of California, suggestive of potential tropical cyclone (TC) activity. This potential TC activity coupled with a wet signal depicted by the CON leads to a slight tilt toward above normal precipitation for parts of the southwestern CONUS. Farther to the north, above normal precipitation is favored for western Alaska due to good agreement between NMME and CON guidance. A weak tilt toward below normal precipitation is indicated for Southeast Alaska due to monthly guidance from the CON, CFSv2, and weeks 3-4 guidance from the ECMWF and GEFS.

SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS (focus on September, October and November)

Temperature

The September-October-November (SON) 2023 temperature outlook favors above-normal seasonal mean temperatures across a majority of the U.S., including all of Alaska, the western CONUS, the Southeast, and the Northeast. The highest probabilities for above-normal temperatures exceed 50 percent for parts of northern Alaska, the Southwest, and the Northeast.

Precipitation

The SON precipitation outlook favors above-normal precipitation for western and northern Alaska and a large area of the southeastern CONUS from Florida to parts of the Mid-Atlantic region. Below-normal precipitation is favored for the Pacific Northwest, the western Great Lakes region, and a small area of the Southwest, including southeastern Arizona, western New Mexico, and a small part of West Texas.

ENSO

El Niño conditions are present and equatorial sea surface temperatures (SSTs) are above average across the central and eastern equatorial Pacific Ocean. El Niño is expected to strengthen and persist through the winter of 2023-24.

BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS

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

The seasonal outlooks relied on dynamical model guidance from the NMME, including hybrid statistical-dynamical guidance from the Calibration, Bridging, and Merging (CBaM) post-processing of the NMME, and a version of the NMME that post-processes and weights the models using Probability Anomaly Correlation (PAC). Statistical tools are also used, such as Constructed Analog, Canonical Correlation Analysis (CCA), and a regression forecast based on the predicted median Niño 3.4 SST anomaly from the CPC SST forecast consolidation with the addition of Optimum Climate Normals (OCN) to represent decadal trends . A final consolidation tool combines the consolidation of NMME model forecasts with a consolidation of statistical tools.

El Niño temperature and precipitation correlations to surface temperatures and precipitation were relied upon from SON 2023 through FMA 2024. Decadal trends in temperature and precipitation were a factor at all time leads but relied upon the most at the longer leads from MAM 2024 onward, when signals  due to El Niño decrease. Current soil moisture conditions were a consideration in the upper Midwest and Southern Plains in the SON temperature outlook.

PROGNOSTIC DISCUSSION OF OUTLOOKS – SON 2023 TO SON 2024

TEMPERATURE

Above-normal temperatures are favored across Alaska and the western CONUS in the SON and OND 2024 temperature outlooks, consistent with dynamical and statistical model guidance and the consolidation of both. The greatest probabilities exceed 50 percent for northern Alaska and parts of the Southwest, due to decadal trends . Above-normal temperatures are also favored eastward across the southern Contiguous United States (CONUS) into the Southeast and northward through the eastern CONUS into the eastern Great Lakes region, consistent with the consolidation of tools. Decadal trends  lead to higher probabilities of above-normal temperatures over the Northeast. Potential El Niño impacts for the SON and OND seasons include cooler temperatures over much of the central CONUS, leading to weaker probabilities for above-normal temperatures in tools that explicitly consider ENSO, such as CBaM. A forecast of Equal Chances (EC) of above-, near-, and below-normal temperatures is indicated over this region for the first two leads.

From the NDJ 2023-24 to the FMA 2024 seasons, as the leads extend through autumn into winter, increasing probabilities of above-normal temperatures are forecast from the Pacific Northwest across the northern tier of the CONUS into the Midwest and Northeast, consistent with increasing impacts of a strengthening El Niño. Equal chances of above, near, and below normal temperatures (EC) is forecast across the southern tier of the CONUS with a tendency towards cooler temperatures arising from El Niño impacts. For MAM 2024 and longer leads through SON 2024, the outlooks relied heavily on the ENSO-OCN tool, and decadal trends  provide most of the available climate signals . The seasonal temperature outlooks evolve toward a similar pattern at the longest leads (MAM 2024 through SON 2024), and the forecasts generally depict a u-shaped pattern of enhanced probabilities of above-normal temperatures over the CONUS, and EC in the northern central CONUS into the central Mississippi valley.

PRECIPITATION

During SON 2023, above-normal precipitation is favored for western and northern Alaska and the southeastern CONUS, while below-normal precipitation is favored for the Pacific Northwest, a small area of the Southwest, and the western Great Lakes region. Enhanced probabilities of above-normal precipitation for parts of Alaska are supported by the consolidation of dynamical and statistical tools. Below-normal precipitation for the Pacific Northwest and Great Lakes and above-normal precipitation for the Southeast are consistent with El Niño impacts, and largely persist through the next several leads. Below-normal precipitation for parts of the Southwest in the SON season is consistent with the persistence of a weaker than normal Southwest monsoon, while the outlook for the month of September favors above-normal precipitation in areas of southern California, Nevada and Arizona to the west due to shorter term moisture flow from the East Pacific.

Below-normal precipitation is forecast across much of the northern tier of the CONUS from parts of the Pacific Northwest to the Great Lakes region from NDJ 2023-24 to FMA 2024, due to the expected strengthening El Niño. The enhanced probabilities of above-normal precipitation for the Southeast increase and extend westward incrementally across the Southern Plains into the Southwest for these leads, also consistent with a strengthening El Niño. For the MAM 2024 season and longer leads, areas of above- and below-normal precipitation coincide with signals  due to decadal trends , including enhanced probabilities for below-normal precipitation in areas of the northwestern CONUS and enhanced probabilities for above-normal precipitation in the lower Mississippi Valley and Mid-Atlantic region.

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