U.S. Summer 2021 Outlook/Climate Discussion

2021 North Atlantic Tropical Cyclone Season Forecast
04/07/2021, 7:49 am EDT
Cool Phase of the Pacific Decadal Oscillation Strengthens
04/16/2021, 7:51 am EDT
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Highlight: Another hot summer, drought advances into the Great Plains. East/Southeast States are wet.

Executive Summary: The Climate Impact Company Summer 2021 climate forecast is updated. The FINAL version will be issued in early May. The forecast is based on a regional SSTA/U.S. soil moisture-based constructed analog. The outlook is very similar to previous outlooks projecting a hotter than normal summer season for the Southwest to Central and Northeast U.S. The Great Plains gain a dry climate pattern and widening drought is increasingly likely. The wet zone is across the East and Southeast U.S. Land-falling hurricanes contribute to the wet forecast in September.

Fig. 1-2: Climate Impact Company constructed analog temperature and precipitation forecast for JUN/JUL/AUG 2021.

Methodology: The Climate Impact Company summer 2021 forecast is based on a constructed analog considering current and forecast regional SSTA across the Pacific and Atlantic Ocean(s) and the current and projected U.S. soil moisture pattern(s). A constructed analog means selecting past years with similar conditions and weighting each year according to agreement between the SSTA patterns and soil moisture conditions. The SSTA patterns considered is El Nino southern oscillation (ENSO), Pacific decadal oscillation (PDO) and Atlantic multi-decadal oscillation (AMO). Also considered due to their unique influence on modern-day climate is the “warm blob” in the Northeast Pacific and the North Atlantic Warm Hole (NAWH) south and southeast of Greenland.

Climate discussion: La Nina 2020-21 is dissipating. Currently, a moderate-strength Madden Julian oscillation (MJO) episode is emerging in the equatorial central Pacific Ocean. Presence of this convective feature is associated with strong negative southern oscillation index (-SOI). Trade winds will ease in the eastern equatorial Pacific and subsurface waters will continue to warm through late April causing surface temperatures to (also) warm. The current borderline cool SSTA pattern in the eastern equatorial Pacific erodes and by late month La Nina has dissipated. The La Nina climate (based on multivariate ENSO index) may linger through May before dissipating.

There is some question as to whether La Nina returns later this year. To best illustrate the 2021 ENSO forecast a depiction of all statistical and dynamic models provided by the International Research Institute for Climate and Society displaying varying Nino34 SSTA forecasts is presented (Fig. 3). The outlook through JUL/AUG/SEP is reasonably confident of neutral ENSO conditions. However, the last third of the year forecast is clearly a low confidence issue. There is historical precedent in the 30-year climatology for moderate-strength La Nina as observed the past 6 months to regenerate. However, given the general very warm global ocean surface, regenerating La Nina is made more difficult.

Fig. 3: A collection of all statistical and dynamic model forecasts of the Nin34 SSTA to project ENSO phase in 2021. The chart is provided by the International Research Institute for Climate and Society.

The International Multi Model Ensemble (IMME) best initiates April global SSTA and is the model of choice to drive the constructed analog forecast. Currently, the IMME identifies a lingering La Nina and is a little strong with that depiction (Fig. 4). However, other regional SSTA important to the climate forecast are well-represented including a cool phase Pacific decadal oscillation and warm phase Atlantic Multi-decadal oscillation. Both the -PDO and +AMO regimes are weak. The -PDO regime is held back by the ever-present Northeast Pacific “warm blob” which is currently displaced farther west while the +AMO pattern is held back by neutral SSTA in the North Atlantic tropics. The cool pool of SSTA southeast of Greenland known as the North Atlantic Warm Hole remains semi-permanent.

The IMME global SSTA forecast for August 2021 indicates ENSO shifts to neutral phase while weak -PDO lingers and the +AMO pattern is a little stronger (Fig. 5). The Northeast Pacific “warm blob” strengthens but avoids surging toward the Canadian West Coast. The NAWH continues to persist. The SSTA component of the constructed analog climate forecast is based on the IMME projections.

Fig. 4-5: The current and August 2021 global SSTA forecast by the International Multi-Model Ensemble (IMME) with regions of influence on North America climate highlighted.

Large regions of dry soils have a tendency to increase anomalous heat risk during the summer season. Conversely, large areas of soil moisture excess suppress summertime heat risk. Currently, drought conditions are overwhelming the U.S. as the warm season approaches. Harsh drought continues across California, the Southwest U.S. and into Texas (Fig. 6). However, new drought areas have emerged stretched across the Northern U.S. The NOAA/CPC drought probability forecast for mid-summer indicates drought conditions are expansive and cover all of the U.S. except the central Great Plains, Missouri Valley and into the Mid-Atlantic States plus western Washington (Fig. 7). Climate Impact Company suspects the aerial coverage and intensity of drought forecast by NOAA/CPC is too conservative. However, the NOAA/CPC July soil moisture probability forecast is the soil moisture component of the constructed analog climate forecast.

Fig. 6-7: Current U.S. soil moisture anomalies identifying drought and excessive soil moisture regions (left) and NOAA/CPC drought projection for July 2021 (right).