Highlight: La Nina could form by summer. If so, watch out! Hot summer followed by dangerous hurricane season.
Executive Summary: The NMME, CFS V2 and GFDL global SSTA forecast models each develop La Nina by summertime. Coupled with much warmer-than-normal middle latitude SSTA in the North Pacific and North Atlantic forecasts there is potential for widespread anomalous heat and dryness this summer season in the U.S. coupled with a dangerous North Atlantic tropical cyclone season.
Fig. 1: The NMME global SSTA forecast indicates La Nina and a warm North Pacific/North Atlantic outlook for JUN/JUL/AUG 2020.
Fig. 2: The NCEP CFS V2 Nino34 SSTA forecast reveals La Nina by late summer.
Fig. 3-4: La Nina temperature and precipitation climatology for JUN/JUL/AUG across the U.S.
Fig. 5: When the North Pacific and North Atlantic are warmer than normal during summertime the risk of drought (in red) increases for much of the U.S.
Discussion: Global SSTA models are increasing the risk of La Nina forming for northern hemisphere summertime. The possible driver of this forecast philosophy change is the cooling of the northeast North Pacific during mid-winter attributed to a large cold trough in the atmosphere related to a vigorous –PNA pattern. The cooling waters are grabbed by the California Ocean Current and propelled south and southwest into the tropics to cool that zone over the next few months causing La Nina by summertime.
The NMME global SSTA forecast rewarms the North Pacific and North Atlantic during summertime. The climatology of summertime temperature is hotter than normal for most of the U.S. when La Nina is present. However, the anomalous heat and attendant dryness could be wider and more intense given the presence of the warm SSTA pattern to the west and east of the U.S. in the mid-latitude oceans.
Additionally, the combination of La Nina and a warmer than normal North Atlantic basin is the recipe for a dramatic hurricane season. The La Nina pattern lowers westerly shear in the upper levels of the atmosphere while warm waters provide the upper ocean heat for intensifying tropical cyclones.
The ECMWF and Australia Bureau of Meteorology SSTA forecasts maintain neutral ENSO. However, the NMME, CFS V2 and GFDL are trending stronger with the flip to La Nina.
Conclusions: The surprise idea of La Nina for mid-to-late 2020 first became noticed about 6 weeks ago with the GFDL forecast model. Currently, the NMME, CFS V2 and GFDL are each showing La Nina for JUN/JUL/AUG 2020. If La Nina forms while the mid-latitudes are warm the U.S. summer climate will be dry and hot! Additionally, a dangerous hurricane season is very likely to occur.