OVERVIEW:  The month of May projections were maintained this week for a below average (-3.0°F to -5.0°F) temperature departure for the month as a whole, with the coolest periods relative to average occurring during the first half of the month. Projections for the month of June were introduced this week for a near to slightly below average (-0.0°F to -2.0°F) temperature departure, using long term climate guidance and global observation trends.

• Another major sudden stratospheric warming event (SSWE) took place last month, leading to a breakdown of the stratospheric polar vortex and notable warming over the polar regions. Stratospheric winds at 10 hPa reversed from westerly to easterly, which defines a major SSW event. Unlike past SSW events this season, this zonal wind reversal is forecast to have sustainability, playing a critical role in maintaining cooler temperature anomaly throughout the month of May… and perhaps lingering effects into June. This is one of the reasons why May (as a whole) is projected to be cooler than average. “Cooler” is of course relative, and the slightly below average temperatures are relative to normal highs at that time, which are getting into the 70s everywhere by the 2nd week of May. Late-season SSWEs are not particularly uncommon, can influence the pattern with cooler temperatures relative to average for several weeks. 

   

• The Madden-Julian Oscillation (MJO) is currently in phase 2 in the Indian Ocean, and we expect a movement into the Circle of Death (COD) or NULL phase from this weekend through at least mid-May. The GEFS-extended expects a re-emergence into phase 7 past mid-month, but does not have support from other guidance for that idea currently. This propagation will contribute to cooler risks through at least the first half May.

   

• A composite average of the SST departures in the ENSO regions places it in El Niño territory currently. From west to east across the Pacific, the Niño 4 (west-central Pacific) is currently +1.0°C, whereas the Niño 3.4 region (east-central Pacific) is at +0.8°C. The eastern regions of Niño 3 (east-central Pacific) is currently +0.8°C, and Niño 1+2 region (near the Peruvian coast) is at +1.1°C as of April 29th. The threshold for El Niño begins at +0.5°C as a composite average of the ENSO regions, and although not officially declared by the CPC, it is the expected transition, and an El Niño Summer and Fall will follow.

   

Average high temperatures are currently in the 68-70°F range from north to south across our coverage region, which is about 2-3°F higher than at this point last week. The rate of increase in average high and low temperatures has accelerated since the vernal equinox. Any above or below average stretches listed in the outlook will be relative to seasonal norms at that time, and it is important to note that cooler/warmer periods will be relative to the time of year we are talking about. For example, the below average temperatures listed for May 7th-8th features highs in the upper 50s to low 60s, coming at a time when average highs are on either side of 70 degrees. Use the average highs/lows chart below providing transparency for what average temperatures are expected to be at each corresponding location through June 15th.

May projections were maintained this week for a below average (-3.0°F to -5.0°F) temperature departure for the month as a whole, with the most notably below average temperatures occurring during the first half of the month. The projections for the month of June were introduced this week for a near to slightly below average (-0.0°F to -2.0°F) temperature departure, using long term climate guidance and global observation trends.