OVERVIEW:  April temperature projections were maintained this week for a slightly above average (+2.0°F to +4.0°F) temperature departure for the month as a whole, with solidly above average temperatures thus far that will only be offset somewhat with some cooler risks at times over the remainder of the month. The month of May projections were reinforced this week for a below average (-3.0°F to -5.0°F) temperature departure for the month as a whole using long term climate guidance and global observation trends.

• Another major sudden stratospheric warming event (SSWE) took place earlier in April, 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 in March, this zonal wind reversal is forecast to have sustainability, playing a critical role in maintaining cooler temperature anomaly risks late April and throughout the month of May…and perhaps June. This is one of the reasons why the end of April and May (as a whole) are 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 well into the 60s and even 70s in the 2nd week of May. Late-season SSWEs are not particularly uncommon, but the cooler temperatures relative to average is a result of high latitude blocking (-NAO) that consolidates over Greenland presently.
  
  
• The Madden-Julian Oscillation (MJO) is currently in phase 1, and we expect a movement into phase 2 through early next week before collapsing into the Circle of Death (COD) or NULL phase. The propagation through phases 8 (previously) as well as phases 1 and 2 will contribute to cooler risks from through most, if not the entirety of 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 +0.9°C, and Niño 1+2 region (near the Peruvian coast) is at +1.9°C as of April 22nd. 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 65-68°F range from north to south across our coverage region, which is about 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 in the May 1st-4th period features highs in the upper 50s to low 60s, coming at a time when average highs are in the upper 60s to near 70 degrees. Use the average highs/lows chart below providing a point of reference and transparency for what average temperatures are expected to be at each corresponding location through June 15th.

April temperature projections were maintained this week for a slightly above average (+2.0°F to +4.0°F) temperature departure for the month as a whole, with solidly above average temps thus far, balancing cooler risks at times over the remainder of April. The month of May projections were also reinforced this week for a below average (-3.0°F to -5.0°F) temperature departure for the month as a whole using long term climate guidance and global observation trends.