OVERVIEW:  April temperature projections were slightly adjusted this week to 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 final two weeks of the month. The month of May projections were maintained this week for a slightly below average (-2.0°F to -4.0°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 last week, 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. 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.

   

• The Madden-Julian Oscillation (MJO) is currently in phase 8, and we expect a movement into phase 1 early next week and maintaining there through the weekend. After that point, a movement into phase 2 is possible for a few days before collapsing into the Circle of Death (COD) or NULL phase. The propagation through phases 8, 1, and 2 will contribute to high-latitude blocking and  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 ENSO neutral territory currently. From west to east across the Pacific, the Niño 4 (west-central Pacific) is currently +0.9°C, whereas the Niño 3.4 region (east-central Pacific) is at +0.4°C. The eastern regions of Niño 3 (east-central Pacific) is currently +0.5°C, and Niño 1+2 region (near the Peruvian coast) is at +1.6°C as of April 15th. The threshold for El Niño begins at +0.5°C as a composite average of the ENSO regions, and that is expected to emerge as early as May. An El Niño Summer and Fall is expected to follow.

   

Average high temperatures are currently in the 62-66°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 slightly below average temperatures listed in the April 27th-May 2nd period features highs in the low to mid 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 1st.

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