High latitude blocking remains very stout on most forecast model guidance through the end of the month of April. The proverbial paint from the stratospheric warming event back in February is certainly still dripping, as the stratospheric/tropospheric coupling has led to dramatic effects. This is almost a 30+ day period of higher than normal heights in the Atlantic higher latitude on rolling averages, obviously with the climax in March when the NAO bottomed out near -5 SD.
Nevertheless, we can use the pattern evolution now and roll forward some analogs over the next few weeks to at least get an idea as to how these things have progressed in the past. I am most interested in 1962 and 1958 as a whole, as they were both analogs heading into DJF and remain relatively strong analogs now - with the ENSO state, Pacific evolution, and higher latitude pattern. Both of these months featured an expansion of ridge heights in the Four Corners during the month of May, both both also featured "opportunities" for severe weather opening up from the tail end of May into early June. 1962 I believe had an event of significance near Memorial Day.
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As we all know, analogs are only a small piece of the puzzle. We can use them to get an idea how the patterns with a similar atmospheric base state have progressed in the past, but they don't offer much in terms of definitive long term forecasting ideas. We have to piece together the forecast based on observations from the past week or two and move them forward using the analogs as a tool. When we consider this, I will be keeping an eye out for a pattern "shift" that allows for more ridging to develop in the Intermountain West during May. While such a development would be a thorn in storm chasers sides, it is the transition period that will be of interest to me.
I wanted to take a moment also to talk about the upcoming severe weather event anticipated around Day 3 -- the MJO is smack in the middle of Phase 2. Matches up quite nicely with the Roundy research that has been floating around. If you haven't read that paper, I strongly recommend it. There is some astounding research available that connects larger outbreaks and tornado activity to the MJO Phase 2 RMM.
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Anyway, I digress. Over the next few weeks, it will be critical to monitor the state of the Tropical Pacific and higher latitudes. I think the overall potential for widespread severe weather activity will remain low. Warm sectors should be modulated by deeper troughs which will be persistent across the Eastern and Northeastern United States, often even progressing into the Ohio Valley. The NPO Ensemble forecasts suggest a brief jet retraction centered around Days 7-10. This will be juxtaposed with MJO weakening and stalling in Phases 2/3 so there is some support for Western USA troughing anomalies during this time.
Still, as evidenced by the above graphic, even the jet retraction itself should be short lived - and the troughing to the Northeast should limit moisture return and warm sector breadth. In the longer term, it will be important to watch how the overall AAM progresses. I suspect we will see some pull back in high latitude blocking by 5/5, which should open the door for more plentiful severe weather opportunities by the second and third week of May.
That seems to be the pattern of late as "DD" drought desperation sets in. I still have faith the RH will work north and west when things settle down and there will be storms from the usual mid-to-late season suspects: disturbances moving off the Rockies, NW flow, etc. None of the models I have looked at lately show any signs of an Omega block or "death ridge" so it's not over yet. Once the moisture is in place, anything can happen.
