AM AG Update: Impacts of a retrograding Pacific pattern

Happy Monday, folks! We hope our morning update finds you well. We’re going to jump straight in to some developments which we communicated over the weekend, and take a bit of a deeper look at their evolution and impacts. About 5 days ago, we discussed at length the pattern which currently existed across the Northern Hemisphere — including why it had become so stagnant. We also discussed the pieces behind the potential for a change during the latter half of the month of August, namely the pattern in the Pacific Ocean. As we move towards the middle of the month,  medium range forecast ensembles are starting to pick up on these gradual changes as well.

Much of what we have been discussing emanates from the Pacific Ocean which, as you will discover over time, is a major pattern driver for the weather here in the United States. In fact, one could argue that it is the single most important feature (amongst many) in determining how the weather will behave in our neighborhood just several days down the road. Accordingly, our team of forecasters pays close attention to the pattern in the Pacific Ocean with regularity, and our eyes remain peeled for possible changes. While many other factors can impact the weather in the United States, changes in the Pacific can often be the first signal for us to pay attention.

Over the past two to three weeks, the pattern in the Pacific Ocean has been stagnant to say the least. In fact, elements of this pattern have been apparent since the middle of the month of July. Namely, tropical forcing in the Pacific has changed to support a ridge building from the Central and Eastern Pacific into parts of the Western United States, while high latitude blocking has formed over Central Canada and parts of the Arctic.

The result has been troughing, with cooler than normal air moving southward into the Plains states, and gradually seeping eastwards into the Mississippi River and Ohio River Valleys. In addition to the cooler than normal temperatures, the troughing has brought about a very active weather pattern, with above normal precipitation the aforementioned regions. This is fairly typical for patterns that feature lower anomalies in the mid levels of the atmosphere. While not everyone has seen a ton of rain, the large-scale pattern has been extremely recognizable and undeniable.

But what can cause these patterns to change? Without getting too technical, adjustments in tropical forcing (in the tropical pacific ocean, several thousand miles to the southwest of the USA) can cause large-scale adjustments of major weather pattern features. This is exactly what is being signaled by medium range ensemble guidance during the 5 to 10 day period. In a post last week we discussed the week of August 21st as one that may feature a pattern transition, and that still looks on track. But what exactly is occurring?

When our team analyzes the pattern in the Pacific Ocean on medium range ensemble guidance, we see a clear signal for retrogression, or movement of features from east to west ( <— ) instead of the typical west to east ( —> ). These retrogressions of major features can cause significant changes to the upstream weather pattern. Both the ECMWF and GEFS ensemble guidance suggest the retrogression of a ridge from the Eastern Pacific into the Central Pacific by the last week of August.

While the results remain somewhat uncertain, the retrogression of this ridge is very likely to allow temperatures to moderate from the Plains into the Mississippi and Ohio Valleys. While the pattern may remain active in terms of precipitation, a rather notable warmer adjustment in temperature compared to seasonable normal appears likely during this time, as heights build higher over the Central United States.

The retrogression of this ridge could also potentially open the door for some tropical weather impacts in the Eastern and Southeastern United States. But that remains too far out to discuss in detail at this time. Stay tuned for more over the coming days as we start to take a deeper dive into the numbers as the pattern change evolves.