PM AG Update: After Moderation, Cool Risk Returns

The retrogression in the Pacific has long been advertised, and it will certainly help a good chunk of the US modify in week 2, and into a decent chunk of the week 3 period as well, as a large ridge builds across much of the US. Now that we have this idea figured out with a decent amount of confidence, it becomes a bit easier to roll forward with expectations in the extended period.

While there will be a lot of modification taking place, the orientation of the retrogression will allow a large piece of a trough to break off in Canada and slide eastward towards the Great Lakes and the Northeast. Thus, despite the overall progression towards modification, there will be a decent shot of cool air for much of the Eastern third of the US to begin week 2, and a potential for a lot of rain to be focused in the Northern Plains, the Great Lakes, and parts of the Northeast in the midst of this trough’s arrival.

Today’s GEFS shows a retrograding Pacific ridge “splitting” a trough in NW Canada, allowing a large chunk of it to go eastward. This has implications moving forward (Tropical Tidbits).

Thus, while we certainly see a lot of modification in the 500mb anomalies for the most part during the above period downstream of an NE Pacific trough (from the retrogression way west in the Pacific) and downstream of this splitting trough, some additional retrogression in the US will allow a bit more amplification to the pattern. This results in a ridge in the Southeast US — as it retrogrades –“merging” with a typical west to east moving ridge in the Pacific Northwest, which further amplifies this ridge. This allows the downstream trough — and the one responsible for a lot of precipitation in the Plains through the Great Lakes and the Northeast — to become more amplified as well, sending a legitimately cool shot of air to these areas.

This is very important to the extended forecast because it already sets the stage for an initially amplified trough –> ridge –> trough pattern on around August 24th. It’s from just off the Pacific Northwest Coast (trough) to the Northern Plains and Central Canada (ridge) through the eastern US (aforementioned big trough). If this does indeed become the initial equilibrium pattern, then when combined with the continued Pacific retrogression discussed yesterday, the ridge in the Central US will have to eventually gradually retrograde westward as well. It may take a few days, but it is evidence that towards the very end of the month, we may be shifting back towards a +PNA pattern (ridge in the Western US) and troughing downstream.

Today’s GEFS in the longer range shows a Central US ridge retrograding westward to being a PNA ridge. This will help cooler risks return for most of the Eastern two thirds of the US (Tropical Tidbits).

This retrograding PNA ridge is further supported by the fact that the models all summer have been too far east with this ridge in the medium range. This is because the models have had trouble resolving the Tropical Forcing in the Pacific, as what was initially interpreted as an El Nino pattern has instead typically resulted in stronger trade winds and thus a more La Nina look.

Speaking of these trade winds and La Nina forcing, one oscillation we like to use is the AAM (Atmospheric Angular Momentum). It represents where Tropical Forcing — activity of showers and thunderstorms — is located, as this can have a big feedback effect on where ridges and troughs are typically located, and is a great indicator of whether we are in an El Nino or La Nina-like state.

The Atmospheric Angular Momentum forecast strongly points towards La Nina conditions taking place (Nicholas Schilardi/Albany).

Without getting too technical, a low AAM means that La Nina conditions are present, and a higher AAM means that an El Nino is present. The above image shows that the GEFS is forecasting a tanking AAM — one that strongly resembles La Nina conditions. A tanking AAM is strongly correlated to have a 2-3 week or longer lag period towards favoring a ridge in the Western US and a trough in the Eastern US during the late Summer and Fall months. When this is combined with the already retrograding Pacific pattern and the trough in the East around August 24th, the ridge in the West may become quite amplified. Wavelengths this time of year are still somewhat short, so the core of the chillier air may end up residing in the Central US — but the theme of below average temperatures in the Eastern two thirds of the US in the extended period is certainly there.

As far as precipitation is concerned, we still favor a strong wet signal in the Northern Plains through the Great Lakes and Northeast in the August 21st through 24th period during this pattern’s transition. Additionally, as the ridge initially in the Southeast retrogrades towards the Corn Belt, some return southerly flow from the Gulf and a strong temperature gradient could result in multiple precipitation events around that same time period. Afterward, once the trough –> ridge –> trough pattern gets established and then further retrogrades, much of the Corn Belt will be on the downstream ridge of the ridge, and thus it may dry out.