Cool risks will return to Central US, September coming into view

Good afternoon! We hope we caught you in the middle of a great Thursday afternoon. To piggyback a bit on our overnight and early morning thoughts, today’s midday update will focus mostly on the evolving pattern across the Central and Eastern states over the next 5 weeks — much of which has come into better view over the past 12 to 24 hours. The weather pattern, largely, has been defined by an Eastern Pacific ridge and high latitude blocking anomaly over the past few weeks, which has led to cooler temperatures and troughing in the Central States and much of the AG Belt.

This pattern has lagged into this week, as a storm system developing over the Central Plains states again drags cold air into the Northern 1/3 of the USA and eventually the Ohio and Mississippi River Valley. Minneapolis, in fact, has seen temperatures below normal averages in 13 of the last 14 days! The well-established pattern is finally forecast to become more transient, as a ridge retrogrades from the Eastern Pacific westward into the Central Pacific.

This weekend, the retrograding ridging will allow for heights in the atmosphere’s mid levels to rise across the Central United States, including a large affected area of the agriculture regions. As these heights in the atmosphere’s mid levels rise, a warming and moderating trend will translate to the surface. Temperatures are expected to rebound towards average or slightly above average for this time of year across a large majority of the mentioned areas.

aug17_agpost_1

Lingering to the north, however, is a persistent and notable feature which will ultimately lead to the demise of this moderation: High latitude blocking. Notice, even in the image above, that despite the retrograding ridge in the Pacific and moderation across the United States, that a large blocking ridge is still lingering in the Arctic and Northern Canada. This ridge will continue to meander until sinking southward and strengthening by the middle of next week.

As the ridging anomaly over the Arctic sinks southward into Canada, tropical forcing and a persistently negative AAM lag will continue to support the re-development of ridging over the Western USA Coast. This will “funnel” cooler than normal air back into the Central, and eventually Eastern United States as we move into the latter days of August and eventually into the early days of September.

Forecast models and ensembles, particularly the GEFS and ECMWF, have converged on a solution that supports this evolution over the past few days. While ensemble muting occurs at this range, we agree with the overall thoughts given the support from teleconnections and tropical forcing evolution. This period may feature highly anomalous temperatures /cooler than normal/ over the Plains States (through 9/5).

aug17_agpost_2

Thereafter, the weather pattern looks likely to re-adjust. Analog years with similar ENSO, tropical pacific, and high latitude/hemispheric conditions support the development of ridging in the Eastern United States gradually throughout the month of September. In fact, these very same analog years support temperature anomalies averaging above normal for September as a whole for a large portion of the country east of the Mississippi River.

While we are still a ways out from getting reliable forecast model or ensemble data for that time period, stay tuned over the next few days as our team of forecasters continues to release information regarding our analogs and supported temperature and precipitation anomaly outlook for the month of September.

Have a great Thursday afternoon!

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