Long Range: Cooler, active pattern ahead for Central US

Far away from the United States, in the equatorial Pacific, a propagating MJO has some ideas as to how the weather pattern will evolve through early October. Okay, maybe that’s not exactly how it works. But the development of an MJO wave will, in fact, aid in the patterns progression over the United States through the end of September. The instra-seasonal traveling pattern of convection more affectionately known as the MJO (Madden Julian Oscillation) has significant impacts on our weather, and this occurrence will be no different. You can read more about the MJO right here.

While the MJO has been stuck in a period of inactivity over the past several weeks, the overall hemispheric pattern has fallen into a bit of a lull. Wash, rinse, repeat has been the overall terminology used. Occasional cold fronts and troughs have brought periods of below normal air to the Central United States while, by and large, the East Coast has remained at the mercy of a large Southeast US and Western Atlantic ridge.

ECMWF model forecasting the MJO to move toward a higher amplitude in Phase 4 over the next few days.

ECMWF model forecasting the MJO to move toward a higher amplitude in Phase 4 over the next few days.

As the MJO strengthens, however briefly, this weekend and moves toward a higher amplitude in Phase 4, a sequence of events will transpire in the hemisphere which will change the pattern over the Central United States. Most notably, ridging on the West Coast of the United States will give way to heightened activity in the northern jet stream coming out of the Pacific. These disturbances are expected to drop east/southeastward toward the Plains States.

Troughs are expected to become sequentially stronger and deeper in the Plains states and eventually Mississippi Valley over the next 10-14 days. Historical analysis of the MJO analogs, within Phase 4 and during Neutral ENSO conditions, reveal a starkly similar pattern to the one which is being advertised on medium and long range ensemble guidance.

Below normal temperature anomalies are likely to become established over the Plains states first, and the presence of increased troughiness will lead to rising precipitation anomalies as well. Troughiness is eventually expected to slip eastward toward the Great Lakes with time, including the Ohio and Mississippi Valley’s by the end of September into early October.

MJO Analogs in Phase 4 during the month of September show troughing and below normal temperatures over the Central US.

MJO Analogs in Phase 4 during the month of September show troughing and below normal temperatures over the Central US.

Much of how things evolve from that point forward will depend on the variability and positioning of the Southeast Ridge. Precipitation may also be dispersed closer to a gradient which sets up between the aforementioned troughs and Southeast Ridge over the Eastern US. Regardless, the main focus of below normal temperatures and troughiness is expected to remain west of the Mississippi River through the end of the month.

Our latest 15 and 30 day outlooks, updated this afternoon, reflect this anticipated pattern evolution. Active times are ahead for the Plains, Mississippi Valley, Corn Belt/Ohio Valley — and potentially East Coast as we move into October.

Our 15 and 30 day outlooks are available on the Long Range Dashboard, which is available to Long Range Outlook subscribers and clients. If you are interested in access, send us a message using the contact form here and mention the Long Range outlooks!