Long Range: Autumn cold blasts, southeast ridge

Meteorological Autumn technically began 9 days ago now. Although Hermine took much of the focus off its start and placed it on tropical weather, the first 9 days of the meteorological season have offered an ironically similar pattern to what had been observed over the last month or so of summer. Warmer temperatures have dominated in the East — especially the Mid Atlantic. Much of this owes to a Southeastern US ridge, which has built back continually over the past 2 months, even when forecast models have suggested otherwise.

Meanwhile, back west, cold fronts and disturbances have continued to drop southward through the Northern Great Plains and into parts of the Great Lakes and Mississippi and Ohio Valleys. By and large, however, the colder than normal temperature anomalies associated with these disturbances have remained west of the Mississippi River. Airmass modification has essentially nullified any below normal temperature anomalies as they move east of there.

Over the next few weeks, the meteorological pattern will move towards Autumn– while remaining within its same overall tendency envelope from the past few months. What exactly does this mean? In essence, the autumn evolution of the pattern will lead to stronger northern stream disturbances and longer wavelengths. This inherently will allow disturbances to dig farther south and with more vigor. Sequentially, we expect stronger disturbances to move through the Northern Plains and towards the Great Lakes and Ohio Valley.

However, we expect them to remain relatively short lived in nature — with no true signs for prolonged and/or maintained below normal temperatures. This will be especially true once one gets east of the Mississippi River and towards the East Coast. Here, significant airmass modification and resistance from the Southeast/Western Atlantic Ridge will provide very little room for sustained autumn air or below normal temperatures.

This airmass modification will be further enhanced by above normal temperatures in the Great Lakes. As air-masses move eastward from the Plains towards the Ohio Valley — especially those originating from Canada — they traverse the Great Lakes and undergo modification to their surroundings.

This year, surface temperatures in the Great Lakes are much above normal, and warmer than the past few years as well. As air masses move over the lakes, they will undergo significant warm modification during the next few months, or at least until the feedback process begins to cool down the waters.

All of this leads to a relatively straight-forward outlook over the next 15-30 days for the United States. A propensity for cold shots will lead to cooler than normal air over the Northern Plains and parts of the Corn Belt at times, but solid below normal anomalies will be hard to come by. Further east, above normal temperatures will remain the likely result of a substantial Southeast Ridge.

Above normal precipitation seems likely along the gradient of the aforementioned southeast ridge and incoming disturbances, especially in the Plains states.