Better Predictability by Sean Sublette

We're getting to the end of astronomical summer, and the weather of the last couple of days certainly emphasizes that fact.

The end of summer also changes the scales of weather predictability. During the summer, the space and time scales of systems that affect the weather at the local level are generally smaller and shorter, respectively. Individual thunderstorms and sea breezes are good examples. As a result, predicting the weather at a certain point for several days in advance is more of a challenge. While general temperature patterns are still largely predictable, there is very little (if any) skill in attempting to forecast a specific thunderstorm at a specific spot 7-10 days in advance.

As cooler airmasses begin to dislodge from Canada early in the fall and make their way into the Lower 48, the primary drivers of weather become larger and tend to last longer. Cold fronts, non-tropical cyclones, and large high pressure areas are all examples. As a result, they can be simulated more easily in both conceptual and mathematical simulations (also known as models).

As a result, we can begin to say with more confidence how the weather may play out a couple of weeks in advance. Not perfectly, but we can get some general ideas. The one theme that seems to be recurring over the last couple of days is that there will be no more truly hot days the rest of this month. While we may have some days that are few degrees above normal, it seems unlikely we will be at or above 90 degrees the rest of the year. Normal high for middle September is 80 degrees and the record high this time of year is in the middle to upper 90s.

You may have heard the El Nio is coming back. El Nio is the abnormal warming of the central and eastern Pacific Ocean along the equator, and it is expected to return this winter. Much of that water is already about 1-2oF above average, and conventional wisdom is that the warming will continue.

The El Nio generally means an enhanced, more active southern (subtropical) jet stream into the southern United States. And while that often means a milder and wetter than average winter, it is not always the case. The winter of 2009-10 featured an El Nio, but cold air was repeatedly sent into the East Coast (due to the negative phase of the North Atlantic Oscillation), yielding much above average snow from the Carolinas to New England.

So, we feel it is a little early to say how the winter will play out. At this point, we feel pretty good that precipitation will be slightly above normal, but whether it is rain or snow is yet to be determined. Either way, it is good news for the water tables, and it would set us up for good planting conditions in the spring of 2013.