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Don Paul: The warmth isn’t over till the fat ridge fades

There is plenty of cold, early autumn air to be found on the continental map; just not around here. Here is a midweek AccuWeather temperature forecast.

The cold air will increase after snow cover begins to expand this month. As of now, snow cover is confined to northern Canada and parts of northern Siberia, as is typical for early October. As you might guess, snow refrigerates the surface air and reflects solar energy back into space. More snow later in October means a larger supply of refrigerated air.

As of now, most of the eastern United States has a buffer in place from the continental polar air mass over central and parts of western Canada. A ridge of warmer high pressure is stacked up in the atmosphere, blocking much penetration of this arctic air to the southeast. That mountainous ridge is easily visible in this forecast for the upper air pattern on Oct. 5.

The wave, or trough, over the interior of the northwest United States is allowing colder air to drop into the northern Rockies, but the colder air is then shunted around the big ridge back to the northeast in Canada, skirting our region entirely.

Eventually, this pattern is going to flip, with a ridge beginning to build with its warmth over the western United States, and more troughing over the eastern United States, allowing colder air to reach the Great Lakes. A hint of this is seen in the recently upgraded Canadian model ensemble.

That kind of pattern, unless it's very amplified, will result in seasonably cool temperatures, such as shown on Oct. 16 in the American GFS model. Nothing extraordinarily cold, just cool.

One of the flies in the ointment for predictability is the great unknown impact from western Pacific typhoons which recurve to the east and northeast toward North America. In this spaghetti-like chart, you’ll see that is quite a common occurrence over time.

When strong, formerly tropical cyclone remnants approach our continent, they can pump up a ridge of high pressure over western North America, increasing the amplitude of the flow coming down from that ridge over the central and eastern United States. This type of change in amplitude is generally not well captured by most models, most times. So, the trend to cooler weather in the latter part of the month may end up being cold, rather than cool. That’s because we have had and will have typhoons in the western Pacific recurving to the eastern Pacific.The NWS Climate Prediction Center does seem to show some confidence in a chillier pattern three to four weeks from now.

However, before that western ridge/eastern trough eventually begin to take shape later in the month, quite the opposite will be occurring by early next week. The ridge in the east will strengthen and look almost like a summerlike pattern. Here is the European ensemble projection for Columbus Day Monday upper air.

There is good agreement on this trend in other ensembles, such as the Canadian.

In such a pattern, with the ridge pumped so far to the northeast, all the colder troughing is out in the interior of the west. In addition, the northeast ridge leaves a weakness near the northwest Caribbean and Gulf of Mexico, which may favor the development of a tropical cyclone. Octobers with warm high pressure to the northeast have sometimes favored late-season tropical development in the region to the south and southwest, so that will bear watching.

For us, though, here are projections of high temperatures on Columbus Day.

Temperatures in the upper 70s look likely, but any kind of a downslope/southerly component to the wind could send some readings into the low 80s. But you’ll have to settle for mere 60s on Sunday, game day. As of this writing, Sunday looks mainly dry.

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