Monday, September 22, 2014

Long Range Lookout: Warm End To September, Chilly Start To October

Model guidance is hinting at the idea of a warm end to October, before a chilly October opener enters the scene.

The two-panel image above shows 500mb height anomalies over North America on the left, with the spread among individual ensemble members on the right. For the image on the left, depressions of contour lines, as well as normalized standard deviations in the shaded areas. On the left panel, darker shades of purple indicate higher and higher uncertainty among ensemble members for what will happen in a given area.

According to the graphic above, valid September 28th, a strong trough looks to be pushing into the Western US, forcing high pressure to build in the Central and East US. Warm and dry weather can be expected from this event, while the West may experience its first period of cold and stormy weather in some time. This ridging of high pressure over the Central and Eastern US should allow for warmer than normal conditions to persist through the end of the month.

By the first week of October, the story changes. The image above shows another ensemble "spread" forecast over the 500mb level, now comprised of multiple North American model guidance systems, and now valid for October 1st. In this forecast, we see 500mb contours collapsing south over the Plains and Rockies, as this trough tries to push east. The ridge we discussed earlier is now weakened and suppressed to the south, as it begins to lose control of the Central and Eastern US.

While all of this is playing out, there's a very interesting story playing out in the Pacific. If we take a look at the northeast Pacific region, in the top-left part of this graphic, we see a large swath of yellows and reds, meaning high uncertainty over the area. The ensemble average for this area shows stormy weather in the Bering Sea, but those red colors mean not all ensemble members agree with this prognosis. The downstream effects of this uncertainty in the Pacific are shown in the Western US, as the stormy weather in the Bering Sea/Northeast Pacific produces high pressure over the Rockies, where some uncertainty is shown by the swath of green colors in that region.

To summarize, a warm end to September is expected, but model guidance is indicating a return to cold weather may be expected to kick off October, when a strong trough tries to push into the Central US.