Saturday, September 27, 2014

Persistent Cold Pattern Expected to Succeed Current Warmth

Another bout of cold weather is expected to replace the nice weather that most of the United States is currently feeling.

The first image we'll analyze above is the North American Ensemble Forecast System, or NAEFS 500mb height forecast. The contour lines show isobars at the 500mb level, with colored regions showing the degree of uncertainty, or "spread" among the ensemble members.

The graphic above, valid October 3rd, shows a deep trough pushing south into the Central US. We can see this well by the NAEFS projected depression of 500mb height contours. A glance around North America tells us this troughing pattern looks to be supported by two areas of ridging. One body of high pressure looks to be positioned in the West US, in a more suppressed version of last winter's ridging episodes, while the other ridge appears to be positioned along the Southeast and East Coast, evident by the arcing motion of the 500mb height contours.

Something we will have to monitor in the days leading up to this outlook of October 3rd is the high uncertainty over the Plains with this trough. The NAEFS outlook shows greens and yellows encompassing the expected placement of the trough, which means ensemble members aren't so sure this will actually end up happening. However, as the trough is actually showing up on this image, it means more members support it than not, a positive sign for this trough being able to form.

This next ensemble projection, from the Canadian ensemble system, shows 500mb height contours valid on October 6th. In agreement with the NAEFS ensemble forecast, we still see that trough centered over the Plains and Midwest, now trying to push a bit east into the East Coast. Sustained ridging is still highlighted over the West US, and a bit still present in the Southeast. A look at the individual members of this ensemble mean image shows only two of the twenty members do not show a large trough in the Central US on this date. Therefore, confidence is rather high that this cooler pattern will play out.

Lastly, we'll take a look at the ECMWF ensembles' outlook for October 3rd, the same time period as the NAEFS graphical outlook. In this forecast, we see a large trough placed over the Central US, echoing the projections made by the NAEFS and CMC ensembles. Adding to the similarities is the rather large spread / degree of uncertainty among ensemble members, as the deeper purple colors on the right-hand panel above show over the Plains. Because it's still showing up in the ensemble mean images here, I would keep confidence in this cold outlook for the start of October, but still be wary for changes in the outlook in the near future.

To summarize, model guidance is agreeing on the potential for a cold blast to kick off October, but individual projections are exhibiting varying degrees of uncertainty as to just how intense the cold may be.