Friday, September 25, 2015

Long Range Outlook: Warm Pattern Expected to Persist

This is the first long range outlook of the winter weather season. In these outlooks, conditions over the next 7-14 days, and possibly beyond, will be discussed.

We begin with an analysis of what's happening at the upper level of the troposphere, with the chart above showing wind speeds at the 150-millibar level as of this past Tuesday. We're going to focus in here on the conditions evolving over the North Pacific. Note how we see a number of disturbances in this area- mainly, two low pressure 'cells' and two high pressure 'cells', each demarcated as needed. One of these high pressure cells rests in the Bering Sea, while the other is found just southeast of Japan. A low pressure anomaly is noted directly south of that high pressure cell in the Bering Sea, while the other is found along the West Coast of North America. 

This is a good example of a Rossby Wave Train, where we see these upper-atmospheric features positioning themselves and becoming nearly stationary for weeks at a time. As a result, the atmospheric pattern will lock in for a prolonged period of time, and that's what we're going to see over the next couple weeks, like we have seen lately. 

That low pressure cell along the coast of California will maintain surface low pressure along the West US, and should provide for warmer than normal conditions in the Central and East US over the next 7-14 days. More on this a little later in the post, but the pattern is pretty clear-cut with respect to these Rossby Waves, in that we're looking at a much better chance for warm weather than cool weather in the eastern 2/3rds of the country.

This image represents forecasted temperature anomalies over the next 8-14 days, based on a number of analog dates as produced by the Climate Prediction Center. Notice the abundance of warm weather across the Midwest, Plains and East US, with well-below-normal temperatures across the West US. This scenario seems rather plausible, given the Rossby Wave pattern supporting surface low development in the West, thus forcing a ridge in the Central and East US.

The precipitation forecast based on the same 8-14 day analogs as those in the temperature graphic highlight a heavy rain event over the South Plains in the next week or two, while mainly dry conditions prevail in the East US. A heavy rain event may also occur in the North Plains, as well as in the Pacific Northwest. I tested this method out this past winter on an irregular basis and found the temperature graphics to verify better than precipitation graphics, so don't take this at face value.

The above covers the 7-14 day period, possibly into the 16 day period. Beyond then, there are some indications of a cool-down for the country, but it's too far out to get a good handle on what could unfold. Latest analog projections as of this morning (I created the majority of this post over the last couple days) show the warmth shifting back west in the 14-ish day range, but whether that happens remains to be seen.

To Summarize:

- Warmer than normal weather is expected to persist across the Central and East US.
- There are signals for a heavy rain event in the South-central US, but confidence is very low.
- A return to more neutral conditions is possible beyond the two-week timeframe.
- A look at the 2-6 week outlook reveals the potential for a cooler than normal pattern setting up, but confidence is very low.