Wednesday, October 22, 2014

Extended Forecast Discussion (Part 1)

This is an extended forecast discussion for the time period of today until approximately one month from today. Due to the length and technicality of this post, there will be multiple parts to this discussion. Today's discussion will focus on the projections for weeks 1 and 2.

We're going to begin with a look at the Atmospheric Angular Momentum readings, or AAM.

The image here shows atmospheric angular momentum transport anomalies over the past few months. We want to focus on the anomalies in the top-right corner, the last few days of recording. On this graphic, we see a swath of yellows and oranges seeming to push diagonally upward along the image. Those yellows and oranges define positive AAM transport anomalies, and they are beginning their long-awaited movement to the north, which will throw a big wrench in our weather pattern.

According to the oranges and yellows on the screen, the positive AAM anomaly transport values have made it as far north as about the 40N parallel. Because positive AAM anomalies generally can mean an enhanced jet stream, we might expect high pressure ridging to build northward with time as well, as the jet stream pushes north with the +AAM transport anomalies. This spells a predominantly warm weather period in the next few weeks, as this strong jet stream will eventually make it to the Arctic and keep the Arctic Oscillation (AO), and by extension the North Atlantic Oscillation (NAO) positive. Such a positive AO, which helps induce warm weather, is already on the forecast from the Climate Prediction Center, as shown below.

The spaghetti-like alignment of red ensemble forecast members above that neutral line suggests the positive Arctic Oscillation phase for the entire forecast period, which extends for 14 days from today.

We're already seeing the effects of the positive AAM transport anomalies in the mid-latitudes, as 250mb zonal winds across the Northern Hemisphere and Southern hemisphere are markedly positive (stronger than normal jet stream) around the 30-60N and 30-60S parallels in each respective hemisphere, per the graphic below.

Let's transition for a moment to the tropics.

The graphic above shows Outgoing Longwave Radiation (OLR) anomalies across the tropics regions, in a global view. Here, we can identify areas of tropical forcing, where enhanced or suppressed convection may be driving or enhancing our weather pattern here at home. Glancing over this image, we do find a swath of negative OLR anomalies, indicating enhanced thunderstorm activity, placed south of India and a bit southwest of the subcontinent as well. This swath of convection has an entirely different meaning for our weather pattern, which will be discussed later on in this discussion. But for now, we'll analyze it as the Madden-Julian Oscillation (MJO).

The MJO states that enhanced or suppressed tropical convection over certain parts of the Equatorial Pacific basin or Indian Ocean can have different effects on the global weather pattern. In this case, when comparing observed negative OLR anomalies in the image above with the Bureau of Meteorology's average negative OLR placement for each MJO phase, we can determine what MJO phase we appear to be in. See if you can figure it out on your own.

If you guessed Phase 2 into Phase 3, you're right! Phase 2 and 3 MJO events typically see enhanced tropical convection just south of India, very similar to the enhanced tropical convection we're seeing now in that JMA graphic above.

Now that we know what phase we're in, let's see if we can identify how this phase MJO is driving our weather pattern.

The image above shows typical 500mb height anomalies for Phase 3 MJO events in the month of October. Blues indicate stormy and cool weather, while greens, yellows and reds highlight warm and quiet weather. In "normal" October Phase 3 MJO events (the word 'normal' placed in quotes, since no MJO phase is really ever 'normal'), we tend to see high pressure extending from the West US into the Central US, with a swath of negative height anomalies from Greenland to the East Coast. Interestingly enough, we're currently seeing that same scenario play out, though not as defined to the naked eye as you might want it to be. Regardless, we are seeing that progressive ridge formation from the West extending into the Central US, with some stormy East US weather as a result. Though we might not make it to Phase 3 of the MJO, and the MJO isn't the only part of these negative OLR anomalies, we're certainly observing Phase 3-like conditions. Consequentially, if this continues, we might expect a continuation of such warm spells in the next week or so in the US.

Going back to the atmospheric angular momentum one last time today, I want to go over the relative AAM values we've seen in recent days. The chart above shows relative AAM anomalies, with greens depicting negative AAM areas and oranges showing positive AAM values. If we look at the bottom panel, we see an average anomaly across the globe of these AAM values, and this is where we want to focus our attention now. In recent days, the AAM has been trending more towards neutral territory, as the AAM was expected to shift into El Nino-like states of high (positive) AAM. Now, however, the AAM has stopped pushing positive, and has leveled off in negative territory. Add to that the AAM tendency is in negative territory once again, and it looks like we'll be more confined to La Nina-like, low AAM stages.

AAM Tendency
I'll go more in-depth into this discussion tomorrow, with our Part 2 segment, but here's the first part of the summary for the next two weeks' outlook.


- Warm weather is expected to continue as MJO Phase 3-like conditions provide a base for ridging in the West, pushing east into the Central US with time.
- This warm weather will also be sustained as positive AAM transport anomalies push north into the mid-latitudes in the next few weeks.

We only got to cover the first week or so in today's post, so weeks 2 & 3 will likely be covered in Part 2.