Monday, January 12, 2015

Long Range Outlook: Dynamic Pattern Unfolding into February

This is the Long Range Outlook, covering the period from mid-January (present day) to around the middle of February.

Tropical Tidbits
The above image shows 500mb geopotential height anomalies across the West Pacific, projected from the GFS ensembles and valid on January 13th. Notice the strong ridge forcing itself north to the west of Japan, likely bringing substantial warmth along with it. If we apply the Typhoon Rule to this situation, which states weather phenomenon occurring in Japan is reciprocated 6-10 days later in the United States, we should expect a period of warm weather commencing around January 15th to 17th, evidenced by a gradual warming trend unfolding in short range forecasts here at home. The Typhoon Rule supports such warmth lasting for about 3-5 days, before a storm system ushers in another wave of cool air. From here, we go to our impending cold blast.

Paul Roundy
A look at the hovmoller diagram above will help us diagnose the atmosphere. We see our current Madden-Julian Oscillation (MJO) wave moving eastward to the 180 degree longitude mark by around the 18th of January. From here, typical MJO phase space diagrams (not shown) show the index moving into territory too weak to identify the phase, but that's just because another MJO wave is forming out by the 50E longitude line, around January 21st. This positioning around the 50E line puts us in a Phase 1 MJO event, as seen below.

This chart shows typical alignment of outgoing longwave radiation (OLR) values by MJO phase in the west-central Equatorial Pacific. Notice that we see negative OLR anomalies (blue colors) in the hovmoller diagram around January 21st near that 50E longitude line, which matches up with Phase 1 (top-left panel) in this graphic immediately above. This, along with some Typhoon Rule support, should enable a sharp Arctic blast to round out January, possibly to begin February. I do have concern that this wintry return may be weaker than what is currently advertised, as stormy weather over Japan only lasts about 24 hours, if that. This should be a cold event lasting anywhere from 2-5 days for the USA.

As we move ahead into February, things get a little dicey.

We saw a notable stratospheric warming event strike to kick off the New Year, with temperatures at the 30hPa level only now settling back down to normal levels, as the graphic above displays. Let me please clarify: This was not a Sudden Stratospheric Warming (SSW) event. In order to deem an event a true SSW, wind direction in the stratosphere must either significantly slow or even reverse, in addition to intense warming of temperatures. We saw the latter, to some degree, while the former wasn't as prevalent as we needed. For this reason, it's referred to as just a notable warming event.

The concept right now is that this warming event will lead to more wintry weather about 2-4 weeks down the road from when the warming occurred, which brings us to that late January-early February period. With unfavorable indications from Japan, though, I worry that the cold (while intense) may not stick around for long. In order for that to happen, we may need another warming event, ideally a true SSW. However, even that prospect is quickly fading, as we see a strengthening of the upper polar vortex in coming days and weeks.

Let's dive into February a bit deeper.

Kyle MacRitchie
The above image once again shows forecasted outgoing longwave radiation (OLR) anomalies, but now forecasted well out into the spring. Due to such low confidence, we're only going to look at the long range OLR forecast as an indicator of what may come, not necessarily as something that's "set in stone". Notice how we see our ongoing MJO wave fading near the 21st of January with the dissipation of negative OLR anomalies, as well as our second wave forming well off to the west. As the Paul Roundy hovmoller showed, this second wave slowly shifts east as we move into February, but then note what this forecast predicts in the middle and late stages of February. We see the MJO stalling out around the 160E longitude mark. Going back to our MJO OLR composite image earlier in this post, we see this correlating to Phase 6, also known as the warm weather phase. Here's a graphic displaying a Phase 6 MJO event in February, to give you an idea of what we could see if this forecast verified.

We see a strong ridge unfolding across the East US into the Ohio Valley and Great Lakes during this February Phase 6 MJO event, with stormy weather impacting the Pacific Northwest. Ridging is shunted south and east across the Northeast Pacific, into the Southwest US. This should be particularly concerning for winter weather fans in the East, because this set-up does have additional support. Model guidance is expecting the atmosphere as a whole to dive into a La Nina-esque pattern, which is shown like the image below.

During a typical La Nina pattern, we tend to see strong high pressure across the Gulf of Alaska, before the jet stream buckles south and delivers cold weather into the North Plains and Northwest. Ridging then is provoked in the East US (the reason why cold weather fans in the East see La Nina's in their nightmares). This, of course, is not a perfect correlation, but you can make key connections between the typical La Nina environment, as well as the projected Phase 6 MJO event in February. This would not bode well for a cold and stormy end to winter for most east of the Rockies, unfortunately.

On a side note, many have asked why I am using the MJO in my forecasts, since we are currently in the pro-warmth Phase 6, but cold weather is prevailing. I continue to use the MJO because it usually provides a good idea as to where we are going in the future. The MJO's effects were being overruled by a very strong ridge along the West Coast; remember that the MJO is not the sole player in this atmosphere, and that's being exhibited well. We are currently seeing warmth return to the US as that ridge breaks down in accordance with the warm MJO phase.

Even when the MJO seemed to be failing, it still managed to reflect itself well in the atmosphere. Shown below is a side-by-side comparison of the last few days of January 500mb geopotential height anomalies (left), as well as typical 500mb height anomalies during a January Phase 6 MJO event, in an ENSO atmosphere reflective of this one. The comparison isn't perfect, by any means, but it's there.

ESRL and Meteonetwork

To summarize:

- Warmer than normal weather is expected around a January 15-23rd period.
- A storm system may impact the country around January 21-25.
- Cooler than normal weather is expected in the final days of January, possibly into February.
- There is increasing potential for a warming trend in the middle of February back to above-normal temperatures.