Wednesday, January 28, 2015

Long Range Outlook (Made January 28, 2015)

This is the long range outlook post, made January 28th, 2015, valid for the next 7-31 days.

We'll begin with a look at what's happened in the last week, as well as a verification check on the calls made in our last long range outlook post.

Over the last week, we saw rather persistent ridging building over the Western United States into southwest Canada. This forced the development of some colder weather in eastern Canada and the northeast United States. The tropospheric polar vortex continued to be in a disrupted state, with multiple bodies of ridging being forced into the Arctic Circle.

In our last outlook, we predicted a period of cooler than normal weather for much of the Central and East US. That call didn't verify as well as we hoped, with warmth taking hold in the Northern Plains, as well as some rather mild conditions across the Southeast. However, the general idea for cold being maximized in the Northeast did work out well.

We'll now go over tropical forcing across the globe, and show how our pattern has been affected by it.

This chart shows a lot of things at once, but for now, we'll take it piece by piece. The first thing to recognize is the blue color shadings on this map. The color shadings are indicative of Outgoing Longwave Radiation (OLR) anomalies, where negative/blue depictions show enhanced convection (thunderstorms), and positive/orange depictions show suppressed convection. Arrows on this image will point away from the blue shadings, as thunderstorms force air up and away, while arrows will compress towards orange shadings, since sinking air (due to lack of convection) drags air down towards the surface. Lastly, the green contours show the intensity of divergence, the action of air being pushed up and away by thunderstorms, while reddish/purple contours show convergence, the action of air being pulled down and compressed towards the surface as the air sinks.

We've begun to see enhanced convection make its way across the waters due south of Eurasia, as our new Madden-Julian Oscillation (MJO) wave forms. This convection is currently located west of India when viewing it from a longitudinal aspect. Such positioning of convection usually favors a colder and stormier pattern in the East US, as was evidenced by the recent blizzard that hit parts of the coastal Northeast. We're still seeing enhanced tropical convection west of the 180 degree longitude line, and this may have led to the unexpected warmth that messed with our call for a cool period this past week. As this convection moves east and dies off, the newly-developing body of convection should take control.

Now that we've discussed what has happened, let's start to go over what will happen.

The image above shows 500mb geopotential height anomalies over the Northern Hemisphere, where blues and purples depict negative height anomalies, usually indicative of cold/stormy weather. Similarly, greens and reds indicate the presence of positive height anomalies, a precursor to warmth and generally quiet weather. In this panel of the ESRL ensemble forecast, valid for 9 days from today, we see that the persistent lobe of the tropospheric polar vortex has retracted north into Greenland, a classic positive North Atlantic Oscillation (NAO) signal. However, due to the blossoming ridge in the West, stormy weather still continues in the nation's midsection. This does not last long, however. This forecast matches our projection in the last Long Range Outlook for a brief cold spell around February 6-7.

By the 336-hour forecast mark, ensembles are in agreement that the positive NAO will permit that ridge in the West US to bleed east, resulting in a warmer than normal pattern for much of the country. Negative height anomalies are shown along the Gulf Coast, but an unfavorable set-up upstream (to the west) of the United States means that warmth should prevail. This matches our earlier call for warmth after about the first week of February. A consolidated tropospheric polar vortex certainly does not help matters for winter weather fans, either.

Now that we've looked over the next 7-14 days, let's start looking out even further.

Tropical Tidbits
The image above shows 500mb geopotential height anomalies, forecasted over the West Pacific on the evening of February 2nd. Note the presence of a trough just to the east Japan, with a very powerful ridge just to the west of the country, arguably influencing the nation more than the trough. This trough appears to be a storm system that may impact us here in the United States down the road, just before this ridge may take hold, but that potential will be investigated in a later post. Using the Typhoon Rule, which states weather phenomenon occurring in the West Pacific is reciprocated in the US about 6-10 days later, we may expect a general warm period, possibly cooler in the East, around February 8th - 12th.

Tropical Tidbits
Long range models have been hinting at the idea that a strong upper level low or deepening trough will slide into Japan around February 6th or 7th. For now, ensembles are taking a more progressive and generally weaker approach with this potential, as is expected. For now, we'll watch a February 12th - 16th period for cooler weather, throwing a wrench into our outlook from the last post.

I want to now go over the teleconnections over the next two weeks, which can help us diagnose the pattern heading into the 14-31 day period.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA, NAO, WPO and EPO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The WPO (West Pacific Oscillation) and EPO (East Pacific Oscillation) are very closely related. In the negative phase of the WPO, a strong ridge exists over the Bering Sea, which can allow for sustained cold weather in the Central and Eastern United States. The negative phase of the EPO gives similar results, though the ridge is positioned in the Gulf of Alaska instead. The positive phase of both the EPO and WPO see warm weather prevail in much of the US, as stormy weather replaces the ridges in each respective region.

The forecast for the PNA includes a decrease in positive values as of right now, before a spike back well into positive territory in the long range. This works well with the ESRL ensembles we were analyzing earlier. The NAO forecast generally stays positive in the long range, a red flag for that positive PNA ridge to shift east into the Central and East US. A dip to negative territory does occur in the January-February transition, which may create a storm threat (upcoming posts will address this), but nothing too serious. Both the WPO and EPO are negative in the long range, which is good for winter weather fans. However, the consensus has been for this -EPO/WPO regime to weaken heading beyond the 16 day period, which may only enhance the threat for warmer weather in the middle of February.

Finally, let's use the skills learned in our OLR analysis earlier, to forecast the long range.

As in the JMA graphic, cooler colors define negative outgoing longwave radiation anomalies, which mean the presence of thunderstorm activity. Warm colors depict positive OLR anomalies, otherwise known as suppressed storm activity. In the 6-10 day forecast panel, we begin to see our new MJO wave really strengthen and define itself as it moves east, just south of the Indian subcontinent. Notice in this same panel that our current MJO wave near the 180 degree longitude line is nearly dissipated by this forecast time period. At the 11-15 day forecast mark, the MJO wave has strengthened considerably, and has now switched to pro-warmth phases. These pro-warmth phases can generally be identified when -OLR anomalies move east of the Indian subcontinent, and this should happen even beyond the 11-15 day forecast period. Beyond then, confidence is too low to accurately produce a forecast.

To summarize:

- A period of rather chilly weather should kick off the month of February.
- A brief period of seasonal to cool weather is expected to impact the Central and East US around February 6-7.
- In mid-February, the atmosphere is in favor of a warm pattern. However, disagreement from the Typhoon Rule means this part of the month could really go either way. We will re-examine this in the next long range post.
- An early look at late February continues to show a predominantly warmth-favoring pattern.