Wednesday, January 21, 2015

Long Range Outlook (Made January 21, 2015)

This is the long range outlook post, made January 21st, 2015. This post will address the upcoming pattern over the next 7-31 days.

We'll begin with an analysis of the pattern over the last several days.

Click images to enlarge
Over the last week, we saw ridging present across the Western US, as exemplified by the green and yellow colors, with that ridge making its way east to provide warmth for many in the Central US. Ridging was also dominant over Greenland and south, into the North Atlantic and Canadian Maritimes. These two ridges combined to force an upper level low into northern Canada, rather than further south towards the United States, a likely scenario if the former ridge out west had been more dominant.

On a more synoptic scale, we recognize high pressure has forced the tropospheric upper-latitude vortex to weaken and splinter, with the prevailing lobe located in western Europe, and other splinters scattered across lower latitudes. This tells us that the cold air up in Canada is more free to move around, rather than maintain its position, locked in the Arctic Circle.

Let's now discuss tropical forcing across the globe to recognize how it has affected our pattern the last several days.

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.

Over the past several days, we've seen an area of enhanced thunderstorms make its way eastward from Oceania, and we now see it beginning to weaken off the western coast of the South and Central Americas. You weather enthusiasts may recognize this as a weakening Madden-Julian Oscillation wave. However, look towards southern Africa. We see a new plume of thunderstorms beginning to develop. It is expected that these storms will propagate eastward and form the new MJO wave that will also push east with time, but that's for discussion later on in this post.

Now that we've gone over the pattern developing in the last several days, let's start looking into the future, beyond the 7 day window.

This is the ESRL ensemble projection of 500mb geopotential height anomalies for 7 days out. Here, we see a strong ridge pumping north in the Western US, resulting in a deep trough (and associated cold weather) in the East US. This fits in well with the expected progression of our new MJO wave east, as the placement of tropical convection just southwest of India supports this type of cold regime. Looking across the northern hemisphere, we can also identify an upper level low over Greenland, a signal for the positive phase of the North Atlantic Oscillation (NAO). The +NAO supports a more low-amplitude jet stream, which is technical-speak for weaker ridges and weaker storms, as well as a faster progression of those ridges and storms to the east. However, at least for now, that +NAO is being overruled by the strong ridge in the West, ensuring a cold period to end January (something we've been discussing for over 2 weeks now).

Going ahead to 11 days from today, the 500mb geopotential height anomaly shows a different picture than the one we analyzed above. The ridge in the West US has been swallowed up by a more dominant ridge stretching across the Bering Sea and into the North-Central Pacific. With support from an inferior ridge along the western North America coastline, it appears a cross-polar flow situation would be in the works, where cold air would be transported from Eurasia, across the Arctic, directly down into the Northeast and Canadian Maritimes, based on the placement of that trough. Things start to improve in the Central US, for those wishing for spring. That +NAO signal we discussed earlier is now in play, as we see the aforementioned West US ridge 'bleeding east' into the Plains. This fits in with the continued progression of the MJO wave east (weather enthusiasts know this part as Phase 3, the image above this one resembled a Phase 2 event). However, I do think we see the ridge retract itself a bit west as the time between this forecast and present day decreases.

We've examined the outlook for the next 7-14 days, so let's start digging into the outlook in the next 14-31 days.

Tropical Tidbits
The image above shows 500mb geopotential height anomalies, forecasted over the West Pacific on the evening of January 22nd. Note the presence of a trough in Japan, in the midst of two ridges on either side of the country. This trough appears to be a storm system that may impact us here in the United States down the road. 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 storm threat around the January 28-February 1 timeframe. As of now, this storm wouldn't be particularly strong, and should be followed by a warm-up. However, it could perk the interest of some severe weather enthusiasts.

Tropical Tidbits
If we fast-forward a bit, we find ourselves looking at 500mb height anomalies on the evening of January 25th, with a much different prognosis than what we saw in the image above. A strong ridge now encompasses all of Japan, bringing a 'heat wave' to the region. This ridge persists for more than a couple of days, which could very well validate my outlook for a mild middle of February. Using the Typhoon Rule here, we look to find ourselves in a warm period around a January 31st - February 4th period, possibly for longer, based on the looks of ensemble guidance further out. This warmth is then interrupted by a brief cool shot before that warmth may return.

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 is positive for the next two weeks, and a sustained strong positive signal at that. This tells me we're looking at that ridge sticking around the West US for a prolonged period of time into February, though it may very well bleed east into the Central US as we already discussed. The NAO forecast is sustained at a moderate level for the entire period, meaning the possibility of that +PNA ridge bleeding east is rather high.
The WPO forecast starts negative, goes positive, and then goes into strong negative territory as that intense ridge on the ESRL ensembles takes over. This should continue for a bit as the MJO wave moves through Phase 3. The EPO follows a nearly-identical path, and both should permit the persistence of cold in the East US. I will refrain from including the Central US in that cold forecast due to the risk of that ridge in the West US bleeding east.

Let's now use tropical forcing to see what we may expect later on in February.

This four-panel image shows OLR anomalies, using the same color definitions as the JMA chart we discussed earlier in this post. In the top panel, we see current OLR anomalies, and that dying MJO wave is observed moving eastward in the next 1-5 days. By the 6-10 day period, our new MJO wave evolves in Phase 2, favoring the cold weather we have discussed earlier in this post. By by the Days 11-15 panel, our MJO wave has shifted east, to Phase 3, favoring a warmer nation as we move into mid-February. As this wave moves eastward over time, it is expected that the wave will go into phases even more favorable for warm weather, which is why I'm maintaining my call for a warm period in mid-late February. Beyond that 31 day benchmark, confidence is too low to forecast further.

To summarize:

- A period of colder than normal weather across the Central and East US is favored next workweek, likely from Tuesday to Friday to round out February. This cold will be maximized in the Northeast.
- A period of warmth may overtake the Central US in the opening days of February, though the Northeast will remain cold.
- Cooler weather should return to the Central and East US for a brief period around February 6th or 7th, before warm weather takes over.
- A warm pattern may persist into the middle portion of February, possibly into the later part of the month, for much of the nation.