Thursday, February 19, 2015

Long Range Outlook (Made February 19, 2015)

This is the updated long range outlook, made on February 19th, and valid for late February into the first half of March.

Tropical Tidbits
Click to enlarge
We're currently seeing a portion of the semi-permanent Arctic low pressure vortex intruding into the United States, with anomalously low 500mb geopotential height values appearing in the colored shadings and analysis in the graphic above. This anomaly has been forced by a strong ridge pushing into Alaska from the northeast Pacific and west coast of North America. Over the next few days, cold air intrusions should continue as this ridge flexes its muscles and persuades additional Arctic air masses to collapse into the lower latitudes.

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 positive PNA and negative EPO have worked in tandem to indicate this strong ridge blossoming into the Gulf of Alaska and general northeast Pacific. In the upcoming couple of weeks, model guidance is indicating we see the ridge shift further offshore to the west, as the PNA dip from positive to negative shows. Over time, ensemble guidance is telling us that this ridge could keep shifting west, resulting in the EPO actually moving positive for the first time in a while. The positive EPO signal, however, is weak, and will have to be watched for a 'false positive', both literally and metaphorically. If this forecast does verify, we can expect a warm-up in the early days of March.

Tropical Tidbits
Ensembles indicate we will see a ridge of high pressure pushing into Japan on the morning of February 21, as the graphic above shows. Using the Typhoon Rule, which states weather phenomena occurring in Japan is reciprocated in the United States 6-10 days later, we can anticipate a warm-up for millions that have undergone a brutal February in the final days of the month, likely into early March. From there, confidence decreases, but a return to an average or cooler than normal pattern may be expected.

You'll notice I haven't used tropical forcing in this post; that's because the ridge in the West is just so overpowering that the Madden Julian Oscillation can't do much of anything.

To summarize:

- A cold pattern is expected to round out February.
- A warm-up is expected for the last days of the month into early March.
- The first half of March should be characterized by a generally average to slightly below-average temperature pattern. There are hints of some stormy weather in early March.