Monday, December 29, 2014

January Long-Range Outlook: La Nina Pattern Expected to Dominate

This is the long-range outlook for the month of January, and is one of the more unfortunate posts I have to write. At this time, it does appear my winter forecast is in trouble, primarily with respect to the Eastern US, based on how January is shaping up. Let's jump right in.

Paul Roundy

The above image shows a long-range Hovmoller forecast, on a time-by-longitude scale. For this chart, we want to focus on the colors, where yellow indicates suppressed tropical convection, and blues represent enhanced tropical convection from an area encompassing 7.5º North latitude to 7.5º South latitude. The other item we want to watch for is the solid red lines, which indicate the presence of a Madden-Julian Oscillation (MJO) wave. These are almost always accompanied by enhanced tropical convection signals (the blue colors).

On this graphic, notice how we see an MJO wave moving from 135E at the start of the New Year, to about 170E before it dies out near the International Date Line at 180º. If you put this into terms of the Madden-Julian Oscillation, it means the MJO will be moving through Phases 4, 5 and 6 before dying out just before reaching Phase 7. There are a total eight phases in the MJO, and Phase 4-6 are classified as 'La Nina pattern' phases. Similarly, Phases 8-2 tend to be associated with 'El Nino pattern' phases; I personally see Phases 7 and 3 as 'transitional phases'. Therefore, we can determine that we will be entrenched in a primarily-La Nina pattern until this MJO wave dies out close to January 20th, and a new wave commences around ~65E longitude to close out January. Long-long range forecasts into February also take this second wave through La Nina phases, a bad signal for winter weather fans in the East.

Nicholas Schiraldi
The image above shows the GFS forecast for atmospheric angular momentum, or AAM over the next 16 days. Notice how the GFS takes the AAM well into low territories, near phases 1-4. These phases and general Low AAM pattern are highly characteristic of a La Nina-dominated weather pattern, and this jives well with the indications we have been receiving from tropical forcing, as was described above.

So, what exactly does a La Nina pattern entail?

The above image shows a general overview of the atmosphere during a La Nina-dominated pattern. During such a pattern, high pressure will take over the Gulf of Alaska and general Northeast Pacific regions, as the polar jet stream rides up into Alaska, before plummeting south into the Rockies. As a result, a general wet and cool pattern takes hold of western Canada and the North Plains. In the United States, high pressure and warm weather will take hold of the Southeast and East US, something we've been seeing, and ought to continue to see for the coming month. This ridge then provides an excellent 'blockade' that can force storm systems from the south to come north, and dump precipitation into the Great Lakes, Midwest, Plains, and even the Ohio Valley at times. That's why we see a 'wet' outline in the areas mentioned above (this outline is centered a bit to the east of where I believe this 'wet' pattern may set up).

Let's split this up into three portions, to better serve our forecasting purposes.

Early January
The early part of the month is likely to favor a cold Central and East US, as a lobe of the polar vortex ventures dangerously close to the United States. This time period is likely to feature multiple winter storm threats, which may affect the East US as well as the Central US. More examination of ridging in the Southeast will be needed, as far as knowing where the individual storms may track.

Middle January
The middle part of the month is likely to feature a warmer than normal East US, but long range model guidance is indicating this warmth may eventually push westward to encompass the Great Lakes and much of the Midwest. Time will tell on how this outlook unfolds, but a warm East and Central US may be a good bet, as the aforementioned MJO wave moves into phases unfavorable for warmth in those areas.

Late January
The closing part of the month is likely to feature a return to some cooler conditions, primarily in the eastern third of the country as a new MJO wave forms in phases favorable for a chill in the East. However, the final days of January and opening days of February could see warmth return to the East, if this new MJO wave moves quickly to La Nina phases.

The parts that would be included in the Summarization have been highlighted in red above.