Tuesday, January 6, 2015

January Outlook: Thaw Gives Way to Final Winter Punch

This is the long range January outlook. In this post, we'll discuss the reasoning behind my expectation for a mid-month thaw, then followed by a return to wintry weather to close January and open February.

Paul Roundy
The above image shows a Hovmoller diagram of forecasted Outgoing Longwave Radiation (OLR) anomalies between 7.5 degrees North and 7.5 degrees South latitude, on a global longitude scale. The blue colors indicate negative OLR anomalies, which depict enhanced convection in that latitude slice, at a given longitude alignment. Similarly, yellow colors indicate suppressed tropical convection, also known as positive OLR anomalies. The MJO waves are shown by solid red lines, with more lines indicating a stronger MJO wave. Dashed lines highlight the suppressed phase of an MJO wave.

Taking a look at this chart, we currently see our Madden-Julian Oscillation wave progressing at a somewhat slow rate eastward, now located around 140 degrees East longitude. Using our MJO OLR composite, with blue and yellow colors having the same meaning as shown in the paragraph above, we can confirm that we are currently in a Phase 5 MJO, just about to switch to Phase 6.

Now, as the Hovmoller diagram shows, we expect our wave to push into that Phase 6 mark before it begins dissipating, and a new wave begins forming way off to the west. The dissipation of our current wave and generation of the new wave should happen around January 21st, meaning the current wave will be free to push through into Phase 6 (possibly Phase 7), generally a phase favoring warmer than normal temperatures in the US.

In late January, the other shoe drops.

The new MJO wave looks to form in time for the final week or so of January, pushing east at a slower rate than our ongoing wave. It will form and strengthen at around the 65 degree East longitude line at this date, which correlates to Phase 1 or Phase 2 on our chart directly above.

The chart above shows 500mb geopotential height anomalies during February, in a Phase 2 MJO wave. Notice the pattern encompassing the Central and Eastern US; strong negative height anomalies are present from south-central Canada into the Northeast. The strength of this anomaly tells me it's more than a simple trough. The longwave pattern here says this may be a piece of the tropospheric polar vortex being shunted south into North America due to blocking high pressure east of Greenland. Luckily, the MJO doesn't drive the whole pattern, so it doesn't automatically mean the vortex will be down to visit again. However, it does give an idea of what late January could be like.

As the MJO wave moves further east in February, the pattern will once again turn warm. But for now, late January and early February are looking wintry across the US.

To summarize:

- A warm pattern is likely in the middle of January.
- It is possible a final burst of wintry weather strikes the Central and Eastern US in the final days of January into early February.