Thursday, September 25, 2014

Long Range Guidance: Atmosphere Primed For Intense Winter

Long range guidance appears to be favoring the Madden-Julian Oscillation (MJO) to remain in its latter phases this winter, something that could make this winter a brutal one.

Let's first define what the Madden-Julian Oscillation is. The MJO is comprised of the placement of enhanced tropical convection over certain parts of the Equatorial Pacific and Indian Ocean. The chart above shows Outgoing Longwave Radiation (OLR) anomalies for each of the eight phases of the MJO; each phase is numbered in the bottom left corner. Blue shadings indicate negative OLR anomalies, which translates to enhanced convection in the area. Similarly, yellows depict positive OLR values, which indicate suppressed tropical convection. You can see how, as the phases progress, the enhanced convection in blues shifts eastward. Phase 1 begins with storminess nearly due south of India, while Phase 8 has that same storminess well east-northeast of Australia.

Let's put what we've learned to the test. The image above shows the precipitation anomaly forecast from the JAMSTEC model, valid over the upcoming winter months of December, January and February. Instead of focusing in on North America, we're going to concentrate on the anomalies over the western Pacific. In this forecast, well above normal precipitation anomalies are targeting the region just north and east of Australia. This is a big clue to us that the enhanced tropical convection we discussed earlier will be centered in those green anomalies.

If we compare the placement of the above-normal precipitation anomalies on the JAMSTEC model to the average negative OLR placement for each MJO phase in the first image of this post, it looks like this graphic is suggesting a predominantly-Phase 6 MJO for this winter. It's expected that the MJO won't remain solely in Phase 6 for the whole winter, but this model says it should stay in Phase 6 more often than not.

This next image is nearly identical to the one we analyzed from the JAMSTEC model. This graphic is from the CFS model, and once again shows projected precipitation anomalies over the December-January-February period for the coming winter. As in the JAMSTEC image, green indicates above-average precipitation, while orange depicts below-normal precipitation.

According to this image, we see a swath of above-normal to well-above-normal rainfall anomalies positioned well to the northeast of Australia. If we do a quick check of the composite image that we first discussed to kick off this post, it's apparent that the CFS model is favoring a mainly-Phase 8 winter.

This final image seems daunting at first, but it's relatively easy to decipher. We'll examine the bottom image for this post, though some of you more experienced weather enthusiasts may know what the top panel means as well.

The bottom panel shows the Japan Meteorological Agency (JMA) three month rainfall outlook over the tropics and mid-latitude. In this graphic, yellows and oranges depict below-normal precipitation, while blues indicate above normal precipitation. According to the JMA, the precipitation anomalies are positioned northeast of Australia, almost a middle ground between the JAMSTEC and CFS models. A glance at the OLR composite and comparing it to the JMA forecast, the MJO appears to be favored to stick around Phases 6 and 7, maybe a bit into Phase 8.

Let's now go over what these different phases mean for the US during the winter.

The image above, from AmericanWx, shows temperature anomalies over the Northern Hemisphere during all amplitudes for Phase 6 MJO events during January (I chose the month as a middle ground, since there were no three-month composites for December-January-February). The image does show cold weather in the western part of Canada, and warm weather in the east. This could translate to warm weather in the East US and chilly weather in the West during Phase 6 MJO events, but we can't make a confirmation on that, since we just don't see it on this chart.

If we now take a look at 500mb height anomalies during a Phase 8 MJO event, we see a much different story than that associated with a Phase 6 event. During Phase 8 events, strong ridging evolves over the Western portion of North America, with strong troughing in the East US and Central US. This translates to warm weather in the west, and cold weather in the east. The anomalies are much more decisive than those with the Phase 6 chart, and if the CFS projection verified from earlier in this post, it's possible another brutal winter might be in store.

To summarize, long range model guidance is hinting at the atmosphere being primed for a pretty harsh winter in coming months, if the Madden-Julian Oscillation cooperates.

More on how the atmosphere is becoming primed in a future post...