Saturday, February 8, 2014

Arctic Cold to Abruptly End in Mid February, Return in March

It's looking like we will see the recent Arctic cold come to a quick end in mid-February.

The image above shows the GFS Ensemble 500mb height anomaly forecast for February 17th over the western Pacific. We see strong ridging present over Japan and much of eastern Asia, with another swath of positive height anomalies displaced further east to the south of Alaska. Using the idea that a storm system in Japan can signal a storm in the US 6-10 days later, we can find that this strong ridge over Japan should also come back to haunt the US 6-10 days later. That would place warmer and quieter weather over the nation in the February 23-27 period. Considering high pressure should be affecting Japan both before and after this February 17th forecast graphic, it would be reasonable to think that this warmer/quieter weather may extend into early March, as well as closing out the last week or so of February.

This warm trend can be seen by the Earth System Research Laboratory's (ESRL) Analog Temperature 8-14 day outlook, as is shown below:

As we move into early March, it looks as though we will be heading into new phases of the Madden Julian Oscillation (MJO). Looking at the March 8th MJO OLR anomaly forecast, in the middle panel of the left side of the images above, we see deep blues spread out across the waters to the southwest and southeast of India. These deep blues represent enhanced tropical convection, and it also means an active phase of the MJO will be evolving.

If we compare that March 8th forecast image to the MJO OLR Composite image from the Australian Bureau of Meteorology, we see that the blues on the forecast image line up the best with MJO Phase 2 and MJO Phase 3, which are seen as the second and third images down from the left-hand side of the panels above. If we look at the low amplitude Phase 2 MJO (which guidance predicts we see when the MJO does enter Phase 2 before transitioning to Phase 3) 500mb height anomaly image below from , we find that deep negative height anomalies are favored over the East US, resulting in colder than normal weather. The troughing in the Gulf of Alaska remains present, with suppressed ridging over the north central Pacific Ocean. Long range ensembles favor this type of set-up for the middle-late portions of February, and it's very possible this continues into March. To clarify, while the set-ups may be similar from mid-February onward, I currently only expect warmer weather for the mid-late parts of February due to East Asian ridging. Beyond that period, into early March, indications of East Asian ridging dissipate, allowing cold weather prospects for early March to flourish.

Low amplitude Phase 2 MJO 500mb height anomaly composite
The 500mb height anomaly composite for all amplitudes of a Phase 3 MJO also favors some cooler weather in the East US, though the cold is more restricted to northern parts of the nation.

So, let's sum it all up.

•We are looking at warmer weather over the United States in the closing week of February, possibly extending into the opening days of March.

•The pattern after the early-early March warm-up looks chillier, with a potentially favorable MJO (keyword is potentially due to the typical long range forecast caveats and inaccuracies) and favorable signs from East Asia after ridging moves out.


1 comment:

Nick Crabtree said...

I'm a little confused looking at the images but does this mean that Michigan will have warmer temps for the second half of February? It would be nice since we have been in a deep freeze since December