© 2019 Terry Swails

THE DECEMBER THAW....

December 5, 2018

Following one of the coldest November's on record (7th coldest in Cedar Rapids) there are strong signals a December thaw is in the works.

The most apparent signal is the MJO (Madden Julien Oscillation) which follows convective trends in the tropical Pacific. the MJO is an eastward moving disturbance of clouds, rainfall, winds, and pressure that traverses the planet in the tropics and returns to its initial starting point in 30 to 60 days, on average. This atmospheric disturbance is distinct from El Nino or La Nina, which once established, is associated with persistent features that can last several seasons over the Pacific Ocean basin. 

 

Until recently the MJO has been in the limited convective phases of 8 and 1 over the Indian Ocean. That's shown below in the brown colors which correlate to below normal precipitation.

Phases 8 and 1 at this time of year analog to colder Midwest temperatures. That's what we've been seeing.

Going forward, you can follow the green dots below observe how the MJO is forecast to progress through the warm phases of 3, 4, and probably 5 leading up to Christmas.

Most of the country is mild in these phases with Arctic air limited. Check out the phase analogs

Notice how 3, 4, and 5 are convective phases in the Indian Ocean, the opposite of 7, 8, and 1 which are not.

In a nutshell, by knowing where convection is forecast to develop in the tropical Pacific, we can get a good idea of where temperatures are likely to trend in the long range period of 10-20 days. We can also use the MJO to forecast precipitation patterns. 

 

So based on what the EURO's MJO cycle is forecast to look like (phases 3, 4, and 5) we have a strong signal that the period mid-December to Christmas will see near to above normal temperatures across the Midwest. Precipitation is likely to be below normal, perhaps well below!

 

Another helpful teleconnection is the EPO (eastern Pacific Oscillation). It's forecast to go strongly positive on the EURO by December 13th.

Here's what that implies. More evidence for a milder brand of weather at that time. 

This is what the GFS has for temperature departures day 5-10 (December 9-14)

Days 10-15 look like this. (December 14-19)

The CFSv2 climate model has this for average 10 day departures leading up to Christmas (December 14-24)

Now just because temperatures may be above normal doesn't mean it can't snow, it just makes it harder. That's something we'll keep an eye on for those of you who like a little frosting for Christmas. Meantime, if you want it consistently cold and wintry, root for that MJO to get back into the colder phases of 8, 1, and 2. It's still likely to happen but it might not occur before Christmas. Roll weather...TS

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