Posted: 5:43 pm Tuesday, October 24th, 2017
By Kirk Mellish
If NOTHING else is considered, here’s a shake down of the winter based ONLY off the current shape of the Pacific in the La Nina zone. And by the way, you never make a forecast by looking at only one input. So this is but one signal, and it may look different by forecast issuance time.
Above are past years (or analogs) that look similar in Pacific to this year… as of now.
RESULTANT UPPER AIR PATTERN:
Ridges west and trough over Great Lakes region.
RESULTANT SURFACE TEMP ANOMALY:
No two are ever exactly alike, but typically in a La Nina December is the warmest and February the coldest compared to average. But most modeling shows the La Nina weakening in February so that will be something to watch.
Last year the warmer than normal blob of water in the Gulf of Alaska turned unexpectedly cool throwing a wrench in the winter outlook. As of now it’s already cool, warm water there favors cooler winter in the East U.S.
WINTER WILD CARDS:
Every year a wild card factor for the winter is a combo of the AO/NAO related to the so-called “Greenland block”. A negative AO/NAO is a signal related to east coast snowstorms and cold shots of air as the jet stream ridge blocks the jet southward:
There are indicators that there MAY be more of these -NAO blocking patterns this winter as a -QBO favors this and there have been some recent indications of a negative NAO heading into November. IF the -QBO is as negative as expected this also favors more SSW (sudden stratospheric warming) events which also favor -NAO/AO blocking. None of these things guarantees, it just favor it.
It will be important to monitor the NAO through November because research shows if we get it in November it often returns at times during the December-March period.
The chart below clearly shows the difference between the -NAO and +NAO phase impact on temperatures during a La Nina…
-NAO CENTER ROW vs. +NAO BOTTOM ROW December-February:
SO, provided a La Nina is in place winters that average a -NAO are colder than those where the NAO is positive on average.
However, here’s the rub… the NAO is not very predictable beyond a couple weeks at a time. When trying to predict it many months in advance its very sketchy.
An anecdotal rule of thumb is that the mean position of the autumn warm ridge is where you get the mean winter cold trough, and where you have the mean autumn cold trough you get the warm winter ridge. But I’ve not seen a scientific paper on it so I don’t know how good a tool that can be. IF true it would suggest a flip from the fall pattern to date.
It has become harder to get very cold winters that predominate all the winter months and parts of the fall and spring. Many but not all 21st century winters have been warmer than 20th century ones. Dating back to 1895, 3/10 warmest winters have occurred in the 2010s and 6 of the 10 warmest have been since 2000. This “background state” makes it a struggle for widespread cold and snow beyond isolated storms.
Day to day, month to month and region to region variability is a normal part of weather and climate. Both the short term and the long term impact people.
More updates to come. FOLLOW ME ON TWITTER @MellishMeterWSB.