11/2/16 Winter Threat Assessment Blog – Targeting a Pattern Change?
Synopsis: Hey guys, thanks for taking the time to check out our very first edition of the FREE winter threat assessment blog, which will be updated 3 times a week as we move into the winter months. The goal of this blog is to target storm systems 7+ days out without touching on specific details (i.e. snow totals, exact temperatures, winds etc). We will also be using a brand new tool we created today that determines a “Threat Level” of these potential storms and the impacts associated with those levels. Below is the graphic we will use for this. Our goal is to keep this as simple as possible, leaving no guessing as to what we feel the impacts will be. Let’s dive right in!
One of the biggest factors in our current pattern as to why we’ve been so warm lately is the influence of the Pacific jet stream extension, which undercuts our Aleutian Low and filters in warmer air to the Heart of the Country. Any “cooler” weather we’ve seen over the past month and a half has been modified and short-lived because of this. So what we are targeting moving forward is for that Pacific jet extension to weaken and push further west. Below is an example of the continued influence from the jet stream over the Pacific Ocean through November 7th vs November 17th. Focus on the weakening and location of the jet in the circled area…this is absolutely necessary for a cooler pattern to set-up going into late November into December.
Now ask yourself, what does it take to break this current cycle that we are in with warm weather, day after day…the answer is we look to the MJO (Madden Julian Oscillation) for a “wave break” or something to influence and weaken our Pacific jet. We believe we’ve found the answer in the latest forecast of the MJO as seen below, which takes us into a Phase 7, 8 and even potentially into a Phase 1…basically this means cold weather finally becomes available. Below also is depicted what these phases look like on the MJO, notice how much “blue” covers the lower-48 in a Phase 1, which means more consistent troughs or cooler weather is able to push into the Plains eastward!
Notice also above how closely Phase 1 of the MJO resembles our top analogs for this upcoming winter (an analog year is a year that had conditions similar to what we are experiencing now)…we have ridging near or west of the West Coast (greens/yellows/oranges), we have a big-time below normal heights (the blues) filtering down into the Heart of the Country with a ridge off the east coast of eastern Canada. By the way our top analog years that we are using now are 1950-51, 1983-84, 1995-96 and 2013-2014 which all coincidentally had above normal snowfall and colder than normal temperatures as shown below.
Another huge factor especially for portions of the Ohio Valley is how warm the Great Lakes waters are…the longer we have sustained heat into November the longer these lakes stay warm. The problem with this is that when a very cold air mass rides over those warm waters it produces big-time lake-effect snow potential, so keep that in mind as we move into December.
The last thing I’d like to cover today is how we use teleconnections to forecast favorable conditions in the medium to long-range for winter weather. Without going into great detail, the 2 teleconnections that will play a big factor in setting up a pattern favorable for winter are the PNA (Pacific/North American Index) and the EPO (Eastern Pacific Oscillation). When the PNA is positive (the reds) or neutral and the EPO is negative (the blues) we get that cooler/stormier weather to filter into the Plains eastward. Here’s a great illustration of this starting to happen, below! We are targeting late month near Thanksgiving onwards for this to occur.
Summary: This pattern change will not happen overnight, it’s a step down process over multiple weeks as we head late month into December. We need to get rid of the strong influence of the Pacific jet stream and we need this warm weather (our ridge) to shift westward (+PNA, -EPO). Right now we are eyeing the last week of November into early December to start feeling these colder changes and for more “winter” weather to start showing up in the Plains to the east.
Confidence and Risk:
- We have above average confidence in the Pacific jet extension to continue to keep temperatures above normal throughout the core of the US over the next week or so
- We have average confidence right now in an MJO wave breaking down that Pacific jet extension
- We have above average confidence that if the MJO enters into a Phase 8 or Phase 1 a cooler pattern will persist
- We have above average confidence in our analog years matching current conditions across the Globe, which suggest potential for below normal temperatures for winter
- We have high confidence that lake effect snow potential will be a concern because of how warm the Lakes are at this time
- We have average confidence in a favorable teleconnection pattern into mid-to-late November that would allow cooler/stormier Canadian air to filter into the Plains eastward
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***And don’t forget our *OFFICIAL* Winter outlook comes November 15th at 8:00pm for BAM clients exclusively!