North American Winter Outlook 2018-19

Hello everybody, sorry for the delay, but here is my final seasonal forecast for Northern America this coming winter. This sums up both my thoughts on Western & Eastern US, and will analyse them together.

Model forecasts:

Let’s try something a bit different, by reviewing a PNA Chart from the ECMWF, and later the UKMET.

ECMWF shows a positive PNA. This indicates Western US blocking, which reduces the snowfall chances for that half of the country. It also correlates with Eastern US troughing, so more snowfall for the Eastern US.

NMME is also playing towards a trend of above average temperatures for the Western US. It also shows neutral temperatures for the Eastern US.

UKMET shows a more neutralised outlook for the PNA, still slightly positive.

CFS (Courtesy of Kyle MacRitchie) shows from about mid December, a +PNA trend, which would bring more cold for the East, until the model run end in Mid February.

So the models are pretty unianimous in giving:

  • Above average temperatures to the Western CONUS
  • Below average temperatures to the Eastern CONUS
  • A +PNA
  • Above average snowfall in general for the Eastern CONUS
  • Below average snowfall in general for the Western CONUS

Climate Driver discussion

ENSO

ENSO is currently neutral, according to the NOAA, with a 80% chance of El Niño developing in the Pacific. My current forecast is for a moderate (+1.0-+1.5) Central Pacific El Niño event (not strict Modoki, important to note). El Niño allows a number of things to occur:

  • a tendency towards a +AAM (will discuss in detail later)
  • a tendency towards a -NAO (broad correlation with more snowfall for the Eastern US)
  • a tendency towards a -AO (broad correlation with more snowfall for the Eastern US
  • -SOI conditions (broad correlation with more troughing on both sides of the continent

The consensus is towards a El Niño in the Niño 3.4 sector from the IRI model consensus chart:

My overall feelings about this El Niño is that it will bring moderately above average snowfall to the West Coast (the more Coastal, the better), but it will bring moderate-strong snowfall conditions to the East Coast, because of downstream patterns, and the support of the Atlantic and Arctic via ENSO.

The AAM

Atmospheric Angular Momentum (which can be demonstrated through it’s Global Wind Oscillation Phases) is often considered a larger part of the ENSO mechanism, and is the way it distributes it’s influence globally. My grand predictions for the AAM is +0.5 – +1.5, with the GWO fluctuating around Phases 5&6, occasionally coming around Phases 7 & 8, back in a cycle through a neutral GWO, to Phase 5. The EAMT (East Asia Mountain Torque) will be largely positive this winter, and I expect the North American Mountain Torque to be largely following through on the +EAMT, with a positive stage a week after typical +EAMT events. A +NAMT brings troughing to the West Coast, and higher heights to the Central and Eastern US. During these events, it will be the opportunity for the West Coast to rack up big snowfall totals. But beside these times, the balance will largely tip in the favour of the East. So watch out for a +EAMT and +NAMT (First post with monitoring links).

Here is the +1.0 +AAM composite just to demonstrate what it does to the atmosphere.

When the +AAM pushes upon the continent’s weather, it favours the Western US in terms of snowfall.

Stratosphere

CFS shows the stratosphere (60N 10hPa wind) weaker than normal throughout the winter. This keeps a roughly -AO (more favourable for the Eastern US) setup going, although the likelihood of strong -AOs are less likely, because of a lack of SSW potential.

https://platform.twitter.com/widgets.jsEC Monthly from last month, shows potential for a relatively strong vortex through December, and then weakening through Jan & Feb, and then lingers through the Spring, potentially leading us towards a late season in North America.

MJO

CFS shows the MJO from about Phase 7-2 throughout this winter. This allows the hitting of the Phases 6 & 7 which are favourable for SSWs, and then favourable for Phases 8-2, which are favourable for the East Coast. Phases 4-6, which are likely going to be missed at times this winter, is more favourable for the West Coast. SST Forcing over the Indonesia and Central Pacific regions is supportive of this.

QBO

The QBO is currently transitioning from negative to positive, and the level of descent is currently between 40-50mb. I expect 50mb to be positive by Mid December. The levels of descent contradict each other, so by that, you would consider a neutral effect, although data does suggest positive descents do correlate with SSWs. I’d expect this effect to lower as the +QBO phase doesn’t favour SSWs.

Snow & Ice

Siberian Snow Cover finished about average in October, and continues to be roughly around that mark. This has a neutral effect on the Northern Hemisphere.

Barents-Kara Sea Ice Cover is still below than normal, which favours SSWs and a -AO. It favours ridging in the Barents-Kara Seas. Below normal ice cover towards the Chukchi Sea will assist ridging in the region (associated with the -EPO).

North Pacific

The North Pacific SSTs are largely warm, with likely -EPO ridging off the coasts of British Columbia and Alaska, and then further down the US Coast. But it is important to acknowledge the somewhat overlooked tripole in the North Pacific between the red (warm) to north, then white (neutral), then red to the south. These differentials could position the STJ into California, along with the +EAMT giving them a lot of snowfall. However the -EPO also supports troughing in the Eastern US, bringing more snowfall towards there.

Solar

We are nearing the solar minimum, so it correlates with a -AO, or more troughing/snowfall for the Eastern US.

Conclusions

Okay, so here is my 500mb heights forecast for Winter 2018-19 for North America.

  • I forecast California to have an above average snowfall season.
  • I forecast Colorado/Utah/NM/AZ to have an average, maybe even slightly above average snowfall season
  • I forecast the PNW and BC to have a below average snowfall season.
  • I forecast the Eastern US (New England and Mid Atlantic) to have a moderately-strongly above average snowfall season, particularly in December and February

This is because of a:

  • +PNA
  • A generally +AAM winter, with periods of +EAMT and +NAMT
  • A moderate Central Pacific El Niño
  • Transition to a +QBO
  • Solar Minimum
  • Neutral Siberian Snow Cover Influence
  • Lower than average Chukchi and Barents-Kara Sea Ice Cover
  • -EPO
  • Stronger Aleutian Low than average (via ENSO and AAM)
  • Weaker than average Stratosphere (More chance for SSWs)
  • Phase 7 – Phase 2 MJO cycle in general
  • The models stated above

And that is my outlook for North America 2018-19.

Disclaimer: Seasonal outlooks tend to have bias and errors, due to the fact that these forecasts are so far out. So don’t use these outlooks to make important decisions. These outlooks are meant to be interesting information, that can help to see what the season might be like.

Okay, that is an another seasonal forecasting season done for another winter. Get ready for the long term outlooks! For now, thanks for reading this North American winter outlook, I’d appreciate a follow on Twitter @longrangesnow and a subscribe to my email list by clicking on the tab on the main header above.

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