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Bob Henson blog: La Nina Declared; Cooldown Nipping Warmest Autumn on Record in NE US

Posted by Chris in Tampa on 11/10/2017, 9:05 am

"For the second winter in a row, La Niña will be in the driver's seat. The NOAA/NWS Climate Prediction Center issued a La Niña Advisory on Thursday morning, declaring that La Niña conditions were now in place in both the atmosphere and ocean. NOAA gives odds of around 65-75% that La Niña conditions will extend at least through the upcoming Northern Hemisphere winter of 2017-18.

To qualify as ongoing La Niña conditions, sea-surface temperatures (SSTs) across a region in the eastern tropical Pacific defined as Niño3.4 must be at least 0.5°C below the seasonal average, with accompanying changes to the atmospheric circulation. For an El Niño, the SST departure must be at least 0.5°C above average. To qualify as an El Niño or La Niña episode in the historical record, these conditions must be sustained for at least five overlapping three-month periods.

As recently as June, long-range forecasts were leaning more toward El Niño vs. La Niña for 2017-18, but such forecasts are often challenging until after the "spring predictability barrier." Climatologically speaking, it's not too much of a shock that La Niña is back with us. La Niña events often linger or recur for two northern winters in a row, sometimes even three. In contrast, El Niño events-especially the strong ones-are often one-year affairs, intensifying in northern autumn and dissipating the next spring or summer.

The La Niña of 2016-17 was a marginal event, just long-lived enough to qualify as a La Niña and never even reaching moderate strength. In fact, it's the only La Niña event in NOAA records going back to 1950 where three-month average SSTs in the Niño3.4 region failed to even get as cool as 0.8°C below average. The current La Niña is expected to be another weak one."

On Wednesday, Jeff Masters had a blog post about the possibility of another storm forming:

"The Atlantic hurricane season officially ends on November 30, but there is a good chance we will see Tropical Storm Sean or Subtropical Storm Sean in the coming weeks. Three of the six years with more ACE than 2017 ended up having at least one more named storm after mid-November, so past history suggests that a busy season like 2017 may see more activity. The Wednesday morning runs of the GFS and European models showed one possible area of tropical cyclone formation early next week: the waters of the east-central Atlantic near 30°N, 30°W, several hundred miles south of the Azores Islands. A non-tropical area of low pressure is expected to form in this region, where waters of about 24°C (75°F) exist. These waters may be warm enough to allow the storm transition to a named subtropical storm. We also need to watch the waters of the Western Caribbean off the coast of Nicaragua in the coming weeks, as well. Sea surface temperatures there are close to 30°C (86°F)--more than 1°C above average for this time of year, and plenty warm enough to support a hurricane--and the long-range GFS model has been predicting low pressure and reduced wind shear favorable for tropical cyclone formation in this region after November 20. Recall that last year, Category 3 Hurricane Otto formed in this region and hit Nicaragua on Thanksgiving Day-November 24."


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Bob Henson blog: La Nina Declared; Cooldown Nipping Warmest Autumn on Record in NE US - Chris in Tampa, 11/10/2017, 9:05 am

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