Sun Jul 16 2017 07:49 PM
Re: 2017: 'Active Era' + Busy Season?

July 16 2017 Update

Since the initial post a month ago on June 14, more things have become clear, and will be covered.

Thanks in advance to the many experts who have taken the time to pour over data and create several of the maps, tweets, etc which I will be including below.

The question, 2017: 'Active Era' + Busy Season?, is an important one, because while 'It only takes one,' and there have been plenty of seasons with below-average totals, but which produced one serious storm with a trajectory that had devastating impacts, it is also more than academic to ponder whether or not 2017 will likely produce more named storms and hurricanes, and whether or not the 'Active Era' is still ongoing.

AMO Phase still unclear

The SST anomaly and weather patterns during the three consecutive years of 2013, 2014 and 2015 do stand out as having looked very much like what one might expect going into a negative phase, but since '16, and now with what we are seeing this year, calls of the end of the active era have been put in doubt. In fact, 2016 saw more Named Storms and Majors in the Atlantic basin than in any year during the last negative phase (1970-1994), and the ACE (Accumulated Cyclone Energy) of 141 was greater than all but one year during that time. (h/t Daulton Bahm).

To me, the current SST and sea level pressure patterns in the Atlantic might best be described as mixed/neutral, with a slight, yet sloppy, positive bias, when considering all of the competing AMO definitions most experts consider. Thus, if my own take on this 'hot topic' (see what I did there?) is right, the AMO should at the very least not have a suppressing effect heading into the peak Atlantic hurricane months of August-September-October, and could, potentially, be supportive of development.

Medium-term Patterns

So with the longer-term state of the AMO still a bit up in the air, although probably not yet negative, what can be said of current trends and forecast conditions for the rest of the season. Here's where things really start to get interesting... I am just cutting and pasting a series of tweets or stories from a variety of experts for your consideration.

With regard to the Saharan Air Layer (dry, dusty, convection-inhibiting low to mid-level air from Africa that travels west across the Atlantic), after a much above average spring, SAL has been running very low so far this summer (h/t Michael Lowry)

JMA's forecast precipitation anomaly for peak Atlantic hurricane months suggest an active Aug-Oct in the tropics, as well as potentially along the US south and east coasts. (h/t Ben Noll)

SSTAs in the Main Development Region (and elsewhere) in the Atlantic have definitely been trending up this year (compare with 2014 and 2015!) .. Source: Michael Lowry

Potential for a peak hurricane season with below normal shear values across the MDR per JMA forecast. Source: Ben Noll

Two other factors that are outsized contributors to Atlantic hurricane seasons are ENSO and Atlantic MDR sea level pressure anomalies.

Sea level pressures in the MDR should be lower than average, if the anomalous warmth verifies.

As for ENSO (potential for a disruptive El Niño event in the Pacific), most forecasts are still looking for either a very mild El Niño, or ENSO neutral conditions to prevail.

The latest CPC update as follows

13 July 2017

ENSO Alert System Status: Not Active

Synopsis: ENSO-neutral is favored (~50 to 55% chance) into the Northern Hemisphere winter 2017-18.

During June, ENSO-neutral continued, although equatorial sea surface temperatures (SSTs) remained above average in the central and east-central Pacific Ocean [Fig. 1]. The latest weekly Niño index values were near +0.5°C in the Niño-4 and Niño-3.4 regions, and closer to zero in the Niño-3 and Niño-1+2 regions [Fig. 2]. The upper-ocean heat content anomaly was above average during June [Fig. 3], reflecting above-average sub-surface temperatures across the central and eastern Pacific [Fig. 4]. In the atmosphere, tropical convection was suppressed over the west-central tropical Pacific and enhanced over the Maritime Continent [Fig. 5]. The lower-level and upper-level winds were near average over most of the tropical Pacific, and the Southern Oscillation Index (SOI) and Equatorial SOI were slightly negative to near-zero. Overall, the ocean and atmosphere system remains consistent with ENSO-neutral.

Some models predict the onset of El Niño (3-month average Niño-3.4 index at or greater than 0.5°C) during the Northern Hemisphere summer [Fig. 6]. However, more than half of the models favor ENSO-neutral through the remainder of 2017. These predictions, along with the near-average atmospheric conditions over the Pacific, lead forecasters to favor ENSO-neutral into the winter (~50 to 55% chance). However, chances for El Niño remain elevated (~35-45%) relative to the long-term average. In summary, ENSO-neutral is favored (~50 to 55% chance) into the Northern Hemisphere winter 2017-18 (click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period)

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