It’s that time. The time to think about (and prepare for) hurricane season, which begins on June 1. Each year, the tropical weather research group at Colorado State University issues an outlook for the season — how many tropical storms and hurricanes the Atlantic may see in the coming months.

This year, the group predicts hurricane season will be just shy of average. They think there will be 11 named storms. Of those, they believe four will become hurricanes, and two will become major hurricanes (Category 3 or stronger). What this prediction doesn’t try to determine is how many of these storms will make landfall.

An average season in the Atlantic consists of 12 named storms, six hurricanes and two major hurricanes. The group’s predictions will be updated on June 1, July 3 and Aug. 2.

Two issues that are critical for determining the severity of the 2017 Atlantic hurricane season are the future state of El Niño as well as the setup of North Atlantic sea surface temperatures.

El Niño

We briefly ventured into weak La Niña over the winter, with associated cooler-than-normal sea surface temperatures (SSTs) in the eastern and central tropical Pacific. Several weeks ago, NOAA officially declared La Niña over, meaning that we now have near-normal SSTs across this portion of the basin.

While there is considerable uncertainty as to whether El Niño (warmer than normal waters in the central and eastern tropical Pacific) will develop, there are indications that this may already be happening. SSTs off the west coast of South America are much warmer than normal right now, and Peru has recently had anomalously heavy rainfall and significant flooding. Both of these conditions are consistent with the development of El Niño. However, SSTs in the central tropical Pacific remain slightly cooler than normal, a vestige of winter’s weak La Niña.

The warming associated with most recent El Niño events has progressed from west to east across the tropical Pacific basin, with strong westerly wind anomalies driving warm water from the western Pacific eastward. What is notable about the current event is that the focus of the warming is in the eastern Pacific, with warming slowly progressing westward.

Low-level winds blowing across the eastern Pacific have been very weak over the past two months, helping to support the warming SSTs in this region. Typically, low-level winds in this region blow out of the east, so westerly anomalies mean weaker winds than normal.

One model that is known to have considerable skill at predicting the future state of El Niño is the European Center for Medium Range Weather Forecasts (ECMWF) model, which is calling for moderate El Niño conditions by September, the peak of hurricane season. Typically, El Niño generates stronger upper-level westerly winds that tear apart Atlantic hurricanes in the tropical Atlantic and especially in the Caribbean.

A wide variety of statistical and dynamic models issue forecasts of the future state of El Niño. There is a considerable spread in the model output for the next few months, with the dynamic models in general calling for more warming than the statistical models. Historically, the Northern Hemisphere spring is known to have a “predictability barrier, in that the models have a more difficult time forecasting El Niño during this portion of the year than during the remainder of the year.

North Atlantic temperatures

The western North Atlantic is currently somewhat warmer than normal, while the far North Atlantic and eastern tropical Atlantic are cooler than normal. SST anomalies in the eastern tropical Atlantic and far North Atlantic have a higher correlation in March with overall Atlantic hurricane activity than do SSTs in the western North Atlantic. The current SST anomaly in the North Atlantic, if it were to persist, could potentially drive a near or slightly below-normal hurricane season, even if El Niño were not to develop.

Typically, colder-than-normal water in the far North Atlantic is indicative of a negative phase of the Atlantic Multi-decadal Oscillation (AMO). When the AMO is negative, it tends to portend quieter periods for Atlantic hurricanes. The cold waters in the far North Atlantic tend to force higher pressure in the tropics and subtropics and associated stronger winds blowing across the tropical Atlantic. This was certainly the case in March, where considerable anomalous cooling occurred across most of the North Atlantic. In general, a colder tropical Atlantic is less conducive for Atlantic hurricane development, due to the colder water providing less fuel for developing hurricanes. In addition, a colder tropical Atlantic is typically associated with both higher pressure and drier middle levels of the atmosphere, both of which suppress the deep thunderstorm activity necessary for hurricane development.

The combination of the potential development of El Niño and the recent cooling across most of the North Atlantic are the primary reasons CSU came out with an outlook for a slightly below-normal hurricane season. A more detailed discussion of the 2017 outlook is available here.