Hurricane researchers at Colorado State University recently released an updated hurricane season outlook, which now calls for an above-average season in the Atlantic Basin. This forecast is significant as we are entering an El Nino phase, which historically has a strong correlation with below-average tropical activity in the Atlantic. 

So why is this hurricane season expected to be more active?

The answer has to do with sea surface temperatures in the tropical and subtropical Atlantic, which are much warmer than average.

Check out the latest global sea surface temperature anomalies:

Taking a closer look at the Atlantic Basin itself, the warm sea surface temperature anomalies are even more obvious:

CSU Hurricane Season Outlook:

Hurricane researchers are now forecasting an above-average Atlantic hurricane season, noting record-warm sea surface temperatures in the Atlantic as the main culprit. 

El Nino conditions tend to result in stronger wind shear (increasing winds with height) in the Caribbean and Atlantic, which can kill off tropical storms by pushing heat and moisture away from the core of the storm. While wind shear is good for severe thunderstorms over land masses, it is bad for tropical storms and hurricanes.

Since the early 1950's, there have only been two years (1969 and 2004) in which El Nino conditions were present during the summer and the number of named storms was above average. 

However, CSU researchers are expecting the anomalous warmth in the Atlantic to counteract the ongoing and strengthening El Nino.

Here is the latest CSU forecast for hurricanes, with above-average activity expected in terms of named storms, hurricanes, and major hurricanes (Category 3 or higher on a 1-5 scale).

Full Colorado State University Hurricane Forecast for 2023

Researchers also note that confidence is lower than usual regarding this forecast, given the conflicting factors between a strengthening El Nino (unfavorable for tropical activity) and anomalously warm sea surface temperatures (favorable for tropical activity).

How Could This Impact Mountain Weather in the Eastern U.S.?

Tropical storms and hurricanes are most widely known for their impacts to coastal regions, and rightly so as hurricanes are most intense at landfall before weakening as they move inland due to the loss of their fuel source (i.e. warm ocean water). 

However, tropical storms have significant impacts inland as well as abundant moisture originating from these storms can produce very heavy rainfall and flooding.

When moisture from tropical systems encounters a mountain range, orographic lift can result in even heavier rainfall compared to adjacent lowland areas. Heavy rain with tropical origins in mountainous regions of the Eastern U.S. can cause major problems, such as flash flooding and landslides.

Also, while winds from tropical storms and hurricanes decrease moving inland when compared to coastal areas, they can still be quite strong by local standards. The uptick in wind speeds in combination with heavy rain and saturated soils can pose a risk of trees being knocked over.

All areas of the Appalachians from Georgia to Maine, and into Southeast Canada, are susceptible to impacts from tropical storms. The location of impacts depends on whether or not a storm makes landfall in the Eastern U.S., where the storm makes landfall, and the direction the storm travels after making landfall.

While hurricane season officially lasts from June to November, the period from August to October is typically when the Eastern U.S. is most susceptible to tropical activity. 

Eastern Pacific Hurricane Season and Impacts on the Western U.S.

While the Western U.S. is not the first area that comes to mind regarding tropical storms or hurricanes, the West can occasionally see the remnants of tropical activity in the Eastern Pacific. 

Unlike the Atlantic, El Nino tends to favor above-average hurricane seasons in the Eastern Pacific as wind shear is typically weaker here during El Nino phases. 

CSU does not issue forecasts for the Eastern Pacific, but NOAA's latest outlook favors an above-average hurricane season.

This will be something to keep an eye on during the late summer and fall months, as tropical moisture from these storms can sneak into the Southwest and West Central U.S. Although somewhat rare, Hawaii can occasionally see more direct impacts from tropical storms and hurricanes as well. 

Currently, sea surface temperatures are below average across the coastal Pacific including the Baja Peninsula, so it's possible that significant tropical activity in the Eastern Pacific may be favored for later in the season (September-October) compared to earlier. 

Depending on the seasonal timing, moisture from the remnants of Eastern Pacific tropical activity can help to enhance the North American Monsoon if it is ongoing, or can act as a source of moisture on its own.

Moisture surges from Eastern Pacific tropical activity can bring more widespread showers and thunderstorms to parts of the Western U.S., leading to an increased risk of lightning and flash flooding, even as late as October. If drought conditions or wildfires are present, however, this moisture could be viewed as beneficial.