Jan 30, 2026

Probabilistic Extra-Tropical Storm Surge model upgraded to version 1.4

In September 2025, the National Oceanic and Atmospheric Administration's National Weather Service (NOAA NWS) Meteorological Development Laboratory (MDL) upgraded its operational Probabilistic Extra-Tropical Storm Surge (P-ETSS) model to version 1.4. P-ETSS leverages wind and pressure output from the United States' 31-member Global Ensemble Forecast System (GEFS) and Canada's 21-member Global Ensemble Prediction System (GEPS) to calculate an ensemble of storm surge results, from which probabilistic storm surge products are subsequently derived.

P-ETSS primarily serves as a tool for analyzing broad extra-tropical storms, as opposed to tighter tropical storms. This distinction arises from the spatial resolution limitations inherent in global ensemble models. Specifically, GEFS produces outputs on 3-hourly, 27.5 km grids, while GEPS employs 3-hourly, 55 km grids. Unfortunately, a 27.5 km grid lacks the resolution required to accurately capture a tropical storm with the same fidelity as a parametric wind model (such as the one utilized by P-Surge). Nevertheless, a significant advantage of utilizing the global models is that P-ETSS can be run four times a day and applied to any computational domain. This makes it ideal for extra-tropical storms and even certain broad tropical storms.

This latest version extends guidance to several new locations, including Seattle, WA; San Francisco, CA; and Puerto Rico and the U.S. Virgin Islands. Additionally, the computational grid for Fort Myers, FL was replaced, and the grid for Kotzebue, AK was corrected. Furthermore, the tidal constituents utilized for the West Coast were increased from 13 to 37, while the wind interpolation method for the East Coast was corrected. Finally, several upgrades were implemented at various stations, and bug fixes were applied.

Feedback from WFO Wakefield, VA Science and Operations Officer Mike Dutter during Hurricane Erin (2025) stated: "This looks to be a huge improvement from the current operational PETSS," suggests the wind interpolation correction lead to improved results at Sewells Point, VA. WFO Wakefield was able to confidently use the updated version of PETSS to provide coastal flood scenarios (including "worst case" and "best case") to decision makers in Hampton Roads.

Feedback from WFO Anchorage, AK Meteorologist Chris Quesada with regards to the use of P-ETSS during forecasts for Ex-Halong's impacts on Alaska stated: "... P-ETSS [continues to be] an invaluable tool for anticipating coastal flooding events in Alaska. It's the primary guidance we look at to assess how high water levels might get during a storm event and whether there is a coastal flooding threat for communities in our forecast area... though we had low confidence regarding actual water levels, during Ex-Halong the (P-)ETSS forecasts were much higher than anything we had seen before and that keyed us into the potential for a highly impactful event. This gave us confidence to alert communities along the Kuskokwim Delta coast of potentially significant coastal flooding, higher than the August 2024 event which was one of the highest water levels seen in recent years in the area." Their confidence in P-ETSS results in saving lives in those communities.

Below are some hydrographs for locations that experienced significant impacts during Ex-Halong, including output several days before and just prior to the event. 

Nome Norton Sound AK forecast from Friday 10/10/2025 00Z:

Nome Norton Sound AK forecast from Sunday 10/12/2025 00Z:

The actual peak for Nome was about 4.3 ft above MHHW at 10/13/2025 09z, which was within the forecast range for the P-ETSS output.

This upgrade was made possible through a collaboration between the NOAA NWS Meteorological Development Laboratory Storm Surge Team, NWS's Eastern Region (for the station improvements), NWS's Alaska Region (for the work on Kotzebue), NWS's Weather Forecast Office in San Juan, PR, and NCEP Central Operations (NCO).

For more information on P-ETSS V1.4, please see the corresponding Service Change Notice [PDF].