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From Dr. Kevin Reed‘s Climate Extremes Modeling GroupEstimating the potential impact of climate change on Hurricane Florence, posted on September 12, 2018.

PLEASE NOTE: The forecasts below are EXPERIMENTAL and will continue to be analyzed in more detail in the coming weeks.

CEM group members have been busy this week watching the path of Hurricane Florence in the North Atlantic. Ph.D. Alyssa Stansfield and Prof. Kevin Reed, along with colleagues at LBNL and NCAR, have for the first time been using a climate model (CAM5) to produce near real-time experimental forecasts of Hurricane Florence to assess how much human induced climate change has altered the anticipated intensity, rainfall and size of the storm. Preliminary results using forecasts initialized Sept. 11 at 00Z (i.e., Sept. 10 at 8PM EDT) are seen below (click HERE for full pdf).

For Hurricane Florence, we present the first advance forecasted attribution statements about the human influence on a tropical cyclone. We find that rainfall will be significantly increased by over 50% in the heaviest precipitating parts of the storm. This increase is substantially larger than expected from thermodynamic considerations alone. We further find that the storm will remain at a high category on the Saffir-Simpson scale for a longer duration and that the storm is approximately 80 km in diameter larger at landfall because of the human interference in the climate system.

For additional information contact Kevin Reed (

These attribution statements are enabled by real-time ensemble forecasts of Hurricane Florence performed using the Community Atmosphere Model (CAM) version 5. Two sets of ensembles forecasts were completed (Initialized Sept 11, 2018 at 00Z):

Standard Forecast: With observed initial atmospheric conditions and sea surface temperatures (SST) adapted from NOAA’s operational Global Forecast System model. This is the forecast of the actual Hurricane Florence.

Modified Forecast: With observed initial conditions modified to remove the estimated climate change signal from the temperature, moisture, and SST fields to represent a world without climate change. This is a counterfactual forecast of Hurricane Florence if it were to occur in a world without human induced global warming.

Through comparison of the standard and modified ensemble forecasts for Hurricane Florence, we quantify the impact of climate change on the storm’s size, rainfall, and intensity.

Intensity: Hurricane Florence is slightly more intense for a longer portion of the forecast period due to climate change according to the forecasted minimum surface pressure.

Individual ensemble forecasts (dashed) and ensemble mean (solid) of Hurricane Florence.
Time evolution of the ensemble average central minimum surface pressure.

Left: Individual ensemble forecasts (dashed) and ensemble mean (solid) of Hurricane Florence.
Right: Time evolution of the ensemble average central minimum surface pressure.
Red: Florence in the world that is. Blue: Florence in the world that might have been without climate change.

Rainfall: The forecasted Hurricane Florence rainfall amounts over the Carolinas are increased by over 50% due to climate change and are linked to warmer sea surface temperatures and available moisture in the atmosphere.

Storm Size: The forecasted size of Hurricane Florence is about 80 km larger due the effect of climate change on the large-scale environment around the storm.

Ensemble average accumulated rainfall Hurricane Florence forecasts.
Evolution of the ensemble average outer storm size (radius at peak wind speed of approximately 18 mph).

Left: Ensemble average accumulated rainfall Hurricane Florence forecasts.
Right: Evolution of the ensemble average outer storm size (radius at peak wind speed of approximately 18 mph).
Red: Florence in the world that is. Blue: Florence in the world that might have been without climate change.

Depiction of the CAM5 variable-resolution computational grid.

Depiction of the CAM5 variable-resolution computational grid.

Methodology. The global atmospheric model CAM5 is set up in a variable-resolution configuration with a base grid spacing of ~100 km, similar to conventional atmospheric general circulation models, and a refined region over the North Atlantic basin with a grid spacing of ~28 km. The model is initialized with atmospheric analyses from NOAA’s Global Forecast System (GFS) following the technique outlined in Zarzycki and Jablonowski (2015) and run for 7 days and the first 5 days are analyzed. For Hurricane Florence, the model is initialized on 9/11 at 00z. To account for model uncertainty in storm characteristics, a 10-member ensemble is created by randomly varying three parameters (c0_ocn, tau, and dmpdz) in the deep convective parameterization (Zhang and McFarlane 1995). TC tracks from the forecast runs are generated using the TempestExtremes algorithm (Ullrich and Zarzycki 2017). For modified forecasts with the climate change signal removed, the methodology follows the the framework of Wehner et al. (2018). In particular, the air temperature, specific humidity, and sea surface temperature from the observed initial conditions are modified to remove climate change effect. Data from the C20C Detection and Attribution project ( define the initial conditions for the counterfactual “storm that might have been”. Differences between global simulations driven by observed boundary conditions and simulations driven by conditions with the human induced climate change removed are calculated for September over the 1996-2016 period and approximate the change in the large scale environment attributable to climate change. Additionally, the greenhouse gas concentrations, solar radiation conditions, ozone concentration, and aerosol concentrations are all set to pre-industrial levels for the modified forecasts.

Hurricane Florence is expected to make landfall along the East Coast on Friday. Check the CEM Group website in the coming days for updated forecasts and analysis!

For questions contact:


Ullrich, P. A., and C. M. Zarzycki (2017), TempestExtremes: A framework for scale-insensitive pointwise feature tracking on unstructured grids, Geosci. Model Dev., 10, 1069-1090, doi:10.5194/gmd-10-1069-2017.

Wehner, M.F., C. M. Zarzycki, and C Patricola (2018). Estimating the human influence on tropical cyclone intensity as the climate changes, In Hurricanes and Climate Change, Springer, Vol. 4., in press.

Zarzycki, C. M. and C. Jablonowski (2015), Experimental tropical cyclone forecasts using a variable-resolution global model. Mon. Weat. Rev., 143(10), 4012–4037. doi:10.1175/MWR-D-15-0159.1.

Zhang, G. J., and N. A. McFarlane (1995), Role of convective scale momentum transport in climate simulation, J. Geophys. Res., 100(D1), 1417–1426, doi: 10.1029/94JD02519.


Additional Press Coverage
New York Times: North Carolina, Warned of Rising Seas, Chose to Favor Development

New York Times: Hurricane Florence’s Path: Winds of Category 2 Storm Begin to Hit Carolina Coast

AXIOS: The ties between Hurricane Florence and climate change

Newsline: Hurricane Florence’s Direction: Class 2 Storm Closes In on Carolina Flit

Technology Review: Man-made climate change is boosting Hurricane Florence’s predicted rainfall by 50 percent

National Geographic: Hurricane Florence’s Rains May Be 50% Worse Thanks to Climate Change

Buzzfeed: Here’s How Climate Change Put Hurricane Florence On Steroids

The Guardian: Climate change means Hurricane Florence will dump 50% more rain

Think Progress: Trump says Florence is just ‘Mother Nature.’ A stunning new climate study says he’s wrong.

Ars Technica: A quick simulation of Hurricane Florence done without climate change

Bloomberg: Florence’s Unique Path From Africa to U.S. Tied to Global Warming

The Post and Courier: Hurricane Florence will unleash a torrent in SC; scientists blame global warming

Science News: Here’s how climate change is fueling Hurricane Florence

Curbed: Hurricane Florence will dump up to 50 percent more rainfall due to climate change

NY Daily news: Hurricane Florence slams North Carolina with rain and wind as storm closes in on southeast coast

EcoWatch: Hurricane Florence: Carolinas to See 50% More Rain Due to Climate Change

Live Science: Hurricane Florence Is 50 Miles Larger, with 50% More Rain, Thanks to Climate Change

Yale Environment 360: Hurricane Florence Expected to Drop 50 Percent More Rain Due to Climate Change

Inside Climate News: Hurricane Florence’s Unusual Extremes Worsened by Climate Change, Scientists Say

Climate Home News: Climate Weekly: Into the storm

Climate Home News: Call for climate justice as millions brace for intense storms

The Science Page: This Unprecedented Study Shows How Different Florence Would Be if Not For Climate Change

Motherboard: Climate Change Exacerbates Hurricane Florence as America Becomes World’s Leading Crude Oil Producer

My Palm Beach Post: Cerabino: Another year, another chance for Hurricane Limbaugh to blow

Latest Nigerian News: An unprecedented study shows how much less severe Hurricane Florence would look without climate change

NPR: What Hurricane Florence Tells Us About Climate Change

Radio National: Scientists prove climate change impact on Hurricane Florence

BBC Newshour: Typhoon Mangkhut Hits Mainland China (starting at 43:45)

Washington Post: Climate change is real. Welcome to the new normal.

New York Times: Humans Are Making Hurricanes Worse. Here’s How.

PBS News Hour: How a warming world may have caused Hurricane Florence to stall

The Weather Channel: Did Climate Change Make Florence Bigger and Wetter? Yes, Says One Early Study

Reuters: Florence portends more massive, slow-moving hurricanes in age of global warming: scientists

Salon: Climate change turned Florence into a monster storm — but mainstream media avoided that story

The Conversation: Trump’s ‘all-out effort’ on climate is derelict and risky

News Trend Today: Flooding is sending unheard amounts of water through the Carolinas 

LIDTime: Trump will visit SC to survey damage from Hurricane Florence

Stony Brook Statesman: Hurricane Florence was worsened by climate change

Newsweek: How Global Warming Is Turbocharging Monster Storms Like Hurricane Florence

BBC World Service: Discovery Heat Wave Part 2


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