Why Hermine Has Caused Extreme Forecast Headaches

September 4,2016
On August 18, the National Hurricane Center designated a tropical wave that emerged off the coast of Africa, Invest 99L. Since then, this tropical system has endured a 4,000-mile journey across the Atlantic, through the Gulf of Mexico and into the southeastern U.S.
(MORE: What is an "Invest"?)
Despite initial forecasts that it would develop into a tropical depression and tropical storm in a short period of time, the process of just becoming a tropical storm was agonizingly slow.
(MORE: 4 Tropical Storms That Overachieved)

Hermine's Odyssey

Hermine began a long journey over two weeks ago.

Hermine's eventual development was delayed for such a long time because of two main factors: dry air and wind shear.
For several days, there was little chance that Invest 99L would develop due to an bundance of dry air in the eastern and central Atlantic, along with significant wind shear. Tropical systems like calm and moist environments in order to develop, even if ocean temperatures are warm enough.
By August 23, 99L was moving into the Lesser Antilles but it remained a tropical wave as it continued to fight wind shear. 99L dumped flooding rain on Puerto Rico and the Dominican Republic.

Computer model ensemble forecast GFS and UKMET August 23, 2016.
(NOAA/ESRL/GSD)

Forecast models indicated the system may develop near the Bahamas, and eventually push into south Florida as a tropical storm or even a hurricane. Wind shear, however, prevailed and the system remained unorganized as it moved through the Bahamas and into the Gulf.
Finally, the wind shear was expected to relax in the Gulf of Mexico and warm water temperatures prevailed.
On Aug. 26-27 forecasters had their eye on satellite and other data in anticipation that it would be designated as Tropical Depression Nine.
The fact it would likely strengthen and become a threat to the U.S. triggerd extreme interest. Comments on social media began to explode.
Finally, 10 days after becoming Invest 99L, the National Hurricane Center designated it as Tropical Depression Nine as it was crossing into the Gulf of Mexico.
Subsequent advisories from the National Hurricane Center forecasted Tropical Depression Nine to strengthen and become Tropical Storm Hermine in short order, but the wait would prove to be quite a long one.

Wind shear (southwesterly winds aloft) slowed development of Tropical Depression 9 earlier this week.

As it turned out, wind shear in the Gulf of Mexico was greater than expected and this hindered development of the tropical depression.
It seemed like the entire meteorological community, and weather enthusiasts on social media, anxiously wanted this storm to get a name.
Finally, on August 31, Tropical Storm Hermine was named. It became a Category 1 hurricane September 1 as it was approaching landfall in the Florida panhandle.
The general track forecasts, that Hermine would gradually intensify and reach the Florida panhandle, were good as a trough (dip in the jet stream) would pull it northward.
Hermine became a Category 1 hurricane on September 1 as it was moving toward the Florida panhandle and made landfall near St. Mark's Florida around 1:10 a.m. of September 2.
(MORE: Gulf of Mexico Hurricane Drought was the Longest in 130 Years)

Forecast Challenges Continue

A (post tropical) Hermine will be trapped under an upper-level ridge for several days, blocking its movement.

Hermine has more up its sleeve for the U.S. over the next several days. Earlier computer models indicated











































that Hermine would cross parts of northern Florida and southern Georgia and the Carolinas before moving out into the Atlantic. From there it would move northeastward and back out to sea before stalling off the mid-Atlantic Coast.
That is exactly what happened, so the models had a good handle on the storm at that time.
The reason for the model shifts and subsequent changes in the forecast had to do with a complex set of factors that came together in the upper atmosphere.
The trough that pulled Hermine northward weakened and moved away, leaving the storm behind
An upper ridge (bulge in the jet stream) built north of Hermine and that blocked its movement, allowing it to slow down.
There are still some questions that need to be resolved.
How close Hermine is to the East Coast during the early part of the week will determine the severity of impacts from the coastal mid-Atlantic to the Northeast U.S.
It would be wise to keep track of any forecast changes and future developments.
(MORE: Hermine Will Leave a Mess of Coastal Flooding, Beach Erosion, Winds Along Northeast Seaboard)

Hermine Projected Path

MORE: Hurricane Hermine-Florida August/September 2016