Monday, September 10, 2018

Tropic Thunder

The tropical Atlantic was fairly quiet this summer. Until last week there were only five named storms, only two of which reached hurricane strength.

Map showing first five tropical systems in the Atlantic in 2018
Tropical Storm Gordon made landfall in the Gulf last week and its remnants brought heavy rain to the Midwest and mid-Atlantic this past weekend.

Since last week, however, three new storms are churning way in the Atlantic, and one area of disturbed weather over the northwestern Caribbean Sea has a 60 percent chance of developing into a tropical system within the next five days.

The focus of all the attention the last few days has been Hurricane Florence. Florence started as an area of low pressure that moved off the African Coast 12 days ago on August 30. The depression became Tropical Storm Florence on September 1. Florence attained hurricane strength for the first time on September 4. The storm encountered an area of upper level shear as it moved west, which weakened Florence considerably and it was downgraded to a Tropical Storm late on September 7. All indications were that Florence would reach hurricane strength again, and yesterday morning it did. The storm underwent rapid intensification today, and as of this writing Hurricane Florence is a category 4 storm (maximum winds 140 mph) heading for the Southeast Coast of the United States. It's possible it could briefly reach Category 5 strength on September 11.

We are also keeping an eye on Hurricane Issac. As of this writing it is a Category 1 storm with top winds of 75 mph. Its projected path will take through the Lesser Antilles and to the south of the Dominican Republic by Saturday as a tropical storm. Its future beyond Saturday is uncertain, but it will be over the warmer waters of the Caribbean Sea.

Hurricane Helene is currently the last in line of the storms in the Atlantic right. This storm is expected to re-curve to the north and weaken by Thursday.

Meanwhile, in the Pacific Tropical Storm Olivia is heading for the Hawaiian Islands. A Flash Flood Watch has been issued for all the Hawaiian Islands though Thursday night. Olivia is forecast to produce total rainfall amounts of 10 to 15 inches. Isolated maximum amounts of 20 inches may occur along the windward sections of Maui County and the Big Island.

Forecast track for Tropical Storm Olivia

Back to Florence, though. The various forecast model projections were the subject of much discussion the past few days. There initially was a wide spread in possible tracks, including the possibility that Florence could re-curve north before reaching the coast. However, as the weekend progressed the models increasingly began to converge on a solution showing landfall somewhere along the Southeast Coast of the U.S. The current forecast (5:00 p.m. EDT 9/10) brings Florence ashore somewhere between Charleston, SC and Cape Hatteras, NC sometime on Thursday likely as a Category 4 storm.

The footprint of Florence is going to be very large, probably as big as South Carolina, North Carolina, and Virginia combined. While winds will certainly be a big concern especially close to landfall, the rain footprint of Florence is what is really scary. Indications are that Florence will stall over the Carolinas blocked by a strong ridge of high pressure over the northeast U.S. The storm's circulation will continue to draw moisture from the Atlantic into the Carolinas, Virginia, and northward. This will mean days of heavy, flooding rain. Here is the latest precipitation forecast for Florence from the Weather Prediction Center.

While the winds with Florence will be significant if the forecast holds, it is the  storm surge and the rain and resultant inland flooding that will make this a notable storm.

CoCoRaHS observers: If you are in the Carolinas and Virginia please remove the funnel and inner cylinder from your gauge as they can become projectiles in high winds. In heavy rain the outer cylinder alone will also be more efficient for catching heavy rain (no splashout from funnel). If you're given evacuation orders, please follow them. Your safety is our number one concern!

For the latest information on Hurricane Florence visit the National Hurricane Center website.