Florence Caveat Forecast

More Hurricanes Than Expected in 2018
09/10/2018, 11:24 am EDT
Hurricane Florence Moves Inland Today SLOWLY as Hurricane Conditions in North Carolina Continue
09/14/2018, 8:02 am EDT
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Making the case for a double landfall for Florence

Not yet formed Joyce may play roll in the Florence weekend track

Discussion: Category 4 Hurricane Florence is barreling west-northwest at 17 mph forecast by NOAA/NHC to strike the North Carolina coast early Friday afternoon. Florence will be a massive storm and despite the slowdown approaching the coast it’s likely the forward momentum of such a large storm will carry Florence slowly inland this weekend. However, there is a caveat presented by both the 12Z ECMWF and 18Z GFS operational forecast models. The new issue is interaction of the subtropical ridge with another evolving tropical storm – Joyce in the western Gulf of Mexico and into the southern Texas coast late Friday.

At the moment, Tropical Depression 95L is located on the northeast tip of the Yucatan Peninsula. NOAA/NHC is forecasting a 70% chance that 95L will become a tropical cyclone after 48 hours in the Gulf of Mexico. The ECMWF model follows-suit developing Joyce to a tropical storm rolling into the southern Texas coast late Friday. This system moves over very warm water and will intensify rapidly. Joyce will take a large mass of convective rains into Texas which is the key to a possible adjustment to the Florence forecast track in the next day or 2.

The forecast is straight-forward through Thursday as the Bermuda High northeast of Florence provides a steering current that glides Florence toward the North Carolina coast likely slowing a bit as the steering current weakens later Thursday (Fig. 1-2).

As Florence strikes the North Carolina coast Friday the steering current pushing Florence northwestward is gone (Fig. 3). The heat release form the Texas convection associated with Joyce releases heat into the middle atmosphere over the Missouri Valley causing a new subtropical high pressure area to develop (Fig. 4).

The new subtropical high pressure ridge provides the new steering current to guide Florence southwestward along the Carolina Coast and inland (again) South Carolina Saturday afternoon. By Sunday the upper ridge crests over the Great Lakes and the east-to-west current guiding Florence pushes her westward across the Southeast States (Fig. 5).

By early next week Florence is a rain zone in the southern Tennessee Valley weakening and beginning the northward recurving process (Fig. 6).

Summary: NOAA/NHC forecast track of Florence into south-coastal North Carolina is well supported by steering guidance provided by the Bermuda High. However, once Florence reaches the coast she’ll rely on forward momentum of a massive storm to ride inland. The steering guidance is lost after landfall and Florence could become stationary. A new subtropical high forms in the Missouri Valley late this week caused by a heat release form a new tropical storm rolling into southern Texas late Friday. The new subtropical high eventually produces a steering current pushing Florence southwest toward South Carolina then west across the Southeast Sates. If Joyce develops a trend by Florence following this ECM and GFS forecast is likely.

Fig. 1: ECMWF projected upper air pattern identifying subtropical ridge steering Florence west-northwestward 2PM EDT WED.

Fig. 2: ECMWF projected upper air pattern identifying subtropical ridge northeast of Florence weakening but still strong enough to guide Florence northwestward toward NC Coast 2 PM THU.

Fig. 3: ECMWF projected upper air pattern identifying lack of established high pressure ridge areas to guide Florence therefore a slowdown on the NC Coast 2PM EDT FRI. Note the release of warm air aloft by convection associated with a new tropical storm (Joyce) into southern Texas Friday causing a new high pressure to form over the Missouri Valley.

Fig. 4: ECMWF projected upper air pattern for 2PM EDT SAT indicates the steering current around the Midwest U.S. high pressure area pushes Florence southwestward.

Fig. 5: ECMWF projected upper air pattern for 2PM EDT SUN indicates a strong high latitude ridge over the Great Lakes region guiding Florence westward across Georgia.

Fig. 6: Remains of Florence shifts into the southern Tennessee Valley early next week pushed by the upper level high pressure center over the Northeast U.S.