Florence aims for Carolinas, but then what?


SOURCE: NASA

Wednesday, Sept. 12, 2018 – 12:55 p.m.

Hurricane Florence remains a powerful hurricane as it approaches the coastal Carolinas, but changes in guidance models since Tuesday are brining new concerns.

CURRENT STATUS

As of the 11 a.m. ET advisory on Wednesday, the storm was moving more northwest (305 degrees) at 13 knots (15 mph). The minimum central pressure dropped slightly to 943 millibars, according to the National Hurricane Center. But maximum sustained winds remained at 130 mph. Florence was 485 miles southeast of Wilmington, N.C.

IMMEDIATE FORECAST

The official forecast has the center of hurricane nearing the coast Thursday night, but it’s important to note that conditions in the eastern Carolinas will deteriorate ahead of the center. Winds and waves will begin to increase along with the outer rain bands making their way inland.

Those in the Carolinas need to have their storm preparations complete by the end of Wednesday.

‘THAT ESCALATED QUICKLY’

Overnight, the ensemble and operational guidance of the GFS and European models have shifted their tracks beyond Thursday a bit farther south.

The top graphic shows the ensemble members of the American GFS (all images are from Weathernerds.org, by the way) while the second graphic shows the latest European model runs. Notice that the ensemble members with the Euro take it just a little farther south than the GFS. My hunch is because of the GFS’ tendency to have a northern bias, which atmospheric scientist Philippe Papin explains well in this twitter thread.

It’s too early to say which exact scenario would happen as ensembles are all over the place. But the Carolinas can still expect storm surge along the coast and adjacent bays and rivers, hurricane-force winds along and close to the coast, and very heavy rain.

The seven-day forecast from the Weather Prediction Center has the heaviest amount of rain over southeastern North Carolina with as much as 20 inches of rain possible.


SOURCE: Weather Prediction Center/NOAA

Heavy rain would spread inland to western North Carolina and parts of Virginia and South Carolina. Of course, isolated higher amounts can’t be ruled out.

BUT WHAT ABOUT THE TRACK?

The National Hurricane Center didn’t make too many adjustments to their official 11 a.m. forecast since the previous one (5 a.m.)


SOURCE: National Hurricane Center/NOAA

But the change since Tuesday night has been the southerly shift. With this cone, there is even the potential that Florence would hug the coast as it moves southwestward before moving inland for real. That scenario would help maintain some of Florence’s intensity as the center would still be over water. It may not be as strong since the upwelling of the water and the hurricane pulling in drier air parcels from the land would limit or even decrease the intensity.

Trends in guidance need to be monitored. New runs from the GFS and Euro should be released mid to late Wednesday afternoon; therefore, we should see if the southerly trend continues or stops (5 PM Wed. UPDATE: The trend has stopped as of the late afternoon model runs).

BUT WHAT’S STEERING THIS THING?

A mid-level high to the north of Florence is steering it northwestward, but as it gets closer to the coastline, it’s expected to run into a mid-level high in the eastern and Midwestern U.S. This high will trap it, and may drift it southward.


SOURCE: Weathernerds.org – This edited model output shows high pressure at 500 millibars over the eastern portion of the U.S. back to the Midwest.

This ridge is complicating the forecast. As indicated by the ensemble guidance, there are a number of scenarios. But it’s necessary to stress to not focus on the line. It’s impacts will be beyond the center of circulation with the concern for heavy rain.

WHAT THEN?

There is some chance that some in eastern Georgia could see tropical-storm-force winds sometime starting Sunday, as noted by the National Weather Service Forecast Office in Peachtree City, Ga.

There is increasing indications that Florence will stall for a period somewhere near the NC to SC coastline then take a track to the SW before heading up somewhere between the SAV river Valley and the center of GA. This will mean increased winds and rain chances for a large portion of the area somewhere in the Sunday to Tuesday timeframe.

The GFS wants to take Florence up the SAV river valley and the European up the center of GA or so. Both models greatly weaken Florence as this occurs from a wind standpoint. The southern Appalachians and portions of N and E GA could become a target for heavy rainfall.

Let me be clear that the details of the forecast are still highly uncertain but there is some increased confidence for a wetter and windier period.

As the last paragraph noted, uncertainty is in the forecast for Georgia.

As for the Big Bend and South Georgia, I have the same thoughts with uncertainty as the NWS in Peachtree City. Any forecast impact at this time regarding Florence would be of low confidence, but will likely be of lower impact for the area (maybe a breeze and some rain). Florence will be monitored over the next few days.

As for the Carolinas, everyone that was ordered to evacuate should have done so. And those who aren’t asked to evacuate should be hunkered down.