Bonnie

A weak Tropical Storm Bonnie continues to head toward a landfall on the South Carolina coast near Charleston. As of the 0300Z NHC advisory, the following information was available on the cyclone:

Wind: 45 mph, with gusts to 60 mph
Location: 31.0°N 79.5°W
Movement: Stationary
Pressure: 1008 mb

Bonnie remains a sheared tropical cyclone, with the deepest convection confined to the northwest of the center due to continued southeasterly shear of at least 25 kt; in addition, water vapor imagery shows dry mid-level air surrounding the cyclone to the south. Since this is not expected to change much, Bonnie should continue to struggle through the remaining time it has over water.

Figure 1. Latest infrared satellite image of Tropical Storm Bonnie. Image credit: NOAA’s Satellite Services Division (SSD).

About the only thing sustaining Bonnie at this point is its presence over the warm waters of the Gulf Stream. With the aforementioned environmental strongholds in mind, no additional strengthening is expected before landfall, and indeed, the cloud pattern even suggests weakening. The models have varying guesses on how long Bonnie will last as a tropical cyclone, which appears directly correlated to how far – or if at all – the cyclone moves inland. The ECMWF keeps Bonnie offshore and parallel to the coast, and thus holds onto the cyclone for five days. The GFS shows Bonnie making landfall and dissipating before 72 hours. As a compromise of these extremes, my forecast shows Bonnie persisting until the 96 hour mark and moving just offshore the North Carolina coast after its initial landfall, but I would definitely not be surprised if the GFS is correct.

After stalling for the last several hours, Bonnie appears to be on the move again, and a little to the right of current NHC forecast; this was not unexpected, as the models have trended to the right on the latest cycle, likely in response to Bonnie’s slow motion enabling the trough over the plains to catch up with it before it makes landfall. Landfall should occur in about 12-18 hours, but it would not surprise me at this rate if it occurred closer to the bottom of that estimate; an alternate possibility is that Bonnie just misses the coast. In any event, steering currents are likely to collapse after the storm makes landfall, and Bonnie will move only very slowly in certainly no more than 24 hours.

Tropical storm force wind gusts will likely impact immediate coastal areas of the South Carolina and possibly far southeastern Georgia coasts through sunrise; these gusts may spread inland with time. Doppler velocities show that winds are not particularly strong on the coast, but offshore buoys show tropical storm force winds lurking just offshore.

The primary hazard from Bonnie will be locally heavy rainfall, but as with any landfalling tropical cyclone event, a tornado threat – albeit quite marginal – exists as well. Upper air soundings from Charleston show a slight increase in cyclonic vorticity along the coast relative to 12 hours ago, and with deep-layer shear of at least 30-40 kt, there remains a conditional threat of tornadoes along immediate coastal areas, perhaps extending as far west as far southeastern Georgia and as far east as extreme southwestern North Carolina.

Bonnie is the second named tropical cyclone to form in the Atlantic this year. This makes 2016 only the fourth year in the historical database to have this occurrence before the official start of the hurricane season on June 1. The others years were 2012, 1908, and 1887, respectively.

Intensity forecast

Initial 05/29 0300Z/11 PM EDT Sat 40 kt 45 mph 31.0°N 79.5°W
12 hour 05/29 1200Z/8 AM EDT Sun 35 kt 40 mph 32.1°N 80.1°W
24 hour 05/30 0000Z/8 PM EDT Sun 35 kt 40 mph 32.8°N 80.0°W: inland
36 hour 05/30 1200Z/8 AM EDT Mon 30 kt 35 mph 33.2°N 79.5°W: inland
48 hour 05/31 0000Z/8 PM EDT Mon 30 kt 35 mph 33.4°N 79.0°W
72 hour 06/01 0000Z/8 PM EDT Tue 25 kt 30 mph: 33.6°N 78.6°W
96 hour 06/02 0000Z/8 PM EDT Wed: dissipated

Track forecast

Figure 2. My forecast track for Bonnie.

Storm information, including watches and warnings (NHC)

000
WTNT32 KNHC 290550
TCPAT2

BULLETIN
TROPICAL STORM BONNIE INTERMEDIATE ADVISORY NUMBER 6A
NWS NATIONAL HURRICANE CENTER MIAMI FL AL022016
200 AM EDT SUN MAY 29 2016

…BONNIE TURNS NORTHWARD…
…RAINBANDS AFFECTING MOST OF THE SOUTH CAROLINA COAST AND EXTREME
EASTERN GEORGIA…

SUMMARY OF 200 AM EDT…0600 UTC…INFORMATION
———————————————-
LOCATION…31.5N 79.4W
ABOUT 100 MI…165 KM SSE OF CHARLESTON SOUTH CAROLINA
ABOUT 155 MI…245 KM S OF MYRTLE BEACH SOUTH CAROLINA
MAXIMUM SUSTAINED WINDS…45 MPH…75 KM/H
PRESENT MOVEMENT…N OR 360 DEGREES AT 6 MPH…9 KM/H
MINIMUM CENTRAL PRESSURE…1008 MB…29.77 INCHES

WATCHES AND WARNINGS
——————–
CHANGES WITH THIS ADVISORY:

None.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT:

A Tropical Storm Warning is in effect for…
* Savannah River to Little River Inlet South Carolina

A Tropical Storm Warning means that tropical storm conditions are
expected somewhere within the warning area, in this case within the
next 12 hours.

For storm information specific to your area, including possible
inland watches and warnings, please monitor products issued by your
local National Weather Service forecast office.

DISCUSSION AND 48-HOUR OUTLOOK
——————————
At 200 AM EDT (0600 UTC), the center of Tropical Storm Bonnie was
estimated to be near latitude 31.5 North, longitude 79.4 West.
Bonnie is now moving toward the north near 6 mph (9 km/h), and this
general motion is expected to continue today. On the forecast
track, the center of Bonnie is expected to move near the coast in
the warning area later today. A slow northeastward motion near the
central and northern coast of South Carolina is expected by tonight
and on Monday.

Maximum sustained winds are near 45 mph (75 km/h) with higher gusts.
Some weakening is expected to begin later today.

Tropical-storm-force winds extend outward up to 70 miles (110 km),
mainly to the northwest of the center.

The estimated minimum central pressure is 1008 mb (29.77 inches).

HAZARDS AFFECTING LAND
———————-
RAINFALL: Bonnie is expected to produce total rainfall accumulations
of 2 to 4 inches with maximum amounts around 6 inches from central
and eastern South Carolina to the Georgia border. Rainfall
accumulations of 1 to 3 inches are expected across southeastern
North Carolina.

WIND: Tropical storm conditions are expected to first reach the
coast within the warning area later this morning.

STORM SURGE: Storm surge inundation of 1 to 2 feet above ground
level is possible within the tropical storm warning area during the
next high tide this morning.

SURF: Bonnie is expected to produce life-threatening surf and rip
current conditions along portions of the southeastern United States
coast through the weekend. Please consult products from your local
weather office.

TORNADOES: An isolated tornado will be possible through early this
morning over the immediate South Carolina coastal region.

NEXT ADVISORY
————-
Next complete advisory at 500 AM EDT.

$$
Forecaster Stewart/Berg