Hurricane warnings for North Carolina for Category 3 Earl

Hurricane warnings are flying for the coast of North Carolina, as Hurricane Earl chugs to the northwest at 17 mph. Earl has weakened some over the past day, thanks to an eyewall replacement cycle and some dry air that got wrapped into the core of the storm. Earl's eye made a direct hit on NOAA buoy 41046 at 4am EDT this morning. The buoy recorded a surface pressure of 943 mb, exactly what the Hurricane Hunters were estimating. The buoy measured winds in the eyewall of 76 mph, gusting to 96 mph. The peak winds of Earl were stronger than this, though, since the buoy only reported measurements once per hour, which is not a fine enough resolution to see the peak winds. The buoy is also located at a height of 5 meters, which is less than the standard ten meter height used to do wind measurements, so an additional upward adjustment needs to be made. Peak waves at the buoy were a remarkable 49 feet.

A recent microwave "radar in space" image (Figure 2) shows that dry air has spiraled into the core of Earl, knocking a gap into the southern eyewall. The latest 9am EDT report from the Hurricane Hunters confirmed that the southwest portion of the eyewall was missing. Top winds seen by the Hurricane Hunters were only Category 2 strength, and Earl may be weaker than the stated 125 mph winds in the 11am NHC advisory.


Figure 1. Image of Hurricane Earl taken by astronaut Douglas Wheelock aboard the International Space Station on Tuesday, Aug. 31, 2010.

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Figure 2. Microwave "radar in space" image of Hurricane Earl taken at 6:45am EDT Wednesday, September 1, 2010. The southern portion of Earl's eyewall was missing, thanks to a slug of dry air (blue colors) that had spiraled into Earl's core.

Intensity forecast for Earl
Recent satellite loops show that upper level outflow is good to the north and east of Earl, but is poor on the southwest side. The latest SHIPS model forecast shows that this is because upper level winds out of the southwest are creating 15 - 20 knots of wind shear on Earl's southwest side. The winds are from a trough of low pressure to Earl's west. This trough is forecast to weaken and move to the west away from Earl, which should reduce the shear to 10 - 15 knots by Thursday morning. If true, the relaxation in shear may give Earl enough time to mix out the dry air it ingested and regain its previous 135 mph Category 4 intensity. Water vapor satellite loops, though, show there is still plenty of dry air on Earl's west side that could potentially wrap into the storm if there is enough wind shear to drive it into Earl's circulation. Ocean temperatures are still very high, a near-record 29.5 - 30°C, and very warm waters extend to great depth, resulting in a total ocean heat content favorable for intensification. It is likely Earl will be a Category 2 or 3 hurricane at its closest approach to North Carolina Thursday night and Friday morning, with a small chance it will be at Category 4 strength. By Friday night, when Earl will be making its closest approach to New England, wind shear will rise to a high 20 - 30 knots and ocean temperatures will plunge to 20°C, resulting in considerable weakening. Earl will still probably be a Category 1 or 2 hurricane on Friday night, when it could potentially make landfall in Massachusetts. Earl is more likely to be a strong tropical storm or weak Category 1 hurricane on Saturday morning, when it could potentially make landfall in Maine or Nova Scotia, Canada.

Impact of Earl on North Carolina
The latest set of computer models runs from 2am EDT (6Z) this morning are very similar to the previous set of runs. The NOGAPS model brings Earl closest to the coast, predicting the west eyewall of the the hurricane will hit the Outer Banks of North Carolina near 2am Friday. If this track verifies, a period of 40+ mph winds will affect coastal North Carolina for a period of 12 - 18 hours beginning at about 6pm EDT Thursday night. Earl's expected radius of hurricane-force winds of 60 miles to the west will bring hurricane conditions as far west as Morehead City and Elizabeth City in North Carolina. Earl's radius of tropical storm-force winds to the west, over land, will probably be about 150 miles, so locations from Wilmington to Norfolk could see sustained winds of 40 mph in this worst-case model scenario. Storm surge would not be significant along the North Carolina coast facing the open ocean, since winds would be offshore. However, a significant storm surge of 3 - 6 feet could occur in Pamlico Sound, due to strong west to north winds. Coastal Highway 12 out of the Outer Banks would likely be blocked by sand and debris or washed out, resulting in a multi-day period where everyone on the Outer Banks would be stranded. Is is possible that the NOGAPS scenario is not the worst case, and that Earl will strike farther west, resulting in the Outer Banks getting the fearsome maximum winds of the storm's right front quadrant. However, it is more likely that Earl will pass just offshore, resulting in North Carolina receiving the weaker west side winds. Since Earl's forward speed will be about 20 mph at that time, the winds on the hurricane's west side will be about 40 mph less than the right front quadrant on the east side. The NHC wind probability forecast is calling for a 23% chance of hurricane-force winds on Cape Hatteras, 7% for Morehead City, and 3% for Norfolk, Virginia.

Impact of Earl on New England
The NOGAPS model brings Earl closest to the coast of New England, predicting the west eyewall of the the hurricane will pass over Nantucket at about 2am Saturday morning, and the tip of Cape Cod a few hours later. If this track verifies, 40+ mph winds would affect southeastern Massachusetts for a period of 6 - 12 hours beginning at about 8pm EDT Friday night. Earl should be a weaker Category 1 or 2 hurricane then, with hurricane-force winds extending 30 miles to the left of its track. Hurricane conditions would then affect the eastern tip of Long Island, coastal Rhode Island, and Southeast Massachusetts. Earl's radius of tropical storm-force winds to the north, over land, will probably be about 150 miles, so locations from Central Long Island to southern Boston would experience sustained winds of 40 mph in this worst-case model scenario. A storm surge of 3 - 5 feet might occur in Long Island Sound, and 2 - 3 feet along the south coast of Long Island. A deviation to the left, with a direct hit on eastern Long Island and Providence, Rhode Island, would probably be a $10 billion disaster, as the hurricane would hit a heavily populated area and drive a drive a 5 - 10 foot storm surge up Buzzards Bay and Narragansett Bay. The odds of this occurring are around 5%, according to the latest NHC wind probability forecast. The forecast is calling for a 25% chance of hurricane-force winds on Nantucket, 8% in Providence, 6% in Boston, and 18% in Hyannis. Keep in mind that the average error in position for a 3-day NHC forecast is 185 miles, which is about how far offshore Earl is predicted to be from New England early Saturday morning.

Impact of Earl on Canada/Maine
Late morning Saturday, Earl is expected to make landfall somewhere between the Maine/New Brunswick border and central Nova Scotia. At that time, Earl should be a strong tropical storm or weak Category 1 hurricane. This won't be another Hurricane Juan, the 2003 Category 2 hurricane which made a direct hit on Halifax, Nova Scotia, causing over $200 million in damage. Earl's impact is likely to be closer to 2008's Hurricane Kyle, which hit near Yarmouth, Nova Scotia, as a Category 1 hurricane with 75 mph winds. Kyle produced a storm surge of 2.6 feet, and did $9 million in damage to Canada. The NHC wind probability forecast is calling for a 29% chance of hurricane-force winds in Yarmouth, 24% in Halifax, and 17% in Eastport, Maine.

Beach erosion
Regardless of Earl's exact track, the U.S. East Coast can expect a long period of high waves beginning on Thursday. Significant beach erosion and dangerous rip currents will be the rule, due to waves that will reach 10 - 15 feet in offshore waters. Beach erosion damage in the mid-Atlantic states will likely run into the millions, but will probably not be as bad as that suffered during Nor'easter Ida in November of 2009. That storm (the remains of Hurricane Ida that developed into a Nor'easter) remained off the coast for several days, resulting in a long-duration pounding of the shore that caused $300 million in damage--$180 million in New Jersey alone.

Record ocean temperatures off the U.S. Mid-Atlantic Coast
The period May - July was the hottest such 3-month period in history for the Northeast and Southeast U.S., according to NOAA's National Climatic Data Center. Most of the hurricane-prone states along the coast, including New Hampshire, Massachusetts, Rhode Island, New Jersey, Delaware, Maryland, Virginia, North Carolina, and South Carolina had their hottest May - July in the 116-year record. These record air temperatures led to record ocean temperatures, according to an analysis I did of monthly average 5x5 degree SST data available from the UK Met Office Hadley Centre.. The region of ocean bounded by 35N - 40N, 75W - 70W, which goes from Cape Hatteras, North Carolina to Central New Jersey, had the warmest July ocean temperatures since records began in 1875--a remarkable 2.12°C (3.8°F) above average. The year 2008 was a distant second place, with temperatures 1.5°C (2.7°F) above average. The ocean region off the Southeast U.S. coast, bounded by 30N - 35N, 80W - 75W, from the Georgia-Florida border to Cape Hatteras, North Carolina, had its 4th warmest July ocean temperatures on record. Temperatures were 0.8°C (1.4°F) above average, which fell short of the record 1.1°C anomaly of 1944. The August numbers are not available yet, but will probably show a similar story.

All this warm water off the East Coast means it is much easier for a major hurricane to make landfall in the mid-Atlantic or Northeast U.S. Usually, ocean temperatures fall below the 26.5°C threshold needed to support a hurricane as soon as a storm pushes north of Cape Hatteras, North Carolina. This year, those temperatures extend all the way to the New Jersey coast (Figure 3.) Such warm ocean temperatures increase the odds of a major hurricane making it to the mid-Atlantic or New England coasts. Since record keeping began in 1851, there have been only 15 major hurricane in U.S. coastal waters north of the North Carolina/Virgina border--about one per decade. The last such storm was Hurricane Alex of August 6, 2004.


Figure 3. Water surface temperatures from AVHRR satellite data for the 6-day period ending August 31, 2010. Ocean temperatures of 26.5°C, capable of supporting a hurricane, stretched almost to Long Island, New York. Image credit: Ocean Remote Sensing Group, Johns Hopkins University Applied Physics Laboratory.

Fiona
Tropical Storm Fiona last night showed us why hurricane forecasting is such a difficult job. The storm made an unexpected slow-down in forward speed. This slow-down resulted in less wind shear affecting Fiona than expected, since the storm is farther from the upper-level outflow of Hurricane Earl. The wind shear analysis from the University of Wisconsin CIMSS group shows just a moderate 10 - 20 knots of shear affecting Fiona, which is low enough that the storm has been able to organize into a respectable 60 mph tropical storm. Martinique radar shows that the outer bands from Fiona are bringing heavy rain squalls to the same islands of the northern Lesser Antilles that were affected by Earl. Our wundermap shows that winds in the islands are all below 20 mph, but winds will increase to 30 - 40 mph later today as Fiona draws closer. Satellite loops show that heavy thunderstorm activity has decreased some in recent hours. This may be due to the fact that Fiona is currently crossing the cold water wake of Earl.

Forecast for Fiona
In the short term, moderate wind shear and dry air should keep Fiona from attaining hurricane status, though we do have several models that predict it could become a Category 1 hurricane. Fiona is likely to come close enough to Bermuda on Saturday or Sunday to pose a threat to that island, though it is possible high wind shear from Earl could kill the storm by then. The long term fate of Fiona remains unclear, with some models calling for dissipation this weekend, and other models calling for Fiona to be left behind by Earl to wander over the ocean near Bermuda early next week.


Figure 4. Morning radar image of Fiona from the Martinique radar. Image credit: Meteo France.

TD 9
Invest 98L gained enough heavy thunderstorm activity to be classified as Tropical Depression Nine this morning. This wil probably be Tropical Storm Gaston by tomorrow morning. The latest SHIPS model forecast predicts shear will remain moderate, 10 - 15 knots, for the next five days, and TD 9 could be a Category 1 hurricane five days from now, as predicted by the GFDL model. The storm will likely pose a threat to the northern Lesser Antilles Islands by Tuesday.

Next post
I'll have an update this afternoon.

Jeff Masters