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Post Tropical Storm Hermine Creates Coastal Flood Risk for Northeast U.S.

Although the structure of Tropical Storm Hermine changed significantly on Saturday and the National Hurricane Center designated it as Post Tropical, it still is creating a risk for coastal flooding for the northeastern U.S.  The size of the circulation of Hermine and its proximity to the U.S. is allowing its winds to push water toward portions of the coast.  The largest immediate risk is for the coasts of Virginia, Delaware and New Jersey.  When Hermine moves north, the greater risk will shift to New York, Rhode Island and Massachusetts.

At 11:00 p.m. EDT on Saturday the center of Post Tropical Storm Hermine was located at latitude 36.5°N and longitude 72.1°W which put it about 205 miles (330 km) southeast of Ocean City, Maryland.  Hermine was moving toward the east-northeast at 13 m.p.h. (20 km/h).  The maximum sustained wind speed was 70 m.p.h. (110 km/h) and there were wind gusts to 85 m.p.h. (135 km/h).  The minimum surface pressure was 998 mb.

A Tropical Storm Warning is in effect for the portion of the coast from Ocracoke Inlet, North Carolina to Watch Hill, Rhode Island including the Chesapeake Bay from Drum Point southward, Delaware Bay, New York City and Long Island.  A Tropical Storm Watch is in effect from Watch Hill, Rhode Island to Sagamore Beach, Massachusetts including Block Island, Martha’s Vineyard and Nantucket.

The circulation of Hermine changed from a tropical cyclone with a tight inner core to structure where area with the maximum wind speed is farther from the center.  In addition all of the thunderstorms near the center dissipated and most of the remaining thunderstorms are well northeast of the center of circulation.  The loss of tropical characteristics are the reason why the National Hurricane Center designated Hermine as Post Tropical.

Future changes of Hermine could continue to be complex.  Dry air has permeated the inner 100 miles (160 km) of the circulation which is devoid of any thunderstorms.  An upper level trough over the northeastern U.S. is forecast to move over the top of Hermine and cut off.  The would create a vertical structure which is the opposite of what is normally found in a tropical cyclone.  However, Hermine is moving over water where the Sea Surface Temperature (SST) is near 30°C.  A combination of cold air aloft in the upper low and warm SSTs could create enough instability to generate the development of new thunderstorms closer to the core of the circulation.  it is possible that Hermine could make a transition back to a more tropical cyclone like structure during the next several days.

The upper level trough is currently steering Hermine toward the east-northeast.  When the trough approaches Hermine, it will steer the storm more toward then north.  However, when the trough moves over the top of Hermine, the steering currents will be weak and Hermine could stall south of Long Island.  It is possible that the counterclockwise rotation in the upper trough could cause the storm to loop once or twice.  If Hermine makes a slow counterclockwise loop, it could move back closer to the coast of New Jersey on Monday.

The wind field in Hermine expanded during the structural changes.  Winds to tropical storm force extend out about 200 miles (320 km).  That makes Hermine about half as big as Hurricane Sandy was in 2012.

Hurricane Hermine Making Landfall in North Florida

After intensifying into a hurricane on Thursday, Hurricane Hermine is making landfall near St. Marks, Florida.  At midnight EDT the center of Hurricane Hermine was located at latitude 29.8°N and longitude 84.2°W which put it about 20 miles (30 km) south of St. Marks, Florida.  Hermine was moving toward the north-northeast at 14 m.p.h. (22 km/h).  The maximum sustained wind speed was 80 m.p.h. (130 km/h) and there were wind gusts to 95 m.p.h. (155 km/h).  The minimum surface pressure was 984 mb.

A Hurricane Warning is in effect from Suwannee River to Mexico Beach, Florida.  A Hurricane Watch is in effect from Anclote River to Suwannee River and from Mexico Beach to the Walton County/Bay County line.  A Tropical Storm Warning is in effect from Englewood to Suwannee River and from Mexico Beach to the Walton County/Bay County line.  A Tropical Storm Warning is also in effect from the Flagler County/Volusia County line to Duck, North Carolina including Pamlico and Albemarle Sounds.  A Tropical Storm Watch is in effect from Duck, North Carolina to Sandy Hook, New Jersey including the Chesapeake Bay from Smith Point southward and southern Delaware Bay.

The circulation of Hurricane Hermine organized quickly on Thursday.  It developed an eye with a mostly complete eyewall.  Spiral rainbands developed with strong winds in the eastern half of the circulation.  Upper level divergence to the east of Hermine pumped out mass and allowed the surface pressure to decrease.  The circulation is still asymmetrical with most of the stronger winds east of the center, but it looks a lot more typical of hurricanes that move toward the northern coast of the Gulf of Mexico.

Hurricane Hermine will weaken after it makes landfall.  It could interact with a cold front moving into the southeastern U.S.  It is possible that Hermine could develop a hybrid structure that is part tropical and part extratropical.  Hermine could have winds to tropical storm force as it passes over the Mid-Atlantic coast.  The stronger winds are likely to be out over the Atlantic Ocean and winds should be weaker farther inland.

An upper level trough is steering Hurricane Hermine toward the north-northeast and a general northeasterly motion is expected to continue for another 36 hours.  Later in the weekend a surface high pressure system could move north of Hermine and stall its progress.  Hermine could be stationary for a time.  On its anticipated track center of Hermine is likely to pass east of Tallahassee, Florida.  The center could pass north of Savannah, Georgia before coming near Charleston, South Carolina.  Hermine is likely to move near Cape Hatteras, North Carolina and out into the Atlantic Ocean.

Hurricane Hermine is capable of causing regional minor wind damage.  It is likely to cause widespread power outages.  The coast of the northeastern Gulf of Mexico is susceptible to storm surges and high water will effect that area overnight.  Locally heavy rainfall will create the potential for flooding.  When rainbands move ashore, wind shear could spin up tornadoes.  As Hermine moves near the Mid-Atlantic coast, easterly winds could cause water rises.

The Hurricane Intensity Index (HII) for Hurricane Hermine is 11.5.  The Hurricane Size Index (HSI) is 14.0.  The Hurricane Wind Intensity SIze Index (HWISI) is 25.5.  These indices are very similar to the ones for Hurricane Isaac before it hit the coast of Louisiana n 2012.  The HII for Isaac was 11.5.  Its HSI was 16.7 and its HWISI was 28.2.  This means that Hurricane Hermine is as strong and just smaller than Hurricane Isaac was just before it made landfall.  Hurricane Isaac did hit a more populated and more built up region.  Hurricane Isaac did 970 million dollars worth of insured damage.  It caused 407 million dollars to be paid out for flood insurance.  It is estimated that Hurricane Isaac caused 2.35 billion dollars worth of damage in the U.S.

Tropical Storm Hermine Strengthens, Hurricane Warning Issued

Tropical Storm Hermine strengthened on Wednesday evening and a Hurricane Warning was issued for a portion of the northern Florida coast.  A Hurricane Warning was in effect for the portion of the coast from Suwannee River to Mexico Beach, Florida.  A Hurricane Watch and a Tropical Storm Warning was in effect for the portions of the coast from Anclote River to Suwannee River and from Mexico Beach to Destin, Florida.  The Tropical Storm Watch was extended farther up the Mid-Atlantic coast.  The Tropical Storm Watch was in effect for the portion of the coast from Marineland, Florida to South Santee River, South Carolina.

At 11:00 p.m. EDT on Wednesday the center of Tropical Storm Hermine was located at latitude 25.8°N and longitude 87.0°W which put it about 295 miles (475 km) south-southwest of Apalachicola, Florida.  Hermine was moving toward the north-northeast at 10 m.p.h. (16 km/h).  The maximum sustained wind speed was 60 m.p.h. (95 km/h) and there were wind gusts to 75 m.p.h. (120 km/h).  The minimum surface pressure was 998 mb.

The circulation of Tropical Storm Hermine became better organized on Wednesday, but it is still not really well organized.  A tighter center of circulation developed.  However, the wind field is still asymmetrical.  The stronger winds are mainly east of the center and the winds are weaker in the western half of the circulation.  An area of strong thunderstorms developed near the center and another cluster of thunderstorms persisted southwest of the center.  There are not many thunderstorms northwest of the center.  There are some spiral rainbands, but they are fragmented.

Tropical Storm Hermine is in an environment that is favorable for intensification.  It is moving over water where the Sea Surface Temperature is near 30°C.  The upper level flow pattern is enhancing the divergence of mass to the northeast of Hermine.  The enhanced upper level divergence pumped out enough mass to allow the surface pressure to decrease by a few millibars on Wednesday evening.  Tropical Storm Hermine is expected to continue to intensify on Thursday and it should become a hurricane before it makes landfall on Thursday night.

The upper level ridge that was blocking a northward motion of Hermine has weakened.  So, the tropical storm has begun to more toward the north-northeast.  An upper level trough is expected to steer Hermine a little faster toward the northeast on Thursday.  On its anticipated track Hermine could make landfall somewhere between Apalachicola and Tarpon Springs, Florida on Thursday night.  After it moves across northeast Florida, Hermine could move near the coast of Carolinas on Friday.

Hermine is likely to be a hurricane when it makes landfall on Thursday night.  The core of the circulation which will contain the highest winds is likely to be fairly small and Hermine is likely to cause localized minor wind damage.  There will undoubtedly be power outages.  The coastline around the northeastern Gulf of Mexico is vulnerable to storm surges and Hermine will also produce a storm surge which could range up to 7 to 8 feet (2 to 2.5 meters) near where the center crosses the coast.  The storm surge will be less farther away from where the center makes landfall.  In addition Hermine will generate locally heavy rain which could cause fresh water flooding.  Directional wind shear associated with rainbands moving inland could spin up tornadoes in the eastern half of Hermine.

Tropical Depression Nine Causes Hurricane Watch for Part of Florida

Although Tropical Depression Nine did not strengthen on Tuesday afternoon, guidance from numerical models suggested it could be stronger when it eventually reaches the coast of Florida.  As a result, the National Hurricane Center at 5:00 p.m. EDT issued a Hurricane Watch for the portion of the Florida coast from Anclote River to Indian Pass.  A Tropical Storm Watch was issued for the portion of the coast from Indian Pass to the Walton County/Bay County line.

At 5:00 p.m. EDT on Tuesday the center of Tropical Depression Nine was located at latitude 24.4°N and longitude 87.3°W which put it about 345 miles (555 km) west of Key West, Florida.  It was moving toward the northwest at 5 m.p.h. (8 km/h).  The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (70 km/h).  The minimum surface pressure was 1004 mb.

The circulation of Tropical Depression Nine is not currently well organized.  The surface center is located northwest of an apparent mid-level center.  Upper level divergence is not well developed.  The development of thunderstorms has been cyclical during the past several days.  Clusters of thunderstorms developed south and east of the center and then they weaken.  A few hours later more thunderstorms would develop, persist for a few hours and weaken again.  It appears that new thunderstorms are developing northeast and southwest of the surface center which would be a change from the previous pattern.

The vertical structure of Tropical Depression Nine has been out of sync.  As mentioned above, the surface center was northwest of the mid-level center.  Both the surface and mid-level centers have been located on the western edge of a larger upper level high.  Clockwise flow around the upper high has been creating vertical wind shear, which has prevented the development of a vertically coherent structure in the depression.  Despite the effect of the wind shear and the lack of a coherent vertical structure, the surface center has managed to persist and become a little more organized each day.

Tropical Depression Nine is moving through an environment that is currently marginal for intensification.  It is moving over water where the Sea Surface Temperature is near 31°C.  So, there is plenty of energy in the upper ocean.  The vertical wind shear mentioned previously is the primary factor inhibiting intensification.  However, as Tropical Depression Nine moves north, if is forecast to move into an area where the upper level winds are not as strong.  If that happens, then the depression will strengthen.  As the depression becomes more organized, the rate of intensification could increase and the depression could become a hurricane over the northeastern Gulf of Mexico.

An upper level ridge over the southeastern U.S. has been blocking the northward movement of Tropical Depression Nine.  An upper level trough approaching from the west is expected to weaken the ridge.  As the ridge weakens, The depression will be able to move north.  Eventually in a couple of days, the upper level trough is expected to steer the depression more toward the northeast.  On its anticipated track the depression could be approaching the northeastern Gulf Coast by Thursday afternoon.

The northeastern Gulf Coast is very susceptible to storm surges and a surge is the greatest risk.  Locally heavy rain could cause some flooding, and there could be some minor wind damage.

Tropical Depressions Form South of Key West & Southeast of Cape Hatteras

Two tropical depressions formed near the U.S. on Sunday.  Tropical Depression Nine formed south of Key West, Florida and Tropical Depression Eight formed southeast of Cape Hatteras, North Carolina.  Both depressions could have an impact on the U.S., but Tropical Depression Nine could pose a greater risk to the southeastern U.S.

At 5:00 p.m. EDT on Sunday the center of Tropical Depression Nine was located at latitude 23.7°N and longitude 81.7°W which put it about 60 miles (95 km) south of Key West, Florida.  It was moving toward the west at 9 m.p.h. (15 km/h).  The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (70 km/h).  The minimum surface pressure was 1009 mb.

At 5:00 p.m. EDT on Sunday the center of Tropical Depression Eight was located at latitude 31.8°N and longitude 70.9°W which put it about 355 miles (570 km) southeast of Cape Hatteras, North Carolina.  It was moving toward the west at 9 m.p.h. (15 km/h).  The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (70 km/h).  The minimum surface pressure was 1010 mb.

The circulation of Tropical Depression Nine is still organizing.  A NOAA plane found a distinct center of circulation, but most of the stronger thunderstorms are south and east of the center.  There is a broader counterclockwise rotation with numerous showers and thunderstorms indicated by the National Weather Service radar in Key West.  An upper level low east of Florida is contributing to easterly winds that are blowing across the northern side of the depression.  The vertical wind shear is inhibiting the development of thunderstorms north and west of the center.

The circulation of Tropical Depression Eight was more well organized earlier today.  Strong easterly winds from the same upper level low that is affecting Tropical Depression Nine are creating significant vertical wind shear.  Those winds and the shear they caused blew the upper portion of the depression’s circulation west of the low level center of circulation.  The low level circulation is presently exposed as seen on visible satellite imagery and by an Air Force Reserve Hurricane Hunter aircraft.

The intensity forecast for Tropical Depression Nine is challenging because it is moving through a complex environment.  The depression is moving over water where the Sea Surface Temperature is near 31°C.  So, there is plenty of energy in the upper ocean.  However, as mentioned above, an upper low is causing vertical wind shear which will inhibit intensification.  Tropical Depression Nine could move into a region where there is less shear when it moves over the Gulf of Mexico.  Tropical Depression Nine has a good chance to intensify into a tropical storm, but it is unclear if it could become a hurricane before it reaches the coast.

There is also significant uncertainty in the track forecast for Tropical Depression Nine.  A ridge over the southeastern U.S. is steering the depression toward the west and that general motion should continue for several more days.  The ridge is forecast to weaken during the middle of the week and that should allow Tropical Depression Nine to turn toward the north.  The timing of that turn and how sharp it will be are still uncertain.

The upper low is expected to continue to generate wind shear over Tropical Depression Eight.  If the upper levels winds remain as strong as they are now, the depression will slowly weaken.  If the upper level winds weaken, then Tropical Depression Eight could strengthen into a minimal tropical storm.  The upper level low is steering the depression west and that general motion is expected to continue in the short term.  The depression could stall just off the coast of North Carolina.

Tropical Depression Nine could intensify and bring wind and rain to the Gulf Coast later this week.  The Gulf Coast is also vulnerable to storm surges.  Given the uncertainty in both the track and intensity, interests along the coast should monitor official sources of information for updates on Tropical Depression Nine.  The primary effects of Tropical Depression Eight are likely to be higher than normal surf, rip currents and some beach erosion along the North Carolina coast.  People along the Mid-Atlantic coast should pay attention to the depression in case the forecast changes.