Cyclone Gita in the South Pacific is a powerful Category 4-equivalent tropical cyclone on the Saffir-Simpson Scale (as of the time of this post). Maximum sustained winds analyzed by the US Joint Typhoon Warning Center are up to 130 mph with gusts to 155 mph. Gita is moving westward and is expected to pass near or over Tonga around 12 UTC Monday (6 am CST or 1 am Tuesday local time). The storm is expected to be at least a powerful Category 4 storm. There is moderate wind shear (increasing winds with height effecting the cyclone and limiting more rapid intensification. However, with water temperatures along the path of 28-29 degrees C (82-84 degrees F) and well-organized structure, Gita will be a potentially catastrophic storm if its eye wall moves over the main island. Gita will also produce very heavy rain (perhaps 6-12 inches) which will lead to flooding. A state of emergency has been declared in the island nation.
After 24 hours, Gita is expected to gradually weaken as sea surface temperatures cool and vertical wind shear increases as the system begins to turn to the southwest into higher latitudes. However, Gita is expected to remain a hurricane-force storm through the end of the week.
New Zealand will need to keep an eye on the remnants of Gita as the dying circulation an moisture plume may curve back southeastward in the mid-latitude westerlies. While, the forecast will certainly change somewhat…such as the position of the upper-level trough of low pressure southwest of New Zealand which will cause the system to curve back towards the country beginning Sunday…any remnant system may lead to locally heavy rainfall for both the North and South Islands early to mid-week next week.
The 2017 North Atlantic Hurricane season was a devastating one in terms of loss of life as well as property damage for the United States and the Caribbean. The National Hurricane Center released its post-season report on Harvey which caused great destruction to parts of Southeast Texas and Southwest Louisiana. What follows is a brief summary and discussion of Harvey based on info from that report as well as other sources related to Harvey’s impacts. The full report is linked at the end of this post in the references.
What became Harvey was originally a tropical disturbance which came off the West Coast of Africa on August 12th. It is common during August and September for land-based thunderstorm complexes known as mesoscale convective systems to move westward off the African coast near or south of the Cape Verde (also known as the Cabo Verde) Islands and later develop into long-lived tropical cyclones. Harvey was a classic “cape-verde” type storm as it would later develop into a tropical depression with a well-defined center on August 16th.
The depression intensified into a storm and given its name 12 hrs after initial development. It peaked over the open Atlantic at 40 knots (~45 mph), moving over the islands of Barbados and St. Vincent on August 18th. However, increasing vertical wind shear (increasing winds with height tilting and blowing the thunderstorms away from the low pressure center) over the central Caribbean Sea lead to Harvey’s dissipation to a remnant low later that day.
The remnant circulation moved over the Yucatan Peninsula on Aug 22nd and redeveloped into a tropical depression over Bay of Campeche on August 23rd, 150 n mi west of Progreso, Yucatan, Mexico.
The initially poor organization of the reformed Harvey transitioned to a period of rapid intensification late on the 23rd as deep convection began to concentrate near the center. This was aided by an environment of light shear, very warm sea surface temperatures and high mid-level moisture. Intensification would continue until landfall on the 26th. Harvey reached Category 3 midday on the 25th and intensified into a Category 4 as it made its landfalls on the Texas coast early August 26th (the evening of the 25th local time). The initial landfall was on San Jose Island, TX as a Category 4 with maximum sustained winds of 130 mph (115 knots) with a second landfall on mainland Texas in northeast Copano Bay as a Category 3 with maximum sustained winds of 120 mph (105 knots). Wind damage was extreme and devastating in Aransas, Nueces, Refugio and the eastern part of San Patricio Counties. 15,000 homes were destroyed and 25,000 homes damaged. The City of Rockport was hit the hardest as the Category 3+ wind field moved into that area causing both extensive wind and surge impacts. The highest surge observed in Harvey was generally in the range of 9-11 ft.
Harvey meandered in light steering currents, “stuck” between a mid-tropospheric high pressure system over the Four Corners states and another mid-troposphere high over the Gulf of Mexico. Torrential rains fell over Houston Metro and the Golden Triangle near a stationary front which formed on the north and east side of Harvey.
The rainfall of Harvey was truly incredible. A storm total of 60.58 inches was confirmed Nederland, TX; 60.54 inches in Groves, TX. Much of the heaviest precipitation fell in the first 72 hrs of the event. Previous continental US record for a tropical cyclone is 48 inches in Medina, TX (1978). The extreme nature of Harvey was displayed in that 18 values over that continental record of 48 inches reported across southeastern TX, with 36-48 inches recorded across the Houston metro area. However, Multi-Sensor Precipitation Estimates (MPE), which includes radar-derived rainfall intensity estimates suggests 65-70 inches where few observations were available or observations failed early in the event. Maximum rainfall measured in Louisiana was 23.71 inches in Vinton, LA, with MPE suggesting a more representative 40 inches as Southeast Southwest LA obs were sparse.
The large-scale or synoptic set up for the Harvey exceptional rainfall event is not particularly unique. Heavy rain bands formed along a modest frontal boundary situated initially near Houston, then the Golden Triangle region in Southeast TX (Beaumont, Port Arthur, Orange, TX area). Enhanced convergence and convective lift with warm cloud droplet precipitation processes allowed for enhanced rainfall rates in abundant thunderstorms. The combination of extremely high rainfall rates of up to 5-7 inches per hour and the stationary nature of the near coastal frontal boundary and Harvey itself contributed to the extreme total accumulation and massive flooding.
NOAA analysis determined that areas of Southeast TX experience a flood with an annual probability of <0.1% (equivalent to a >1000 year flood event). I believe this is one of the most important parts of the National Hurricane Center report, so I’ll quote it:
While established records of this nature are not kept, given the exceptional exceedance probabilities, it is unlikely the United States has ever seen such a sizable area of excessive tropical cyclone rainfall totals as it did from Harvey.
In addition to storm surge, wind and flooding rains, Harvey produced 57 tornadoes (many in the Houston Metro area) and killed 68 people directly with an additional 35 indirect deaths. All direct deaths were in Texas and it was the deadliest tropical cyclone for Texas since 1919. All but three direct deaths were caused by freshwater flooding.
According to NOAA, preliminary damage analysis suggests estimated damages of $125 billion, making Harvey the second-costliest hurricane on record in the North Atlantic basin, only behind Hurricane Katrina, when adjusted for inflation.
Connection to Anthropogenic (human-caused) Climate Change
During and immediately following the events of Hurricane Harvey, there was intense controversy over even discussing climate change as it related to the extreme events related to Hurricane Harvey. Even mentioning climate change in reference to an individual extreme weather event. A lot of opinions were thrown about, but the science of climate change has evolved dramatically in the past 10 years and climate researchers have a much better understanding of many of the connections between climate variables and the statistics of weather which make up the recent past and current climate. From this, attribution studies can be conducted to determine a likelihood of connection to the changing climate regime. A attribution study was done by World Weather Attribution (#2 below) and the probabilistic statistical analysis determined that the record rainfall from Harvey was approximately a) 3 times more likely and b) 15% more intense in terms of rainfall rate because of climate change. One location witnessed a return period for extreme rainfall of 9000 years with a high degree of statistical confidence. The impacts were consistent with what would be expected with 1 degree C+ of global warming since the late 19th century (the world has thought to have begun warming because of humanity since the mid 18th century). I did an extensive post previously during this most recent hurricane season on the climate change connection with includes references to numerous recent peer reviewed papers HERE.
Five years ago today (October 29, 2012), the post-tropical remnants of what was Hurricane Sandy made landfall on the New Jersey coastline as a hurricane-force windstorm, causing destructive straight-line winds and historic, damaging surge from the North Atlantic extending from the Jersey coast north into the New York City Metro Area, with historic flooding of lower Manhattan.
Sandy produced widespread wind gusts of 75-90 mph across portions of New York and New Jersey with heavy rainfall totals of 7-10 inches across parts of New Jersey, Delaware, and Maryland. Storm surge was Sandy’s main cause of significant damage, with wind damage and flooding rainfall additional impacts. The post-tropical “superstorm” caused a 10-13 ft storm surge which damaged and destroyed homes and businesses along the Jersey Shore and Hudson Waterfront, with a record 13.88 ft water rise reported at Battery Park in Lower Manhattan.
Sandy’s expansive storm surge was more intense by multiple factors. As it came poleward, it grew significantly in size, a typical phenomenon for tropical cyclones moving into the mid-latitudes. However, Sandy’s weakening and mid-latitude interactions caused it become the largest North Atlantic tropical cyclone on record, producing a huge fetch (extensive wind over long stretch of open water). This fetch allowed for the building of significant ocean waves and piling up of water toward the shallow continental shelf of the Atlantic coast of the US. And although Sandy weakened somewhat and became “non-tropical”, this did not matter as the very large wind field remained and forward momentum of the very heavy ocean could not settle down in time before pounding the coastline with destructive surge.
In addition, Sandy made landfall at high tide, enhancing the storm’s ability to flood dry land areas and cause direct damage with battering waves. I will also note that this “flood reach” was even greater because of climate change-induced sea level rise. Global sea levels have risen 9 inches since 1880 and while the Intergovernmental Panel on Climate Change (UN) continues to indicate a likely sea level rise of up to 3.2 ft by 2100, many other reputable scientists have suggested the possibility of multi-foot sea level rise occurring this century as the result of exponential glacial melt feedbacks in Greenland and Antarctica. Perhaps as high as 6.5-16.5 feet by 2100 (see references #1-2 below). This, of course would be catastrophic for vulnerable coastal cities for both livability but initially for any places already exposed to storm surges. New York City is one most at risk.
Sea level rise has also been locally enhanced along the Northeast US Coast because of abnormally warm waters building offshore for years, leading to increased thermal expansion of the water surface upward. This may also be a result of climate change-induced weakening (#3) of the Atlantic Meridional Overturning Circulation (AMOC). While Superstorm Sandy wasn’t “caused” by climate change, it was part of an increasing regime of more extreme weather events (and events with with more extreme hazard variables) and a prelude to what will be far more frequent in the coming decades.
Sandy was retired after the 2012 Hurricane Season, causing 233 deaths from the Caribbean to the United States and producing an incredible $75 billion in damages (only 2nd to Hurricane Katrina). An incredible and devastating meteorological event which we can hope we continue to recover from and our country will be better prepared to mitigate against next time.
Post-Tropical Cyclone Ophelia is quickly approaching Ireland with hurricane-force strength. It’s a fully non-tropical frontal system, but powerful one. Gusts of 45-55 mph are already occurring over the south coast of the country. My forecast for Ireland is HERE at the bottom, no significant changes since 24 hrs ago. Lots of wind and of course high surf. Stay safe if you’re located in Ireland or Scotland.
Ophelia appears to have nearly completed the process to Post-Tropical based on satellite imagery, with the whole arrangement of frontal boundaries and more asymmetric wind field and lack of any significant tropical characteristics outside of some convection (thunderstorm activity) northeast of the center. Ophelia is still a hurricane-force cyclone (likely top sustained winds 75-85 mph) and impacts still expected to quickly increase over Ireland Monday morning with rain, damaging winds and dangerous surf and coastal flooding.
Hurricane Ophelia…at least it was still considered one at 11 am AST…is quickly transitioning to a hybrid post-tropical cyclone. I made up a schematic using current infrared satellite imagery. You can clearly see the transitioning hurricane becoming surrounded by cold, dry air on its’ back side, with its own warm, moist tropical air mass contributing to warm air advection ahead of it. And you can the developing frontal structure…cold front developing offshore Portugal and warm frontal cloud structure fanning out far to the north of the low center and offshore Ireland. The cyclone itself should be fully post-tropical in the next few hours, if it can’t be considered so already. Impacts (moderate to heavy rain and damaging winds) begin their arrival Monday morning. My forecast for Ireland (written last night) can be found HERE.
Hurricane Ophelia set two records: 1) The highest latitude major hurricane on record in the North Atlantic Basin, set beginning at 35.9 N and 2) the most easterly major hurricane on record in the basin, set beginning at 26.6W. It will likely weaken below major hurricane force by Sunday morning as it begins to undergo transition into a frontal cyclone from its interaction with the jet stream and further reduction of sea surface temperatures below 72 degrees F/22 degrees C. However, it will be one for the record books.
Fortunately, Ireland and the United Kingdom will not need to worry about a major hurricane hitting them. They will need to worry about a likely damaging windstorm from a post-tropical hybrid cyclone. The post-tropical incarnation will develop frontal characteristics as it initially weakens, but its strong inner warm-core will continue to release some heat into the system, re-intensifying it as it becomes fully embedded in the mid-latitude westerlies and races into Ireland and the UK Monday afternoon and evening. My updated forecast for Ireland is below. Still expecting winds capable of downing trees and causing major power disruptions. The forecast for intense winds is high in confidence as computer models hone in on the center of the storm either coming ashore the southern tip of Ireland or just grazing the western shore. This is favorable for a “big blow” over the entire island. Residents need to be prepared to stay indoors and stay safe during the day Monday.
Ireland Forecast for Post-Tropical Cyclone Ophelia:
Monday Morning (After 7 am local time): For the southern half of the island, wind gusts of 40-50 mph (64-80 km/h) will develop during the morning, increasing to 60-85 mph (97-137 km/h) by mid to late morning from the coast, northward. The strongest gusts will be along the coastal areas, especially the south shores where isolated gusts may approach 100 mph (161 km/h). For the northern half of the island, wind gusts to 40 mph will develop mid morning , increasing to 50-60 mph late morning, from south to north.
Monday Afternoon (After noon): For the south, wind gusts of 60-85 mph (97-137 km/h) early afternoon with isolated to 100 mph/161 km/h along the south/southeast shores). For the north, wind gusts of 50-60 mph (80-97 km/h) early afternoon will increase to 60-85 mph by mid afternoon with isolated gusts to 100 mph along the northeast shores, spreading from south to north into the late afternoon.
Monday Night (after 5 pm local time): For the south, wind gusts will gradually decrease to 40-55 mph (64-89 km/h) during the early evening from south to north. For the north, wind gusts will gradually decrease to 40-55 mph during the mid to late evening (after 7 pm) from south to north.
Sea conditions will be hazardous all around Ireland with wind gusts in excess of 100 mph (161 km/h) likely in the south coastal waters and in the Irish Sea.
Hurricane Ophelia has strengthened into a Category 3 hurricane with maximum sustained winds of 115 mph as it moves south of the Azores. It is moving over prime atmospheric conditions, even as it overcomes waters of only 25 degrees C/77 degrees F. In normal tropical environments, tropical cyclones need water temperatures of 26 degrees C/79 degrees F to maintain themselves and warmer to significantly strengthen. However, the colder temperatures in the upper-atmosphere associated with the mid-latitude troposphere is providing Ophelia with ample atmospheric instability (warm, moist air rising into cold air aloft intensifying thunderstorm activity). In addition, mid-latitude dynamics are playing a role…the approaching frontal system and associated upper-level trough of low pressure approaching Ophelia is giving the system a “poleward outflow jet” to pull air away from the system and allow the surface low to strengthen.
See my previous post from late last night for my wind forecast for Ireland. Strong winds should begin to impact the island midday Monday (local time), with stormy conditions lasting into Monday night. The southeastern Azores will see some gusty winds and 1-3 inches of rain as it passes by this evening and night.