The topic of my blog is severe weather. My posts will talk about severe weather that is occurring in the United States and/or around the world. I chose this topic because I am fascinated by severe weather and because I am pursuing a degree in meteorology and therefore I am knowledgeable about the topic. I also chose this subject because we are currently in the hurricane season and I will be able to blog about tropical cyclones.
Overview
The Atlantic hurricane season officially ended on November 30. The season was fairly active with 15 named storms, 7 hurricanes, and 3 major (category 3 or higher) hurricanes. An average (based on 30-year average) hurricane season would have 12 named storms, 6 hurricanes, and 3 major hurricanes. Another way to measure the total tropical activity is by using a metric called accumulated cyclone energy (ACE). The ACE is the combined wind energy of all the named storms in a particular hurricane season. The ACE for 2016 was approximately 132 which was higher than the mean of about 93. This statistic along with the number of storms led to this season being classified as above normal. The first named storm formed very early in the year on January 12 while the last storm dissipated on November 25. The season caused 1,765 deaths and over 11.5 billion dollars in damage. The strongest tropical cyclone was category 5 hurricane Matthew with maximum sustained winds of 160 mph. It is possible that there could be more storms this year but it is unlikely.
Tracks of all Atlantic tropical cyclones from 2016 with colored intensity dots (redder dots correspond to higher wind speeds). Image Credit: Wikipedia
Tropical cyclones that made landfall in the U.S.
The United States saw five named storms make landfall in 2016. Two storms hit South Carolina and three other storms hit Florida. One of the cyclones that made landfall in South Carolina was Hurricane Matthew, the most intense storm of the season. According to the National Oceanic and Atmospheric Administration (NOAA), “Matthew was the first category 5 hurricane in the Atlantic basin since Felix in 2007.” Matthew wreaked havoc on several states and countries including the U.S., Haiti, Cuba, and the Bahamas. Also, category 1 Hurricane Hermine made landfall on the panhandle of Florida and it marked the first time that a hurricane made landfall in the state since 2005.
Satellite image of Hurricane Matthew, the strongest storm of the season. Image credit: NASA and Wikipedia
How did the predictions for tropical activity do?
Colorado State University predicted in April that there were going to be 13 named storms, 6 hurricanes, and 2 major hurricanes. This forecast was slightly below the actual number of storms in each category. NOAA’s outlook in late May called for between 10 and 16 named storms, 4-8 hurricanes, and 1-4 major hurricanes. The actual totals did end up being in these ranges. So overall, the predictions were not way off this year.
Summary
The 2016 Atlantic hurricane season was above normal in tropical activity. It caused billions of dollars in damage and over 1,500 deaths. Five named storms made landfall in the U.S., including Hurricane Matthew which just itself produced over 10 billion dollars in damage and hundreds of fatalities. This shows you how one storm can have a huge impact. A season could be way below average but generate a powerful hurricane that makes landfall and devastates communities. Therefore, you should be aware and ready every year for a bad tropical cyclone, regardless of the forecast for the season.
Sources
https://en.wikipedia.org/wiki/2016_Atlantic_hurricane_season
https://en.wikipedia.org/wiki/Accumulated_cyclone_energy
https://en.wikipedia.org/wiki/Hurricane_Hermine
https://en.wikipedia.org/wiki/Hurricane_Matthew
A tornado is defined as a rapidly rotating column of air that is connected to a cloud and touching the ground. Tornado intensity is rated by wind speed and can range from 65 mph (EF0) to greater than 200 mph (EF5). From the year 1950-1994, approximately ¾ of all tornadoes were rated F0-F1, ¼ were rated F2-F3, and only 1% were F4-F5’s. Although, the F4-F5 tornadoes caused 67% of the deaths from tornadoes.
Most tornadoes occur in the U.S. because the necessary set of ingredients are commonly present there. These ingredients include strong vertical wind shear, high atmospheric instability, and a lifting mechanism such as a cold front. These are the same environmental conditions that are required for severe thunderstorms (although the levels of the conditions may not need to be as high). Tornadoes have been recorded in every U.S. state. The most concentrated area of tornado occurrences in the U.S. is the central Great Plains and it is called tornado alley.
Both supercell and non-supercell thunderstorms can spawn tornadoes. Tornadoes that are generated from supercells are more common and are usually the ones that cause the most destruction.
There is a special type of tornado called a multiple-vortex tornado that is particularly dangerous. These kinds of tornadoes have a main funnel that contains smaller, individual vortices within it. So the wind speed can be dramatically higher in multi-vortex tornadoes because you have the combined speed of the large vortex with a smaller vortex. In the strongest multi-vortex tornadoes, the wind speed has gotten up to near 300 mph.
A multi-vortex tornado. Image credit: Wikipedia
Appearance
You can also classify tornadoes by their shape and appearance. One tornado can have multiple shapes over its lifetime. First, there is what is called a rope tornado. A rope tornado is long and skinny like a rope and it usually signifies the dissipation stage of a tornado. Another type is a cone tornado, which looks like a cone with the narrower end at the ground. A third way that a tornado can appear is in the shape of a stovepipe. A stovepipe tornado isn’t that much narrower at the ground than it is at the cloud base and it can be pretty wide. The last tornado shape that I will mention is a wedge. A wedge tornado is very wide and it appears as wide or wider than it is tall. There are several other ways that a tornado can look.
A rope tornado. Image credit: Wikipedia
Tornadoes are rapidly spinning funnels of air that can have extreme wind speeds of over 200 mph. They mostly occur in the U.S., and particularly in the area known as tornado alley due to the presence of favorable conditions there. Tornadoes can come from supercell thunderstorms or other types of thunderstorms, although supercell tornadoes are generally stronger and larger. A multi-vortex tornado has smaller vortices that rotate within the main, larger vortex. Also, there are various appearances that a tornado can have, such as rope, cone, stovepipe, and wedge shapes. Spring is the most active season for tornadoes in the U.S, although tornadoes can occur at any time of the year. Therefore, you should stay aware of any severe weather, especially if you live in a tornado-prone area like tornado alley. If you are under a tornado watch or warning, you should follow the safety guidelines that are laid out here: https://www.ready.gov/tornadoes.
Meteorology by Steven A. Ackerman and John A. Knox
https://en.wikipedia.org/wiki/Tornadoes_in_the_United_States
https://en.wikipedia.org/wiki/Tornado
http://www.nssl.noaa.gov/education/svrwx101/tornadoes/types/
http://theweatherprediction.com/habyhints2/714/
http://www.spc.noaa.gov/wcm/permonth_by_state/
There are several types of thunderstorms that have different structures and require different environmental conditions to form. The three basic types are single-cell, multi-cell, and supercell thunderstorms.
Normal Single-Cell Thunderstorms
These kinds of thunderstorms do not last very long (less than 1 hour) and they generally do not produce severe weather. They form in unstable environments with weak wind shear. Surface heating by the sun leads to upward vertical motion of warm and moist air (the updraft). The air cools as it rises and eventually becomes saturated and condenses into tiny water droplets or ice crystals. These water droplets and ice crystals form the cloud and they grow through various processes into raindrops or snowflakes. The thunderstorm starts to precipitate and the downdraft forms. The downdraft eventually squashes the updraft and that leads to the dissipation of the thunderstorm. These garden-variety storms can still produce gusty winds, heavy rain, and lightning.
Multi-cell Thunderstorms
Multi-cell thunderstorms are made up of a collection of single-cell storms and they require moderate amounts of vertical wind shear. Also, they can last for several hours and can produce severe weather. The wind shear tilts the updraft and that means that it doesn’t get choked off by the downdraft. This allows multi-cell storms to sustain themselves and strengthen. Two common types of multi-cell thunderstorms are squall lines and mesoscale convective complexes (MCC’s). A squall line is a line of strong thunderstorm cells and it is often caused by a cold front. A MCC is made up of many individual single-cell thunderstorms and it looks like a round blob of clouds on satellite imagery. Multi-cell thunderstorms can produce severe weather, including strong winds, frequent lightning, flash flooding, hail, and tornadoes.
A radar image of a squall line. Image credit: Lyndon State College
Supercell Thunderstorms
Supercell thunderstorms are big single-cell storms that rotate and can last for hours. For supercells to form, they need an environment that has high instability and high wind shear. The shear creates horizontal rotation that then gets tilted into the vertical by the strong updraft. Supercell thunderstorms are probably the most severe type of thunderstorms because they can produce the whole array of dangerous weather. They can generate all the weather that multi-cell storms can produce but with more severity. They are especially known to spawn strong and violent tornadoes due to the rotating nature of the storms.
A tornado in El Reno, Oklahoma that was generated by a supercell thunderstorm. Image credit: Wikipedia
Summary
There are three main types of thunderstorms: single-cell, multi-cell, and supercell. Single-cell storms are usually the weakest of the three and are short-lived. Multi-cell storms can be fairly intense and last for several hours. Supercells can be very strong storms that can last for hours and produce deadly tornadoes. All these types of thunderstorms can generate dangerous weather so you should stay aware of the forecast for storms. You don’t want to be caught outside when a thunderstorm comes roaring through.
Sources:
Meteorology by Steven A. Ackerman and John A. Knox
https://en.wikipedia.org/wiki/Air-mass_thunderstorm
https://en.wikipedia.org/wiki/Supercell#Effects
https://commons.wikimedia.org/wiki/File:El_Reno,_OK_EF-5_Tornado_2013-05-31.jpg
http://apollo.lsc.vsc.edu/classes/met130/notes/chapter14/squall_lines.html
A severe thunderstorm is defined by the NWS as a thunderstorm that generates wind gusts of 58 mph or more and/or hail of 1-inch diameter or larger and/or a tornado. There are 3 main ingredients that have to be present for severe thunderstorms to form. These ingredients are instability, vertical wind shear, and a lifting mechanism.
Instability
Instability is a measure of whether a parcel of air will keep rising once initially lifted. More unstable environments promote the upward vertical motion of air parcels while stable environments inhibit upward vertical motion. In an unstable environment, a parcel of air will keep rising because it will remain warmer and less dense than its environment. Moisture is also a factor in instability due to the fact that moister air is less dense and will rise more readily. Instability is important to thunderstorms because when air rises, the water vapor condenses and clouds and precipitation form. The rising air essentially creates the thunderstorm. Severe thunderstorms generally require a more unstable environment than regular thunderstorms. A way to measure instability is with Convective Available Potential Energy or CAPE. Higher values of CAPE correspond to more instability.
Vertical Wind Shear
Vertical wind shear is another important factor determining whether a severe thunderstorm will form. Vertical wind shear is defined as the change in wind speed and direction with height through the atmosphere. Severe thunderstorms need wind shear to tilt the updraft so that the precipitation or downdraft does not squash it. The updraft feeds the thunderstorm the warm, moist air and if that gets choked off, the thunderstorm will die. Another effect that wind shear can have on thunderstorms is that it can cause them to rotate. Rotating thunderstorms are called supercells and they can produce tornadoes.
A supercell thunderstorm. Image credit: Wikipedia
Lifting Mechanism
A lifting mechanism is necessary for severe thunderstorms because it is the thing that initially starts the upward vertical motion of air. A lifting mechanism could be solar heating of the ground, a dryline, or a front. Cold fronts commonly serve as lifting mechanisms for severe thunderstorms. The warm and moist air ahead of the front is forced up and over the cold and dry air behind the front. The warm and moist air rises, cools, and condenses into clouds which then precipitate rain, hail, etc.
Summary
Severe thunderstorm formation requires three main ingredients. The ingredients are instability, vertical wind shear, and a lifting mechanism. The lifting mechanism triggers the initial ascent of air and then the instability causes the air to keep rising. The rising air eventually condenses and forms a thunderstorm. Vertical wind shear tilts the updraft of the storm and allows the storm to sustain itself and become more severe. The shear can also cause the storm to rotate. People should be especially aware of severe thunderstorms because they can create dangerous weather such as damaging straight-line winds, hail, lightning, flash flooding, and tornadoes.
A thunderstorm that is possibly severe. Image credit: Wikipedia
Sources:
http://www.atmo.arizona.edu/students/courselinks/spring11/atmo436a/Lectures/Lecture32.pdf
https://en.wikipedia.org/wiki/Atmospheric_instability
https://upload.wikimedia.org/wikipedia/commons/4/4f/Supercell_storm.jpg
https://en.wikipedia.org/wiki/Wall_cloud#/media/File:Rolling-thunder-cloud.jpg
Meteorology by Steven A. Ackerman and John A. Knox
As of October 24, the number of tornadoes that have occurred in the U.S. this year is 897. This is significantly lower than the 20-year average of 1,134 tornadoes through October 24. There have also been a lot less tornado reports this year than usual. The month of June had a tornado count that was less than half of the normal number of tornadoes for that month. Not all the months had below average activity though. February had almost 3 times as many tornadoes as average for the month and March and August were also above average. But overall, the statistics put 2016 on track to be one of the least active tornado years (in terms of number of tornadoes) in the past decade.
Largest Tornado Outbreak and Strongest Tornadoes of the Year
The tornado outbreak of May 22-26 was the biggest of 2016 with 73 tornadoes occurring. There was 1 EF4 tornado, 8 EF3’s, and many more EF2 and below tornadoes. The EF4 (winds between 166 and 200 mph) that touched down on May 25 was one of only two of that strength for 2016 so far. This EF4 tornado was a half-mile wide wedge tornado that tore through parts of Kansas. According to Wikipedia, the “tornado obliterated farm homes, debarked trees, bent railroad tracks, and mangled farm machinery and vehicles beyond recognition, though no injuries or fatalities occurred along the path.” The other EF4 tornado of 2016 touched down in Oklahoma on May 9. This EF4 destroyed homes, dug up the ground, and killed one person.
Destruction of a home by the EF4 Kansas tornado of May 25. Image Credit: Wikipedia
An example of a wedge tornado, which is a tornado that appears as wide or wider than it is tall. This one occurred in Binger, Oklahoma in 1981. Image Credit: Wikipedia
One tornado in New Hampshire this year
One weak tornado touched down in New Hampshire this year on July 18th. It was on the ground in Pittsburg, NH for about 2 miles and lasted approximately 4 minutes. The tornado was given an EF0 rating, the lowest classification, with winds estimated to be about 75 mph. The maximum width of the tornado was 200 yards. There were no deaths, injuries, or property damages from the tornado, although numerous trees were knocked down. One tornado is actually the average tornado count for New Hampshire per year (averaged 1991-2010), so this was a typical year tornado-wise.
Summary
It has been a slow year so far for tornadoes in the U.S., but it has been an average year for New Hampshire. The biggest tornado outbreak of the year was in May and it resulted in a total of 73 tornadoes. Also, the strongest tornadoes in the U.S. this year have been the 2 EF4’s in May, and there has been one tornado in New Hampshire which was an EF0.
Even though there has been below-average tornadic activity so far this year, that does not mean that there will not be any more outbreaks for the rest of 2016. Unlike with hurricanes, there is no specific tornado season. Tornadoes occur in every month of the year and can cause death and destruction anytime of the year. Therefore, anyone in a tornado-prone region of the U.S. should always stay alert to severe weather and the potential for tornadoes. In November and December, the highest threat area is in the southern U.S. and so people in this region should pay the most attention to the weather forecast for the remainder of the year.
Sources:
http://www.spc.noaa.gov/faq/tornado/ef-scale.html
https://www.wunderground.com/news/tornadoes-low-2016-fall-update
https://en.wikipedia.org/wiki/Tornado
https://en.wikipedia.org/wiki/Tornadoes_of_2016
https://www.ncdc.noaa.gov/stormevents/eventdetails.jsp?id=653509
https://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology
https://weather.com/storms/tornado/news/tornado-risk-by-month-20130315
http://www.accuweather.com/en/features/trend/what-is-a-wedge-tornado/6235796
I am basing the intensity rankings of the tropical cyclones on the lowest central pressure that they attained in their lifetimes, with the lower the pressure the stronger the storm.
The strongest hurricane in the Atlantic basin was Hurricane Matthew which was active from September 28 to October 9. Matthew reached category 5 status with a minimum pressure of 934 mb and 1-minute maximum sustained winds of 160 mph. Matthew started out near the Lesser Antilles Islands and then moved west and rapidly intensified into a category 5 hurricane. It was only briefly a category 5 storm because it weakened, turned north, and eventually made landfall in Haiti and Cuba. It then traveled through the Bahamas and passed just off the coast of Florida as a category 3 hurricane. It eventually made landfall in South Carolina as a category 1 hurricane and then became post-tropical shortly thereafter.
How does Matthew’s strength compare to Atlantic hurricanes in years past?
Well, the most powerful hurricane in the Atlantic basin in recorded history was Hurricane Wilma of 2005 which had a pressure of 882 mb and winds of 185 mph. Even though Matthew was very intense, it was a low end category 5 storm and was not nearly as strong as Wilma or many other tropical cyclones in history.
Map showing Matthew’s track throughout its lifespan with the colored circles indicating its strength [goes from blue (weakest) to red (strongest)]. Image Credit: Wikipedia
The strongest West Pacific tropical cyclone of 2016 was Super Typhoon Meranti which was active from September 8 to September 16. The typhoon’s pressure got down to 890 mb and the winds got up to 190 mph. The storm formed well west of the Philippines and strengthened as it moved west-northwest. It moved just south of Taiwan and then made landfall in China as a typhoon with category 2 equivalent winds.
How does Meranti’s strength compare to Western Pacific typhoons in years past?
Super Typhoon Meranti is tied for 15th place on the list of Northwest Pacific typhoons with the lowest pressures. This means that Meranti was a very powerful storm, even by western pacific standards (the West Pacific basin usually gets more and stronger tropical cyclones than the Atlantic basin in a given year).
Satellite image of Super Typhoon Meranti. Image Credit: Wikipedia and NASA
The most intense Atlantic hurricane of 2016 so far was Hurricane Matthew and the most intense West Pacific typhoon of 2016 so far was Super Typhoon Meranti. I doubt that there will be any stronger tropical cyclones than these that develop in their respective basins by the end of this tropical season.
There is currently one tropical disturbance in the Atlantic that has a chance to develop. It has been given 2-day and 5-day odds of development of 50% by the NHC. I think it is unlikely that the system will develop given all the dry air around it and the wind shear that it is facing.
https://www.wunderground.com/hurricane/atlantic/2016/Post-Tropical-Cyclone-Matthew
https://en.wikipedia.org/wiki/2016_Atlantic_hurricane_season
https://en.wikipedia.org/wiki/List_of_the_most_intense_tropical_cyclones
https://en.wikipedia.org/wiki/2016_Pacific_typhoon_season
http://tropic.ssec.wisc.edu/real-time/windmain.php?&basin=atlantic&sat=wg8&prod=shr&zoom
Category 3 Hurricane Nicole’s eye passed over Bermuda at around 11:00 AM this morning. The island experienced strong winds (with gusts over 100 mph), heavy rain, storm surge, and high waves. The storm caused more than 20,000 people to lose power and likely damaged some structures and knocked down trees. Storm surge was expected to be 6-8 feet and rainfall was supposed to total 5-8 inches. On Tuesday and Wednesday, Hurricane Nicole underwent rapid intensification from a tropical storm to a category 4 hurricane with winds of 130 mph. Nicole weakened to a category 3 storm before hitting Bermuda due to wind shear. It is very rare for a storm this powerful to make landfall in Bermuda. A big reason why the storm strengthened so much is because of the record warm sea surface temperatures that the storm moved over. Hurricane Nicole is now moving away from Bermuda and is expected to continue to weaken over the coming days. My advice to the people of Bermuda would be to stay informed about the storm and listen to your local authorities and public safety officials. You should stay inside and shelter until the storm passes and the all clear is given.
Hurricane Nicole Forecast Track Cone. Image Credit: NHC
Satellite image of Hurricane Nicole before hitting Bermuda. Image credit: NASA
Impacts from Hurricane Matthew
Hurricane Matthew did not make landfall in Florida but did make landfall in South Carolina as a category 1 storm with 75 mph winds. Although Matthew did not make landfall in Florida, it still had significant negative impacts on the state. There was flooding from storm surge in various places including in St. Augustine. High winds affected the eastern coast of Florida and caused power outages and damage to structures. Also, Florida received heavy rainfall from the storm. Similar impacts were felt in Georgia, the Carolinas, Virginia, and the Northeast (with lesser severity). There was major flooding in North and South Carolina due to high rainfall totals that exceeded 10 inches in some places. The damage done by Hurricane Matthew in the U.S. totals at least $6 billion.
Summary
Hurricane Nicole made landfall in Bermuda as a category 3 storm and has brought the whole range of impacts that come with hurricanes to Bermuda. This unusually strong storm (for its path) will likely cause significant damage to the island.
Hurricane Matthew is not around anymore but has left a path of devastation in its wake. Its winds, flooding rains, and storm surge flooding did billions of dollars in damage along the U.S. east coast and especially in the southeast.
Finally, there is not likely to be any tropical development in the next five days. Based on my recollection of previous hurricane seasons, I think that there probably will only be a few more named tropical cyclones this season.
Sources:
https://en.wikipedia.org/wiki/Hurricane_Nicole_(2016)
https://www.wunderground.com/blog/JeffMasters/category-3-hurricane-nicole-pounding-bermuda
https://weather.com/storms/hurricane/news/major-hurricane-nicole-atlantic-bermuda
http://www.nasa.gov/feature/goddard/2016/nicole-atlantic-ocean
https://en.wikipedia.org/wiki/Hurricane_Matthew#United_States_2
Hurricane Matthew as of the 2 PM EDT National Hurricane Center (NHC) advisory
Hurricane Matthew is a category 4 storm with winds of 140 mph and a central pressure of 939 mb. The hurricane is moving northwest at 14 mph and is located in the area of the northern Bahama islands, east of south Florida. Currently, satellite imagery shows a visible eye and a fairly well-organized storm with stronger convection off to the northeast and less strong convection to the northwest. The storm has been pounding the Bahamas and now has its sights set on Florida. Matthew is forecast to move north-northwest and skim the eastern coast of Florida. The center of Matthew should get very close to the Port Canaveral area, if not make landfall, on Friday morning with winds of 145 mph. Note that the cone of uncertainty shows that it could make landfall in Florida or it could stay off the coast. The exact track will determine the severity of impacts, with a more eastern track being potentially significantly less damaging than a more western track.
The storm will then go almost parallel to the Florida coast and then turn to the right and stay farther offshore of Georgia, South Carolina, and North Carolina. But it will still be close enough to have significant impacts in those states. Matthew will weaken as it moves up the coast due to interactions with land, higher wind shear, and dry air.
Hurricane Matthew Forecast Track Cone. Image Credit: NHC
Impacts from Major Hurricane Matthew
Hurricane warnings are in effect for almost all of eastern Florida except for the southern tip (where tropical storm warnings are in effect) and for the coasts of Georgia and southern South Carolina. There are also tropical storm warnings in effect for inland areas of these states. Impacts from Hurricane Matthew will likely include significant storm surge, very high waves, very heavy rainfall, very strong winds, and potentially some tornadoes.
Storm surge could get as high as 7-11 feet from the coast of central Florida to southern South Carolina and 4-6 feet for a good distance south and north of that area. The storm surge combined with the high wave heights will likely cause significant beach erosion and flooding of the coastline.
Rainfall through Sunday is forecast to be 12-18 inches along parts of the Georgia, South Carolina and North Carolina coast. Also, large swaths of the southeast will get under 12 inches of rain. This heavy rainfall will likely lead to some flooding.
Hurricane-force winds are likely to impact the coast of central Florida and tropical storm force winds are likely to impact a large part of Florida and some of Georgia, South Carolina, and North Carolina. These winds could damage houses, knock down trees, and cause widespread power outages.
Finally, there will also be the chance of isolated tornadoes along the southeast U.S. coast.
Summary
Category 4 Hurricane Matthew is a dangerous storm and could cause significant destruction to the Southeast and especially Florida. I would suggest that anyone located in the threat zone heed all warnings and follow the advice of public safety officials. You should stay away from the beach for the duration of the storm and be careful of flooding and high winds. Also, you should probably stay inside if you can and monitor the progress of the hurricane so you know when it is okay to carry on with your normal activities.
Radar image of Hurricane Matthew moving towards Florida. Image Credit: NWS
Sources:
Water vapor image of the Atlantic Ocean with Hurricane Matthew located north of Venezuela in the eastern Caribbean Sea. Image Credit: National Hurricane Center
Current Conditions
Matthew is currently a category 1 hurricane with 75 mph maximum sustained winds and a minimum central pressure of 987 mb. The hurricane is located north of Venezuela in the eastern Caribbean Sea. The storm is moving west and looks decently well put together on visible satellite imagery, although its strong convection is getting blown to the north of the circulation center. This shearing is more apparent on water vapor imagery and a wind shear analysis shows a fairly high 20-25 knots of shear affecting the hurricane, especially on its northern side. The storm has good outflow to its north and its west which is likely helping the storm to strengthen. Also, the water vapor imagery shows some dry air to the west of the tropical cyclone, which is in the direction that it is moving. Sea surface temperatures (SST’s) are 29°C-30°C in the vicinity of Hurricane Matthew and relative humidities in the middle levels of the atmosphere are 50-55%.
Forecast Track Cone for Hurricane Matthew. Image Credit: National Hurricane Center
Short-Term Forecast
The current wind shear is not favorable for intensification of Hurricane Matthew. The SST’s are conducive for strengthening and the relative humidity and dry air are marginally favorable for further development of Matthew. In the near term, Hurricane Matthew looks like it will stay over warm waters but will travel into an area that currently has some dry air and high wind shear (although the wind shear should decrease some). In my post last week, I talked about the environmental conditions necessary for tropical cyclogenesis and that even one unfavorable condition could prevent strengthening and even cause weakening. So based on this fact, my own opinion would be that I would not expect much intensification of Matthew in the next couple of days. The National Hurricane Center (NHC) forecast calls for the storm to move just south of west and then make a sharp right turn on Saturday and move north after that. The NHC also forecasts a gradual strengthening with an increase in wind speed to 100 mph by Saturday.
Longer-Term Forecast and Summary
The NHC track forecast has the hurricane passing over Cuba and then continuing north. After that, Matthew could potentially hit the U.S. but the uncertainty is high. In summary, Hurricane Matthew is in unfavorable environmental conditions right now, but it will move approximately west and the conditions will likely get better so the storm should gradually strengthen. Then the storm will take a right turn and move north and affect Caribbean islands such as Cuba. In the longer term, there is a lot of uncertainty but the storm has the potential to impact the U.S. So I would recommend that anyone living on or visiting the East Coast should stay informed about the path of the storm so that they can prepare if it comes their way.
Sources:
http://tropic.ssec.wisc.edu/real-time/windmain.php?&basin=atlantic&sat=wg8&prod=shr&zoom
For a tropical cyclone to form and strengthen (called tropical cyclogenesis), several environmental conditions need to be present. The factors that determine whether there is tropical formation are the existence of an atmospheric disturbance, sea surface temperatures, moisture, wind shear, instability, and distance from the equator. I will talk about each of these factors and what level of each produces favorable conditions for tropical cyclones.
Pre-existing Tropical Disturbance
The most integral requirement for the development of a tropical cyclone is that there be an initial weak disturbance that has enough rotation and low-level convergence. An example of this would be a tropical or easterly wave. Tropical waves have low pressure and have an area of disorganized thunderstorms associated with them. The waves come off of northwest Africa and move into the Atlantic Ocean where they then can develop into a tropical cyclone. Most tropical storms and hurricanes start out as easterly waves, but just because there is a tropical wave does not mean a tropical cyclone will form. In fact, only about 20% of these waves develop into tropical cyclones. So there are clearly other factors at play in determining formation of storms. One of these is sea surface temperatures.
Sea Surface Temperatures
Sea surface temperatures (SST) are a big factor in tropical cyclone development because evaporation of seawater provides the energy that drives a tropical cyclone. SST have to be 26.5°C (80°F) or higher through a depth of the ocean of at least 50 meters (150 ft). The higher the SST, the greater the chance that a tropical cyclone will form or intensify.
Tropospheric Moisture
There needs to be a sufficient amount of moisture in the troposphere so that condensation occurs and clouds and thunderstorms form. The value of relative humidity required for tropical cyclones is usually about 50-60% near the middle troposphere (approx. 5 km up). Dry air will kill the thunderstorms and prevent the development of a tropical cyclone.
Wind Shear
Vertical wind shear (change in wind speed and direction with height) has to be low (less than 20 knots) for tropical cyclogenesis to occur. If wind shear is too high, a tropical disturbance or cyclone will get ripped apart and the thunderstorms will be displaced from the circulation. Also, the shear can bring dry air into a storm which, as I noted already, is detrimental to the formation or strengthening of a tropical system.
Instability
The atmosphere has to be unstable for a tropical disturbance to develop or for a tropical cyclone to strengthen. Instability means that a parcel of air will rise due to it being less dense than the environment around it. This is related to tropical cyclones because they need the evaporated ocean water to rise until it condenses and forms clouds, precipitation, and thunderstorms. More instability means that thunderstorms will be more numerous and stronger which promotes cyclogenesis.
Coriolis Force
Finally, tropical disturbances must be at least 5° of latitude away from the equator to develop into cyclones. This is because of the Coriolis force which causes weather systems like tropical cyclones to rotate. The Coriolis force is zero at the equator and increases as you get closer to the poles. Tropical disturbances have to spin to be able to maintain their low pressure.
Summary
As you can see, there are several conditions that need to be present for cyclogenesis to occur. Even if all these conditions are met, that still does not mean that a tropical cyclone will form for sure. There is also likely a possibility that a tropical cyclone could form without a completely favorable environment. But the factors that I laid out are generally what is required for the formation and intensification of tropical cyclones. With all these factors determining whether cyclogenesis occurs, you can see how tropical cyclones can be very sensitive to changes in their environments. An example that I remember is Hurricane Irene of 2011, which was forecast at one time to strengthen to a Category 4 hurricane as it moved north just off the southeast U.S. coast. But dry air over the land got wrapped into Irene and weakened the storm to a Category 1 hurricane before it made landfall in North Carolina. This case shows how one unfavorable environmental factor can have a big impact on the intensity of a tropical cyclone.
Category 3 Hurricane Irene. Image Credit: NASA and Wikipedia
Tropical Storm Irene making landfall in New York City. Image credit: NASA and Wikipedia
Sources:
http://weather.about.com/od/t/g/tropical_wave.htm
https://en.wikipedia.org/wiki/Tropical_cyclogenesis#Requirements_for_tropical_cyclone_formation
http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/pubref/References/GUIDE/chap2/se302.htm
http://www.goes-r.gov/users/comet/tropical/textbook_2nd_edition/navmenu.php_tab_9_page_3.1.0.htm
http://www.aoml.noaa.gov/hrd/tcfaq/A15.html
http://www.theweatherprediction.com/severe/ingredients/instability/
https://en.wikipedia.org/wiki/Hurricane_Irene
Severe Weather 
