Monster North Pacific Storm Sets Record

Satellite image

NOAA Visual Satellite image of Monster North Pacific Storm

As 2020 transitioned into 2021, a rapidly deepening and intense hurricane-force storm over the western North Pacific has set an all-time record low central pressure of 921 mb breaking the previous record of 924 mb recorded on Nov 8th, 2014 over the Bering and tied in Dec 2015.

High Seas Forecast

The NOAA NWS High Seas Forecast earlier today was predicting winds of 60-95 knots within 300 south of the low center with significant wave heights up to 60 feet(18.3 meters). I cannot remember ever seeing a forecast up to 95 knots wind for an extratropical storm.

NOAA OPC North Pacific Surface Analysis 18Z 31 Dec 2020

Hurricane-force Storms

These winter storms are extratopical cyclones, storm systems that get energy from horizontal temperature gradients and are often associated with frontal zones. Tropical cyclones, in contrast, are generated by the energy released as clouds and rain form in warm, moist, tropical air masses.
Extratropical cyclones occur throughout the year and can vary widely in size from under 100 nautical miles to over 2500 nautical miles. On average, extra-tropical cyclones last about 5 days, however, hurricane-force wind events associated with these systems typically occur during the rapidly deepening phase of the cyclone and that hurricane force conditions were short lived, on average lasting less than 24 hours in duration.
During the cold weather season Arctic air masses will move out over both the North Pacific and North Atlantic Oceans and interact with low latitude tropical air to produce large temperature gradients and strong frontal boundaries. Low pressure centers will then intensify using the temperature contrast as one of the main ingredients for development. When surface winds reach 64 knots the system is classified as “hurricane force”.

Warm ocean currents like the Gulf Stream in the Atlantic and the Kuroshio current in the North Pacific enhance the temperature contrast and thus add significant energy to these developing storm systems.

NOAA OPC Significant Wave Height 00Z 01 Jan 2021

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Case Study: Marine Weather Reconstruction, Arabian Sea

Beaufort Force 9

I was asked to review a marine weather analysis report of conditions in the northern Arabian Sea, east of Socotra Island during July of 2008 that was presented by a weather consulting company on behalf of cargo interests.

The report stated that the weather conditions at a specific location from 4th July 2008 through 8th July 2008 were from the Southwest and gradually strengthened from Beaufort Force 6 to Force 7 with the significant wave heights increasing from around 4 meters to between 5.0 and 5.5 meters.  The peak wave period increased from 9-9.5 seconds to 10-10.5 seconds and those conditions were very typical of the SW monsoon. The report concludes that Beaufort Force 9 conditions with 8 meter waves did not occur as was reported by the master of the vessel.

Reviewed in that report were:

  • 3 hourly wind data from the archive from the NCEP (USA) operational global forecast    model
  • 3 hour wave data from NOAA Wave Watch III Model
  • 3 hour interval NCEP Hi Resolution (fine mesh) Reanalysis models
  • 6 hourly re-analysis project model by the European Centre for Medium Range Forecasts (ECMWF)
  • Derived surface wind data from QuikSCAT and WindSat satellites estimating winds over a 12.5 or 25 km squares and rounded to the nearest 5 knots.

Initial Research
The most accurate source of surface marine weather data are instrumented weather buoys. From my research there were no such buoys near the coordinates at the time of the incident, so to establish the prevailing wind and sea conditions, other sources, such as ship weather reports, must be considered along with any pertinent computer model data.

Ship Observations
Ship observations are usually taken by experienced seaman with wind speed and direction either measured directly with allowances made for ship motion or by estimating those conditions by viewing the sea state.  Wave observations taken from onboard ships are, for the most part, estimated. Ship observations are, however, actual observations and as such are an essential source of information to be considered along with other data sources. Since, in this particular case there were no buoys reporting wind or sea conditions then the only available actual observations were the ship based observations.

Computer Models
The model data outputs used in the report are highly dependent on the accuracy of the initial conditions and the grid spacing used when running the model. The initial conditions will include all available weather observations and available satellite derived data. The initial data inputs were not available in this report so I cannot tell how many, if any, ship observations were used in formulating the initial conditions.

The conclusions in the report regarding the prevailing wind and wave conditions were based on calculations made by various atmospheric and wave model algorithms.  As stated in the report “These are not direct measurements. They are extracted from charts based on the analysis fields generated by the numerical model”.  Since the model grid points can vary from 12.5 km up to 50 km they can sometimes miss smaller atmospheric features so any actual marine weather observations must also be considered along with  model data when making such an analysis.

Research
In order to review the actual observations, I downloaded the ship weather observation for the time period 4 July to 8 July, 2008. These observations were obtained from the NOAA National Climatic Data Center (NCDC) website and included all the ship weather observations available for the area from 10N to 20N latitude between 50E and 60E longitude for the time period in question.  Below are the position reports of all gale force or higher observations:

Conclusions
The listing of actual ship observations obtained from NOAA NCDC included a total of 4 observations of Beaufort force 8 wind conditions and 6 observations of ships reporting Beaufort force 9 wind conditions in the general vicinity of the vessel during this time period 4 July to 7 July 2008 with significant wave heights reported of between 4 and 6 meters.

The full spectrum of wave heights and wave periods in the open sea can be extremely complex with a mix of individual waves interfering with each other so that you can get wave peaks one on top of another adding to a combined higher wave height as well as wave peaks being canceled by wave troughs and reducing the combined height. The significant wave height is defined as the average height of the highest one-third waves in a wave spectrum and this happens to correlate very well with the wave height a skilled mariner perceives in a wave spectrum.

Since the significant wave height represents the average of the 1/3 highest waves then some waves will be higher than the reported significant wave height.  Based on past statistical studies, a significant wave height of 6 meters means that on average, about 10 waves out of 100 within this wave spectrum will reach a height about 7-8 meters and 1 out of 100 waves will reach a height of about 10 meters.

Based on the above, it is my opinion that the vessel likely did encounter winds up to Beaufort force 9 and likely encountered some waves in the range of 7 to 10 meters. There is also the possibility that wind speeds would have exceeded this level in stronger gusts. In general, over open ocean waters, gusts can exceed the average wind speeds by 20-25 percent.

Shipboard Anemometers
We note that ship anemometers tend to be mounted higher than the standard 10 meter wind observation height of land-based weather stations and could lead to some over reporting of the wind speed, however, the difference in heights of various ship anemometers only creates a relatively small effect.  Another consideration with the use of anemometers is that some inexperienced observers might record the apparent wind speed and direction without subtracting out the ship motion so that ships moving into the wind will report higher winds while those moving with the wind will report lower speeds. This is a function of experience and training. The superstructure of ships, like buildings, can also interfere with the wind flow so the placement of the anemometer is important.  Anemometers located above the bridge of tankers/bulk carriers for example have been shown to have the wind accelerated by up to 10 % or decelerated by as much as 100%. It is more likely than not that this results in under recording,  not over recording wind speed.

Climatology
July is well-known for heavy weather in this area of the Arabian Sea. Various sources offer the risk for gale force (BF-8) or higher winds from 16 % to about 30% depending on how small or large an area is considered.  The Atlas of Pilot Charts shows for July that the this area experiences winds from the southwest 83% of the time with the most frequent wind observation at BF -7.  The Atlas of Pilot Charts also shows that there is a 50-60% occurrence of wave heights of 12 feet (3.65 meters) or more in that area during July.

The US Navy forecast handbook states that “Surface winds reach a maximum in July with the highest speeds (up to 50-60 knots) reported northeast of Socotra Island where winds greater than 33 knots occur more than 30 % of the time.”

 

Fred Pickhardt
Ocean Weather Services

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Ship and Yacht Weather Routing Services

Basic Ship and Yacht Weather Routing Services

Ship Weather Routing

Ship and Yacht Weather Routing

Ocean weather routing (Optimum ship routing) provides for a “best route” for ocean transits based on the existing weather forecasts, ship characteristics, ocean currents and special cargo requirements. For most transits this will mean the minimum transit time that avoids significant risk to the vessel, crew and cargo. Other routing considerations might include passenger comfort, fuel savings or schedule keeping. The goal is not to avoid all adverse weather but to find the best balance to minimize time of transit and fuel consumption without placing the vessel at risk of weather damage or crew injury.

A preliminary routing message is transmitted to the master of a vessel prior to departure with a detailed forecast of expected storm tracks, an initial route proposal with reasoning behind the recommendation or plus any alternate routes to be considered. In addition a forecast of the expected weather conditions to be encountered along that route (wind, sea and swell). This allows the master to better plan his route and offers an opportunity to communicate with the routing service any special concerns that he or she might have due to special cargo requirements or ship condition. Once the vessel departs, the vessel’s progress is monitored closely with weather and route updates sent as needed, on average, about every 2-3 days.

Learn more about the benefits of ship routing here: http://www.oceanweatherservices.com/benefits_of_optimum_shi

Contact us for pricing http://www.oceanweatherservices.com/contact_us

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Florida West Coast Hurricanes 

1921 Hurricane Track

September and October are the two months when the Florida West Coast is most at risk of encountering landfalling hurricanes.  As the cold season approaches, the prevailing westerlies begin to increase and migrate southward across the US and start to encroach out over the Gulf of Mexico.

When tropical cyclones move across the Caribbean Sea or form over the Western Caribbean and then move northwestward through the Yucatan Channel during the early Autumn, they will often  feel the effects of the westerlies and as a result, will turn north then northeastward towards the West Coast of Florida.  A few notable examples where Hurricanes Donna (Sept 1960)Wilma (Oct 2005) and Irma (Sept 2017).

Tampa Bay Major Hurricane Landfalls 

The first major hurricane to make landfall in the Tampa Bay region was the “Great Gale of 48”, which was a major hurricane that hit in late Sept of 1848.  The September 1848 storm was an intense hurricane with estimated maximum winds of between 101-135 mph at landfall near Clearwater during the early afternoon of September  25th  with an estimated minimum pressure of about 945mb.  

Thomas B Garland driven ashore – Credit:
Florida State Archives collection

The second major hurricane to make landfall near Tampa was also the most significant hurricane to affect the area, making landfall on October 25th of 1921.  During the night of the 24th and the morning of the 25th the hurricane turned toward the north-northeast then later northeast finally making landfall near Tarpon Springs, Florida where a minimum barometer reading of 28.12 inches (952 mb) was recorded at 2:15 PM. This reading suggests that a max wind at landfall was about 110 knots (125 mph) which would make this storm a Cat 3 hurricane.  After landfall, the storm tracked east-northeast across Florida exiting near Daytona Beach early on the 26th as a Cat.1 hurricane.

Climatology

During the latter half of the hurricane season, the prevailing westerlies begin to increase and migrate southward across the US and start to encroach over the Gulf of Mexico.  Tropical cyclones that move into the southeastern Gulf of Mexico often will feel the effects of the westerlies causing the storms to recurve north then northeastward and thus threaten the west coast of Florida.

During September the primary tracks will cause these storms to make landfall mostly between the Mississippi Delta to the Florida Panhandle or Florida’s Nature Coast.  During October, the storms will tend to recurve sooner targeting the West-Central and Southwestern Florida Coasts. 

Prevailing Hurricane Tracks during September Source NHC

Prevailing Hurricane Tracks during October. Source NHC

 

 

 

 

 

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Gulf of Tehuantepec Gales

The Gulf of Tehuantepec

I first heard about the Gulf of Tehuantepec gales from my father who sailed with the United Fruit Company’s Great White Fleet during the late 1940’s and 1950’s. The Gulf of Tehuantepec (Spanish: Golfo de Tehuantepec) is a large body of water on the Pacific coast of the Isthmus of Tehuantepec in southeastern Mexico.  The gulf extends approximately 300 miles from Puerto Angel, in southern Oaxaca state, southeastward to Barra del Suchiate, in southeastern Chiapas state, and measures approximately 100 miles across its mouth.

The onset of winter brings frequent cold outbreaks across the central and eastern US that can often result in sudden gale to storm force winds over the Gulf of Tehuantepec. The northerly winds funnel through gaps in the Sierra Madre Mountains and spill out over the Gulf of Tehuantepec and out over the eastern North Pacific.

Geography

The Sierra Madre mountains extend southeastward through Mexico and Central America and separates the Gulf of Mexico, the Bay of Campeche´ and Caribbean Sea from the Pacific Ocean. Several mountain gaps allow air to flow across Mexico and the most prominent gap is the Chievela Pass which allows strong cold air surges to pass into the Gulf of Tehuantepec on average about 15 times each winter season with about 2 of these strong enough to reach storm force conditions.

Meteorology

The winds are produced when there is a strong pressure gradient between the Gulf of Mexico to the north and the eastern North Pacific to the south. Northerly winds can increase to storm or even hurricane force during the more extreme events. The first event of each cool season normally occurs in mid-October with the last event occurring in late March or early April.

Gulf of Tehuantepec Surface Analysis

At the outset of gale events, surface pressures reach a maximum value of about 1028mb at Brownsville and 1024 mb at Coatzacoalcos. During storm events the pressure at Brownsville are about 4 mb higher and at Coatzacoalcos about 3 mb more. During Tehuantepec events, the track of the high pressure center is often more critical than the maximum pressure at the center. The path that the anticyclone takes drives the northerly fetch down the coast of Mexico and setting up the strong pressure gradient across the Isthmus of Tehuantepec.

As a practical matter, whenever the Brownsville pressure exceeds 1020 mb there is a good chance that a Gulf of Tehuantepec event may occur so mariners expecting to pass across the Gulf of Tehuantepec should monitor the surface forecast charts for the Western Gulf of Mexico as an indicator.

During gale events, the center of the high could track as far north as the Tennessee and Ohio River valleys while storm events require the high center to track into Mexico or the western Gulf of Mexico. Storm events are also frequently correlated to strong 500 mb upper level troughs.

Hurricane Force Storm event over the Gulf of Tehuantepec

 

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Does global climate change have an influence on tropical cyclone activity?

NOAA Satellite image of Major Hurricane Dorian

Does global climate change have an influence on tropical cyclone activity and can these influences be detected?  This is important given the high risk to populations along coastal regions. This question was tackled in a recent assessment published in The Bulletin of the American Meteorological Society (BAMS):  Tropical Cyclones and Climate Change Assessment (1)

 

In this assessment, the authors focused on the question: “Can an anthropogenic influence on tropical cyclone activity be detected in past data?”  The paper reviewed a number of published case studies about possible detectable anthropogenic influence on tropical cyclones and concluded that there was:

at least a low to medium confidence that the observed poleward migration of the latitude of maximum intensity in the western North Pacific is detectable, or highly unusual compared to expected natural variability.”

The opinions on the team were divided, however, on whether any observed tropical cyclone changes could be attributed directly to anthropogenic influence and these opinions are summarized here:  https://journals.ametsoc.org/view-large/12351541

Storm Surge and Extreme Rainfall 

A storm surge causes tides to quickly rise while rough waves pound the concrete seawall along the shores of Lake Pontchartrain. Hurricane Isaac made landfall along the Gulf Coast and now threatens New Orleans.

Regarding storm surge, the paper indicating that:

a widespread worsening of total inundation levels during storms is occurring because of the global mean sea level rise associated with anthropogenic warming, assuming all other factors equal, although we note that no TC climate change signal has been convincingly detected in sea level extremes data. To date, there is not convincing evidence of a detectable anthropogenic influence on hurricane precipitation rates, in contrast to the case for extreme precipitation in general, where some anthropogenic influence has been detected.”

Reference

  1. Knutson, T., and Coauthors, 2019: Tropical Cyclones and Climate Change       Assessment:    Part I: Detection and Attribution. Bull. Amer. Meteor. Soc., 100, 1987–2007, https://doi.org/10.1175/BAMS-D-18-0189.1.

 

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Tampa Bay Hurricane Storm Surge Scenarios

Hurricane Isaac Storm Surge along the Gulf Coast. Image Credit NOAA

The 2020 hurricane season is off to an early start so it is not too early to prepare for the possibility of the Tampa Bay area experiencing a direct hurricane hit. Tampa has experienced strong storm surges in the past and the question is when will it happen again?

Will History Repeat? 

Since Tampa was a small town associated with a US Army post, Fort Brooke in the 1820s, there have been only two major hurricanes that directly impacted the Tampa Bay area with high winds and storm surges.  The highest reported storm tide in Tampa Bay occurred in September of 1848 during the “Great Tampa Gale” when the tide was reported to have been 15 feet above low tide or about 12.75 feet above MHHW.  During the October Hurricane of 1921 the tide was reported to have reached 10.5 feet above low water or about 8.25 feet above MHHW.  Both of these storms were category 3 hurricanes and so far, Tampa Bay has escaped a direct hit from any category 4 or 5 storms.   Hurricane Elena,  in August of 1985, although not directly hitting Tampa Bay, did create a storm surge that was 4.0 feet above Mean Higher High Water (MHHW) or 6.26 feet above Mean Lower Low Water (MLLW) as Elena hovered offshore over the Gulf of Mexico to the northwest of Tampa Bay. 

Although these catestrophic events are rare, they can and will happen again.  I would estimate that there is a 1-2% risk for a major hurricane impacting the Tampa Bay area in any given year and the question is will it be this year?  Even though there has never been a Category 4 or 5 hurricane to directly hit Tampa Bay, it is not impossible and the devastation would be extreme.

Storm Surge Simulations

In the paper:  Hurricane Storm Surge Simulations for Tampa Bay (1), authors Robert H. Weisberg and Lianyuan Zheng  took a close look at the flooding potential caused by wind stress and atmospheric pressure induced storm surge for a category 2 hurricane and for a catastrophic category 4 hurricane where storm surges of 4-6 meters (13-20 ft) are likely. The paper describes 11 different scenarios with 8 different tracks.  Storm tracks E1, E2, E3, and E4 are for eastward moving hurricanes making landfall at Indian Rocks Beach, Sarasota, Tampa Bay mouth, and Tarpon Springs, respectively. Tracks D2, D3, D4, and D5 are for hurricanes paralleling the axis of the bay or paralleling the coastline from the northwest or the southeast, respectively.

Storm surge hurricane tracks

Proposed Hurricane Track Scenarios Image: (Weisberg and Zheng (1))

Most people believe that a hurricane moving northeastward directly up Tampa Bay would be the “worst case scenario”, however, this is not the case. The paper, in fact, shows that a storm transiting up the bay axis from southwest to northeast actually results in the smallest surge!  

Storm Surge (meters) associated with a category 2 storm moving up the axis of Tampa Bay.  Image: (Weisberg and Zheng (1))

While no storm scenario is a good one for the Tampa Bay area, the worst case is when the hurricane center is located north of the Tampa Bay entrance such that the maximum winds associated with the eye wall occur at the mouth of the bay with storms making landfall farther north result in lower surges.  Storms that make landfall to the south of the bay tend to reduce storm surge along Pinellas County beaches, although localized surges can still occur within the bay.  

Storm surge is also sensitive to the direction that storms approach the bay.  Storms that approach from the south produce lower surges than those approaching from the north. The speed at which the hurricane approaches is also important.  If a storm moves fast enough, the surge may not have enough time to reach the full storm surge potential.  Wind speed is also key as surge height tends to increase with the square of the wind speed.  A category 2 storm might produce limited flooding, while the flooding potential for a category 4 storm could be “catastrophic”. 

Additional factors in surge development include the effects of tides, rivers, and waves. Tide ranges for Tampa Bay tend to be small (0.5 m to 1 m) when compared to the surge potential by winds and pressure. Flooding by heavy rains can be locally important with additional inundation and damage caused by waves.  

Below is the storm surge expected by a Category 2 hurricane, approaching from the west at 5 m/s (10 knots)  and making landfall at Indian Rocks Beach. The asterisk denotes the landfall location while the filled circles show the storm center. The bold lines are surge elevation contours at 1-m intervals.

Cat 2 Hurricane landfall Indian Beach

Storm surge expected by a Category 2 hurricane making landfall at Indian Rocks Beach  Image: (Weisberg & Zheng (1))

 

Cat. 4 Hurricane Scenari0

Below is the storm surge associated with a Cat 4 hurricane making landfall at Indian Rocks Beach. The asterisk denotes the landfall location while the filled circles show the storm center. The bold lines are surge elevation contours at 1-m intervals.

Cat. 4 Hurricane Storm Surge

Storm surge associated with a Cat 4 hurricane making landfall at Indian Rocks Beach  Image: (Weisberg & Zheng (1))

How do Waves add to Storm Surge?

During hurricanes Ivan in 2004 (Alabama) and Katrina in 2005 (SE Louisiana and Mississippi) heavy waves combined with storm surge destroyed sections of bridges. The contributions of the waves to the storm surge and the increased water elevation by storm surge to the waves was described in another paper: Coupling of surge and waves for an Ivan‐like hurricane impacting the Tampa Bay, Florida region (2)

The paper describes the important  interaction between waves and storm surge in shoaling water where the surge height will be elevated by the wave‐induced forces and where changes in water level by storm surge will greatly impact the wave field. It was found that storm surge is increased by wave stress by 0.3–0.5 m.  In addition, the greatest surge enhancement by waves occurs in advance of the actual peak in the storm surge because the wave speed exceeds the storm speed of advance. 

On the other hand, there is also an effect of storm surge on waves. The Significant Wave Height  (the average of the ⅓ highest waves) can increase with surge elevation by about a 1.0–1.5 m and hence perhaps as much as a 2–3 m increase in the maximum wave height.  Such enhancement of waves for a major hurricane could be sufficient to initiate large‐scale damage within the Tampa Bay region prior to the onset of hurricane strength winds. About 3 hours prior to landfall, both the inundation and the significant wave heights would be high enough to begin causing massive damage to low-lying buildings that are open water exposure. 

How Vulnerable is Tampa Bay?

Particularly vulnerable would be the northeast section of St. Petersburg and the northern sections of Old Tampa Bay, including the Courtney Campbell Causeway, the Howard Frankland Bridge, and the Gandy Bridge. As the storm makes landfall, the situation worsens as the inundation continues to increase along with the significant wave heights, even until the  center passes Tampa Bay. 

Therefore, it is not only the wind speed (Saffir‐Simpson category) that matters, rather it is the complex mix of wind, tide, surge and waves that combine to cause major destruction. The entire coastline of Tampa Bay with low elevation and open to water exposure is susceptible to such damage. The eastern shore of Tampa Bay is at risk, even after the storm center passes beyond the Bay since the bay would already be filled with storm surge water and waves which would then shift direction to attack the eastern shore.  

It is also very likely that the wave forces acting vertically beneath bridge spans would destroy spans of the Courtney Campbell Causeway, the Howard Frankland Bridge, and the Gandy Bridge that are closest to the water. McDill Air Force Base and Tampa International Airport would be damaged along with Tampa General Hospital, as well as some other critical areas of infrastructure. 

References:

  1. Weisberg, R.H., Zheng, L. Hurricane storm surge simulations for Tampa Bay. Estuaries and Coasts: J ERF 29, 899–913 (2006).
    https://www.researchgate.net/publication/226176569_Hurricane_storm_surge_simulation_for_Tampa_Bay
  2. Huang, Yong & Weisberg, Robert & Zheng, Lianyuan. (2010). Coupling of surge and waves for an Ivan‐like hurricane impacting the Tampa Bay, Florida region. J. Geophys. Res. 115. 0.1029/2009JC006090.
    https://www.researchgate.net/publication/228973196_Coupling_of_surge_and_waves_for_an_Ivan-like_hurricane_impacting_the_Tampa_Bay_Florida_region

Other Links:

Hurricane Storm Surge Simulations for Florida’s Tampa Bay Region   http://ocgweb.marine.usf.edu/Posters/TBHurricane.pdf

Mariner’s Wave Guide  http://oceanweatherservices.com/featured_blog_posts/a_mariners_guide_to_wavesStorm

Surge Overview, NHC:
https://www.nhc.noaa.gov/surge

NOAA Blog Inside the Eye
Storm Surge–Plain and Simple (Part 1)

Ocean Weather Services
http://www.oceanweatherservices.com

 

 

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What is a Storm Surge?

Storm Surge NOAA NWS

Storm Surge is the abnormal sea level elevations (or depressions) caused by winds and atmospheric pressure. The components are:

1. Coastal set up (down) by the along shore wind stress. 
In deep water, the Earth’s rotation causes a water to move at a right angle the wind stress. This sets up a sea level slope against the coast and an alongshore current in geostrophic balance. With the current limited by friction the sea level set up is less than a meter.
2. Coastal set up (down) by atmospheric pressure.
Atmospheric pressure operates like an inverted barometer. Each mb of pressure drop (increase) raises (lowers) sea level by 1 cm. The largest hurricanes with pressure drops of 100 mb can cause a 1 m surge by this mechanism.
3. Coastal set up (down) by the across shore wind stress. 
In shallow water, and because of friction, the wind stress drives water downwind and piles it up against the coastline. The resulting sea surface slope (tending to balance the across shore wind stress) is the largest contributor to coastal storm surge and can exceed several m.

Other Factors

4. Coastal geometry.
By varying fetch and direction relative to a hurricane the embayment geometry is very important, as are the water depths and land elevations.
5. Continental shelf width.
In shallow water the sea surface slope required to balance the across shelf wind stress is inversely proportional to water depth. Hence wide, shallow shelves are prone to larger storm surges.
6. Tides.
Water level will be higher (lower) at high (low) tide. Since tides in Tampa Bay are about plus and minus 1.5’ this is small relative to the storm surge.
7. Water density.
By being lighter, warmer water in summer stands higher than colder water in winter. This can amount to about 1’.
8. Waves.
Waves are additive to surge. Theoretically a solitary wave can be 1.8 times the water depth. While this is not naturally realized, waves can have a huge impact. Imagine the surf zone on a very rough day displaced to Gulf Blvd.
Sources:
R.H. Weisberg and L. Zheng (College of Marine Science at USF St. Petersburg, Ocean Circulation Group)
http://ocgweb.marine.usf.edu/
National Hurricane Center: Storm Surge Overview https://www.nhc.noaa.gov/surge/
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Tropical Cyclone Wind Speed probability graphic

Tropical Cyclone Wind Speed probability graphic

In addition to the familiar National Hurricane Center’s (NHC) hurricane track forecast with it’s iconic cone, the NHC also produces a wind speed probabilities graphic which provides probabilities (in percent) that wind speeds of at least 34 kt (39 mph, tropical storm force), 50 kt (58 mph), or 64 kt (74 mph, hurricane force) will occur during cumulative time periods at each specific point on the map. These cumulative probabilities indicate the overall chances that the indicated wind speed will occur at any specific location on the map during the period between hour 0 and the forecast hour.

These cumulative probabilities tell decision-makers the chances that the event will happen at any point on the map within the forecast time period stated.  The wind speed probabilities also help users to understand forecast uncertainties, such that they are not surprised by any relatively small changes in the track. This graphic also shows why it is crucial to make proper preparations when a watch or warning is issued for your area, even if the exact track forecast does not go over your area.

Read full explanation here: https://www.nhc.noaa.gov/aboutnhcprobs3.shtml

NOAA NHC Wind Probabilities for 50 knots or higher for Hurricane Katrina

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2020 Atlantic Hurricane Outlook issued by Colorado State University

Hurricane Dorian at its peak Image credit NOAA

The Department of Atmospheric Science Colorado State University has issued its “Extended Range Forecast for the 2020 Atlantic Hurricane Season.  

The outlook anticipates that the 2020 Atlantic basin hurricane season will have above-normal activity. The report points to the current warm neutral ENSO conditions to likely transition to cool neutral ENSO or potentially even weak La Niña conditions by this summer and/or /fall with sea surface temperatures over most of the tropical Atlantic warmer than normal. 

 

 

As a result, an above-average probability for major hurricanes making landfall along the continental United States coastline and in the Caribbean is anticipated. Read full report https://tropical.colostate.edu/media/sites/111/2020/04/2020-04.pdf

 

Colorado State 2020 Hurricane Outlook

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