TROPICAL UPDATE…SUBTROPICAL STORM ONE…ANALYSIS SYNOPSIS…ISSUED MAY 13, 2023…11:30 A.M. EDT8 min read
Disclaimer: This is not affiliated with the National Hurricane Center, Hurricane Hunters, Storm Prediction Center, or National Weather Service. ALL forecasts herein are the result of my analysis, (to which you will see me at times, insert excerpts from various agencies due to the nature of the importance of the information) and I am solely responsible for the content. As ALWAYS, follow the National Hurricane Center, National Weather Service, and your local Emergency Management officials for emergency decisions. In addition, this is strictly a FORECAST OFFICE. I CANNOT make decisions regarding travel plans, etc. My purpose, is to provide you the information, based solely on information I analyze, and the accuracy of the information at hand of the time of analysis, so you may make informed decisions.
(T. F. “Storm” Walsh)
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I will reiterate, my forecasts are based on the available information at the time of analysis, and are only as accurate as the information analyzed and the solutions provided.
The following is my outlook forecast for the upcoming 2023 Atlantic Hurricane Season. These totals are based on changes to some of the updated climate model data.
STORM W PRE-SEASON FORECAST
TOTAL NAMED STORMS: 11– 14
TOTAL HURRICANES : 5 – 6
MAJOR HURRICANES: 2 – 3
AVERAGE HURRICANE SEASON:
TOTAL NAMED STORMS: 14
TOTAL HURRICANES: 7
MAJOR HURRICANES: 3
The following are the storm names for the 2023 hurricane season. As each storm is named, they will be colored in red in order to keep track of the used names in the list:
Arlene Bret Cindy Don Emily Franklin Gert Harold Idalia Jose Katia
Lee Margot Nigel Ophelia Philippe Rina Sean Tammy Vince Whitney
Today’s update will be in regard to the NHC Public Information Statement issued 2 days ago, stating that re-assessment of the area of low pressure that developed 300 miles north of Bermuda, off the U.S. east coast, should be designated as a subtropical storm, in which the NHC has done so. Going back and performing my analysis with the current information archives, I am going to point out WHY I believe this storm was not subtropical in nature. First, here is the information statement from the NHC:
Public Information Statement NWS National Hurricane Center Miami FL 1100 AM EDT Thu May 11 2023 ...NHC Determines That a Subtropical Storm Formed in the Atlantic Basin in Mid-January 2023... Through the course of typical re-assessment of weather systems in the National Hurricane Center's (NHC) area of responsibility, NHC hurricane specialists have determined that an area of low pressure that formed off the northeastern coast of the United States in mid-January should be designated as a subtropical storm. Specific information on the justification for the subtropical storm designation, as well as the system's synoptic history and impacts, will be available in a Tropical Cyclone Report, which will likely be issued during the next couple of months. This subtropical storm is being numbered as the first cyclone of 2023 in the Atlantic basin and will be given AL012023 as its system ID. As a result, the next system that forms in 2023 in the Atlantic basin will be designated as AL022023, and advisories will be issued in AWIPS bin 2 (e.g., Public Advisories will be issued under AWIPS header TCPAT2 and WMO header WTNT32 KNHC). If the system begins as a tropical depression, then it would be given the designation 'TROPICAL DEPRESSION TWO', and if it becomes a tropical storm, it would be given the name 'ARLENE'. National Weather Service policy (through NWS Instruction 10-607, Section 1) allows for marginal subtropical systems to be handled in real-time as non-tropical gale or storm events in NWS High Seas Forecast products. This was the procedure followed for the unnamed subtropical storm in mid-January. However, the lack of real-time issuance of advisories does not preclude NHC from retroactively designating these systems as a subtropical cyclones in post-analysis, if necessary. $ Hurricane Specialist Unit
The following was the only Tropical Weather Outlook issued for the system:
Special Tropical Weather Outlook
NWS National Hurricane Center Miami FL 1005 AM EST Mon Jan 16 2023 For the North Atlantic...Caribbean Sea and the Gulf of Mexico: Special Tropical Weather Outlook issued to discuss the potential for subtropical development over the northwest Atlantic. Northwestern Atlantic: A non-tropical low pressure system centered over the northwestern Atlantic Ocean about 300 miles north of Bermuda is producing storm-force winds. Although the cyclone is producing some thunderstorm activity near the center, it is embedded in a cold air mass with nearby frontal boundaries. The low is expected to move northeastward today and northward tonight, bringing the system over much colder waters and across Atlantic Canada by early Tuesday. Therefore, it is unlikely that the low will transition to a subtropical or tropical cyclone. Nevertheless, the system is expected to remain a strong non-tropical low during the next day or so, and additional information, including storm-force wind warnings, can be found in High Seas Forecasts issued by the National Weather Service. No additional Special Tropical Weather Outlooks are scheduled for this system. Regularly scheduled Tropical Weather Outlooks will resume on May 15, 2023, while Special Tropical Weather Outlooks will be issued as necessary during the off-season. * Formation chance through 48 hours...low...near 0 percent. * Formation chance through 5 days...low...near 0 percent. && High Seas Forecasts issued by the National Weather Service can be found under AWIPS header NFDHSFAT1, WMO header FZNT01 KWBC, and online at ocean.weather.gov/shtml/NFDHSFAT1.php $ Forecaster Cangialosi NNNN
Based on my careful analysis of information available to me at the moment, I have to disagree on this decision by the NHC on this system being a subtropical entity. If you note in the TWO issued by the NHC, they mention the storm being embedded in a cold airmass, with nearby frontal boundaries. Based on this, I went back through the NHC TAFB surface analysis maps and analyzed the maps for both 16 and 17 Jan. 2023. Analysis revealed that this system in fact had frontal boundaries running through the storm, meaning the system was attached to a frontal boundary. Based on this alone, it would disqualify the storm from being designated as subtropical. The following is a definition of subtropical:
There are two definitions currently used for subtropical cyclones depending on their location. Across the north Atlantic and southwest Indian Ocean, they require some central convection fairly near the center surrounding a warming core existing in the mid-levels of the troposphere. Across the eastern half of the northern Pacific however, they require a mid-tropospheric cyclone to be cut off from the main belt of the westerlies and with only a weak surface circulation. Subtropical cyclones have wider wind fields with the maximum sustained winds located further from the center than typical tropical cyclones, and have no weather fronts linked into their center.Since they form from initially extratropical cyclones which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required for their formation are lower than the tropical cyclone threshold (around 26.5°C (79.7°F)) by 3°C (5°F), lying around 23 °C (73 °F). This also means that subtropical cyclones are more likely to form outside the traditional bounds of the North Atlantic hurricane season and at higher latitudes. Subtropical cyclones are also observed to form in the South Atlantic, where subtropical cyclones are observed in all months.
You will notice the frontal boundary which does connect to the center of the low:
NHC TAFB SURFACE ANALYSIS MAPS JAN. 16 – 17 2023
In the second graphic, it is hard to make out, but the occlusion does wrap around into the center of the northern most low pressure marking.
Based on the statement from the NHC TWO of this having been embedded in a cold air mass, I kind of doubt that the system had a warm core, or even a partial warm core. In regard to warm core development, SST’s in general need to be around 21C to 23C for the system to take advantage of latent heat of evaporation. From weather.gov:
1) An extratropical cyclone forms. Extratropical cyclones have cold air at their core, and derive their energy from the release of potential energy when cold and warm air masses interact. These storms always have one or more fronts connected to them, and can occur over land or ocean. An extratropical cyclone can have winds as weak as a tropical depression, or as strong as a hurricane. Examples of extratropical cyclones include blizzards, Nor’easters, and the ordinary low pressure systems that give the continents at mid-latitudes much of their precipitation.
2) If the waters under the extratropical cyclone are at least 21C (70F), thunderstorm activity will gradually build inside the storm and moisten and warm the lower levels. Over time, the core of the storm may gradually go from cold to warm, and the storm will start getting some of its energy from “latent heat”, which is the energy released when water vapor that has evaporated from warm ocean waters condenses into liquid water. Latent heat is what powers tropical cyclones. At this point, the storm is called subtropical. If the winds are already more than 39 mph (as happened in the case 2007’s Subtropical Storm Andrea), it is called a subtropical storm. If the winds are less than 39 mph, then it is called a subtropical depression. So, you don’t need to start with a subtropical depression in order to get a subtropical storm.
Analysis of archived SST charts indicate the SST at the time of development, based on co-ordinates in satellite imagery (41.5N; 61.0W), was below the required temperature for the ocean to impart evaporation to produce latent heat. As the storm moved NE, it kept moving into colder water. So again, I do not feel this system attained a warm core.
SST CHART FOR JAN 16, 2023
Based on these analyses, I cannot concur with the NHC finding in that this was a subtropical storm.
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Have a blessed weekend!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
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