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Re: Why is the Wind Speed Probabilities in two pieces? Other comments too.

Posted by Chris in Tampa on 10/6/2017, 11:01 pm

Tropical-storm-force wind speed probabilities:
http://www.nhc.noaa.gov/refresh/graphics_at1+shtml/?tswind120#contents

It's not forecast to strengthen at that time, wind is forecast to be 30mph, gusting to 40mph.

Description of product:

"These graphics show probabilities of sustained (1-minute average) surface wind speeds equal to or exceeding 34 kt (39 mph). These wind speed probability graphics are based on the official National Hurricane Center (NHC) track, intensity, and wind radii forecasts, and on NHC forecast error statistics for those forecast variables during recent years. Each graphic provides cumulative probabilities that wind speeds of at least 39 mph will occur during cumulative time periods at each specific point on the map. The cumulative periods begin at the start of the forecast period and extend through the entire 5-day forecast period at cumulative 12-hour intervals (i.e., 0-12 h, 0-24 h, 0-36 h, ... , 0-120 h). An individual graphic is produced for each cumulative interval, and the capability to zoom and animate through the periods is provided. To assess the overall risk of experiencing winds of at least 39 mph at any location, the 120-h graphics are recommended."

It's not that they are forecasting it to have those wind there, but there is a 20% to 30% chance that 39mph+ sustained wind will be seen to the right of the track due to its movement, winds are higher on that side of the storm so if there are to be sustained tropical storm winds, that's the location that has the highest chance. Also, less friction over water for areas directly along coast to have greater winds.

50 knot wind (58mph) probabilities:
http://www.nhc.noaa.gov/refresh/graphics_at1+shtml/?50wind120#wcontents

Those are not the forecast wind fields it is important to note. Only probabilities color coded to the percent shown on the image.

See Forecast Advisory for Nate:
http://www.nhc.noaa.gov/text/refresh/MIATCMAT1+shtml/

The NHC does give forecast wind fields for Nate, but they are not shown on those images.

The NHC only ever gives the forecast hurricane force wind field through 36 hours and the tropical storm force and 50 knot forecast wind field through 3 days. Days 4 and 5 never have a forecast wind field. Take a look at a forecast for Irma for example.

They currently have forecast tropical storm force wind fields for Nate through 48 hours because it is forecast to weaken after that. If it were to last longer, they would have a forecast wind field for 72 hours, but by then it is forecast to be a depression. But in case it is not, they have the wind speed probabilities giving the NE a heads up.

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In this thread:

Why is the Wind Speed Probabilities in two pieces? Other comments too. - AquaRN, 10/6/2017, 10:35 pm

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