Baseline Set
Primary Set #1
Primary Set #2
Secondary Set #1
Secondary Set #2
Tertiary Set #1
Tutorial Index
Expectations
Forecasts
Landfall
Outlooks
Science Myths
Storm Surge
Hurr. Dialogues
Lessons of Ike
Lessons of Charley
Gulf Coast Index
Katrina Mississippi
Galveston After Ike
Nola 4 Yrs Later
Galveston Before Ike
Miss. 3 Yrs Later
Historic Indianola
La. Coastal Wetlands
Key West, Florida
Front Page Items
Overheard in Media
1900 Storm Tribute
Tech. Terms Defined
Contact/Feedback
Sitemap
Privacy Policy
Wind Radii / Size Forecasts
The forecast below is for Hurricane Rita as a Category-5 hurricane (max winds = 145 kt = 167 mph = Category-5 hurricane). Its current (zero-hour) estimate is that Rita's hurricane-force (64 kt / 74 mph or higher) wind field extends out to a distance of about 75 nautical miles (86 miles) from center in the storm's northeast, southeast, and northwest quadrants, and only about 40 nautical miles (46 miles) out in its southwest quadrant.
|
HURRICANE CENTER LOCATED NEAR 25.4N 88.7W AT 22/1500Z PRESENT MOVEMENT TOWARD THE WEST-NORTHWEST OR 295 DEGREES AT 8 KT ESTIMATED MINIMUM CENTRAL PRESSURE 907 MB MAX SUSTAINED WINDS 145 KT WITH GUSTS TO 175 KT. WINDS ... VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.... ....FORECAST VALID 23/0000Z.... FORECAST VALID 23/1200Z.... FORECAST VALID 24/0000Z.... FORECAST VALID 24/1200Z ... INLAND.... FORECAST VALID 25/1200Z ... INLAND.... |
Forecasts are issued for the outer extent of hurricane-force or 64 kt (74 mph or higher) winds for FORECAST VALID times (forecast periods) only through 36h, and for 50 kt (58 mph or higher) and tropical storm-force or 34 kt (39 mph or higher) winds only through 72h. As these size forecasts are issued only out through a maximum of three days, there is no EXTENDED OUTLOOK for the wind radii.
The diagram below is a graphical illustration of a wind field size forecast for 2004 Hurricane Charley for a 12h forecast period beginning on the early morning of September 13, 2004, together with the excerpt from the official text advisory (#16) that it is meant to illustrate. At this point, Charley was a Category-3 hurricane (105 kt max winds = 120 mph = Category-3) getting ready to intensify to a Category-4 and make landfall. The forecasted radius of its hurricane-force or 64 kt (74 mph or higher) winds is marked in red in both the text and the diagram to show the correspondence; the tropical-storm force or 34 kt (39 mph or higher) wind field forecast is in blue (50 kt wind field not illustrated).
 |
Although this was a Category-3 storm, the forecast was projecting Charley's hurricane-force (Category-1 or higher) winds to extend only 35 miles outward from center at best. Its tropical storm-force winds were projected to have a much farther reach, 138 miles from center at maximum. By contrast, the forecast for Category-5 Rita (Forecast Advisory #20 excerpts, top of page) was estimating that storm's hurricane-force winds as having an estimated radius of more than twice that distance (75 nautical miles = 86 miles out from center) in three of its four quadrants. Wind radii vary quite a bit from storm to storm, and within the same storm during the different phases of its life cycle.
These estimates and forecasts, like all estimates and forecasts, contain uncertainty. However, unlike the track and intensity forecasts, the uncertainties in the size forecast are difficult if not impossible to quantify. Due to the difficulties in physically measuring the wind field in real time at the end of every forecast period, forecast errors cannot be adequately recorded, catalogued, tracked, averaged, or compared to the forecasts of prior years. The actual radii at present cannot be measured, only broadly estimated, and without reliable observational data, the forecasts cannnot be adequately verified, and the post-mortem accuracy of the forecasts cannot be properly quantified.
In the northern hemisphere, the fastest, most destructive winds are usually found in the right side of the storm, especially in the storm's right front quadrant, relative to its direction of motion. This is because the circulating winds on that side are moving in the same direction as the storm's forward motion; the two speeds are additive on that side of the storm. On the left side of the storm, the circulating winds are flowing in opposition to the storm's motion rather than into it, such that the speed of motion is subtracted from the speed of the circulating winds to give the net maximum wind speed on that side of the storm. This built-in asymmetry is a result of the counterclockwise rotation of the winds around the center in northern hemisphere hurricanes; in their southern hemisphere equivalents, the circulating winds rotate in the opposite direction (clockwise around low pressure) and the situation is exactly reversed.
Next Section - Landfalling Hurricanes
Previous Page - Hurricane Quadrants
Back to Tutorial Home