Intensity as a Broad Directional Estimate

Because of the large uncertanties involved, the intensity forecast is best understood as a broad, directional estimate of future storm intensity, a prediction as to whether a storm is expected to intensify, de-intensify, or stay about the same during the forecast interval.

Wind speed numbers in the intensity forecast may be interpreted as rough estimates of what the hurricane is likely to do over the near term; what should be noticed is the trend in the numbers from one forecast to the next, and the magnitude of the approximations. Are the actual and projected maximum sustained wind speeds increasing or decreasing over time, or are they relatively stable, and are the wind speeds themselves high or low? Any wind speed in the triple digits is well above hurricane intensity, and any storm that is consistently dropping its central pressure is undergoing a process of intensification.

These are excerpts of a forecast for a tropical storm getting ready to intensify into a hurricane. The forecast was issued during the early stages of what became Hurricane Ivan of 2004, which briefly threatened New Orleans before eventually making landfall as a Category-4 hurricane at Pensacola, located in the Florida panhandle.

Excerpts from Intensity Forecast for Tropical Storm Ivan (Forecast Advisory #8), September 4, 2004, 11:00 a.m. EDT

....ESTIMATED MINIMUM CENTRAL PRESSURE 997 MB
MAX SUSTAINED WINDS 50 KT WITH GUSTS TO 60 KT
....FORECAST VALID 05/0000Z MAX WIND 55 KT...GUSTS 65 KT....
....FORECAST VALID 05/1200Z MAX WIND 60 KT...GUSTS 75 KT....
....FORECAST VALID 06/0000Z MAX WIND 65 KT...GUSTS 80 KT....
....FORECAST VALID 06/1200Z MAX WIND 70 KT...GUSTS 85 KT....
....FORECAST VALID 07/1200Z MAX WIND 80 KT...GUSTS 100 KT....
EXTENDED OUTLOOK. NOTE...ERRORS FOR TRACK HAVE AVERAGED NEAR 250 NM ON DAY 4 AND 325 NM ON DAY 5...AND FOR INTENSITY NEAR 20 KT EACH DAY
....OUTLOOK VALID 08/1200Z MAX WIND 90 KT...GUSTS 110 KT....
....OUTLOOK VALID 09/1200Z MAX WIND 100 KT...GUSTS 120 KT....

As in the track forecast, the 96h (4-day) and 120h (5-day) forecast periods are labeled EXTENDED OUTLOOK because of the significant decline in forecasting skill after 72 hours.

Central barometric pressure is an indicator of storm intensity and is measured in millibars (mb). There is no forecast for central pressure, only a current estimate. The lower the pressure, the more intense the storm. Normal sea level pressure is 1013.25 mb. A typical winter storm (frontal system) in the midlatitudes will dip below 1000 mb but only the most intense among them will have pressures below around 970 mb. Atlantic tropical cyclones are quite a bit more intense: they operate throughout the 900-1000 mb range, with the less intense storms - like Ivan in this example during its tropical storm stage - just below 1000 mb and the most intense hurricanes just above 900 mb. Any storm dipping below 900 mb during any time in its existence is in the record books of most intense hurricanes, probably as a permanent entry. Thus far, no Atlantic hurricane has been recorded below 880 mb. Current record-holders that have fallen into the 880-900 mb range are limited to a small handful of Category-5 hurricanes over water, three of which are from the 2005 season.

Maximum sustained wind speed, the official indicator of hurricane intensity, is measured in knots (kt), which stands for "nautical miles per hour"; multiplying knots by 1.15 converts to miles per hour. Hurricane wind speeds are always rounded to the nearest 5 knots. The present intensity estimate of top winds of 50 kt (58 mph) in this forecast indicates that Ivan was still a tropical storm when the advisory was issued. The third item on the FORECAST VALID list, corresponding to the 36h forecast period, predicted Ivan's maximum sustained winds to reach 65 kt (75 mph), with subsequent increases for later forecast periods, meaning that the system was forecast to intensify to hurricane status within approximately the subsequent 36 hours and to continue intensifying thereafter.

Close-up of a cone of uncertainty to highlight the plotted points from the track forecast

Intensity forecast information is also contained within the cone of uncertainty - primarily a track forecasting product - but only in the form of a broad guideline. This is the cone corresponding to the Ivan text advisory above. Each of its plotted points corresponds to one of the standard forecast periods (e.g., 12h, 36h, 48h), each tagged as either H or S to denote whether the winds anywhere in Ivan were expected to reach the hurricane-force or merely the tropical storm-force wind speed thresholds at the end of the forecast period.

The initial time is represented by the 0h forecast point, far right in the above cone, labeled 11 AM SAT . It corresponds to the initial line in the text forecast referencing current estimated intensity: ESTIMATED MINIMUM CENTRAL PRESSURE 997 MB; MAX SUSTAINED WINDS 50 KT WITH GUSTS TO 60 KT . The forecast called for Ivan's maximum sustained winds to remain at tropical storm intensity through the 12h and 24h forecast periods, at 55 kt (63 mph) and 60 kt (69 mph), respectively, in the text advisory. On the cone, these forecast points are represented by the next two points working westward from the initial point and ending with the point labeled 8 AM SUN . Both the 12h and 24h forecast points are thus designated S for Tropical Storm.

Once the maximum winds have reached or exceeded hurricane force (74 mph), the system is officially designated a hurricane. As Ivan's winds were forecast to have done this by the end of the 36h forecast period (the unlabeled point matching 11 AM Sunday), that point is the first to be tagged as H for hurricane. Ivan was expected to remain a hurricane throughout the rest of the three-day period, so the remaining two forecast points in this 3-day cone, the two westernmost points corresponding to the 48h and 72h forecast periods, are also stamped with H . These points correspond to the last two FORECAST VALID lines in the test advisory, listing the 48h and 72h MAX WINDS numbers to complete the three-day period covered by this cone; those numbers, at 70 kt (80 mph) and 80 kt (92 mph), were above the hurricane threshhold and continuing to increase.

A 5-day cone would include two additional forecast points to stand for the EXTENDED OUTLOOK , which covers the 96h and 120h forecast periods (4-day and 5-day, respectively). These would be labeled H for Hurricane and M for Major Hurricane (Category-3 or higher) respectively, as the extended outlook was clearly projecting that Ivan would continue to intensify through the five-day period.

This addition of intensity guidelines on the uncertainty cone is of relatively recent vintage; through 2003 the cone had only solid black points. What has now been introduced into the cone is still a very rough estimate. It does not label the forecast points with specific Saffir-Simpson category numbers, just H for "hurricane," S for "tropical storm", M for "major hurricane" (Category-3 or higher), or D for "tropical depression," to indicate which general intensity classification the storm's maximum winds are predicted to reach.

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