Tropical Weather Outlooks & Disturbances Explained

Long before a hurricane gets a name, the National Hurricane Center (NHC) is watching the area of disorganized showers and storms that might become one. Those early-stage systems are what RadarBug's Hurricane Outlook layer shows. Understanding the outlook helps you tell the difference between a disturbance worth watching and one that's mostly a curiosity, and gives you days of extra lead time to prepare when one does develop.

From disturbance to hurricane

Tropical cyclones grow out of weaker systems. The terms you'll see along the way:

• Tropical wave: a ripple in the easterly winds that blow across the tropics, often starting over Africa. Dozens cross the Atlantic every season, and a minority develop.
• Disturbance: a general term for an organized area of showers and thunderstorms that has persisted for a day or more but has no closed circulation.
• Invest: short for "investigative area," a disturbance the NHC has decided to study more closely. It's given a number from 90 to 99 with a letter for its basin, such as Invest 94L in the Atlantic (L) or 91E in the eastern Pacific (E). Labeling something an invest triggers specialized computer model runs and sometimes reconnaissance flights. It doesn't mean development is likely.
• Tropical depression: once the system has a closed, well-defined circulation and organized thunderstorms near its center, it becomes a numbered depression, with sustained winds under 39 mph.
• Tropical storm and hurricane: names are assigned at 39 mph, and it becomes a hurricane at 74 mph. See Hurricane Categories & Forecast Cones Explained.

Reading the outlook

During hurricane season the NHC issues the Tropical Weather Outlook four times a day, around 2 a.m., 8 a.m., 2 p.m., and 8 p.m. Eastern time. It gives every disturbance two numbers:

• the chance it becomes a tropical depression or storm within the next 48 hours, and
• the chance it does so within the next 7 days.

The chances are grouped into colored categories: low (under 40 percent, yellow), medium (40 to 60 percent, orange), and high (over 60 percent, red). On the outlook map, an X marks a disturbance's current location, and the shaded or hatched area shows where it could develop over the next seven days. That area is not the storm's size or a forecast track. A long shaded area stretching toward the coast means formation could happen anywhere along that path as it drifts, not that the coast will be hit.

A few practical rules of thumb for reading the outlook:

• The 48-hour number matters more for near-term action. A system with a low 2-day chance and a high 7-day chance has time for things to change.
• Watch the trend, not just the latest number. A disturbance climbing from 20 to 50 to 80 percent over a couple of days is behaving very differently from one that's stuck at 30.
• Close to land, small chances still deserve attention. A disturbance in the Gulf of Mexico can become a tropical storm and reach the coast within a day or two, faster than one that forms far out in the Atlantic.
• Heavy rain doesn't wait for a name. A disturbance that never becomes a depression can still drop flooding rain, so check local forecasts even if formation chances are low.

Potential Tropical Cyclones

Sometimes a disturbance is expected to bring tropical-storm or hurricane conditions to land within 48 hours but hasn't formed yet. Without a named system, there would be no advisory and no watches or warnings. To close that gap, the NHC can designate a Potential Tropical Cyclone (PTC) with a number, for example "Potential Tropical Cyclone Three," and issue full advisories, a forecast track, and coastal watches and warnings before formation. When you see a PTC near your area, treat it with the same seriousness as a named storm.

Advisories once a storm forms

Once a depression or storm exists, the NHC issues a full advisory every six hours, at 5 and 11 a.m. and p.m. Eastern time, with a track forecast and cone, intensity forecast, and watches and warnings. When watches or warnings are in effect for land, intermediate advisories update the storm's position and intensity every three hours in between. RadarBug's storm tracking layer shows the official track, forecast cone, and current position from these advisories.

How the season unfolds

The Atlantic hurricane season officially runs from June 1 to November 30, and the eastern Pacific season from May 15 to November 30. Activity is far from even across those months:

• June and July: storms are usually fewer and weaker and tend to form close to home, in the Gulf of Mexico, the western Caribbean, or off the Southeast coast, often from old fronts or a broad area of low pressure over Central America.
• August through October: the heart of the season. Ocean temperatures are warmest and long-track storms form from tropical waves off Africa. The statistical peak is around September 10, and the large majority of major hurricanes form from August through October.
• October and November: activity shifts back toward the western Caribbean and Gulf of Mexico, where late-season storms can strengthen rapidly and often curve north toward Florida.

Storms occasionally form outside the official season, which is why the NHC sometimes issues special outlooks in May or December.

Names and retired names

Atlantic storm names come from six lists that rotate, so a list used this year comes back six years later. When a storm is so deadly or costly that reusing its name would be insensitive, the World Meteorological Organization retires the name and replaces it, as happened with Katrina, Sandy, Harvey, Maria, and Ian. If a season uses up every name on its list, storms take names from a supplemental list.

Using the outlook on RadarBug

• Turn on Hurricane Outlook to see current disturbances and their development areas, and Storm Tracking for named storms with their cone and track.
• During the peak months, a quick daily look at the outlook is enough most of the time. Pay closer attention once a disturbance's 7-day chance reaches the medium or high category with a development area near you.
• Use the outlook to start low-cost preparations early, such as checking supplies, reviewing your evacuation zone, and filling prescriptions, when a system is still days away. See Severe Weather Preparedness Checklist.