Weather Blog: September 27 2025 12 PM (Tropics)

RISK OF FLOODING RAINS OVER PORTIONS OF THE SOUTHEAST U.S.

A developing tropical cyclone over the Bahamas will likely move close enough to the South Carolina coast to produce river flooding and small stream and urban flash flooding across portions of South Carolina, North Carolina, Georgia, and possibly southern Virginia.  Will discuss that further but first an update on Humberto.

Humberto

As of this issuance, Humberto has developed into a major hurricane and may maintain category 3 or 4 status for the next few days.  Humberto should turn more northward early next week and pass west and then north of Bermuda.  As Humberto passes northwest of Bermuda, it will encounter increasing shear from an upper-level trough passing off the Canadian Maritimes and begin to weaken.  However, the wind field (both hurricane force and tropical storm force) will also begin to expand as Humberto gains latitude.  Bermuda will likely have fringe effects of gusty winds and high surf.  Swells from Humberto will also likely cause high surf and dangerous rip currents along the east coast of the U.S.  including and especially the outer banks area of North Carolina, the south side of Long Island, and the exposed ocean shorelines of the Cape and Islands next week.  Mariners planning a trip far offshore of New England toward Bermuda should monitor the latest forecasts from the National Weather Service closely and be ready to adjust plans accordingly.

Tropical Depression Nine (Likely to become Imelda This Weekend)

Satellite imagery is depicting increasing and better organized convection near the center of this system, and it has been upgraded to a tropical depression.  It will likely intensify into a tropical storm over the Bahamas this weekend.  This system may develop further into a hurricane as it moves north or northwest of the Bahamas.  Most guidance does not show this system intensifying beyond a strong tropical storm or category 1 hurricane due to the proximity of land masses and moderate shear.  Along with the moderate shear could be the risk of dryer air being entrained into the southern part of this storm as it heads north/northwest of the Bahamas.  On the other hand, upper-level divergence may aid intensification and counter some of the effects of moderate shear.  Upper divergence refers to upper-level winds that tend to spread out or diverge, which in turn promotes greater upward motion and storm intensification.  This effect is something we often see in stronger extratropical coastal storms off the mid-Atlantic or New England coast.

There remains considerable uncertainty in the track of this system after about 3 days.  To a large extent, the track will be determined by how quickly the system moves to the north northwest and remains north or south of Humberto churning along to the east.  If this tropical cyclone remains north of Humberto’s latitude, then it has a higher probability of making landfall somewhere along the South Carolina coast as it scoots along the east side of a weak but elongated upper trough over the southern Appalachians.  However, if the system ends up adjacent or south of Humberto’s latitude, then there is likely to be greater interaction with Humberto.  In that case Humberto would have an opportunity to break down the subtropical ridge east of potential Imelda and try to draw the system eastward and offshore.  There is a tendency when two tropical cyclones are relatively close together for them to partially “orbit” one another.  Further complicating all of this, potential Imelda may be “pulled” offshore but only a short distance before possibly losing the connection with Humberto as that storm begins to accelerate north and then northeast.  Potential Imelda could then (as shown by some models and model ensembles) shift back to the west or northwest and still make landfall in South Carolina or North Carolina.  Or potential Imelda might get pulled far enough offshore to become embedded in the same circulation about the Bermuda High that is steering Humberto and in essence become another “fish storm.”  It is important to note, however, that regardless of the eventual track scenario, a tropical storm or hurricane will likely move close enough to the South Carolina coast to impact portions of the Southeast U.S. with heavy rain, strong wind gusts, and possibly some storm surge flooding.

Possible Impacts to the Southeast U.S.

The greatest impact is likely to be heavy rain for a fairly extended period early to mid-week, especially since the storm may stall near the coast whether it makes landfall or not.  This may cause serious river flooding as well as small stream and urban flash flooding over portions of South Carolina, North Carolina, Georgia, and possibly southern Virginia.  This area has already received considerable rain with many areas experiencing saturated or near saturated soils.  Depending upon the intensity of Imelda as it approaches the South Carolina coast, it may also produce strong wind gusts and some storm surge flooding along portions of the South Carolina and North Carolina coastline.  Should the storm stall offshore or move a short distance offshore only to move back to the northwest, then the heavy rain threat may spread to more of North Carolina and into at least southern Virginia (but that is only one of several possible scenarios).

There is also a risk of at least tropical storm force gusts along the immediate east coast of central Florida early next week (most likely Monday), and a Tropical Storm Watch is in effect there.

Although uncertainty remains in the exact peak intensity of this storm and even greater uncertainty in its ultimate track, people in the Southeast U.S. should have a plan of action especially if the heavy rain and flooding begin to materialize.

Next Message

This message will be updated as greater clarity on potential Imelda’s track and intensity unfolds.  Meanwhile, interests in the Southeast U.S. should monitor later advisories from the National Hurricane Center (nhc.noaa.gov) and from local National Weather Service offices.

About this Blog and Disclaimer

The National Weather Service monitors the weather 24/7 and updates forecasts, watches, and warnings as necessary.  For most of southern New England, refer to your local Weather Forecast Office for the latest forecast/warning information at weather.gov/box.  For information from other Weather Forecast Offices, go to weather.gov and click on the location of interest on the map.  For the latest information on Atlantic basin hurricane activity, access the National Hurricane Center website at nhc.noaa.gov.

A blog is issued when there is a potential risk of hazardous weather to southern New England and serve to provide background information regarding the significant weather with somewhat of an educational slant.  In the case of hurricanes, the author will attempt to issue a message when any portion of the United States coastline may be seriously impacted.  It’s intended to complement and not serve as an alternative to official forecasts issued by the National Weather Service.  Readers are urged to check National Weather Service forecasts and warnings for the most timely and updated weather information.  This weather message is solely produced by the listed author and does not constitute an official product of the Blue Hill Observatory and Science Center (BHOSC).

The frequency of this blog will depend upon the nature of the weather threat and availability of the author.  Thus, for the latest information on weather forecasts and warnings, refer to the National Weather Service, which monitors and forecasts the weather 24/7.

Blue Hill Observatory and Science Center (BHOSC)

The BHOSC maintains a continuous daily observation record that dates from February 1, 1885 without interruption and provides educational information on weather and climate to area students and the community at large.  Your support of BHOSC is much appreciated and enables this nonprofit organization to maintain the long-term climate record at the Blue Hill Observatory (longest continuous record in North America) and provide valuable educational resources on weather and climate via outreach to schools, site visits/tours, webinars, and conferences.  For more information on the Blue Hill Observatory and Science Center, explore bluehill.org.

Message issued September 27, 2025 by:

Bob Thompson

Retired National Weather Service Meteorologist

Blue Hill Observatory and Science Center Board member