Hurricane Isaias Heads Towards The Bahamas

Hurricane Isaias strengthened from a tropical storm on Wednesday night, the earliest “I” named storm in the Atlantic basin history. The previous record for the earliest ‘I’ named storm in the Atlantic basin is Irene on August 7, 2005.

Isaias is also the 5th named storm to form this July, tying the 2005 record for the most Atlantic named storms to form in July since 1851. It is also the first time on record (since 1851), the Atlantic basin has had two #hurricane formations in the last week of July (July 25-31).

At 5:00 PM AST, the center of Hurricane Isaias was located by satellite imagery and Bahamas radar data near latitude 22.6 North, longitude 75.7 West. Isaias is moving toward the northwest near 15 MPH (24 KM/H), and a general northwestward motion with some decrease in forward speed is expected for the next day or so, followed by a turn toward the north-northwest by late Sunday.

On the forecast track, the center of Isaias will continue to move near or over the Southeastern Bahamas this afternoon and evening. Isaias is forecast to be near the Central Bahamas tonight, and move near or over the northwestern Bahamas Saturday and near the east coast of the Florida Peninsula Saturday afternoon through Sunday.

Maximum sustained winds are near 75 MPH (120 KM/H) with higher gusts. Strengthening is expected later tonight and early Saturday, and Isaias is forecast to remain a hurricane for the next couple of days.

Hurricane-force winds extend outward up to 35 miles (55 KM) from the center and tropical-storm-force winds extend outward up to 175 miles (280 km). A private weather observing station at Pitts Town Point, Bahamas, recently reported a sustained wind of 37 MPH (59 KM/H).

The estimated minimum central pressure is 991 millibars.

Hurricane Isaias Forecast Cone as of 5:00 PM AST Friday 31st July 2020. (National Hurricane Center)
Hurricane Isaias Forecast Cone as of 5:00 PM AST Friday 31st July 2020. (National Hurricane Center)

Watches & Warnings

Hurricane Isaias Watches and Warnings as of 5:00 PM AST Friday 31st July 2020. (National Hurricane Center)
Hurricane Isaias Watches and Warnings as of 5:00 PM AST Friday 31st July 2020. (National Hurricane Center)

A Hurricane Warning is in effect for:

  • Boca Raton to the Volusia/Brevard County Line Florida
  • Northwestern Bahamas
  • Southeastern Bahamas
  • Central Bahamas

A Hurricane Watch is in effect for:

  • Hallendale Beach to south of Boca Raton Florida
  • Volusia-Brevard County Line to the Flagler/Volusia County Line

A Storm Surge Watch is in effect for:

  • Jupiter Inlet to Ponte Vedre Beach Florida

A Tropical Storm Warning is in effect for:

  • Turks and Caicos Islands
  • North of Ocean Reef to south of Boca Raton Florida
  • Lake Okeechobee

A Tropical Storm Watch is in effect for:

  • Flagler/Volusia County Line to Ponte Vedre Beach Florida

Interests elsewhere along the southeast coast of the United States should monitor the progress of Isaias. Additional watches or warnings may be required later tonight and Saturday.

A Hurricane Warning means that hurricane conditions are expected somewhere within the warning area. A warning is typically issued 36 hours before the anticipated first occurrence of tropical-storm-force winds, conditions that make outside preparations difficult or dangerous. Preparations to protect life and property should be rushed to completion.

A Hurricane Watch means that hurricane conditions are possible within the watch area. A watch is typically issued 48 hours before the anticipated first occurrence of tropical-storm-force winds, conditions that make outside preparations difficult or dangerous.

A Storm Surge Watch means there is a possibility of life-threatening inundation, from rising water moving inland from the coastline, in the indicated locations during the next 48 hours. For a depiction of areas at risk, please see the National Weather Service Storm Surge Watch/Warning Graphic, available at hurricanes.gov.

A Tropical Storm Warning means that tropical storm conditions are expected somewhere within the warning area within 36 hours.

A Tropical Storm Watch means that tropical storm conditions are possible within the watch area, generally within 48 hours.

For storm information specific to your area in the United States, including possible inland watches and warnings, please monitor products issued by your local National Weather Service forecast office. For storm information specific to your area outside of the United States, please monitor products issued by your national meteorological service.

Hazards Affecting Land

Hurricane Isaias as of 5:00 PM Friday 31st July 2020. (Weathernerds)
Hurricane Isaias as of 5:00 PM Friday 31st July 2020. (Weathernerds)

WIND: Hurricane conditions will continue to spread northwestward into the central and northwestern Bahamas tonight and Saturday.

Hurricane conditions are expected to reach the coast within the hurricane warning area in Florida late Saturday and Saturday night. Winds are expected to first reach tropical storm strength on Saturday, making outside preparations difficult or dangerous. Preparations to protect life and property should be rushed to completion.

Hurricane Isaias Rainfall Forecast as of 5:00 PM Friday 31st July 2020. (NHC)
Hurricane Isaias Rainfall Forecast as of 5:00 PM Friday 31st July 2020. (NHC)

RAINFALL: Isaias is expected to produce the following rain accumulations:

  • Bahamas, Turks and Caicos: 4 to 8 inches.
  • Cuba: 1 to 2 inches, with isolated maximum totals of 4 inches.

These rainfall amounts could lead to life-threatening flash and urban flooding, especially in low-lying and poorly drained areas.

From Friday night through Tuesday:

  • South Florida into east-Central Florida: 2 to 4 inches, with isolated maximum totals of 6 inches.
  • Northeast Florida into coastal Georgia: 1 to 2 inches.
  • Carolinas into the mid-Atlantic, including the southern and central Appalachians: 2 to 4 inches, with isolated maximum totals of 6 inches.

Heavy rainfall from Isaias could result in isolated flash and urban flooding, especially in low-lying and poorly drained areas. Isolated minor river flooding is possible across the eastern Carolinas and into Virginia.

Hurricane Isaias Storm Surge Forecast as of 5:00 PM Friday 31st July 2020. (NHC)
Hurricane Isaias Storm Surge Forecast as of 5:00 PM Friday 31st July 2020. (NHC)

SURF: Swells generated by Isaias are affecting portions of Hispaniola, eastern Cuba, the Turks and Caicos, and the southeastern and central Bahamas. These swells will spread along the east coast of Florida and the southeastern United States coast on Saturday. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office.

This system is of no direct threat to the Windward Islands, including Trinidad and Tobago.

Hurricane Isaias Forecast Discussion

Deep convection, with occasional overshooting cloud tops of -85C to -90C just north of the center, has continued to develop during the normal diurnal convective minimum period, which is quite impressive. The most recent Air Force Reserve recon flight-level wind data, along with ASCAT surface wind data, indicate that the inner-core and outer wind field have both contracted in size. Furthermore, radar data from the Bahamas and a 1810Z AMSR-2 microwave pass also indicate that a small 10-nmi-wide mid-level eye is forming. The last recon central pressure was 991 mb and the 700 mb height had decreased by 30 meters since the earlier maximum height around 1230Z. These data indicate that Isaias is getting better organized. The initial intensity remains 65 kt based on an earlier 700-mb
flight-level wind speed of 72 kt, which reduces to a 65-kt surface wind speed using a 90-percent adjustment factor.

The initial motion remains northwestward or 305/12 kt. The 12Z global models have once again made a westward shift due to the ridge to the north of Isaias not weakening as quickly as expected. This is partly due to the ridge being stronger than expected and a shortwave trough over the central United States moving a little slower into the southeastern U.S. than previously indicated. The UKMET and ECMWF explicitly show Isaias making landfall in 36-48 hours along the southeast Florida coast, but appear to weaken the system below hurricane strength. The GFS similarly brings the cyclone close to the southeast and east-central Florida coasts, but also as a somewhat weaker system.

In the 48 to 60-hour period, the cyclone is forecast to move slowly north-northwestward and northward through a break in the subtropical ridge extending westward from the Atlantic across Florida and into the northern Gulf of Mexico. By that time, however, Isaias is expected to weaken below hurricane strength due to the combination of strong southwesterly vertical wind shear and interaction the Florida peninsula. Around 72 hours, the cyclone should accelerate northeastward and possibly strengthen some before passing over eastern North Carolina on day 4, and across eastern New England on day 5. The NHC track forecast lies close to a blend of the consensus models TVCA and NOAA-HCCA and is east of the UKMET and ECMWF with the system forecast to be stronger than those models indicate. Due to the westward shift in the NHC forecast track, a Hurricane Warning and Storm Surge Watch have been issued for portions of the Florida east coast.

The center of Isaias is now located in the center of an expanding CDO feature. The improved inner-core wind field and aforementioned convective structure, along with very warm SSTs near 30C, should support some strengthening overnight and early Saturday morning. However, increasing southwesterly vertical wind shear is expected to cause a gradual decrease in intensity by Sunday and continue into early next week. The new official intensity forecast is a little lower than the previous advisory and is near the higher end of the intensity guidance.

Key Messages:

  1. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas through Saturday, and Hurricane Warnings are in effect.
  2. Hurricane conditions are expected along portions of the Florida east coast late Saturday and Saturday night, and a Hurricane Warning has been issued. Preparations to protect life and property should be rushed to completion.
  3. Dangerous storm surge is possible along the Florida east coast from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4 feet above ground level are possible along the immediate coastline and adjacent waterways. Residents in these areas should follow advice given by local emergency officials.
  4. Isaias will produce heavy rains and potentially life-threatening flash and urban flooding, especially in low-lying and poorly drained areas across south to east-central Florida, and across the Carolinas to the mid Atlantic. Isolated minor river flooding is possible across the eastern Carolinas and into Virginia early next week.
  5. There is a risk of impacts from winds, heavy rainfall, and storm surge spreading along much of the the U.S. east coast through early next week, and interests there should monitor the progress of Isaias and updates to the forecast.

Forecast Discussion by Forecaster Stewart from the NHC.

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