BULLETIN
Hurricane Florence Advisory Number 42 NWS National Hurricane Center Miami FL AL062018 500 PM AST Sun Sep 09 2018 …NOAA HURRICANE HUNTER FINDS FLORENCE STRENGTHENING… …FORECAST TO BECOME A MAJOR HURRICANE ON MONDAY… SUMMARY OF 500 PM AST…2100 UTC…INFORMATION —LOCATION…24.4N 57.0W ABOUT 720 MI…1160 KM SE OF BERMUDA ABOUT 580 MI…935 KM NE OF THE NORTHERN LEEWARD ISLANDS MAXIMUM SUSTAINED WINDS…85 MPH…140 KM/H PRESENT MOVEMENT…W OR 270 DEGREES AT 7 MPH…11 KM/H MINIMUM CENTRAL PRESSURE…975 MB…28.80 INCHES WATCHES AND WARNINGS –There are no coastal watches or warnings in effect. Interests in the southeastern and mid-Atlantic states should monitor the progress of Florence.
MODEL ANIMATIONS
TOTAL PRECIP MODEL
CMC-CANADIAN
DISCUSSION AND OUTLOOK –At 500 PM AST (2100 UTC), the center of Hurricane Florence was located near latitude 24.4 North, longitude 57.0 West. Florence is moving toward the west near 7 mph (11 km/h), and this general motion is expected to continue today. A west-northwestward motion with an increase in forward speed is expected by Monday, and that motion is forecast to continue through mid-week. On the forecast track, the center of Florence will move over the southwestern Atlantic Ocean between Bermuda and the Bahamas Tuesday and Wednesday, and approach the southeastern coast of the United States on Thursday. Aircraft and satellite data indicate that maximum sustained winds have increased to near 85 mph (140 km/h) with higher gusts. Florence is forecast to rapidly strengthen to a major hurricane by Monday, and is expected to remain an extremely dangerous major hurricane through Thursday. Hurricane-force winds extend outward up to 25 miles (35 km) from the center and tropical-storm-force winds extend outward up to 125 miles (205 km). Florence is forecast to become larger over the next few days. The estimated minimum central pressure from NOAA Hurricane Hunter and satellite data is 975 mb (28.80 inches).
HAZARDS AFFECTING LAND —SURF: Swells generated by Florence are affecting Bermuda and portions of the U.S. East Coast. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
FLORENCE SURGE
Hurricane Florence Discussion Number 42
NWS National Hurricane Center Miami FL AL062018 500 PM AST Sun Sep 09 2018 The last reconnaissance fix indicated that the pressure had fallen to about 978 mb with Florence, a drop of about 6 mb in 4 hours, with uncontaminated SFMR winds of about 70 kt. Since then, the satellite presentation has continued to improve, with intensifying deep convection near the center. Thus the wind speed is set to 75 kt on this advisory. Radar data from the NOAA Hurricane Hunter indicated that there were still a few breaks in the eyewall, possibly allowing some dry air to mix into the core. This structure has potentially kept Florence from intensifying rapidly so far. It is likely, however, that the eyewall will close soon with low environmental shear present, resulting in a faster intensification rate. The guidance is remarkably well clustered, with all of the hurricane models showing category 4 strength within 48 hours, which is uncommon given the current intensity. The new forecast continues to show rapid intensification and is increased 5 or 10 kt at most time periods, but it is still not as high as the Florida State Superensemble or the NOAA corrected consensus models.
It is worth noting that the model guidance is also significantly increasing the size of Florence’s wind field over the next few days, and the official forecast reflects this trend. The fixes from the aircraft indicate that Florence hasn’t gained much latitude yet, and continues moving westward at about 6 kt. There is no change to the synoptic reason as a very strong ridge is forecast to build over the northwestern Atlantic during the next few days. This pattern should steer Florence west-northwestward at a much faster forward speed by Tuesday. On Wednesday, the hurricane will likely turn northwestward and slow down somewhat due to another ridge forming over the Ohio Valley. The most notable change from the previous advisory is that most of the models are showing a faster motion in 3 or 4 days, which unfortunately increases the risk of a destructive hurricane landfall. The GFS and its ensemble mean are outliers from the main model envelope, showing the system near the Outer Banks, while the rest of the guidance is well to the southwest. While a near-miss isn’t impossible given the spread of the ECMWF Ensemble, there is much more support for a track to the southwest, especially considering the poleward bias of the GFS thus far this season. Therefore, the official track forecast is shifted a little bit to the southwest, and is very close to the corrected-consensus models. The NOAA G-IV jet is conducting a synoptic surveillance mission this afternoon to gather data near and around Florence for assimilation into the numerical models, and these missions will continue through Tuesday. In addition, upper-air stations across portions of the central and eastern U.S. are conducting special 0600 UTC and 1800 UTC radiosonde launches to collect extra data for the numerical models. Hopefully these data will help improve the track and intensity forecasts.
Key Messages:
1. There is an increasing risk of two life-threatening impacts from Florence: storm surge at the coast and freshwater flooding from a prolonged heavy rainfall event inland. While it is too soon to determine the exact timing, location, and magnitude of these impacts, interests at the coast and inland from South Carolina into the mid-Atlantic region should closely monitor the progress of Florence, ensure they have their hurricane plan in place, and follow any advice given by local officials.
2. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week. These swells will result in life-threatening surf and rip currents.
TROPICAL STORM ISAAC
BULLETIN
Tropical Storm Isaac Advisory Number 9 NWS National Hurricane Center Miami FL AL092018 500 PM AST Sun Sep 09 2018 …ISAAC ALMOST A HURRICANE… …MOVING FASTER TOWARD THE WEST OVER THE TROPICAL ATLANTIC… SUMMARY OF 500 PM AST…2100 UTC…INFORMATION –LOCATION…14.5N 40.3W ABOUT 1390 MI…2240 KM E OF THE WINDWARD ISLANDS MAXIMUM SUSTAINED WINDS…70 MPH…110 KM/H PRESENT MOVEMENT…W OR 275 DEGREES AT 12 MPH…19 KM/H MINIMUM CENTRAL PRESSURE…997 MB…29.44 INCHES
WATCHES AND WARNINGS—- There are no coastal watches or warnings in effect. Interests in the Lesser Antilles should monitor the progress of Isaac.
DISCUSSION AND OUTLOOK —At 500 PM AST (2100 UTC), the center of Tropical Storm Isaac was located near latitude 14.5 North, longitude 40.3 West. Isaac is moving toward the west near 12 mph (19 km/h) and is expected to accelerate during the next 36 hours. A westward motion is forecast to continue through the end of the week, with Isaac expected to move across the Lesser Antilles and into the eastern Caribbean Sea Wednesday night or Thursday. Maximum sustained winds have increased to near 70 mph (110 km/h) with higher gusts. Additional strengthening is forecast, and Isaac is expected to become a hurricane tonight. Weakening is anticipated to begin by the middle of the week while Isaac approaches the Lesser Antilles. Tropical-storm-force winds extend outward up to 45 miles (75 km) from the center. The estimated minimum central pressure is 997 mb (29.44 inches).
HAZARDS AFFECTING LAND- NONE RIGHT NOW
Tropical Storm Isaac Discussion Number 9
NWS National Hurricane Center Miami FL AL092018 500 PM AST Sun Sep 09 2018 Isaac is a tiny tropical cyclone, but even still, recent SSMIS data showed that it has developed a mid-level microwave eye feature. At the time of the pass (1800 UTC), the convection was a little thin on the southern side, but geostationary satellite images suggest that it has filled in since that time. Dvorak intensity estimates are T4.0 from TAFB and T3.5 from SAB, so the maximum winds are estimated to be 60 kt. The intensity forecast philosophy has not changed since this morning. Isaac is strengthening in an environment of low shear and over warm sea surface temperatures, and for the next 36-48 hours, the NHC intensity forecast is above the bulk of the intensity models. The main reason for this is that Isaac’s tiny size could allow the intensity to increase quickly within the favorable environment.
However, the cyclone’s tiny size will likely also be to its detriment after 48 hours when northwesterly vertical shear is expected to develop and increase to 20-30 kt. The shear will probably easily decouple the small system, causing the intensity to decrease much faster than suggested by most of the intensity models. For that reason, the official NHC intensity forecast is below the intensity consensus on days 3 through 5. It should be noted that both the GFS and ECMWF models show a weakening cyclone moving into the eastern Caribbean Sea by days 4 and 5, with the GFS even making the system an open wave by the end of the forecast period. These global model solutions lend credence to the belief that it may be difficult for Isaac to maintain hurricane status while it approaches the Lesser Antilles. Isaac is accelerating toward the west with an initial motion of 275/10 kt. Ridging to the north is expected to keep the cyclone on a westward trajectory for the entire forecast period, with acceleration continuing for the next 36 hours. The UKMET remains the biggest outlier, showing Isaac turning northwestward and northward into the central Atlantic after 48 hours. That still appears to be unlikely at this time, and the NHC track forecast is still close to the center of the guidance envelope. Only a slight northward shift was made to the new forecast based on the latest model solutions.
Key Messages:
1. Isaac is expected to become a hurricane tonight or on Monday while it moves westward across the tropical Atlantic Ocean.
2. While Isaac is forecast to begin weakening by Tuesday while it approaches the Lesser Antilles, it is still expected to be at or near hurricane intensity when it reaches the islands, and the uncertainty in the intensity forecast is higher than usual. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days.
HURRICANE HELENE
BULLETIN
Hurricane Helene Advisory Number 10 NWS National Hurricane Center Miami FL AL082018 500 PM AST Sun Sep 09 2018 …HELENE BECOMES A HURRICANE SOUTHWEST OF THE CABO VERDE ISLANDS… SUMMARY OF 500 PM AST…2100 UTC…INFORMATION —LOCATION…13.4N 26.0W ABOUT 145 MI…230 KM SW OF THE SOUTHERNMOST CABO VERDE ISLANDS MAXIMUM SUSTAINED WINDS…75 MPH…120 KM/H PRESENT MOVEMENT…W OR 280 DEGREES AT 13 MPH…20 KM/H MINIMUM CENTRAL PRESSURE…992 MB…29.30 INCHES
WATCHES AND WARNINGS —CHANGES WITH THIS ADVISORY: The government of the Cabo Verde Islands has discontinued the Hurricane Watch and Tropical Storm Warning for the Cabo Verde islands of Santiago, Fogo, and Brava. SUMMARY OF WATCHES AND WARNINGS IN EFFECT: There are no coastal watches or warnings in effect.
DISCUSSION AND OUTLOOK –At 500 PM AST (2100 UTC), the center of Hurricane Helene was located near latitude 13.4 North, longitude 26.0 West. Helene is moving toward the west near 13 mph (20 km/h), and a turn toward the west-northwest is expected on Monday, with a continued west-northwestward motion through Tuesday. On the forecast track, the center of Helene will continue to move away from the Cabo Verde Islands. Maximum sustained winds have increased to near 75 mph (120 km/h) with higher gusts. Additional strengthening is forecast during the next 48 hours. Hurricane-force winds extend outward up to 15 miles (30 km) from the center and tropical-storm-force winds extend outward up to 80 miles (130 km). The estimated minimum central pressure is 992 mb (29.30 inches).
HAZARDS AFFECTING LAND –RAINFALL: Helene is expected to produce total rain accumulations of 2 to 4 inches over the southern Cabo Verde Islands through today, with isolated maximum amounts of 6 inches. This rainfall may produce life-threatening flash floods.
Hurricane Helene Discussion Number 10
NWS National Hurricane Center Miami FL AL082018 500 PM AST Sun Sep 09 2018 Deep convection has increased in coverage and intensity over the southern portion of the circulation, and microwave imagery suggests that an inner core is beginning to develop. Helene’s maximum winds are boosted to hurricane strength, 65 kt, based on Dvorak Current Intensity numbers from SAB and TAFB. The cyclone should continue to traverse warm waters and be in a low-shear environment for the next couple of days. Therefore, additional intensification is called for during the next 48 hours. Afterwards, increasing south-southwesterly shear should cause gradual weakening. The official intensity forecast is close to the model consensus, IVCN. Helene is moving just north of due west, or 280/11 kt.
There are no significant changes to the track forecast philosophy for this advisory. Helene should move generally west-northwestward along the south side of a weak mid-level ridge for the next couple of days. After that, a mid-level trough digging along 40W-45W longitude should induce a turn toward the northwest and eventually north. The official track forecast is very similar to the one from the previous cycle through 72 hours, and is nudged a little to the east after that time. This is fairly close to the latest corrected consensus guidance, HCCA.
OTHER WAVES IN THE ATLANTIC
Tropical Weather Outlook NWS National Hurricane Center Miami FL 800 PM EDT Sun Sep 9 2018 For the North Atlantic…Caribbean Sea and the Gulf of Mexico:
The National Hurricane Center is issuing advisories on Hurricane Florence, located over the west-central Atlantic Ocean, on recently upgraded Hurricane Helene, located southwest of the Cabo Verde Islands, and on Tropical Storm Isaac, located over the central tropical Atlantic.
1. The combination of an upper-level trough and a tropical wave over the northwestern Caribbean Sea is producing a large area of disorganized showers and thunderstorms between Cuba and Honduras. The disturbance is forecast to move slowly west-northwestward to northwestward across the northwestern Caribbean Sea and the Yucatan peninsula during the next couple of days, with little or no development expected due to unfavorable upper-level winds and land interaction. However, upper-level winds are forecast to become a little more conducive for development when the system moves across the western Gulf of Mexico Wednesday through Friday.
* Formation chance through 48 hours…low…near 0 percent.
* Formation chance through 5 days…low…30 percent.
2. A non-tropical area of low pressure is forecast to form in the northeastern Atlantic Ocean several hundred miles west-southwest of the Azores in two or three days. Some slow tropical or subtropical development is possible by the end of the week while the low moves southwestward.
* Formation chance through 48 hours…low…near 0 percent.
* Formation chance through 5 days…medium…40 percent.
