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Hurricane Helene Forecast Discussion Number 16

2018-09-11 10:33:59| National Hurricane Center (Atlantic)

Issued at 500 AM AST Tue Sep 11 2018 592 WTNT43 KNHC 110833 TCDAT3 Hurricane Helene Discussion Number 16 NWS National Hurricane Center Miami FL AL082018 500 AM AST Tue Sep 11 2018 An Advanced Microwave Technology Sounder (ATMS) 165GHZ overpass on Helene indicated a well-developed closed eyewall and a deep convective outer band with associated cold cloud tops of -76C wrapping around the south through east quadrants of the cyclone. The initial intensity is held at 95 kt for this advisory and is based on a blend of the subjective and objective T-numbers. There still appears to be a small window of opportunity for Helene to strengthen, during the next 12 hours or so, before the ocean and the upper wind environment become less conducive. Afterward, the SSTs decrease significantly and the vertical shear increases, which should induce gradual weakening. By day 3, the sea surface temperatures increase, however, the southwesterly shear persists and the mid-level atmospheric moisture decreases. Interestingly enough, the global models show Helene either maintaining tropical storm strength or even intensifying by the end of the period, possibly due to some mid-latitude dynamic forcing influences. It's also worth noting that the Florida State Cyclone Phase Evolution analysis and forecast product shows the system retaining a relatively symmetric warm core through the entire forecast. The initial motion is estimated to be west-northwestward, or 300/12 kt. A turn more toward the northwest is forecast early on Wednesday. Beyond mid-period, the large-scale models agree that a mid- to upper tropospheric trough over the northeast Atlantic will dig southward inducing a weakness in the eastern portion of the subtropical ridge. In response to this change in the synoptic steering pattern, Helene should slowly turn northward and then north-northeastward during the next 72 to 96 hours, followed by a turn toward the northeast on day 5. The official forecast is close to the previous one, and lies near the TVCN consensus model and the GFS ensemble mean. FORECAST POSITIONS AND MAX WINDS INIT 11/0900Z 16.0N 33.6W 95 KT 110 MPH 12H 11/1800Z 16.8N 35.0W 100 KT 115 MPH 24H 12/0600Z 18.0N 36.5W 85 KT 100 MPH 36H 12/1800Z 19.5N 37.6W 75 KT 85 MPH 48H 13/0600Z 21.4N 38.3W 65 KT 75 MPH 72H 14/0600Z 25.7N 38.8W 55 KT 65 MPH 96H 15/0600Z 31.4N 36.6W 50 KT 60 MPH 120H 16/0600Z 37.3N 31.2W 50 KT 60 MPH $$ Forecaster Roberts

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Tropical Storm Isaac Forecast Discussion Number 15

2018-09-11 10:31:59| National Hurricane Center (Atlantic)

Issued at 500 AM AST Tue Sep 11 2018 608 WTNT44 KNHC 110831 TCDAT4 Tropical Storm Isaac Discussion Number 15 NWS National Hurricane Center Miami FL AL092018 500 AM AST Tue Sep 11 2018 Isaac has not changed much overnight. Satellite images indicate that the strong tropical storm still has a central dense overcast pattern and a limited amount of outer bands. Earlier microwave data indicated that the center was not located in the middle of the convection, however, likely due to some westerly shear. Since the cyclone has changed little during the past several hours, the initial intensity is held at 60 kt, which is an average of the TAFB and SAB Dvorak final T- and CI-numbers. A nearby NOAA buoy shows that the wind field of Isaac is very compact, with tropical-storm- force winds only extending up to 40 n mi from the center. Satellite fixes suggest that Isaac continues to move westward at 13 kt. Although there remains a fair amount of spread in the models, the usually more reliable GFS and ECMWF models continue to show a westward motion during the next several days as the storm moves on the south side of a strengthening mid-level ridge. The NHC track forecast continues to follow that theme, and this forecast lies near a blend of the GFS and ECMWF aids. This forecast is a little slower than the previous one, and takes Isaac across the Lesser Antilles and into the eastern Caribbean in 2 to 3 days. Predicting Isaac's intensity has been challenging. Isaac appeared to have been in relatively favorable conditions during the past day or so, but it did not strengthen. The upper-level pattern should remain relatively favorable for another day, so modest strengthening back to a hurricane is possible during that time. Thereafter, however, an increase in shear first from a trough in the central Atlantic and then from the outflow of Florence should cause some weakening when the storm moves into the Caribbean. The models are in a little better agreement this cycle as the HWRF is no longer showing Isaac becoming a major hurricane. The NHC intensity forecast is very similar to the previous one and is fairly close to the consensus aids. Key Messages: 1. Isaac is expected to be at or near hurricane intensity when it approaches the Lesser Antilles. However, confidence in the forecast is lower than normal. 2. Interests in the Lesser Antilles should continue to monitor Isaac during the next couple of days. Watches will likely be required for portions of the Lesser Antilles later today. FORECAST POSITIONS AND MAX WINDS INIT 11/0900Z 14.6N 48.1W 60 KT 70 MPH 12H 11/1800Z 14.6N 50.1W 60 KT 70 MPH 24H 12/0600Z 14.6N 52.6W 65 KT 75 MPH 36H 12/1800Z 14.8N 55.0W 65 KT 75 MPH 48H 13/0600Z 15.1N 57.6W 60 KT 70 MPH 72H 14/0600Z 15.5N 62.8W 55 KT 65 MPH 96H 15/0600Z 15.6N 67.4W 55 KT 65 MPH 120H 16/0600Z 15.8N 72.0W 50 KT 60 MPH $$ Forecaster Cangialosi

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Tropical Depression Paul Forecast Discussion Number 11

2018-09-11 04:46:25| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Mon Sep 10 2018 317 WTPZ43 KNHC 110246 TCDEP3 Tropical Depression Paul Discussion Number 11 NWS National Hurricane Center Miami FL EP182018 800 PM PDT Mon Sep 10 2018 Paul remains a sheared tropical cyclone with the associated deep convection limited to the western portion of the circulation. The center of the system is completely exposed and the Dvorak classifications have decreased from all agencies. Based on that data, Paul is now downgraded to a tropical depression with an initial intensity of 30 kt. Although the wind shear is expected to lessen during the next couple of days, Paul will soon cross the 26 deg C isotherm and it will be headed for even cooler waters and a drier airmass during the next few days. These conditions support continued weakening, and Paul will likely become a remnant low in a couple of days or less. The depression has turned toward the west-northwest, with the initial motion estimated to be 300/8 kt. A gradual turn toward the west is expected during the next few days as the cyclone becomes increasingly shallow and is steered by the low-level flow. The new NHC track forecast is adjusted a little to the south of the previous one to come into better agreement with the latest consensus models. FORECAST POSITIONS AND MAX WINDS INIT 11/0300Z 21.6N 122.0W 30 KT 35 MPH 12H 11/1200Z 21.9N 123.6W 30 KT 35 MPH 24H 12/0000Z 22.3N 125.5W 30 KT 35 MPH 36H 12/1200Z 22.6N 127.1W 30 KT 35 MPH 48H 13/0000Z 22.9N 128.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 14/0000Z 23.2N 130.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 15/0000Z 23.2N 131.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 16/0000Z 23.0N 133.5W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Cangialosi

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Tropical Storm Isaac Forecast Discussion Number 14

2018-09-11 04:43:56| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Mon Sep 10 2018 269 WTNT44 KNHC 110243 TCDAT4 Tropical Storm Isaac Discussion Number 14 NWS National Hurricane Center Miami FL AL092018 1100 PM AST Mon Sep 10 2018 Several microwave images that arrived after the issuance of the last advisory revealed that the low-level center of Isaac has raced ahead of the main convective mass, and the system is poorly vertically aligned. The system also recently passed very near NOAA buoy 41041, which confirmed that the center is on the western side of the central dense overcast. The maximum winds observed by the buoy were only about 30 kt, with a minimum pressure of near 1001 mb. Due to the western adjustment of the center location, the latest Dvorak classification from TAFB is lower, and a blend of the TAFB final-T and CI-numbers supports a lower initial intensity of 60 kt. It is worth noting that this estimate is more uncertain than normal due to limitations of the Dvorak technique, and most of the other intensity estimates are higher. Given the high pressure and light winds at the buoy, I am inclined to favor the lower estimate at this time. Confidence in the track forecast is slightly higher now, and the UKMET is the only outlier that does not bring Isaac westward into the Caribbean within the next 72 h or so. There is still some disagreement on the speed of Isaac, especially once it crosses the Leeward Islands, and this seems largely related to the cyclone's intensity. The ECMWF and GFS depict a weakening storm that simply continues westward, while the HWRF and CTCI show strengthening as Isaac slows down and turns more west-northwestward. The NHC track forecast for now continues to favor the typically more reliable global models and shows a steady westward motion through day 5. The updated NHC forecast is a little faster than the previous advisory, due primarily to the westward adjustment of the initial position, and remains close to HCCA. A slight adjustment has been made to the intensity forecast, but confidence in the intensity forecast remains quite low. Due to the current structure of Isaac, it seems less likely that the tropical storm will intensify more than what is shown by the intensity guidance over the next 24-48 h. The NHC forecast has therefore been lowered slightly, but it is still near the top end of the guidance, and calls for Isaac to be a hurricane when it approaches the Leeward Islands later this week. It is also worth noting that the small size of Isaac could make it more susceptible to sudden short-term changes in intensity that are nearly impossible to forecast. The intensity spread is tremendous by the end of the forecast period, with multiple models showing Isaac becoming a major hurricane around day 4 or 5, and others showing outright dissipation. The NHC forecasts have been favoring a weaker solution thus far due to an expected increase in shear beginning in a couple of days, and I see no reason to make a dramatic change at this point. Key Messages: 1. Isaac is expected to be at or near hurricane intensity when it approaches the Lesser Antilles. However, confidence in the forecast is lower than normal. 2. Interests in the Lesser Antilles should continue to monitor Isaac during the next few days. Watches will likely be required for portions of the Lesser Antilles tomorrow. FORECAST POSITIONS AND MAX WINDS INIT 11/0300Z 14.5N 46.9W 60 KT 70 MPH 12H 11/1200Z 14.5N 49.0W 60 KT 70 MPH 24H 12/0000Z 14.6N 51.7W 65 KT 75 MPH 36H 12/1200Z 14.8N 54.3W 65 KT 75 MPH 48H 13/0000Z 15.0N 56.9W 65 KT 75 MPH 72H 14/0000Z 15.4N 62.4W 60 KT 70 MPH 96H 15/0000Z 15.5N 68.0W 55 KT 65 MPH 120H 16/0000Z 15.5N 72.5W 50 KT 60 MPH $$ Forecaster Zelinsky

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Hurricane Florence Forecast Discussion Number 47

2018-09-11 04:42:59| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Mon Sep 10 2018 808 WTNT41 KNHC 110242 TCDAT1 Hurricane Florence Discussion Number 47 NWS National Hurricane Center Miami FL AL062018 1100 PM AST Mon Sep 10 2018 The rapid intensification of Florence ended just after the last advisory, with the central pressure falling to near 939 mb. Since that time, the eyewall convection has become a bit ragged and the latest central pressure from the Air Force Reserve Hurricane Hunter aircraft is near 944 mb. The initial intensity will remain a possibly generous 120 kt for this advisory based on the aircraft winds. Microwave imagery suggests that Florence may be starting an eyewall replacement cycle. However, the winds from the Hurricane Hunter did not clearly indicate the presence of an outer eyewall. Florence should remain in a light shear environment and over sea surface temperatures near 29C for at least the next 48 h. Thus, there is little other than eyewall replacement cycles to keep the hurricane from intensifying further as indicated by all of the intensity guidance. The new intensity forecast calls for continued strengthening to near category 5 strength, although at a slower rate than what occurred during the last 30 h. Florence is expected to encounter southwesterly shear near the 72 h point, which could cause slight weakening before landfall. However, there remains high confidence that Florence will be a large and extremely dangerous hurricane, regardless of its exact intensity. The initial motion is 290/11. A building mid-level ridge over the northwestern Atlantic is expected to steer Florence west-northwestward to northwestward with an increase in forward speed during the next 48 h. After that time, a marked decrease in forward speed is likely as another ridge builds over the Great Lakes to the north of Florence. The track guidance continues to show some spread between the ECMWF on the left side of the envelope and the GFS on the right side. Overall, though, the guidance has again shifted a little to the right, and the 72-96 h points are nudged just a little to the right from the previous forecast. It is important not to focus on the exact forecast track as average NHC errors at days 3, 4, and 5 are about 100, 140 and 180 n mi, respectively, and dangerous hazards will extend well away from the center. The NOAA G-IV jet will continue to conduct synoptic surveillance missions every 12 h through at least Wednesday. In addition, special 0600 UTC and 1800 UTC radiosonde launches have been expanded to additional upper-air stations across the U.S. to collect extra data for the numerical models. The next Air Force Reserve Hurricane Hunter mission into Florence is scheduled for near 12Z. Key Messages: 1. A life-threatening storm surge is likely along portions of the coastlines of South Carolina, North Carolina, and Virginia, and a Storm Surge Watch will likely be issued for some of these areas by Tuesday morning. All interests from South Carolina into the mid- Atlantic region should ensure they have their hurricane plan in place and follow any advice given by local officials. 2. Life-threatening freshwater flooding is likely from a prolonged and exceptionally heavy rainfall event, which may extend inland over the Carolinas and Mid Atlantic for hundreds of miles as Florence is expected to slow down as it approaches the coast and moves inland. 3. Damaging hurricane-force winds are likely along portions of the coasts of South Carolina and North Carolina, and a Hurricane Watch will likely be issued by Tuesday morning. Damaging winds could also spread well inland into portions of the Carolinas and Virginia. 4. Large swells affecting Bermuda and portions of the U.S. East Coast will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 11/0300Z 25.9N 62.4W 120 KT 140 MPH 12H 11/1200Z 26.5N 64.5W 125 KT 145 MPH 24H 12/0000Z 27.9N 67.5W 130 KT 150 MPH 36H 12/1200Z 29.6N 70.4W 135 KT 155 MPH 48H 13/0000Z 31.3N 73.2W 130 KT 150 MPH 72H 14/0000Z 34.0N 76.5W 120 KT 140 MPH 96H 15/0000Z 35.5N 78.0W 50 KT 60 MPH...INLAND 120H 16/0000Z 36.5N 79.0W 25 KT 30 MPH...INLAND $$ Forecaster Beven

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