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Tropical Depression Isaac Forecast Discussion Number 27

2018-09-14 10:32:53| National Hurricane Center (Atlantic)

Issued at 500 AM AST Fri Sep 14 2018 207 WTNT44 KNHC 140832 TCDAT4 Tropical Depression Isaac Discussion Number 27 NWS National Hurricane Center Miami FL AL092018 500 AM AST Fri Sep 14 2018 Northwesterly shear continues to take a toll on Isaac. Although the system is still producing areas of deep convection, an ASCAT overpass that arrived shortly after the release of the previous advisory, indicated that the circulation had become even less defined. The ASCAT data also showed that the winds are below tropical storm strength, therefore the initial intensity has been reduced to 30 kt. Moderate to strong northwesterly shear and dry mid-level air are likely to cause additional weakening, and Isaac is forecast to degenerate into an open wave within the next several days, but this could occur much sooner if the current trends continue. When the system reaches the western Caribbean Sea the shear may relax somewhat and moisture levels are expected to be higher. These conditions could allow for regeneration of the system, but the ECMWF and GFS both show Issac weakening and moving over the western Caribbean as an open wave, and the NHC forecast follows this scenario. A low- to mid-level ridge over the southwestern Atlantic continues to steer Isaac westward, and this general motion should continue until dissipation occur. The new track forecast is once again essentially an update of the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 15.0N 65.5W 30 KT 35 MPH 12H 14/1800Z 15.1N 67.3W 30 KT 35 MPH 24H 15/0600Z 15.2N 69.7W 30 KT 35 MPH 36H 15/1800Z 15.2N 72.0W 25 KT 30 MPH 48H 16/0600Z 15.4N 73.8W 25 KT 30 MPH 72H 17/0600Z 16.4N 77.7W 25 KT 30 MPH 96H 18/0600Z...DISSIPATED $$ Forecaster Brown

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Hurricane Florence Forecast Discussion Number 59

2018-09-14 04:58:21| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Thu Sep 13 2018 486 WTNT41 KNHC 140258 TCDAT1 Hurricane Florence Discussion Number 59 NWS National Hurricane Center Miami FL AL062018 1100 PM EDT Thu Sep 13 2018 Radar imagery and data from an Air Force Reserve Hurricane Hunter flight indicate that Florence has an inner eye that is open on the east side, and a secondary wind maximum that has already begun to spread inland across southeastern North Carolina. The highest flight-level wind (102 kt) was actually measured within the outer band that is moving onshore in the Wilmington area. However, the highest SFMR-measured wind was 73 kt, and the highest land observation so far was a sustained wind of 72 kt at Cape Lookout, North Carolina. On top of that, NOAA Doppler radar velocities have been running around 100 kt at about 5,000 feet. All of these data support reducing the initial intensity to 80 kt, but based on the higher flight-level winds, the gust factor is being set a little higher than is typical for an 80-kt hurricane. Florence has been wobbling a little since the previous advisory, but the 12-hour motion is northwestward, or 305/5 kt. Nearly all of the track models agree that Florence will turn westward or west-southwestward during the next 36 hours, bringing the hurricane's center inland over extreme southern North Carolina and extreme eastern South Carolina Friday and Friday night. After 48 hours, Florence is expected to recurve across the western Carolinas and the central Appalachian Mountains around a western Atlantic ridge. No major changes were required to the NHC official track forecast, and it is generally close to the multi-model consensus aids. It should be noted that the ECMWF, HCCA, and Florida State Superensemble still take Florence on a more southern track, straddling the coast of South Carolina before turning inland. While that is not shown by the official forecast, it cannot yet be ruled out as a possibility. Florence's intensity is not likely to change much in the 12 hours or so it has left over water. Once the center moves inland, the intensity is forecast to decrease. However, since a good portion of Florence's circulation will remain over water for the next 36-48 hours, the NHC intensity forecast is a blend of the regular SHIPS model (which assumes a storm staying over water) and the Decay-SHIPS model (which assumes the storm has moved inland). Based on that, Florence should weaken to a tropical storm just after 24 hours and then below tropical storm strength after 48 hours. Florence remains a large hurricane. Life-threatening storm surge, heavy rainfall, and damaging wind will cover a large area regardless of exactly where the center of Florence moves. Key Messages: 1. A life-threatening storm surge is already occurring along portions of the North Carolina coast and will continue through Friday. This surge is also likely along portions of the South Carolina coast. The greatest storm surge inundation is expected between Cape Fear and Cape Hatteras, including the Neuse and Pamlico Rivers and western Pamlico Sound. 2. Life-threatening, catastrophic flash flooding and prolonged significant river flooding are likely over portions of the Carolinas and the southern and central Appalachians through early next week, as Florence is expected to slow down as it approaches the coast and moves inland. 3. Damaging hurricane-force winds are occurring along portions of the North Carolina coast and are expected to spread to portions of the South Carolina coast on Friday. Strong winds could also spread inland into portions of the Carolinas over the next couple of days. 4. Large swells affecting Bermuda, portions of the U.S. East Coast, and the northwestern and central Bahamas will continue this week, resulting in life-threatening surf and rip currents. FORECAST POSITIONS AND MAX WINDS INIT 14/0300Z 34.0N 76.8W 80 KT 90 MPH 12H 14/1200Z 34.2N 77.6W 75 KT 85 MPH 24H 15/0000Z 34.0N 78.5W 65 KT 75 MPH...INLAND 36H 15/1200Z 33.8N 79.2W 50 KT 60 MPH...INLAND 48H 16/0000Z 33.8N 80.2W 40 KT 45 MPH...INLAND 72H 17/0000Z 35.5N 82.9W 25 KT 30 MPH...INLAND 96H 18/0000Z 39.5N 82.0W 20 KT 25 MPH...POST-TROP/INLAND 120H 19/0000Z 44.0N 72.5W 25 KT 30 MPH...POST-TROP/INLAND $$ Forecaster Berg

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Tropical Storm Joyce Forecast Discussion Number 6

2018-09-14 04:38:22| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Thu Sep 13 2018 466 WTNT45 KNHC 140238 TCDAT5 Tropical Storm Joyce Discussion Number 6 NWS National Hurricane Center Miami FL AL102018 1100 PM AST Thu Sep 13 2018 Beginning around 6 hours ago, convection redeveloped near the center of Joyce. Since then, the cyclone has become displaced from the upper-level low, and an ASCAT overpass around 2330 UTC showed that Joyce has a small radius of maximum winds of around 30 n mi. Based on these factors, it appears Joyce has become a tropical cyclone. The ASCAT pass showed maximum winds of just over 30 kt, but its possible the instrument slightly under-sampled the maximum winds of the small cyclone. In addition, convection near the center increased somewhat after the time of the ASCAT pass. The initial intensity has therefore been held at 35 kt, making Joyce a tropical storm. Joyce has accelerated a little toward the west-southwest, perhaps more than previously anticipated. As a result, the forecast has been shifted south at most forecast hours, but not quite as far as the consensus models would suggest. There is otherwise no change to the track forecast reasoning, and Joyce is still expected to move slowly generally southward while Helene passes to the east until accelerating toward the northeast ahead of a mid-latitude trough in a day or two. Despite the recent, and perhaps temporary, resurgence of convection, no change has been made to the intensity forecast. None of the guidance shows much intensification, and the NHC forecast continues to rely on a blend of the intensity consensus and the GFS and ECMWF, which only show gradual weakening throughout the forecast period. Considering that the cyclone has only gradually weakened to this point, this still seems like a reasonable forecast. FORECAST POSITIONS AND MAX WINDS INIT 14/0300Z 32.9N 44.4W 35 KT 40 MPH 12H 14/1200Z 32.2N 44.7W 35 KT 40 MPH 24H 15/0000Z 31.7N 44.2W 35 KT 40 MPH 36H 15/1200Z 32.2N 43.3W 35 KT 40 MPH 48H 16/0000Z 33.3N 42.1W 30 KT 35 MPH 72H 17/0000Z 36.0N 37.5W 30 KT 35 MPH 96H 18/0000Z...DISSIPATED $$ Forecaster Zelinsky

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Tropical Storm Isaac Forecast Discussion Number 26

2018-09-14 04:37:24| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Thu Sep 13 2018 073 WTNT44 KNHC 140237 TCDAT4 Tropical Storm Isaac Discussion Number 26 NWS National Hurricane Center Miami FL AL092018 1100 PM AST Thu Sep 13 2018 A burst of deep convection has developed and covered the previously exposed center of Isaac this evening. However, there is still evidence of northwesterly shear over the cyclone as there is a rather sharp edge to the convective mass and the cirrus is quickly blowing off toward the southeast. A recent ASCAT pass shows that the circulation is not particularly well defined, but it did show some light west or west-southwesterly winds, enough to keep the system classified as a tropical cyclone. The ASCAT also revealed winds of 30-32 kt, so the initial wind speed is held at 35 kt, which is also an average of the latest Dvorak current intensity numbers. Moderate to strong northwesterly shear and dry mid-level air are likely to cause Isaac to weaken to a tropical depression or even become an open wave within the next day or so. When Isaac reaches the west-central Caribbean Sea in a couple of days, the shear may relax somewhat and the global models suggest that there will be an increase in mid-level moisture. These conditions could allow for regeneration of the system, but given the uncertainty in whether Isaac will survive long enough to take advantage of these conditions, the official forecast still calls for dissipation like most of the guidance. Isaac continues moving westward at about 14 kt. The system should move generally westward over the next couple of days as it is steered by a low- to mid-level ridge to the north. Later in the period, the models show that Isaac could gain some latitude, but the official forecast is along the southern side of the guidance since a weaker system is likely to move more westward. The new NHC track forecast is essentially an update of the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 14/0300Z 14.9N 64.4W 35 KT 40 MPH 12H 14/1200Z 15.0N 66.5W 35 KT 40 MPH 24H 15/0000Z 15.0N 69.1W 30 KT 35 MPH 36H 15/1200Z 15.0N 71.4W 30 KT 35 MPH 48H 16/0000Z 15.2N 73.3W 30 KT 35 MPH 72H 17/0000Z 16.3N 77.5W 30 KT 35 MPH 96H 18/0000Z 17.7N 82.0W 30 KT 35 MPH 120H 19/0000Z...DISSIPATED $$ Forecaster Brown

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Tropical Storm Helene Forecast Discussion Number 27

2018-09-14 04:34:22| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Thu Sep 13 2018 558 WTNT43 KNHC 140234 TCDAT3 Tropical Storm Helene Discussion Number 27 NWS National Hurricane Center Miami FL AL082018 1100 PM AST Thu Sep 13 2018 Helene's cloud pattern is beginning to have the appearance of a cyclone in extratropical transition with the rain shield limited to the northwest quadrant. The Dvorak numbers have decreased, but very recent scatterometer data show winds of around 55 kt associated with Helene's circulation, and this is the intensity value used in this advisory. Most of the guidance suggests that Helene's strength should remain steady through the forecast period, perhaps due to baroclinic forcing. On this basis, the NHC forecast keeps Helene with the same intensity through the forecast period, but as a post-tropical cyclone beyond 3 days. Helene has increased its forward speed and is now moving toward the north or 010 degrees at about 18 kt. The flow ahead of a sharp mid-level trough is well established, and this pattern will continue to steer Helene toward the north and then northeast during the next few days. This is consistent with most of the track guidance which is indeed tightly clustered. The NHC forecast is not very different from the previous one, and is in between the corrected consensus HCCA and the multi-model consensus aids. Given the increase in forward speed and the expansion of the wind field, a tropical storm warning will likely be required for a portion of the Azores early Friday. FORECAST POSITIONS AND MAX WINDS INIT 14/0300Z 28.6N 36.5W 55 KT 65 MPH 12H 14/1200Z 31.4N 35.9W 55 KT 65 MPH 24H 15/0000Z 34.8N 34.7W 50 KT 60 MPH 36H 15/1200Z 37.8N 32.8W 50 KT 60 MPH 48H 16/0000Z 40.0N 29.0W 50 KT 60 MPH 72H 17/0000Z 43.0N 21.5W 50 KT 60 MPH...POST-TROP/EXTRATROP 96H 18/0000Z 46.5N 15.0W 50 KT 60 MPH...POST-TROP/EXTRATROP 120H 19/0000Z 53.0N 5.0W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Avila

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