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Hurricane Larry Forecast Discussion Number 20

2021-09-05 16:45:32| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sun Sep 05 2021 000 WTNT42 KNHC 051445 TCDAT2 Hurricane Larry Discussion Number 20 NWS National Hurricane Center Miami FL AL122021 1100 AM AST Sun Sep 05 2021 Larry continues as a large and intense hurricane this morning, with a 40 n mi diameter eye surround by cloud tops to near -80 deg C. There are, however, some breaks in the cold tops over the northern portion of the circulation. The current intensity is adjusted slightly to 110 kt, based on a blend of subjective Dvorak estimates from TAFB and SAB along with objective ADT estimates from UW-CIMSS. Upper-level outflow remains somewhat restricted over the southwestern quadrant of the system, indicative of at least slight vertical shear. This shear is probably being caused by the flow to the east of an upper-level low to the north of Puerto Rico. The low is forecast to move northwestward over the next few days, possibly lessening the shear over Larry. The hurricane is moving northwestward at a slightly slower speed, or 310/11 kt. Larry is likely to continue its northwestward trek along the southwestern periphery of a mid-tropospheric anticyclone during the next 2-3 days, with only a slight reduction in forward speed. Around 96 hours, the hurricane is forecast to turn toward the north-northwest and north while rounding the western side of the ridge. By the end of the forecast period, Larry is expected to begin its recurvature into the mid-latitude westerlies. The new official forecast track essentially follows the previous one, and is also very close to the latest NOAA corrected consensus and Florida State University Superensemble guidance. Larry's large eye suggests that no rapid changes in strength are likely during the short term. Since vertical shear is not expected to increase and the system will be traversing warm waters, the hurricane should be able to more or less maintain its intensity during the next few days. One inhibiting factor is the presence of relatively dry mid-level air in the environment. The official intensity forecast is on the high side of the model guidance and keeps Larry as a major hurricane through 72 hours. Key Messages: 1. Large swells generated by Larry are expected to reach the Lesser Antilles today and will spread to portions of the Greater Antilles, the Bahamas, and Bermuda on Monday and Tuesday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by midweek. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials this week. 2. Larry is forecast to approach Bermuda during the next several days, likely as a major hurricane, bringing a risk of strong winds, heavy rainfall, and coastal flooding to the island by the middle of this week. While it is too soon to determine the magnitude of these hazards and potential impacts on Bermuda, interests there should closely monitor the latest forecast updates during the next several days. FORECAST POSITIONS AND MAX WINDS INIT 05/1500Z 19.5N 49.7W 110 KT 125 MPH 12H 06/0000Z 20.5N 51.1W 110 KT 125 MPH 24H 06/1200Z 21.9N 52.7W 110 KT 125 MPH 36H 07/0000Z 23.0N 54.0W 110 KT 125 MPH 48H 07/1200Z 24.3N 55.4W 105 KT 120 MPH 60H 08/0000Z 25.7N 56.7W 105 KT 120 MPH 72H 08/1200Z 27.3N 58.1W 105 KT 120 MPH 96H 09/1200Z 31.1N 61.1W 95 KT 110 MPH 120H 10/1200Z 37.4N 60.6W 85 KT 100 MPH $$ Forecaster Pasch

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Hurricane Larry Forecast Discussion Number 19

2021-09-05 10:55:28| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sun Sep 05 2021 000 WTNT42 KNHC 050855 TCDAT2 Hurricane Larry Discussion Number 19 NWS National Hurricane Center Miami FL AL122021 500 AM AST Sun Sep 05 2021 Larry remains a powerful hurricane this morning. The system has a warm, well-defined eye with a large diameter of around 40 n mi. Larry's central dense overcast has become more symmetric overnight, although the upper-level outflow is still somewhat restricted on the southwestern side of the hurricane. This is likely an indication of some vertical wind shear impinging on the cyclone. However, recent AMSR2 microwave data indicate that the vortex is still well-aligned vertically, with a pronounced eyewall that slopes outward with height. The initial intensity is held at 105 kt for this advisory based on consensus T5.5/102 kt Dvorak classifications from TAFB and SAB, and a recent UW-CIMSS ADT estimate of 105 kt. The initial motion of Larry is northwestward, or 305/12 kt. Larry is expected to continue moving northwestward for the next several days around the southwestern periphery of a mid-level ridge over the central Atlantic. By Thursday, the cyclone is forecast to accelerate northward ahead of a deep-layer trough that will move off the east coast of the United States later this week. The latest track guidance is very tightly clustered, and the official NHC forecast is similar to the previous one and lies near the multi-model consensus aids. Confidence remains high in Larry's track forecast given the excellent model agreement. The hurricane is still expected to make its closest approach to Bermuda at days 4 and 5, but it remains too early to pinpoint how close it will come to the island. Even if the center passes east of Bermuda as forecast, it could still be large enough to produce some impacts on the island. Over the next few days, some fluctuations in Larry's intensity are possible. The oceanic heat content along the forecast track is certainly sufficient for some intensification. However, vertical wind shear associated with an upper-level trough/low to the west of Larry may inhibit strengthening at times during the next couple of days, especially if the stronger shear values in the ECMWF-based SHIPS guidance are realized. Given Larry's large eye size, it does not appear as if the cyclone is poised to significantly intensify. The official NHC intensity forecast shows slight strengthening in the near-term, but then levels off the intensity through 48 h. The NHC forecast keeps Larry at major hurricane intensity through day 4, as the environment appears favorable enough between 48-96 h for Larry to maintain its organization, even as the cyclone gains latitude. Overall, this forecast lies on the high end of the intensity guidance, above the HFIP corrected consensus approach (HCCA) and the other multi-model consensus aids. Key Messages: 1. Large swells generated by Larry are expected to reach the Lesser Antilles today and will spread to portions of the Greater Antilles, the Bahamas, and Bermuda on Monday and Tuesday. Significant swells will likely reach the east coast of the United States and Atlantic Canada by midweek. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials this week. 2. Larry is forecast to approach Bermuda during the next several days, possibly as a major hurricane, bringing a risk of strong winds, heavy rainfall, and coastal flooding to the island by the middle of this week. While it is too soon to determine the magnitude of these hazards and potential impacts on Bermuda, interests there should closely monitor the latest forecast updates during the next several days. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 18.8N 49.0W 105 KT 120 MPH 12H 05/1800Z 19.9N 50.4W 110 KT 125 MPH 24H 06/0600Z 21.2N 52.1W 110 KT 125 MPH 36H 06/1800Z 22.4N 53.6W 110 KT 125 MPH 48H 07/0600Z 23.6N 55.0W 110 KT 125 MPH 60H 07/1800Z 25.0N 56.4W 105 KT 120 MPH 72H 08/0600Z 26.5N 57.8W 105 KT 120 MPH 96H 09/0600Z 30.4N 60.8W 100 KT 115 MPH 120H 10/0600Z 36.5N 61.0W 90 KT 105 MPH $$ Forecaster Reinhart

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Hurricane Larry Forecast Discussion Number 17

2021-09-04 22:56:16| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 04 2021 000 WTNT42 KNHC 042056 TCDAT2 Hurricane Larry Discussion Number 17 NWS National Hurricane Center Miami FL AL122021 500 PM AST Sat Sep 04 2021 Larry remains a formidable hurricane this afternoon. Both visible and infrared satellite bands show the hurricane has a well-defined and warm eye (greater than 10 C) surrounded by a cold ring of eyewall convection (-60 to -70 C). An AMSR2 microwave pass received at 1630 UTC showed the well-defined eye of Larry, though the 89 GHz channel hinted that the eyewall was weaker on its eastern side. Taking a look at the high-density atmospheric motion vectors (AMVs), available thanks to a GOES-16 1-minute updating domain over Larry, there is some restriction of Larry's outflow to the southwest. Flight-level wind data from the NASA-DC8 aircraft conducting a research mission around Larry also showed some light southwesterly flow between 10-20 kt just a few degrees to the south and west of the storm center. These data suggest that the upper-level wind environment is not as pristine as earlier suggested by SHIPS guidance, with the southwesterly flow likely restricting Larry's outflow in that quadrant. The SAB/TAFB subjective Dvorak intensity estimates are unchanged from this morning, and the latest UW-CIMSS ADT and SATCON estimates have also plateaued in the 105-110 kt range. Therefore, Larry was maintained as a 110 kt hurricane this advisory. Larry remains on a west-northwest track this afternoon, but a bit slower at 300/12 kt. There has been little change to the track philosophy over the next several days, as the guidance is in good agreement that Larry will maintain a continued west-northwest heading while gradually slowing down as it rounds the southern periphery of a large mid-level ridge. However, there has been a notable eastward shift in the track guidance in the short-term. The latest ECMWF run, which had previously been on the southwest side of the track guidance envelope, is now very similar or even a tad east of the latest GFS run. This shift has also resulted in an eastward adjustment in the consensus aids this afternoon. The latest NHC track forecast was adjusted a bit to the right early on, but not as far right as the HCCA and TVCA aids. After 72 hours, the track guidance actually converges very close to the previous track forecast, and few changes were needed after this time period. Based on this forecast, Larry will continue moving across the central Atlantic in the coming days, and be approaching Bermuda from the southeast in the day 4 to 5 forecast period. The existence of some light upper-level southwesterly flow ahead of Larry today was bit of a surprise, since the SHIPS guidance from the last few days suggested the shear-vector would be out of the east and weak. Indeed, the latest ECMWF-SHIPS guidance now shows moderate southwesterly vertical wind shear beginning earlier, and peaking between 20-25 knots in 36 to 48 hours. While the GFS-SHIPS shear remains much lower, given what I'm seeing from the latest upper-level flow in front of Larry, the ECMWF seems closer to correct. For this reason, the latest NHC intensity guidance now shows a bit of weakening after 24 hours, when the shear magnitude is expected to peak as the hurricane interacts with a large tropical upper-tropospheric trough (TUTT) located to its northwest. However, Larry has a large and vertically-deep circulation, and ultimately it will win the battle against the more vertically shallow TUTT, which is forecast to cut off and move away from the hurricane's expansive upper-level outflow. By 60 hours, this change in the synoptic pattern should once again reduce the vertical wind shear over Larry, and it will have a chance to achieve a secondary peak between the 60-84 hour time-frame. However, eyewall replacement cycles could also occur at any time over the next 2-5 days, providing additional intensity fluctuations that make this a challenging intensity forecast. The latest NHC intensity forecast is lower than the previous one, but still remains higher than the latest HCCA consensus aid. Regardless of the details, Larry is expected to remain a large major hurricane over the next 3-4 days. Key Messages: 1. Large swells generated by Larry are expected to first reach the Lesser Antilles on Sunday and will spread to portions of the Greater Antilles, the Bahamas, and Bermuda Monday and Tuesday. Significant swells will likely reach the east coast of the United States and Atlantic Canada around midweek. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the advice of lifeguards and local officials through the upcoming week. 2. Larry is forecast to approach Bermuda during the next several days, possibly as a major hurricane, bringing a risk of strong winds, heavy rainfall, and coastal flooding to the island by the middle of next week. While it is too soon to determine the magnitude of these hazards and potential impacts on Bermuda, interests there should monitor changes to the forecast during the next several days. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 17.4N 47.1W 110 KT 125 MPH 12H 05/0600Z 18.4N 48.6W 115 KT 130 MPH 24H 05/1800Z 19.8N 50.6W 120 KT 140 MPH 36H 06/0600Z 21.0N 52.3W 115 KT 130 MPH 48H 06/1800Z 22.2N 54.0W 110 KT 125 MPH 60H 07/0600Z 23.4N 55.6W 110 KT 125 MPH 72H 07/1800Z 24.7N 57.0W 115 KT 130 MPH 96H 08/1800Z 28.3N 59.9W 110 KT 125 MPH 120H 09/1800Z 33.4N 61.9W 95 KT 110 MPH $$ Forecaster Papin

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Hurricane Larry Forecast Discussion Number 16

2021-09-04 16:59:11| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Sep 04 2021 000 WTNT42 KNHC 041459 TCDAT2 Hurricane Larry Discussion Number 16 NWS National Hurricane Center Miami FL AL122021 1100 AM AST Sat Sep 04 2021 Overnight and this morning, Larry appears to have gone through a rather quick eyewall replacement cycle (ERC). The most recent 1000 UTC GMI microwave pass suggests that Larry now has a much larger eye and surrounding eyewall, with less evidence of concentric bands as noted yesterday. On geostationary satellite the larger eye is also becoming apparent on visible and infrared channels, though some left over inner eyewall debris remains. Consistent with the larger eye, an ASCAT-B pass received at 1226 UTC indicated that the 34-, 50-, and 64-kt wind radii have expanded with Larry this morning. While the most recent SAB and TAFB subjective Dvorak intensity estimates have remain at 90 kt and 100 kt respectively, the objective estimates are higher, with an earlier UW-CIMSS SATCON estimate up to 112 kt, though it has recently been adjusted lower. With the eye beginning to clear out on the most recent satellite images, I am favoring the higher objective estimates, bringing Larry up to 110 kt for this advisory. Larry's track has remained fairly steady toward the west-northwest, but just a touch slower and more poleward with the latest estimated motion at 300/13 kt. This west-northwest motion is expected to continue for the next several days with only a very gradual poleward bend as Larry remains positioned to the south of an expansive mid-level ridge extending westward ahead of Larry's expected path. As has been the case the last several days, the track guidance is in excellent agreement over the next 36-48 hours with more spread becoming apparent thereafter. As discussed yesterday, the increase in spread in the latter portion of the forecast appears to be related to how much mid-level ridging stays north of Larry. The ECMWF and its ensembles maintain more ridging that results in a slower and more westward track, while the GFS and its ensembles place more ridging east of Larry that results in a faster more eastward track. The UKMET favors a solution closer to the ECMWF while the Canadian favors a solution closer to the GFS. For now, the latest NHC track has elected to remain close to the HFIP corrected consensus approach (HCCA) which slightly favors the ECMWF solution over the GFS. This latest track forecast is quite similar to the previous one, but just a touch slower. Based on this forecast, Larry will continue moving across the central Atlantic in the coming days, and be approaching the latitude of Bermuda by the end of the forecast period. Conditions continue to remain favorable for additional intensification over the next few days, with vertical wind shear remaining under 10 kts, sea-surface temperatures gradually increasing, and mid-level relatively humidity staying fairly moist. However, by 48 hours, Larry will be impinging on the eastern edge of a large upper-level mid-oceanic trough, which could result in an increase in westerly vertical wind shear over the hurricane. However, the models handle Larry's interaction with this synoptic feature differently. The GFS suggests this upper-level trough will fracture to the west fairly quickly, keeping a more favorable upper-level pattern over Larry, while the ECMWF has a stronger trough that takes longer to give way to Larry's upper-level outflow, providing more westerly shear over the hurricane. Additional eyewall replacement cycles are also possible in the coming days, which could result in additional intensity fluctuations that are challenging to predict ahead of time. For now, the latest NHC intensity forecast keeps Larry intensifying over the next 24-36 hours, with only a very gradual decay thereafter due to the somewhat less favorable dynamical environment. The latest intensity forecast remains on the high end of the guidance, noting that there still remain higher outliers making Larry stronger than the 125 kt peak intensity (HAFS-B, COAMPS-TC). Regardless of the details, Larry is expected to remain a large major hurricane through the forecast period. Significant ocean swells generated by Larry's growing wind field are expected to reach the Lesser Antilles tomorrow, and then spread westward to portions of the Greater Antilles, the Bahamas, and Bermuda on Monday and Tuesday. These swells could cause life-threatening rip currents and high surf conditions. Large swells are also likely to spread to the east coast of the United States by midweek. FORECAST POSITIONS AND MAX WINDS INIT 04/1500Z 16.8N 45.8W 110 KT 125 MPH 12H 05/0000Z 17.7N 47.5W 115 KT 130 MPH 24H 05/1200Z 19.1N 49.6W 120 KT 140 MPH 36H 06/0000Z 20.3N 51.5W 125 KT 145 MPH 48H 06/1200Z 21.5N 53.2W 120 KT 140 MPH 60H 07/0000Z 22.6N 54.8W 120 KT 140 MPH 72H 07/1200Z 23.8N 56.4W 115 KT 130 MPH 96H 08/1200Z 27.0N 59.4W 110 KT 125 MPH 120H 09/1200Z 31.6N 62.0W 100 KT 115 MPH $$ Forecaster Papin

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Hurricane Larry Forecast Discussion Number 14

2021-09-04 04:37:23| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Sep 03 2021 078 WTNT42 KNHC 040237 TCDAT2 Hurricane Larry Discussion Number 14 NWS National Hurricane Center Miami FL AL122021 1100 PM AST Fri Sep 03 2021 Larry has had a distinct but slightly ragged 15 n mi wide eye during the past few hours, while convective cloud tops within the eyewall have been gradually cooling. Intensity estimates have responded, somewhat significantly, to the improved structure, and TAFB and SAB Dvorak fixes at 0000 UTC were T5.5/102 kt and T5.0/90 kt, respectively. In addition, objective UW-CIMSS ADT and SATCON estimates are near 105 kt. Larry has become a major hurricane, the third of the Atlantic season, with estimated 100-kt sustained winds. The hurricane is located due south of a mid-tropospheric high centered over the central Atlantic, and continues to move toward the west-northwest (285/14 kt). Larry is generally expected to move around the southwestern periphery of the high, turning northwestward at a slower speed by Sunday and then north-northwestward by Wednesday as a deep-layer trough moves eastward across the eastern United States. There is fairly high confidence in the track forecast, with model guidance showing a below- to near-normal amount of spread through day 5. The new NHC track forecast is right along the forecast from the previous advisory through day 3, and then nudged slightly westward on days 4 and 5. It should be noted that a few of the consensus aids, including HCCA, are still a little bit west of the official forecast at day 5, which might portend additional westward nudges in future advisories. By strict definition, Larry hasn't quite rapidly intensified since this time yesterday, but it has still strengthened quickly over waters that are considered only marginally warm (26-27 degrees Celsius). For the next couple of days, low shear, higher oceanic heat content, and a more unstable environment should favor additional intensification. However, there are still indications that Larry could run into an environment of higher shear and less upper-level divergence in 2-3 days as it approaches a mid-/upper-level trough currently located north and northeast of the Leeward Islands. In addition, internal processes within the hurricane's core itself, such as eyewall replacement cycles, could affect the intensity. In light of all these factors, the NHC intensity forecast relies on persistence to show additional strengthening during the next 36 hours, and then holds Larry steady through 60 hours at an intensity that is near the upper end of the guidance. Very gradual weakening is anticipated on days 3 through 5 while Larry heads in the direction of higher latitudes, yet the hurricane is forecast to remain at major hurricane intensity for the entire 5-day forecast period. Significant ocean swells generated by Larry's growing wind field are expected to reach the Lesser Antilles on Sunday, and then spread westward to portions of the Greater Antilles, the Bahamas, and Bermuda on Monday and Tuesday. These swells could cause life-threatening rip currents and high surf conditions. Large swells are also likely to spread to the east coast of the United States by midweek. FORECAST POSITIONS AND MAX WINDS INIT 04/0300Z 15.5N 43.3W 100 KT 115 MPH 12H 04/1200Z 16.2N 45.3W 110 KT 125 MPH 24H 05/0000Z 17.4N 47.6W 115 KT 130 MPH 36H 05/1200Z 18.7N 49.7W 120 KT 140 MPH 48H 06/0000Z 20.0N 51.7W 120 KT 140 MPH 60H 06/1200Z 21.2N 53.6W 120 KT 140 MPH 72H 07/0000Z 22.4N 55.3W 115 KT 130 MPH 96H 08/0000Z 25.4N 58.5W 115 KT 130 MPH 120H 09/0000Z 29.5N 61.5W 105 KT 120 MPH $$ Forecaster Berg

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