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Hurricane Larry Forecast Discussion Number 7

2021-09-02 10:46:13| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Sep 02 2021 000 WTNT42 KNHC 020846 TCDAT2 Hurricane Larry Discussion Number 7 NWS National Hurricane Center Miami FL AL122021 500 AM AST Thu Sep 02 2021 Larry's cloud pattern has gradually become better organized, with deep convection increasing in coverage and intensity near the center and a banding-type eye trying to form. Upper-level outflow is most prominent over the western and northern portions of the circulation. A well-defined low-level eye was apparent on an AMSR-2 image from a few hours ago. Dvorak final T- and Current Intensity-numbers from both TAFB and SAB are 4.0/4.0, corresponding to an intensity of 65 kt. Based on these estimates, Larry is upgraded to a hurricane, the fifth of the 2021 Atlantic season. The hurricane is moving just slightly north of west, or 280/17 kt. There are no important changes to the track forecast scenario from the previous advisory. Larry is likely to move along the southern and southwestern periphery of a strong deep-layer subtropical high pressure area over the east-central Atlantic during the next few days. This should result in a gradual turn toward the west-northwest during the next 48-72 hours. A turn toward the northwest is likely in 4-5 days as the system approaches a weakness in the ridge. The track models are fairly well clustered on this future direction of motion with some differences in forward speed. The official track forecast is quite similar to the previous one and is also very close to the latest dynamical model consensus, TVCA. Larry is expected to remain in an environment conducive for strengthening over the next few days, with marginally warm SSTs, low vertical shear, and surrounded by broad-scale anticyclonic upper-level flow. Therefore, strengthening is likely, and the official forecast calls for Larry to become a major hurricane in 48 hours or so. Some fluctuations in intensity due to eyewall replacements are possible around and after that time. Late in the forecast period, slightly drier air in the environment could limit further intensification. The official intensity forecast is close to the latest NOAA corrected consensus, HCCA, guidance. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 13.0N 32.3W 65 KT 75 MPH 12H 02/1800Z 13.4N 34.7W 75 KT 85 MPH 24H 03/0600Z 14.0N 37.9W 85 KT 100 MPH 36H 03/1800Z 14.8N 40.9W 95 KT 110 MPH 48H 04/0600Z 15.8N 43.6W 100 KT 115 MPH 60H 04/1800Z 16.9N 46.1W 105 KT 120 MPH 72H 05/0600Z 18.3N 48.3W 110 KT 125 MPH 96H 06/0600Z 20.9N 52.1W 115 KT 130 MPH 120H 07/0600Z 23.8N 55.4W 110 KT 125 MPH $$ Forecaster Pasch

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Tropical Storm Larry Forecast Discussion Number 6

2021-09-02 04:40:42| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Wed Sep 01 2021 000 WTNT42 KNHC 020240 TCDAT2 Tropical Storm Larry Discussion Number 6 NWS National Hurricane Center Miami FL AL122021 1100 PM AST Wed Sep 01 2021 Larry's strengthening phase appears to have taken a pause this evening. While an earlier 2055 UTC GMI microwave overpass indicated that the low-level structure was fairly impressive with a tight low-level eye-like feature, the deep convection waned briefly, but has since returned in a band around the eastern portion of the circulation. The various satellite-based intensity estimates have an unusually large spread from about 45 to 75 kt this evening. At the higher end of the estimates are UW/CIMSS ADT T-numbers of T4.5 and at the lower end were scatterometer wind data showing peak wind retrievals of about 45 kt. But given the small-inner core seen in the microwave data, there is likely some undersampling occuring with the ASCAT instrument. Subjective Dvorak data T-numbers from both TAFB and SAB were T3.5 (55 kt), and the latest SATCON estimate is 57 kt. The NHC initial intensity estimate leans toward the subjective Dvorak numbers and SATCON, and therefore remains at 60 kt, but there is larger-than-normal uncertainty regarding the initial intensity at this time. Larry is moving westward or 275/18 kt. The track forecast philosophy has not changed this evening. Larry is expected to move around the south and southwestern portion of a strong mid-level ridge that is anchored over the east-central Atlantic. This should steer the tropical cyclone steadily westward to west-northwestward during the next few days, with a turn toward the northwest by days 4 and 5 as Larry nears the southwestern portion of the ridge. The latest model envelope is largely unchanged through 120 hours, and the updated official forecast is very similar to the previous advisory. Although the track guidance is fairly tightly clustered there are some differences in how fast Larry will move across the tropical Atlantic, with the UKMET depicting the fastest forward speed. The NHC track forecast is slightly slower than the consensus aids to be in better agreement with the bulk of the guidance and the GFS ensemble mean. Conditions are expected to remain favorable for steady to rapid strengthening. Despite the recent pause in intensification, Larry is likely to resume strengthening later tonight, and the short-term portion of the intensity forecast is unchanged from before. Larry is forecast to become a hurricane by Thursday morning, and a major hurricane in 36-48 hours. That portion of the intensity forecast is in good agreement with the SHIPS guidance and is supported by the global model guidance which significantly deepen Larry over the next couple of days. After that time, there are some mixed signals on the amount of shear over the system with the GFS-based SHIPS guidance predicting moderate shear and some dry air, while the ECMWF-based SHIPS model diagnoses more favorable conditions. For now, the NHC intensity forecast shows a peak slightly higher than before by 72 hours, and then indicates little overall change in strength thereafter as some fluctuations in intensity could occur due to eyewall replacement cycles. The latter portion of the forecast is in best agreement with the HFIP corrected consensus aid. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 12.8N 30.9W 60 KT 70 MPH 12H 02/1200Z 13.0N 33.3W 70 KT 80 MPH 24H 03/0000Z 13.6N 36.5W 85 KT 100 MPH 36H 03/1200Z 14.2N 39.5W 95 KT 110 MPH 48H 04/0000Z 15.1N 42.2W 100 KT 115 MPH 60H 04/1200Z 16.2N 44.8W 105 KT 120 MPH 72H 05/0000Z 17.4N 47.3W 110 KT 125 MPH 96H 06/0000Z 20.1N 51.4W 110 KT 125 MPH 120H 07/0000Z 22.9N 55.0W 110 KT 125 MPH $$ Forecaster Brown

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Remnants of Kate Forecast Discussion Number 19

2021-09-01 22:42:12| National Hurricane Center (Atlantic)

Issued at 500 PM AST Wed Sep 01 2021 000 WTNT45 KNHC 012042 TCDAT5 Remnants Of Kate Discussion Number 19 NWS National Hurricane Center Miami FL AL102021 500 PM AST Wed Sep 01 2021 Kate's structure this afternoon has deteriorated further. While occasional bursts of deep convection are still occuring to the south of a broad area of low-level cyclonic rotation, this activity lacks much organization. Recent visible satellite imagery also suggests that the low-level circulation is in the process of opening up into a trough, with little if any northerly cloud motions being observed to the west of the estimated center. In addition, I have been fortunate to receive some in-situ data from the NASA-DC8 aircraft that earlier preformed a research mission into Kate. Dropsonde data launched near the center indicated that the surface pressure was near 1012 mb, which is only a few millibars lower than the environmental ambient pressure. The dropsondes launched west of the center also failed to find any northerly surface winds. The combination of these data suggest that Kate's center is losing definition and no longer possesses a well-defined circulation. Therefore, the system no longer meets the definition of a tropical cyclone, and this will be the final advisory. Maximum sustained winds have also been lowered to 25-kt based on the surface winds from dropsonde data provided by the DC8 aircraft to the east of the center. The remnants of Kate have accelerated to the north-northwest today with the estimated motion at 340/13 kt, likely as the low-level vorticity maxima has become fully decoupled from the mid-level vortex located well to the south and east. This motion should continue until Kate completely fades away while embedded in the synoptic environment near a low-level subtropical ridge. FORECAST POSITIONS AND MAX WINDS INIT 01/2100Z 28.5N 52.9W 25 KT 30 MPH 12H 02/0600Z...DISSIPATED $$ Forecaster Papin

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Tropical Storm Larry Forecast Discussion Number 5

2021-09-01 22:35:59| National Hurricane Center (Atlantic)

Issued at 800 PM CVT Wed Sep 01 2021 000 WTNT42 KNHC 012035 TCDAT2 Tropical Storm Larry Discussion Number 5 NWS National Hurricane Center Miami FL AL122021 800 PM CVT Wed Sep 01 2021 Afternoon satellite imagery indicates that Larry continues to become better organized, with visible imagery hinting at eye formation and a partial SSM/IS overpass suggesting that a 37 GHz convective ring is present around the center. Satellite intensity estimates are now 65 kt from SAB and 55 kt from TAFB, and the initial intensity is increased to 60 kt, which is a 30 kt increase since this time yesterday satisfying the 24-h definition of rapid intensification. The initial motion is now 275/19 kt. Larry is expected to move around the south and southwest sides of a subtropical ridge during the next five days, with a general westward motion for 24-36 h, followed by a turn toward the west-northwest and, by 96-120 h, a turn toward the northwest. The spread in the track guidance after 36 h has decreased a little in the latest model runs. However, the guidance envelope has again shifted a little to the west. The new forecast track will also be shifted a little westward, and it lies on the southern edge of the various consensus models. Conditions appear favorable for additional rapid intensification during at the next 36-48 h, and the intensity forecast now calls for Larry to become a hurricane in less than 12 h and become a major hurricane in about 48 h. Given current trends, this part of the intensity forecast could be conservative. There is lower confidence in the intensity forecast after 48 h. Larry is expected to encounter somewhat increased shear, and around the 72 h point it is expected to encounter some dry air. After 96 h, moderate shear is forecast to continue, but the cyclone is expected to move into a more moist air mass and over increasing sea surface temperatures. On top of these factors, there is the good chance that Larry will have fluctuations in intensity after it becomes a major hurricane due to eyewall replacement cycles. The intensity guidance basically keeps a steady intensity from 60-120 h, and the official forecast follows that trend. However, this part of the intensity forecast is in the middle of the guidance, and it is possible Larry could be stronger than forecast during this time. FORECAST POSITIONS AND MAX WINDS INIT 01/2100Z 12.5N 29.4W 60 KT 70 MPH 12H 02/0600Z 12.6N 31.8W 70 KT 80 MPH 24H 02/1800Z 13.0N 34.9W 85 KT 100 MPH 36H 03/0600Z 13.7N 38.0W 95 KT 110 MPH 48H 03/1800Z 14.6N 40.9W 100 KT 115 MPH 60H 04/0600Z 15.6N 43.5W 105 KT 120 MPH 72H 04/1800Z 16.9N 46.2W 105 KT 120 MPH 96H 05/1800Z 19.5N 50.5W 105 KT 120 MPH 120H 06/1800Z 22.5N 54.0W 105 KT 120 MPH $$ Forecaster Beven

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Tropical Depression Kate Forecast Discussion Number 18

2021-09-01 17:00:05| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Wed Sep 01 2021 448 WTNT45 KNHC 011459 TCDAT5 Tropical Depression Kate Discussion Number 18 NWS National Hurricane Center Miami FL AL102021 1100 AM AST Wed Sep 01 2021 It has been difficult to pinpoint where the low-level circulation of Kate is this morning. After last night's diurnal convective maximum, the remaining convection has taken on a very disorganized structure, with a mid-level vortex being left behind to the south, while deeper, but more outflow driven convection is racing off to the north, ahead of the estimated low-level center position. A recently received 1211 UTC ASCAT-A pass indicated that Kate's low-level circulation is still closed, but just barely. The scatterometer wind data supports maintaining the current intensity at 30-kt, which also agrees with the latest subjective and objective Dvorak estimates. The estimated motion is continuing off to the north-northwest, at 340/9 kt. Kate appears to now be primarily steered by the low-level flow around a subtropical ridge located to its east. A general north-northwest motion is expected to continue today, followed by a turn to the north and north-northeast around the periphery of this ridge. The latest NHC track guidance has been adjusted a bit right of the previous track, shifting towards the latest consensus guidance (TVCN) that can still track the cyclone beyond 24 hours. A 0958 UTC SSMIS microwave pass suggested that the better organized structure observed last night has decayed, with the low- and mid-level centers quite misaligned. Vertical wind shear diagnosed by SHIPS is now between 15-20 kt out of the north. This shear is likely contributing to the vortex tilt, while also helping to import very dry mid-level air, preventing Kate's convective activity from organizing. The bulk of the intensity guidance is in agreement that gradual spin down of the low-level circulation will occur over the next several days, with the deterministic ECMWF model suggesting Kate could open up to a trough as soon as tomorrow. The latest NHC intensity forecast makes Kate a remnant low in 36 hours, with dissipation after 48 hours. However, given the current structure, this could occur sooner than forecast. FORECAST POSITIONS AND MAX WINDS INIT 01/1500Z 26.8N 52.3W 30 KT 35 MPH 12H 02/0000Z 27.9N 52.7W 25 KT 30 MPH 24H 02/1200Z 29.6N 53.1W 25 KT 30 MPH 36H 03/0000Z 31.1N 53.2W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 03/1200Z 32.4N 52.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 04/0000Z...DISSIPATED $$ Forecaster Papin

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