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Tropical Depression Twelve Forecast Discussion Number 1

2021-08-31 22:35:19| National Hurricane Center (Atlantic)

Issued at 800 PM CVT Tue Aug 31 2021 142 WTNT42 KNHC 312035 TCDAT2 Tropical Depression Twelve Discussion Number 1 NWS National Hurricane Center Miami FL AL122021 800 PM CVT Tue Aug 31 2021 Satellite imagery, along with earlier scatterometer data, indicates that the low pressure area over the eastern tropical Atlantic has a well-defined circulation and sufficient organized convection to be considered a tropical depression. Thus, advisories are being initiated on Tropical Depression Twelve. The initial intensity is set at 30 kt based on satellite intensity estimates from TAFB and SAB as well as the scatterometer data. The initial motion is a somewhat uncertain 285/14. A strong low- to mid-level ridge to the north should cause the cyclone to move westward at a faster forward speed for the next 36 h or so, followed by a gradual turn toward the west-northwest and northwest by the end of the period as the depression moves along the southwest side of the ridge. There is some spread in the guidance after 36-48 h, with the GFS and GFS ensemble mean showing a more northward motion while the UKMET and the UKMET ensemble mean show a more westward motion. The official forecast is near the middle of the guidance envelope and is close to the HFIP corrected consensus model, which is a little to the south of the other consensus models. Conditions appear favorable for intensification during the next 72 h or so as the cyclone is in a moist environment with light shear and warm sea surface temperatures. The intensity forecast thus calls for the cyclone to become a tropical storm in 12 h or so and reach hurricane strength in 48-60 h. By the end of the forecast period, the forecast track takes the cyclone over slightly cooler water and into a drier air mass. Based on this, the intensity forecast calls for a slower development rate. The official intensity forecast is in the middle of the intensity guidance, and it is possibly conservative given that many of the global models are showing the development of a large and powerful tropical cyclone. FORECAST POSITIONS AND MAX WINDS INIT 31/2100Z 11.2N 21.1W 30 KT 35 MPH 12H 01/0600Z 11.6N 23.9W 35 KT 40 MPH 24H 01/1800Z 12.0N 27.5W 40 KT 45 MPH 36H 02/0600Z 12.1N 30.7W 50 KT 60 MPH 48H 02/1800Z 12.5N 33.7W 60 KT 70 MPH 60H 03/0600Z 13.3N 36.5W 70 KT 80 MPH 72H 03/1800Z 14.3N 39.0W 80 KT 90 MPH 96H 04/1800Z 16.5N 43.5W 85 KT 100 MPH 120H 05/1800Z 19.5N 47.5W 90 KT 105 MPH $$ Forecaster Beven

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Tropical Depression Kate Forecast Discussion Number 15

2021-08-31 22:31:53| National Hurricane Center (Atlantic)

Issued at 500 PM AST Tue Aug 31 2021 000 WTNT45 KNHC 312031 TCDAT5 Tropical Depression Kate Discussion Number 15 NWS National Hurricane Center Miami FL AL102021 500 PM AST Tue Aug 31 2021 Kate continues to produce small, sporadic bursts of convection near and to the east of its partially exposed low-level center this afternoon. The cyclone has moved north of the subtropical jet stream that it has been centered under for the past couple of days, so the mid- to upper-level vertical wind shear is beginning to diminish. Unfortunately, all three ASCAT passes this morning missed Kate's small circulation. The current intensity Dvorak estimates from TAFB and SAB support maintaining the initial intensity at 30 kt for this advisory. The cyclone is moving a little faster to the north, or 360/6 kt. The track forecast reasoning is largely unchanged. Kate is expected to begin moving north-northwestward tonight as a mid-level ridge builds to the east and northeast of the cyclone. This general motion should continue through early Thursday, before an approaching mid- to upper-level trough begins steering Kate more northward through Friday. The official NHC track forecast is essentially an update of the previous one and lies near the center of the guidance envelope, remaining close to the multi-model consensus aids. The intensity forecast is a bit more challenging, with mixed signals noted from the various intensity models. On the one hand, satellite imagery indicates the vertical wind shear is obviously diminishing over Kate, and the cyclone is forecast to move over SSTs of around 28.5 deg C for the next couple of days. However, the cyclone remains embedded within a dry mid-level environment that is clearly making it difficult for Kate to produce organized convection. The official NHC intensity forecast still shows no explicit intensity change and remains closest to the HFIP corrected consensus approach (HCCA) and IVDR aids. However, some short-term intensity fluctuations are certainly possible, which is notable since Kate's intensity is hovering around the tropical depression/storm threshold. By Thursday, Kate will be moving into an even drier, more subsident environment that will make it even more difficult to sustain deep convection. Thus, the NHC forecast shows Kate becoming a remnant low completely devoid of convection by Friday, with dissipation shortly thereafter ahead of an approaching frontal system. FORECAST POSITIONS AND MAX WINDS INIT 31/2100Z 24.6N 50.9W 30 KT 35 MPH 12H 01/0600Z 25.5N 51.5W 30 KT 35 MPH 24H 01/1800Z 26.9N 52.6W 30 KT 35 MPH 36H 02/0600Z 28.6N 53.5W 30 KT 35 MPH 48H 02/1800Z 30.3N 54.2W 30 KT 35 MPH 60H 03/0600Z 32.0N 54.2W 30 KT 35 MPH 72H 03/1800Z 33.7N 53.8W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 04/1800Z...DISSIPATED $$ Forecaster Reinhart

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Tropical Depression Kate Forecast Discussion Number 14

2021-08-31 16:45:46| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Tue Aug 31 2021 000 WTNT45 KNHC 311445 TCDAT5 Tropical Depression Kate Discussion Number 14 NWS National Hurricane Center Miami FL AL102021 1100 AM AST Tue Aug 31 2021 Strong west-northwesterly shear continues to displace the limited convective activity associated with Kate well to the east of its center, which is fully exposed and becoming less well-defined in visible satellite imagery. A small burst of deep convection from earlier this morning appears to be waning, as infrared cloud top temperatures are warming to the east of Kate's center. Consensus T2.0/30 kt Dvorak classifications from TAFB and SAB, in addition to the latest objective UW-CIMSS ADT estimates, suggest that Kate is now likely a tropical depression. Therefore, the initial intensity is lowered to 30 kt for this advisory. Kate's center has jogged a little east of due north this morning, and its initial estimated 12-h motion is northward, or 010/4 kt. A mid-level ridge is forecast to build to the northeast and east of Kate later today through Wednesday, which should steer the cyclone more north-northwestward through midweek. Then, Kate should turn toward the north on Thursday night and early Friday within the flow associated with a mid- to upper-level trough that will be moving across the western Atlantic. The official NHC forecast track is very similar to the previous one, and it lies near the reliable consensus aids TVCA and HCCA. The only minor track adjustment has Kate turning a little earlier and faster toward the north late this week. Even though the vertical shear is expected to diminish over Kate during the next 12-24 h, the cyclone remains embedded within a relatively dry mid-level environment. This will make it difficult for Kate to sustain enough organized convection to strengthen much. In fact, numerous models including the GFS, HWRF, and HMON show the low weakening in the coming days, as the model simulated satellite imagery suggests only sporadic bursts of convection will occur even after the shear subsides. However, the possibility of Kate briefly re-intensifying into a weak tropical storm cannot be completely ruled out, since the cyclone will remain over warm SSTs around 28 deg C for at least the next couple of days. The official NHC intensity forecast remains consistent with the consensus aids IVCN and HCCA and shows no explicit change in intensity, although some fluctuation is possible. If Kate survives this week, the global models indicate that it will likely open up into a trough by Saturday morning, shortly before being absorbed by an approaching frontal system. FORECAST POSITIONS AND MAX WINDS INIT 31/1500Z 23.5N 50.8W 30 KT 35 MPH 12H 01/0000Z 24.3N 51.0W 30 KT 35 MPH 24H 01/1200Z 25.7N 51.7W 30 KT 35 MPH 36H 02/0000Z 27.3N 52.7W 30 KT 35 MPH 48H 02/1200Z 29.0N 53.6W 30 KT 35 MPH 60H 03/0000Z 30.7N 54.1W 30 KT 35 MPH 72H 03/1200Z 32.5N 53.9W 30 KT 35 MPH 96H 04/1200Z...DISSIPATED $$ Forecaster Reinhart

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Tropical Storm Kate Forecast Discussion Number 13

2021-08-31 10:44:43| National Hurricane Center (Atlantic)

Issued at 500 AM AST Tue Aug 31 2021 000 WTNT45 KNHC 310844 TCDAT5 Tropical Storm Kate Discussion Number 13 NWS National Hurricane Center Miami FL AL102021 500 AM AST Tue Aug 31 2021 Kate remains a strongly sheared tropical storm with the associated deep convection occurring in intermittent bursts in the eastern semicircle of the cyclone, owing to 850-200-mb west-northwesterly vertical wind shear of at least 30 kt. The most recent Dvorak satellite current intensity (CI) estimates remain at 35 kt, and that is the initial intensity set for this advisory. This intensity is consistent with an earlier SSMI-S passive microwave satellite overpass that showed wind speeds of 30-35 kt on the west side of Kate where no convection/rain was present. The initial motion estimate is 360/04 kt. Kate's forward motion has slowed to as low 2 kt during the psst 6 hours, likely due to the hindering effects of the west-northwesterly shear direction and weak steering currents since Kate has recently moved into a weakness in the low- mid-level subtropical ridge. However, the latest NHC track guidance is in good agreement that the cyclone will begin moving northward at a slightly faster forward speed as Kate moves through a break in the ridge. In about 24 hours, a mid-level ridge to the northeast and east of Kate is forecast to build westward, forcing the cyclone on a more northwestward trajectory that should continue through Thursday. By Friday, a mid- to upper-level trough moving eastward over the western Atlantic is forecast to gradually lift out Kate toward the north, and accelerate the cyclone toward the northeast on Saturday. The new NHC forecast track is similar to but slightly east or right of the previous advisory track, and lies near the left edge of the tightly packed simple- and corrected-consensus track-model envelope. The intensity forecast is not as straightforward as the track forecast. The aforementioned hostile shear conditions are predicted by the regional and global models, especially the ECMWF model, to gradually abate over the next 18-24 hours, with the magnitude of the shear to decrease to less than 10 kt by 24-36 hours when Kate will be moving over 28.5 deg C water temperatures. If Kate can survive the next 18 hours or so, which is possible since the 850-300-mb shear is much less at only 10-15 kt, then some restrengthening could occur, especially in the 24-48-hour period when the cyclone will be located underneath a small upper-level anticyclone. Countering that, however, is the somewhat dry air (near-50-percent humidity) in the mid-levels of the atmosphere that could limit the formation of inner-core convection. Thus for now, the new official intensity forecast remains similar to the previous advisory, which shows Kate remaining as a low-end tropical storm through the 96-hour period. This scenario is consistent with the IVCN and HCCA intensity consensus models. On day 5, Kate is forecast to dissipate when the cyclone or its remnants are expected to merge with an extratropical low and associated frontal system. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 23.0N 50.9W 35 KT 40 MPH 12H 31/1800Z 23.9N 50.9W 35 KT 40 MPH 24H 01/0600Z 25.2N 51.5W 35 KT 40 MPH 36H 01/1800Z 26.6N 52.4W 35 KT 40 MPH 48H 02/0600Z 28.3N 53.4W 35 KT 40 MPH 60H 02/1800Z 29.9N 54.2W 35 KT 40 MPH 72H 03/0600Z 31.5N 54.8W 35 KT 40 MPH 96H 04/0600Z 35.1N 53.6W 35 KT 40 MPH 120H 05/0600Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Kate Forecast Discussion Number 12

2021-08-31 04:33:38| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Mon Aug 30 2021 000 WTNT45 KNHC 310233 TCDAT5 Tropical Storm Kate Discussion Number 12 NWS National Hurricane Center Miami FL AL102021 1100 PM AST Mon Aug 30 2021 Kate is a strongly sheared and poorly organized tropical cyclone. Satellite images show an exposed low-level center with a few patches of deep convection on the system's east side. The latest Dvorak classifications range from 25-35 kt, and the initial intensity is held at a possibly generous 35 kt for this advisory. Unfortunately, ASCAT missed the circulation this evening. Strong west-northwesterly shear of about 30 kt is expected to persist into early Tuesday, and that could cause some weakening in the short term. Although the shear is expected to lessen after that, Kate will be moving into a drier and more stable airmass. None of the intensity models show much strengthening, and the global models suggest that Kate could dissipate sometime within the next couple of days. The official forecast is again nudged downward and generally shows little change in strength during the next 4 days. Kate, or its remnants, are likely to merge with an extratropical low and associated front in 4 to 5 days. The tropical storm has moved little during the past few hours, but a 12-hour motion yields an estimate of 360/5 kt. The storm is moving toward a weakness in the ridge caused by a deep-layer low over the North Atlantic, and that motion should continue into Tuesday. By Wednesday, mid-level ridging building to the northeast of the system should cause it to turn northwestward. However, another trough moving eastward over the western Atlantic is expected to cause Kate to turn northward again toward the end of the week. The NHC track forecast follows the various consensus aids and is similar to the previous one. FORECAST POSITIONS AND MAX WINDS INIT 31/0300Z 22.7N 50.9W 35 KT 40 MPH 12H 31/1200Z 23.6N 50.7W 30 KT 35 MPH 24H 01/0000Z 24.8N 50.9W 30 KT 35 MPH 36H 01/1200Z 26.1N 51.8W 35 KT 40 MPH 48H 02/0000Z 27.6N 52.9W 35 KT 40 MPH 60H 02/1200Z 29.1N 54.0W 35 KT 40 MPH 72H 03/0000Z 30.4N 54.8W 35 KT 40 MPH 96H 04/0000Z 33.9N 54.6W 35 KT 40 MPH 120H 05/0000Z...DISSIPATED $$ Forecaster Cangialosi

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