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Tropical Storm Maria Forecast Discussion Number 51

2017-09-28 22:39:33| National Hurricane Center (Atlantic)

Issued at 500 PM AST Thu Sep 28 2017 000 WTNT45 KNHC 282039 TCDAT5 Tropical Storm Maria Discussion Number 51 NWS National Hurricane Center Miami FL AL152017 500 PM AST Thu Sep 28 2017 After the earlier burst of convection near the center of Maria, the overall convective pattern has become more ragged and cloud tops have warmed. Westerly vertical shear has also displaced most of the convective cloud shield to the east of the center. The most recent Dvorak satellite intensity estimate from TAFB has decreased to 45 kt, and the latest UW-CIMSS ADT and SATCON values have decreased to 54 kt and 55 kt, respectively. Therefore, Maria's initial intensity has been lowered to 55 kt for this advisory. Despite the ragged satellite appearance, a 1431Z AMSU overpass indicated that Maria is still a tropical cyclone based on a deep warm core that extends from near the surface to above the 200 mb level, accompanied by a warm anomaly of at least 2.5 deg C in the middle- and upper-levels of the cyclone. The initial motion estimate is 090/11 kt. Maria is forecast to move in an easterly direction for the next 12 h or along the northern edge of a strong deep-layer ridge located to its south. By 24 h, acceleration and a turn toward the east-northeast is expected ahead an approaching deep-layer trough. By 36-48 h, Maria is forecast to turn toward the northeast and further accelerate, reaching forward speeds of 30-35 kt when the cyclone will be moving over the far North Atlantic. The latest track guidance continues to remain in excellent agreement on this developing track scenario, and only a slight nudge to the south of the previous forecast was required. Little change in intensity is forecast during the next 36 h or so while Maria remains over marginal SSTs of 25-26C and within a low to moderate vertical wind shear environment. By 36 h and beyond, Maria will be moving over sub-24C SSTs, reaching 21 deg C water by 48 h or so. Increasing westerly wind shear of more than 40 kt should induce a slow weakening trend, and Maria is now expected to become extratropical by 48 h due to the aforementioned unfavorable conditions. The extratropical low is forecast to be absorbed by a larger low or frontal system over the northeastern Atlantic by 96 h, before the low moves across Ireland. The intensity and wind radii forecasts at 48 and 72 h are based on guidance provided by the NOAA Ocean Prediction Center. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 36.8N 67.8W 55 KT 65 MPH 12H 29/0600Z 37.1N 64.4W 55 KT 65 MPH 24H 29/1800Z 38.1N 58.8W 55 KT 65 MPH 36H 30/0600Z 40.0N 52.0W 55 KT 65 MPH 48H 30/1800Z 42.7N 44.4W 55 KT 65 MPH...POST-TROP/EXTRATROP 72H 01/1800Z 48.8N 26.7W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 02/1800Z...DISSIPATED $$ Forecaster Stewart

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Hurricane Lee Forecast Discussion Number 43

2017-09-28 22:36:27| National Hurricane Center (Atlantic)

Issued at 500 PM AST Thu Sep 28 2017 000 WTNT44 KNHC 282036 TCDAT4 Hurricane Lee Discussion Number 43 NWS National Hurricane Center Miami FL AL142017 500 PM AST Thu Sep 28 2017 The satellite presentation of Lee has continued to degrade over the past six hours. The IR eye has opened on the northern side, and the low-level center of circulation appears to be displaced to the north-northwest of the mid-level center. Outflow in the northwest quadrant has also become severely restricted. A blend of Dvorak current intensity estimates from TAFB, SAB, and the U-W CIMSS at 1800 UTC was used as the basis for the initial intensity of 80 kt, but it is possible this is generous, given the continued degradation of the satellite presentation since that time. No significant changes were made to the track or intensity forecast. Lee is now estimated to be moving toward the north-northeast, or 25 degrees, around 15 kt. Lee should continue to accelerate toward the north-northeast to northeast ahead of a deep-layer mid-latitude trough for the next couple of days. The dynamical models are still tightly clustered, and the NHC track forecast remains close to the track consensus. Along this track, increasing shear and cooler SSTs will cause Lee to continue to weaken. Between 36 and 48 hours, a combination of weakening and faster forward motion should cause Lee to open up into a trough and dissipate. The NHC intensity forecast follows the relatively fast weakening trend of DSHP for the first 12 hours, and is close to the intensity consensus after that. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 35.1N 55.8W 80 KT 90 MPH 12H 29/0600Z 37.4N 53.4W 65 KT 75 MPH 24H 29/1800Z 41.2N 47.9W 55 KT 65 MPH 36H 30/0600Z 45.6N 39.2W 50 KT 60 MPH 48H 30/1800Z...DISSIPATED $$ Forecaster Zelinsky

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Tropical Storm Maria Forecast Discussion Number 50

2017-09-28 16:50:53| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Sep 28 2017 000 WTNT45 KNHC 281450 TCDAT5 Tropical Storm Maria Discussion Number 50 NWS National Hurricane Center Miami FL AL152017 1100 AM AST Thu Sep 28 2017 A burst of strong convection containing cloud top temperatures colder than -70C has developed just east of the center since the previous advisory. Based on the recent inner-core convective development and a Dvorak current intensity estimate of 3.5/55 kt, the initial intensity estimate is being maintained at 60 kt for this advisory. Also, the last AMSU overpass around 0200 UTC indicated that Maria has retained tropical characteristics based on a deep warm core that extends from near the surface to above the 200 mb level, accompanied by a warm anomaly of more than 2.5 deg C, present in the upper-levels of the cyclone. Maria is now situated on the north side of a strong deep-layer ridge, and the initial motion estimate is now toward the east or 085/11 kt. The cyclone is expected to move in a general easterly direction for the next 12-24 h along the northern edge of the aforementioned ridge. Afterwards, Maria is forecast to get caught up in west-southwesterly flow ahead of an approaching deep-layer trough that is currently located over the Great Lakes region, causing the cyclone to accelerate toward the east-northeast at forward speeds in excess of 30 kt by 36 h. The new track guidance remains in excellent agreement with cross-track differences of only about 60 nmi and speed differences of just a couple of knots. As a result, little change was made to the previous advisory track, and the new NHC forecast lies down the middle of the guidance envelope, close to the various consensus models. Little change in intensity is forecast during the next 36 h or so while Maria remains over SSTs of 26C-27C and the vertical shear is less than 10 kt. By 36 h and beyond, Maria will be moving over sub-25C SSTs, reaching 21 deg C water by 48 h. The colder water, along with increasing wind shear and dry mid-level humidity values of less than 40 percent, should induce some weakening. Maria is expected to become extratropical by 72 h due to these unfavorable dynamic and thermodynamic conditions. However, given the very cold water that will exist beneath the cyclone, it is possible that Maria could become extratropical as early as 48 h. The extratropical low is expected to be absorbed by a larger low or frontal system over the northeastern Atlantic by 96 h, before the low moves across Ireland. The intensity forecast remains in agreement with guidance provided by the NOAA Ocean Prediction Center. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 36.8N 69.3W 60 KT 70 MPH 12H 29/0000Z 36.9N 66.7W 55 KT 65 MPH 24H 29/1200Z 37.4N 62.1W 55 KT 65 MPH 36H 30/0000Z 38.8N 56.1W 55 KT 65 MPH 48H 30/1200Z 41.3N 48.8W 55 KT 65 MPH 72H 01/1200Z 47.9N 31.2W 45 KT 50 MPH...POST-TROP/EXTRATROP 96H 02/1200Z...DISSIPATED $$ Forecaster Stewart

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Hurricane Lee Forecast Discussion Number 42

2017-09-28 16:47:55| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Sep 28 2017 000 WTNT44 KNHC 281447 TCDAT4 Hurricane Lee Discussion Number 42 NWS National Hurricane Center Miami FL AL142017 1100 AM AST Thu Sep 28 2017 Northerly shear continues to adversely affect the organization of Lee. The central dense overcast has become fairly asymmetric, with most of the cold cloud tops limited to the southern semicircle, and the eye is also losing definition. All of the various objective and subjective satellite-based intensity estimates have decreased, and the initial intensity has been lowered to 85 kt, based on a blend of these data. The global models continue to indicate that the northerly shear will get stronger over the next two days. This shear, combined with progressively cooler SSTs along the forecast track of Lee, should result in continued weakening. The new intensity forecast shows a slightly faster weakening rate than before, in line with the latest intensity guidance. The dynamical models all forecast Lee to dissipate near a frontal zone around 48 h. The NHC forecast conservatively shows Lee as a tropical cyclone at that point, but it would not be surprising if the cyclone dissipated or became a remnant low between 36 and 48 h. Lee is beginning to accelerate toward the north, and the initial motion estimate is 010/10 kt. The hurricane is forecast to turn toward the northeast while accelerating in the fast mid-latitude flow until it eventually opens up into a trough in a couple of days. Very little change was made to the NHC forecast, which remains close to HCCA and near the center of the tightly clustered global models. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 33.7N 57.0W 85 KT 100 MPH 12H 29/0000Z 35.7N 55.3W 75 KT 85 MPH 24H 29/1200Z 39.0N 51.2W 60 KT 70 MPH 36H 30/0000Z 42.9N 44.0W 50 KT 60 MPH 48H 30/1200Z 47.0N 34.5W 45 KT 50 MPH 72H 01/1200Z...DISSIPATED $$ Forecaster Zelinsky

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Hurricane Lee Forecast Discussion Number 41

2017-09-28 10:37:56| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Sep 28 2017 000 WTNT44 KNHC 280837 TCDAT4 Hurricane Lee Discussion Number 41 NWS National Hurricane Center Miami FL AL142017 500 AM AST Thu Sep 28 2017 Although Lee remains a powerful hurricane, the cloud pattern appears to be gradually losing some organization. The eye has become ragged at times, and the convective pattern is now more asymmetric with convection becoming limited over the northwestern quadrant. The initial wind speed is held at 95 kt, based on an average of the Dvorak CI-numbers from TAFB/SAB and CIMSS at the University of Wisconsin, but this could be a little generous. Lee is headed toward an environment of strong wind shear and cooler waters. These more hostile conditions should cause the system to steadily weaken during the next couple of days, and Lee will likely fall below hurricane strength in 36 to 48 hours. The system is expected to lose its tropical characteristics in a little more than 2 days when it will be over SSTs below 20 deg C and in an environment of about 30 kt of westerly shear. Dissipation is now predicted to occur by day 3, in agreement with the latest runs of the GFS and ECMWF models. The NHC intensity forecast is an update of the previous one, and it remains in good agreement with the ICON and HCCA consensus aids. Lee has turned to the north at 8 kt on the western side of a mid-level ridge. The hurricane is forecast to begin accelerating to the northeast later today when it becomes embedded in fast mid-latitude flow, and it should continue moving in that direction until it dissipates. The models are tightly clustered, and only minor changes were made to the previous NHC track forecast. FORECAST POSITIONS AND MAX WINDS INIT 28/0900Z 32.5N 57.2W 95 KT 110 MPH 12H 28/1800Z 34.5N 56.3W 90 KT 105 MPH 24H 29/0600Z 37.4N 53.4W 75 KT 85 MPH 36H 29/1800Z 41.0N 47.7W 65 KT 75 MPH 48H 30/0600Z 44.7N 39.4W 55 KT 65 MPH 72H 01/0600Z...DISSIPATED $$ Forecaster Cangialosi

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