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Tropical Storm Theta Forecast Discussion Number 4

2020-11-10 21:37:17| National Hurricane Center (Atlantic)

Issued at 900 PM GMT Tue Nov 10 2020 000 WTNT45 KNHC 102037 TCDAT5 Tropical Storm Theta Discussion Number 4 NWS National Hurricane Center Miami FL AL302020 900 PM GMT Tue Nov 10 2020 A central dense overcast has persisted near the center of Theta since yesterday, and although the clouds tops have warmed over the past several hours there remains a distinct curved band feature over the northern portion of the circulation. Water vapor imagery shows the mid- upper-level trough that was over the Azores is beginning to leave the cyclone behind. Based on these factors, and the earlier indications of a compact radius of maximum winds, it appears that Theta has completed a transition to a tropical cyclone. Even though the clouds tops have recently warmed, the overall structure of Theta has not degraded by much since the previous advisory. Therefore, the initial intensity is being held at 60 kt. The storm is moving 075/12 kt around the north side of a mid-level ridge. This feature should continue to steer Theta in a generally east-northeastward motion for the next few days. Late in the forecast period, the ridge will begin to weaken as another trough approaches from the northwest. This should cause the cyclone to slow its forward motion and begin to turn to the northeast. The latest model guidance has once again slowed down and the new NHC track forecast now lies between the previous one and the various consensus models. Although Theta will be moving over progressively cooler SSTs and within moderate wind shear conditions, the air mass is expected to remain unstable for the next day or so, which should be supportive of deep convection. Therefore, little change is strength is indicated during that time. Some weakening is anticipated later in the forecast period as the airmass becomes more stable. By the end of the 5-day forecast period, the combination of the cooler waters and an approaching trough should result in extratropical transition. The NHC intensity forecast has changed little from the previous one and remains near the IVCN and HCCA consensus models. FORECAST POSITIONS AND MAX WINDS INIT 10/2100Z 29.4N 36.2W 60 KT 70 MPH 12H 11/0600Z 29.8N 34.7W 60 KT 70 MPH 24H 11/1800Z 30.3N 32.8W 55 KT 65 MPH 36H 12/0600Z 30.8N 30.5W 55 KT 65 MPH 48H 12/1800Z 31.5N 28.0W 50 KT 60 MPH 60H 13/0600Z 32.1N 25.7W 50 KT 60 MPH 72H 13/1800Z 32.6N 23.8W 45 KT 50 MPH 96H 14/1800Z 33.2N 20.5W 40 KT 45 MPH 120H 15/1800Z 34.4N 19.0W 35 KT 40 MPH...POST-TROP/EXTRATROP $$ Forecaster Latto

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Tropical Storm Eta Forecast Discussion Number 41

2020-11-10 15:55:27| National Hurricane Center (Atlantic)

Issued at 900 AM CST Tue Nov 10 2020 000 WTNT44 KNHC 101455 TCDAT4 Tropical Storm Eta Discussion Number 41 NWS National Hurricane Center Miami FL AL292020 900 AM CST Tue Nov 10 2020 A significant increase in deep convection has occurred since the previous advisory this morning, including a CDO-like feature with cloud tops of -85C to -87C very near the low-level center. However, recent passive microwave satellite images indicate that the center is displaced to the northwest of the coldest cloud tops due to modest northwesterly mid- and upper-level vertical wind shear. The initial intensity has been increased to 50 kt, which is based on an average of Dvorak satellite intensity estimates of T3.0/45 kt from TAFB and T3.5/55 kt from SAB. Although the initial motion estimate is 360/02 kt, Eta has essentially been stationary for the past 9 hours or so. Radar data from Casablanca, Cuba, and satellite fixes suggest that Eta has been making a small cyclonic/counter-clockwise loop within the larger gyre in which the small center is embedded. Until the gyre breaks down or moves northward, there will be little poleward motion by Eta today. By tonight, however, the global and regional models are in good agreement on a broad mid-tropospheric trough moving eastward across the central and eastern United States, which is expected to erode a subtropical ridge to the north of the Eta, allowing both the larger gyre and Eta to move slowly northward. This steering pattern is expected to continue through about 72 hours. Thereafter, the model guidance diverges significantly between motions ranging from westward (GFS/GFS-ensemble) to northward (ECMWF) to northeastward (COAMPS-TC). The 96-120 hour motions are directly related to the strength of the cyclone, with a much weaker Eta forecast to move westward and a stronger hurricane solution moving northeastward. The latter scenario seems unlikely given that the vertical shear is forecast to increase from the northwest and west at more than 25 kt, which acts to weaken Eta and also impart a slight eastward tug on the system. As a result, the official forecast track calls for Eta to basically move slowly northward through the 120-h forecast period and gradually weaken into a shallow cyclone that drifts northward. The new NHC track forecast is similar to the previous advisory track, and lies close to a blend of the consensus models TVCA, NOAA-HCCA, and the Florida Superensemble (FSSE). Eta is expected to remain in a low-to-moderate vertical wind shear environment and over warm SSTs of 27-28 deg C for the next couple of days. Although the surrounding environment is expected to be somewhat dry, the other two favorable environmental factors should allow for some strengthening into Thursday, Thereafter, increasing shear from the northwest and west, along with drier mid-level air and cooler SSTs are likely to cause Eta to weaken. The rate of this weakening remains uncertain, and depends heavily on how much Eta re-intensifies over the next couple of days. The new intensity forecast is essentially the same as the previous advisory, and is a little below the consensus models IVCN, HCCA, and FSSE, all of which make Eta a hurricane again by 36 hours. Key Messages: 1. Heavy rainfall from Eta will continue across western Cuba and South Florida today and tonight. Additional flash and urban flooding, especially across previously inundated areas, will be possible in South Florida. Flash and urban flooding will also be possible for western Cuba. 2. Eta could approach the northeastern or north-central U.S. Gulf Coast later this week as a tropical storm, and possibly bring impacts from rain, wind, and storm surge. Interests in this area should continue to monitor the progress of Eta and updates to the forecast this week. FORECAST POSITIONS AND MAX WINDS INIT 10/1500Z 22.7N 85.3W 50 KT 60 MPH 12H 11/0000Z 23.1N 85.4W 55 KT 65 MPH 24H 11/1200Z 24.0N 85.3W 60 KT 70 MPH 36H 12/0000Z 25.0N 85.2W 60 KT 70 MPH 48H 12/1200Z 25.9N 85.1W 55 KT 65 MPH 60H 13/0000Z 26.6N 85.2W 45 KT 50 MPH 72H 13/1200Z 27.2N 85.5W 40 KT 45 MPH 96H 14/1200Z 28.6N 86.4W 35 KT 40 MPH 120H 15/1200Z 30.1N 86.4W 30 KT 35 MPH $$ Forecaster Stewart

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Subtropical Storm Theta Forecast Discussion Number 3

2020-11-10 15:52:55| National Hurricane Center (Atlantic)

Issued at 300 PM GMT Tue Nov 10 2020 000 WTNT45 KNHC 101452 TCDAT5 Subtropical Storm Theta Discussion Number 3 NWS National Hurricane Center Miami FL AL302020 300 PM GMT Tue Nov 10 2020 Theta continues to exhibit a mix of tropical and subtropical characteristics. The cyclone has a fairly compact radius of maximum winds with a central dense overcast over the northern portion of the circulation. However, there is a strong subtropical jet located just to the south of Theta, and strong upper-level winds in the surrounding environment in the presence of an upper trough. Based on this inconclusive data, Theta is being maintained as a subtropical storm for this advisory. An ASCAT-B overpass this morning revealed 50-kt winds in the northwest and southwest quadrants, with several 55-60 kt wind vectors, some of those vectors outside of the deep convection making them more believable. Based on this, the initial advisory intensity has been set to 60 kt. The storm is moving just north of due east, or 085/13 kt. Theta should continue to be steered eastward to east-northeastward during the next several days on the north side of a mid-level ridge. This motion should take the cyclone across the subtropical eastern Atlantic throughout the forecast period. The latest model guidance is a little faster and slightly north of the previous forecast track, and so the latest NHC forecast has been adjusted slightly as well. Theta is still expected to transition to a tropical storm later today as it separates from the upper-level trough. Although Theta will be tracking over progressively cooler SSTs and within moderate wind shear conditions, the air mass is expected to remain unstable for the next couple of days, which should be supportive of deep convection. Therefore, little change is strength is indicated during that time. Some weakening is anticipated later in the forecast period as the airmass becomes more stable. The NHC intensity forecast is higher than the previous one primarily due to the higher initial intensity, and is near the IVCN and HCCA consensus models. FORECAST POSITIONS AND MAX WINDS INIT 10/1500Z 29.0N 37.4W 60 KT 70 MPH 12H 11/0000Z 29.1N 35.7W 60 KT 70 MPH 24H 11/1200Z 29.7N 33.4W 60 KT 70 MPH 36H 12/0000Z 30.4N 31.3W 55 KT 65 MPH 48H 12/1200Z 31.1N 28.7W 55 KT 65 MPH 60H 13/0000Z 31.7N 26.0W 50 KT 60 MPH 72H 13/1200Z 32.2N 23.7W 45 KT 50 MPH 96H 14/1200Z 33.0N 19.5W 40 KT 45 MPH 120H 15/1200Z 34.3N 17.0W 35 KT 40 MPH...POST-TROP/EXTRATROP $$ Forecaster Latto

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Subtropical Storm Theta Forecast Discussion Number 2

2020-11-10 09:48:19| National Hurricane Center (Atlantic)

Issued at 900 AM GMT Tue Nov 10 2020 000 WTNT45 KNHC 100848 TCDAT5 Subtropical Storm Theta Discussion Number 2 NWS National Hurricane Center Miami FL AL302020 900 AM GMT Tue Nov 10 2020 Theta has generally changed little during the past several hours. The cyclone has some characteristics of a tropical cyclone, including a central dense overcast and a relatively small radius of maximum wind. However, the storm is still entangled with an upper-level trough, and based on all of these factors, Theta is being maintained as a subtropical storm. The initial wind speed remains 45 kt, which is at the high end of the satellite intensity estimates. The storm is moving due eastward at about 10 kt. The track forecast reasoning seems fairly straightforward. A trough to the north of the subtropical storm is expected to lift out, and Theta should be steered eastward to east-northeastward during the next several days on the north side of a mid-level ridge. This motion should take the cyclone across the subtropical eastern Atlantic throughout the forecast period. The models are in fairly good agreement, and the NHC track forecast lies near the various consensus aids. Theta should transition to a tropical storm later today as it separates from the upper-level trough. Little change in intensity is expected during the next several days due to mixed environmental conditions. Although Theta will be tracking over progressively cooler SSTs and within moderate wind shear conditions, the air mass is expected to remain unstable, which should be supportive of deep convection. The NHC intensity forecast is similar to the previous one and lies near the high end of the model guidance. FORECAST POSITIONS AND MAX WINDS INIT 10/0900Z 28.8N 39.5W 45 KT 50 MPH 12H 10/1800Z 28.9N 37.8W 50 KT 60 MPH 24H 11/0600Z 29.2N 35.7W 50 KT 60 MPH 36H 11/1800Z 29.7N 33.9W 50 KT 60 MPH 48H 12/0600Z 30.4N 31.6W 50 KT 60 MPH 60H 12/1800Z 31.0N 29.0W 50 KT 60 MPH 72H 13/0600Z 31.4N 26.7W 45 KT 50 MPH 96H 14/0600Z 32.2N 22.8W 45 KT 50 MPH 120H 15/0600Z 33.1N 19.9W 45 KT 50 MPH $$ Forecaster Cangialosi

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Tropical Storm Eta Forecast Discussion Number 40

2020-11-10 09:39:18| National Hurricane Center (Atlantic)

Issued at 300 AM CST Tue Nov 10 2020 000 WTNT44 KNHC 100839 TCDAT4 Tropical Storm Eta Discussion Number 40 NWS National Hurricane Center Miami FL AL292020 300 AM CST Tue Nov 10 2020 There hasn't been a lot of change to the cloud pattern of Eta overnight, with the center apparently located on the northwestern side of a small central dense overcast. Scatterometer data from earlier indicated 35-40 kt winds, and with the typical undersampling from that instrument, along with consensus 45-kt estimates from TAFB/SAB, so Eta's initial wind speed is kept at 45 kt. A NOAA Hurricane Hunter aircraft should be in the storm this morning. Eta is forecast to be in a low-to-moderate shear environment over warm SSTs for the next couple of days. Gradual strengthening is expected through early Thursday, with the main restrictive factor being nearby dry air. After that time, increasing shear and drier mid-level air are likely to cause Eta to weaken. The rate of this weakening is pretty uncertain, and a lot depends on how much Eta can re-intensify over the next couple of days. The new forecast is similar to the previous one, which is quite close to the consensus guidance. The storm has basically become stationary overnight. Eta should drift to north today due to steering flow changes with a nearby upper-level low losing influence while ridging east of Florida builds southward. A slow northward track is then anticipated for a couple of days over the east-central Gulf of Mexico in this flow pattern. Beyond that time, model guidance is in rather poor agreement, ranging from a stronger system moving faster to the northeast due to broad troughing over the south-central United States, to a weaker tropical cyclone getting caught under a narrow ridge over the Gulf of Mexico and turning west-southwestward. Since the new forecast expects Eta to be weakening, the NHC forecast will be west of the consensus and west of the previous track. I have little confidence in the long-range track forecast due to its seeming dependence on the intensity, and the wide spread in the guidance. Key Messages: 1. Heavy rainfall from Eta will continue across western Cuba and South Florida today and tonight. Additional flash and urban flooding, especially across previously inundated areas, will be possible in South Florida. Flash and urban flooding will also be possible for western Cuba. 2. Eta could approach the Florida Gulf Coast later this week as a tropical storm, and possibly bring impacts from rain, wind, and storm surge. Interests in this area should continue to monitor the progress of Eta and updates to the forecast this week. FORECAST POSITIONS AND MAX WINDS INIT 10/0900Z 22.9N 85.5W 45 KT 50 MPH 12H 10/1800Z 23.1N 85.6W 50 KT 60 MPH 24H 11/0600Z 24.0N 85.6W 55 KT 65 MPH 36H 11/1800Z 25.0N 85.4W 60 KT 70 MPH 48H 12/0600Z 26.0N 85.3W 60 KT 70 MPH 60H 12/1800Z 26.8N 85.3W 50 KT 60 MPH 72H 13/0600Z 27.3N 85.4W 40 KT 45 MPH 96H 14/0600Z 28.8N 86.5W 35 KT 40 MPH 120H 15/0600Z 30.0N 87.0W 30 KT 35 MPH $$ Forecaster Blake

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