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Tropical Storm Theta Forecast Discussion Number 18

2020-11-14 09:36:56| National Hurricane Center (Atlantic)

Issued at 900 AM GMT Sat Nov 14 2020 806 WTNT45 KNHC 140836 TCDAT5 Tropical Storm Theta Discussion Number 18 NWS National Hurricane Center Miami FL AL302020 900 AM GMT Sat Nov 14 2020 Nearly all of the deep convection associated with Theta has dissipated since the previous advisory, with only cold, high cirrus clouds remaining in the northeastern quadrant. A late-arriving 13/2255Z partial ASCAT-C pass indicated a few 35-kt wind vectors were present in the southeastern quadrant, so the initial intensity has been lowered to 35 kt for this advisory, which is consistent the latest TAFB current intensity estimate of T2.5/35 kt. The initial motion estimate is due east, or 090/08 kt. Theta is expected to move eastward along the northern periphery of a deep-layer ridge for the next 24 hours or so, a motion that will also be accompanied by a gradual decrease in forward speed. By 36 hours, a much weaker and more vertically shallow Theta is forecast to turn sharply northward ahead of a frontal system. In the 48-72 hour period, the weakening cyclone expected to accelerate northeastward ahead of the front, passing to the north of the Canary Islands this weekend. Theta is forecast to dissipate by 96 hours near the Madeira Island. The new NHC track forecast is similar to the previous advisory, and lies close to the consensus model TVCN. Strong shear in excess of 35 kt currently affecting Theta is forecast to steadily increase to more than 40 kt by 12 h and beyond. In addition, entrainment of very dry mid-level air should combine with the increasing shear and prevent the reformation of persistent organized deep convection near the center. Although the global models show that some marginal instability is expected to linger through the forecast period, only intermittent small bursts of modest convection away from the center could develop. However, the general trend in the model guidance calls for Theta to slowly spin down, and the official intensity forecast follows that scenario, calling for the cyclone to weaken to a remnant low later today, then becoming extratropical before dissipating in 3-4 days. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 32.0N 20.8W 35 KT 40 MPH 12H 14/1800Z 31.8N 19.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 24H 15/0600Z 31.8N 18.3W 30 KT 35 MPH...POST-TROP/EXTRATROP 36H 15/1800Z 32.2N 17.7W 25 KT 30 MPH...POST-TROP/EXTRATROP 48H 16/0600Z 33.2N 17.4W 20 KT 25 MPH...POST-TROP/EXTRATROP 60H 16/1800Z 34.9N 16.6W 20 KT 25 MPH...POST-TROP/EXTRATROP 72H 17/0600Z 37.2N 15.6W 20 KT 25 MPH...POST-TROP/EXTRATROP 96H 18/0600Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Iota Forecast Discussion Number 3

2020-11-14 03:40:31| National Hurricane Center (Atlantic)

Issued at 1000 PM EST Fri Nov 13 2020 000 WTNT41 KNHC 140240 TCDAT1 Tropical Storm Iota Discussion Number 3 NWS National Hurricane Center Miami FL AL312020 1000 PM EST Fri Nov 13 2020 Satellite images show that Iota has changed little during the past several hours. Deep convection is organized in fragmented bands around the center, except in the northwestern quadrant where there is a dry slot. All of the satellite intensity estimates are around 35 kt, and therefore, the initial intensity is held at that value. Iota has been drifting to the south-southwest during the past several hours, however, the steering currents are expected to become more pronounced soon. A strong and sprawling mid-level ridge currently centered over the Gulf of Mexico will shift eastward and build to the north of the tropical cyclone. Since this ridging is expected to remain well established to the north of Iota during the next several days, a general westward motion is expected through the forecast period. The NHC track forecast takes the core of Iota to the Nicaragua and Honduras coasts in about 3 days. However, conditions are expected to deteriorate along that part of the coast before the center arrives. The latest models are in fairly good agreement, except for the HWRF which is a northern outlier, and the NHC track forecast lies near the middle of the guidance envelope. This forecast is a little slower than the previous one. An upper-level trough to the west of Iota is expected to move west and dissipate on Saturday. This should leave the storm in favorable conditions of 29-30 C waters, and in an air mass of low wind shear and high moisture this weekend and early next week. These conditions support steady to possibly rapid intensification until Iota makes landfall. The main question is how strong will Iota become before it crosses the coast. The SHIPS Rapid Intensification Index shows a 55 percent chance of Iota strengthening 65 kt or more before it makes landfall, which is about 11 times the climatological mean. The NHC intensity forecast is an update of the previous one, and shows Iota strengthening to a major hurricane in about 60 hours. Key Messages: 1. Iota is expected to strengthen and be a major hurricane when it approaches the coast of Central America. There is a risk of dangerous wind, storm surge, and rainfall impacts across portions of Nicaragua and Honduras beginning Monday. Hurricane Watches will likely be issued for a portion of this area on Saturday. 2. Through Wednesday, heavy rainfall from Tropical Storm Iota will likely lead to life-threatening flash flooding and river flooding across portions of northern Colombia and Central America. Flooding and mudslides in Honduras and Nicaragua could be exacerbated by Hurricane Eta's recent effects there. FORECAST POSITIONS AND MAX WINDS INIT 14/0300Z 13.6N 74.2W 35 KT 40 MPH 12H 14/1200Z 13.5N 74.9W 45 KT 50 MPH 24H 15/0000Z 13.7N 76.0W 55 KT 65 MPH 36H 15/1200Z 14.0N 77.6W 70 KT 80 MPH 48H 16/0000Z 14.3N 79.5W 85 KT 100 MPH 60H 16/1200Z 14.5N 81.2W 100 KT 115 MPH 72H 17/0000Z 14.7N 82.7W 105 KT 120 MPH 96H 18/0000Z 14.8N 85.4W 50 KT 60 MPH...INLAND 120H 19/0000Z 14.3N 88.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Cangialosi

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Tropical Storm Iota Forecast Discussion Number 2

2020-11-13 21:44:48| National Hurricane Center (Atlantic)

Issued at 400 PM EST Fri Nov 13 2020 000 WTNT41 KNHC 132044 TCDAT1 Tropical Storm Iota Discussion Number 2 NWS National Hurricane Center Miami FL AL312020 400 PM EST Fri Nov 13 2020 Banding features over the eastern and southeastern portions of the cyclone's circulation have increased since this morning, and the overall organization of the system continues to quickly improve. Earlier ASCAT data indicated that there was a fairly large area of light winds near the center, and that the low-level center was displaced to the northwest of the mid-level center seen in visible satellite imagery. Since the system is still in its formative stage, the low-level center may reform closer to the mid-level feature, and the advisory position is a compromise between the low- and mid-level circulations. The earlier ASCAT data indicated peak winds of around 30 kt with several higher rain-inflated vectors. Based on the continued increase in organization, and Dvorak T-numbers of T2.5 from both TAFB and SAB, the initial intensity is raised to 35 kt. Iota becomes the 30th named storm of the recording-breaking 2020 hurricane season. The environment ahead of Iota appears to be quite conducive for intensification. The system will be moving over warm waters, in a moist atmosphere, and within an area of very low vertical wind shear. As a result, steady to rapid strengthening appears likely over the next few days. The NHC intensity forecast calls for Iota to reach hurricane status within 36 h, and now calls for the system to be a major hurricane when it approaches the coast of Central America. The NHC intensity foreast is in good agreement with the HFIP corrected consensus model, and the 70-kt increase in intensity over the next 72 hours is supported by the SHIPS Rapid Intensification Index that shows a nearly 60 percent chance of a 65 kt increase in intensity during that time period. The tropical storm has not moved very much today, and the initial motion estimate is a somewhat uncertain 255/3 kt. A strong mid-level ridge that extends across Florida and the western Atlantic is forecast to slide eastward over the next few days causing the cyclone to move faster toward the west or west-northwestward. The track guidance has come into a bit better agreement this afternoon, with only the HWRF showing a track farther north over the northwestern Caribbean Sea. The latest consensus aids were very close to the previous official forecast, and no significant adjustments to the earlier track forecast were required. Key Messages: 1. Iota is expected to strengthen and be a major hurricane when it approaches the coast of Central America. There is a risk of dangerous wind, storm surge, and rainfall impacts across portions of Nicaragua and Honduras beginning Sunday night or early Monday. Hurricane Watches will likely be issued for a portion of this area tonight or early Saturday. 2. Through Wednesday morning, heavy rainfall from Iota may lead to life-threatening flash flooding and river flooding across portions of Haiti, Jamaica and Central America. Flooding and landslides from heavy rainfall could be significant across Central America given recovery efforts underway after Hurricane Eta. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 13.8N 74.3W 35 KT 40 MPH 12H 14/0600Z 13.7N 75.1W 45 KT 50 MPH 24H 14/1800Z 13.7N 76.2W 55 KT 65 MPH 36H 15/0600Z 13.9N 77.5W 70 KT 80 MPH 48H 15/1800Z 14.2N 79.3W 85 KT 100 MPH 60H 16/0600Z 14.6N 81.2W 95 KT 110 MPH 72H 16/1800Z 15.0N 82.8W 105 KT 120 MPH 96H 17/1800Z 15.2N 85.5W 50 KT 60 MPH...INLAND 120H 18/1800Z 14.7N 88.5W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Brown

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Tropical Storm Theta Forecast Discussion Number 16

2020-11-13 21:38:13| National Hurricane Center (Atlantic)

Issued at 900 PM GMT Fri Nov 13 2020 000 WTNT45 KNHC 132038 TCDAT5 Tropical Storm Theta Discussion Number 16 NWS National Hurricane Center Miami FL AL302020 900 PM GMT Fri Nov 13 2020 Theta appears to be holding its own still, with a large curved band feature characterizing the bulk of the deep convection, and a smaller area of convection intermittently developing over the center. Like yesterday, there has been a lack of observational data over the system's core since last night. However, since Theta's organization has not appeared to change much since that time, the initial intensity is being kept at a somewhat uncertain 50 kt. Theta is forecast to change little in intensity into tonight as the cyclone continues moving in a relatively low shear zone associated with a mid- to upper-level trough axis. By tomorrow, strong northerly shear is forecast to begin impacting the system while stable air gets entrained into its circulation. As a result, Theta is expected to spin down over the weekend, degenerate to a remnant low on Sunday, and dissipate by the middle of next week. Theta is moving eastward at around 9 kt, steered by a mid-level ridge extending westward from west Africa to the eastern tropical Atlantic. A turn to the east-southeast along with a gradual slowing of the forward motion is expected overnight and continuing through the weekend as the ridge weakens, and mid- to upper level northerly winds impart on the cyclone. Early next week, what remains of Theta are expected to turn northward and then accelerate northeastward as a mid-latitude cyclone passes to the north of the region. The NHC forecast is little changed from the previous one and lies near the various consensus aids. On the forecast track, the center of Theta is forecast to track very near the westernmost Canary Islands. However, the system is forecast to weaken to a remnant low before reaching those islands. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 31.9N 22.6W 50 KT 60 MPH 12H 14/0600Z 31.7N 21.0W 50 KT 60 MPH 24H 14/1800Z 31.4N 19.2W 45 KT 50 MPH 36H 15/0600Z 31.3N 18.2W 40 KT 45 MPH 48H 15/1800Z 31.7N 17.7W 30 KT 35 MPH...POST-TROP/REMNT LOW 60H 16/0600Z 32.5N 17.7W 30 KT 35 MPH...POST-TROP/REMNT LOW 72H 16/1800Z 34.4N 17.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 17/1800Z 39.2N 12.8W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 18/1800Z...DISSIPATED $$ Forecaster Latto

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Tropical Depression Thirty-One Forecast Discussion Number 1

2020-11-13 16:05:07| National Hurricane Center (Atlantic)

Issued at 1000 AM EST Fri Nov 13 2020 000 WTNT41 KNHC 131504 CCA TCDAT1 Tropical Depression Thirty-One Discussion Number 1...Corrected NWS National Hurricane Center Miami FL AL312020 1000 AM EST Fri Nov 13 2020 Corrected the time from EDT to EST Deep convection associated with the area of low pressure over the central Caribbean Sea has increased and become more concentrated since yesterday. One-minute GOES-16 visible satellite imagery shows that the circulation has also become better defined, with a westerly component seen in the low-cloud motion near the southwestern edge of the primary convective mass. Subjective Dvorak intensity estimates from both TAFB and SAB are T2.0, indicating that the convection has become sufficiently organized for the system to be classified as a tropical depression. The initial wind speed is set at 30 kt, in line with the Dvorak classifications. Environmental conditions of low vertical wind shear, warm sea surface temperatures, and a moist atmosphere favor intensification over the next few days. Given the current broad and sprawling structure of the system, strengthening may begin as gradual today, but once an inner core organizes, steady-to-rapid strengthening appears likely. While the SHIPS rapid intensification index does not show very high chances of rapid strengthening for any one 24-h period over the next few days, it does indicate a 50/50 chance (nearly 10 times the climatological mean) of a 65-kt increase in wind speed over the next 72 hours. As a result, the NHC forecast calls for significant strengthening during the 24 to 72 hour time period, and the system could approach the coast of Central America as a major hurricane in a few days. The NHC track forecast at days 4 and 5 shows the system weakening over land, however some of the track guidance keeps the system just off the coast on Honduras at that time. If a more northern track occurs, the system could be stronger at 96 and 120 h if it remains over water. The depression is moving west-southwestward at about 6 kt. A strong mid-level ridge that lies over Florida and the western Atlantic should steer the cyclone west-southwestward during the next 12 to 24 hours. After that time, the ridge is forecast to begin sliding eastward, and a westward to west-northwestward motion should begin. On the foreast track, the cyclone is expected to approach the coast of Central America in 60-72 h. The track guidance is in good agreement through the first couple of days, but there is increasing cross-track spread after that time. The HMON, HWRF, and GFS show a track near or north of the coast of Honduras after 72 hours, while the ECMWF and UKMET are farther south. For now, the NHC track is near the middle of the guidance envelope between the HFIP corrected consensus model and the TCVA multi-model consensus. Key Messages: 1. The depression is expected to strengthen to a hurricane while it approaches the coast of Central America, and there is a risk of dangerous wind, storm surge, and rainfall impacts across portions of Nicaragua and Honduras beginning Sunday night. Hurricane Watches will likely be issued for a portion of this area tonight. 2. Through Wednesday morning, heavy rainfall from Tropical Depression Thirty-One may lead to life-threatening flash flooding and river flooding across portions of Haiti, Jamaica and Central America. Flooding and landslides from heavy rainfall could be significant across Central America given recovery efforts underway after Hurricane Eta. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 14.2N 74.3W 30 KT 35 MPH 12H 14/0000Z 14.0N 75.0W 35 KT 40 MPH 24H 14/1200Z 13.8N 76.0W 45 KT 50 MPH 36H 15/0000Z 13.9N 77.3W 60 KT 70 MPH 48H 15/1200Z 14.1N 79.0W 70 KT 80 MPH 60H 16/0000Z 14.5N 80.9W 85 KT 100 MPH 72H 16/1200Z 14.9N 82.7W 95 KT 110 MPH 96H 17/1200Z 15.2N 85.7W 50 KT 60 MPH...INLAND 120H 18/1200Z 14.7N 88.3W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Brown

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