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Subtropical Storm Theta Forecast Discussion Number 1

2020-11-10 03:59:41| National Hurricane Center (Atlantic)

Issued at 300 AM GMT Tue Nov 10 2020 000 WTNT45 KNHC 100259 TCDAT5 Subtropical Storm Theta Discussion Number 1 NWS National Hurricane Center Miami FL AL302020 300 AM GMT Tue Nov 10 2020 Over the past 12 h convection has slowly consolidated near the center of a non-tropical area of low pressure over the far northeast Atlantic. Earlier, there was a 2325 UTC ASCAT-B pass that showed that the low-level circulation had become more detached from the warm frontal boundary located to its northeast. In addition, the ASCAT pass showed that winds have increased to near 45 kt on the southeast portion of the circulation near where coldest convective cloud tops were located. Both a 2137 UTC SSMIS and 2324 UTC AMSU microwave pass showed convective banding under the cirrus canopy near where the highest winds were observed. However, water vapor satellite imagery still shows the presence of a broad upper-level low tangled up with the low-level circulation embedded in the convection. Therefore, Theta has been designated as a subtropical storm with maximum wind of 45 kt. Theta has been on a near due east heading over the last 12 h at 90/13 kt. Currently the cyclone is located south of a broad deep-layer longwave trough. This trough is expected to bypass the cyclone to the east and Theta will primarily be steered by southwesterly flow on the northern periphery of a mid-level ridge centered over the Cape Verde islands. This flow will steer Theta slowly to the east-northeast over the next 3-5 days. The track guidance is in good agreement with this solution, with only minor differences in across-track spread. There are somewhat larger differences in the along-track spread related to different forward motion, but the latest NHC track forecast elected to stay close to the track consensus at this time. Thetas structure has evolved from a frontal cyclone to a subtropical cyclone, though a frontal boundary remains nearby on the northeast side of the circulation. This boundary is expected to gradually dissipate and Theta is expected to fully transition to a tropical storm in 24 h as convection erodes the upper-level cyclonic flow overhead. The maximum sustained winds were increased slightly in the first 12 h of the forecast but are kept at 50 kt through 120 h which is on the high end of the intensity guidance. Theta is the 29th named storm in the record-breaking 2020 Atlantic Hurricane Season, breaking the previous record of 28th named storms in 2005. FORECAST POSITIONS AND MAX WINDS INIT 10/0300Z 28.8N 40.3W 45 KT 50 MPH...SUBTROPICAL STORM 12H 10/1200Z 29.0N 38.6W 50 KT 60 MPH...SUBTROPICAL STORM 24H 11/0000Z 29.1N 36.3W 50 KT 60 MPH...TROPICAL STORM 36H 11/1200Z 29.5N 34.3W 50 KT 60 MPH 48H 12/0000Z 30.0N 32.6W 50 KT 60 MPH 60H 12/1200Z 30.5N 30.7W 50 KT 60 MPH 72H 13/0000Z 31.0N 28.7W 50 KT 60 MPH 96H 14/0000Z 32.2N 24.3W 50 KT 60 MPH 120H 15/0000Z 33.3N 20.6W 50 KT 60 MPH $$ Forecaster Papin/Brown

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Tropical Storm Eta Forecast Discussion Number 39

2020-11-10 03:53:43| National Hurricane Center (Atlantic)

Issued at 900 PM CST Mon Nov 09 2020 000 WTNT44 KNHC 100253 TCDAT4 Tropical Storm Eta Discussion Number 39 NWS National Hurricane Center Miami FL AL292020 900 PM CST Mon Nov 09 2020 Eta's convective structure appears to be gradually becoming better organized this evening. The center is located near the northern edge of a band that wraps around the southeastern and southern portions of the circulation. Recent subjective Dvorak data T-numbers were 3.0, which still support an initial intensity of 45 kt. With Eta's structure gradually improving and a forecast for the cyclone to remain over SSTs of around 28C and in generally low vertical wind shear conditions, some re-strengthening is likely during the next 24-36 hours. Although the NHC intensity forecast shows Eta remaining just below hurricane strength, there is a possibility that the storm will briefly regain hurricane status over the southeastern Gulf of Mexico within the next day or so. By 48 hours, when the cyclone moves over the east-central Gulf of Mexico, increasing vertical wind shear and dry mid-level air are likely to result in weakening, and like the previous forecast, the new NHC forecast indicates that Eta could weaken to a tropical depression by the end of the period. Some of the global model guidance suggests that Eta could weaken even faster than indicated below after 72 hours. Recent satellite fixes indicate that Eta is moving southwestward but a little slower than before at around 8 kt. Eta should slow its forward progress overnight and then meander over the southeastern Gulf of Mexico on Tuesday as steering current collapse. In 36-48 hours, Eta should begin moving northward between a mid-level ridge over the western Atlantic and a broad trough over the central United States. After 72 hours, the spread in the guidance increases when Eta is expected to be weaker and be steered by the low-level flow. Most of the guidance shows a northwestward to northward motion at that time period but have varying forward speed. The NHC track shows a slow north-northwest motion late in the period to account for these differences. There is lower than normal confidence in the latter portion of the track forecast given the large spread in the guidance. Key Messages: 1. Heavy rainfall from Eta will continue across portions of Cuba, the Bahamas, and southern Florida and spread north into central Florida. Additional flash and urban flooding, especially across previously inundated areas, will be possible in South Florida tonight. Flash and urban flooding will also be possible for Cuba, the Bahamas and the remainder of southern Florida over the next several days. 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 10/0300Z 23.2N 85.2W 45 KT 50 MPH 12H 10/1200Z 22.8N 85.5W 50 KT 60 MPH 24H 11/0000Z 23.4N 85.5W 60 KT 70 MPH 36H 11/1200Z 24.6N 85.3W 60 KT 70 MPH 48H 12/0000Z 25.8N 85.2W 55 KT 65 MPH 60H 12/1200Z 26.6N 85.2W 50 KT 60 MPH 72H 13/0000Z 27.0N 85.3W 45 KT 50 MPH 96H 14/0000Z 27.7N 85.6W 35 KT 40 MPH 120H 15/0000Z 29.0N 86.0W 30 KT 35 MPH $$ Forecaster Brown

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Tropical Storm Eta Forecast Discussion Number 38

2020-11-09 21:51:44| National Hurricane Center (Atlantic)

Issued at 400 PM EST Mon Nov 09 2020 000 WTNT44 KNHC 092051 TCDAT4 Tropical Storm Eta Discussion Number 38 NWS National Hurricane Center Miami FL AL292020 400 PM EST Mon Nov 09 2020 Eta's convective pattern now consists of mainly a compact ring of inner-core convection with cloud tops of -60C to -70C. The earlier main outer convective band located in the northeast quadrant has weakened considerably, and the inflow into that feature is now being shunted westward into Eta's inner-core region. Visible and water vapor satellite images also indicate that weak anticyclonic cirrus outflow has recently developed over the inner core. The last recon pass through Eta a few hours ago showed a pressure rise to 997 mb that was followed by a pressure decrease to 995 mb on the last pass. Both flight-level and SFMR-derived surface winds had also decreased and only supported around 45 kt, which is the initial intensity used for this advisory. The initial motion estimate is southwestward, or 235/14 kt. Mid- and upper-level water vapor images show a cut-off low located over the extreme northwestern Caribbean Sea near the Isle of Youth. This feature, in conjunction with a deep-layer ridge extending across Florida and the Gulf of Mexico, is expected to produce northeasterly flow that will keep Eta moving southwestward for the next 24-36 hours. However, the cyclone will gradually slow down during that time as a broad deep-layer trough moving across the central and south-central U.S. weakens the ridge over the Gulf, causing Eta to stall or make a small loop around 36 hours. By 48-60 hours and beyond, the aforementioned trough is forecast to lift out to the northeast, allowing at least some of the Gulf ridge to build back in, slowing down Eta's poleward progress or even possibly trapping the cyclone over the northeastern Gulf of Mexico. The global and regional models are in very good agreement on the developing track scenario through about 72 hours, and then diverge significantly thereafter, with the bulk of the guidance taking a much weaker Eta northwestward or northward into strong shear conditions. However, the Navy COAMPS-TC model strengthens Eta to near major hurricane status and takes the cyclone northeastward, while the HMON model also intensifies the cyclone into a major hurricane, but leaves it trapped over the southeastern Gulf of Mexico. The latter two scenarios are considered to be outliers due to the abundance of very dry air surrounding Eta and an expected increase in the deep-layer shear to more than 25 kt by 96 and 120 hours. The new official forecast track is to the left or west of the previous advisory track, but not as far west as the consensus models, which take a significantly weaker and more shallow cyclone toward the north-central Gulf coast. Some re-strengthening appears more likely now that Eta has shed a lot of outer convective baggage and has become more compact, and has developed a donut ring of inner-core convection and some modest upper-level outflow in all quadrants. Eta's best opportunity for intensification should come during the next 36 hours when the cyclone will be moving over the warm waters of the Gulf of Mexico Loop Current and the deep-layer vertical wind shear gradually decreases to less than 10 kt. Although occasional intrusions of very dry air will prevent rapid intensification from occurring, some gradual strengthening seems to be in order given the other favorable environmental conditions and the cyclone's new smaller size. By 48 hours and beyond, increasing vertical wind shear and dry air entrainment should cause steady weakening of the cyclone through end of the forecast period. However, if Eta takes a more northwestward track like some of the NHC model guidance is indicating, then the cyclone will get sheared more and weaken sooner than indicated in the official forecast. The new NHC intensity forecast is similar to the previous advisory, and but is lower than intensity consensus models IVCN and HCCA, which re-strengthen Eta to a 65-70 kt hurricane. Key Messages: 1. Heavy rainfall from Eta will continue across portions of Cuba, the Bahamas, and southern Florida and spread north into central Florida. Additional flash flooding is possible across inundated urban areas of southeast Florida today. Flash and urban flooding will also be possible for Cuba, the Bahamas and the remainder of southern Florida over the next several days. 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 monitor the progress of Eta and updates to the forecast this week. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 23.7N 84.8W 45 KT 50 MPH 12H 10/0600Z 23.2N 85.3W 50 KT 60 MPH 24H 10/1800Z 23.4N 85.5W 55 KT 65 MPH 36H 11/0600Z 24.3N 85.5W 60 KT 70 MPH 48H 11/1800Z 25.6N 85.4W 55 KT 65 MPH 60H 12/0600Z 26.5N 85.6W 50 KT 60 MPH 72H 12/1800Z 27.2N 85.4W 45 KT 50 MPH 96H 13/1800Z 28.0N 85.4W 40 KT 45 MPH 120H 14/1800Z 28.6N 85.6W 30 KT 35 MPH $$ Forecaster Stewart

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Tropical Storm Eta Forecast Discussion Number 37

2020-11-09 15:50:08| National Hurricane Center (Atlantic)

Issued at 1000 AM EST Mon Nov 09 2020 000 WTNT44 KNHC 091449 TCDAT4 Tropical Storm Eta Discussion Number 37 NWS National Hurricane Center Miami FL AL292020 1000 AM EST Mon Nov 09 2020 Eta's overall appearance in satellite radar imagery has become quite fragmented, with inner-core convection having become vertically shallow and broken due to mid-level dry air entrainment. Some of the recent Air Force Reserve reconnaissance flight-level data indicate that dry air has worked way down to the 850-mb level based dewpoint spreads of more than 8 deg C. In the northeastern quadrant, a large curved convective band persists that has produced widespread rainfall amounts exceeding 10 inches along with strong gusty winds across portions of the southern Florida peninsula. However, even that band of thunderstorms has become less pronounced in both satellite radar imagery over the past few hours. Doppler velocities over land and over water have also come decreased significantly, and the latest reconnaissance flight-level (58 kt) and SFMR surface wind data (49 kt) supports lowering the intensity to 50 kt. The decrease in intensity is also supported by the gradual rise in the central pressure, which is now up to 994 mb. Eta has finally made the expected southwestward turn, and the initial motion is now 235/12 kt. A strong deep-layer ridge across the Gulf of Mexico, Florida, and near the U.S. east coast is expected to keep Eta moving southwestward for the next 24-36 hours, accompanied by a steady decrease in forward speed. Steering currents are forecast to collapse by 36-48 hours, causing Eta to possibly stall and/or make a small loop just northwest of western Cuba. By 60 hours and beyond, a broad mid-latitude trough currently located over the Rocky Mountains is forecast to move eastward and gradually erode the portion of the ridge over the Gulf of Mexico, allowing Eta slowly move northward to north-northeastward through 20 hours. However, there remains considerable divergence between the global models on days 4 and 5 with regards to how far north Eta will move, with the ECMWF showing a more northward progression while the GFS and UKMET models show the trough lifting out and not eroding the ridge as much. For now, the official forecast track remains a compromise of these two extremes, and shows a slow poleward progression on days 3-5, similar to the consensus models TCVA and NOAA-HCCA. Eta could still re-strengthen some during the 24-48 hour period when the cyclone will be moving over the warm waters of the Gulf of Mexico Loop Current and the wind shear gradually decrease to less than 10 kt. However, occasional intrusions of very dry air will likely continue to plague the system, which would prevent any rapid intensification from occurring and could keep Eta from regaining hurricane status. By 48 hours and beyond, increasing vertical wind shear and dry air should cause a gradual weakening of the cyclone through end of the forecast period. The new NHC intensity forecast is a little lower than the previous advisory, and closely follows the intensity consensus models IVCN and HCCA. Key Messages: 1. Heavy rainfall from Eta will continue across portions of Cuba, the Bahamas, and southern and central Florida. Life-threatening flash flooding will be possible across urban areas of southeast Florida today. Flash and urban flooding will also be possible for Cuba, the Bahamas and the remainder of southern Florida, along with potential minor river flooding in central Florida. 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 monitor the progress of Eta and updates to the forecast this week. FORECAST POSITIONS AND MAX WINDS INIT 09/1500Z 24.6N 83.4W 50 KT 60 MPH 12H 10/0000Z 23.9N 84.7W 50 KT 60 MPH 24H 10/1200Z 23.6N 85.3W 55 KT 65 MPH 36H 11/0000Z 24.5N 85.4W 60 KT 70 MPH 48H 11/1200Z 25.6N 85.5W 55 KT 65 MPH 60H 12/0000Z 26.5N 85.3W 55 KT 65 MPH 72H 12/1200Z 27.2N 85.1W 50 KT 60 MPH 96H 13/1200Z 28.4N 84.4W 45 KT 50 MPH 120H 14/1200Z 29.6N 83.7W 35 KT 40 MPH...INLAND $$ Forecaster Stewart

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Tropical Storm Eta Forecast Discussion Number 36

2020-11-09 10:03:02| National Hurricane Center (Atlantic)

Issued at 400 AM EST Mon Nov 09 2020 000 WTNT44 KNHC 090902 CCA TCDAT4 Tropical Storm Eta Discussion Number 36...Corrected NWS National Hurricane Center Miami FL AL292020 400 AM EST Mon Nov 09 2020 Corrected a typo in the second paragraph Tropical Storm Eta made landfall in the middle Florida Keys around 11 pm EST with maximum winds of about 55 kt. Since then it has moved across Florida Bay and is now located off the coast of extreme southwestern Florida. The storm has generally changed little in strength overnight with maximum winds estimated to be near 55 kt based on NOAA Hurricane Hunter and Doppler radar data. Bands of heavy rain and tropical-storm-force winds continue over portions of the Florida Keys, and south and central Florida. Eta is now moving west-northwestward at 12 kt. The upper-level trough that Eta is entangled with and a mid-level high pressure system off the southeast U.S. coast should steer the storm westward to west-southwestward away from south Florida and the Keys through tonight. The steering currents surrounding Eta are expected to collapse on Tuesday, and most of the guidance responds by showing Eta meandering over the southeastern Gulf of Mexico in the 24-48 hour time period. After that time, the models diverge significantly with the ECMWF showing a turn to the north or northeast in response to an upper trough that is expected to move into the northwestern Gulf of Mexico. However, the latest runs of GFS and UKMET models show the trough lifting out and provided little steering for Eta, which causes a continued slow and erratic motion over the Gulf in those models. The new track forecast shows a slower northward to northeastward motion compared to the previous one from days 3-5 as a compromise of the latest models and continuity. Due to the poor model agreement beyond a couple of days, the track forecast at the longer range is of low confidence, and large changes are possible if the models converge toward the GFS/UKMET solutions. The storm will have an opportunity to strengthen a little during the next couple of days and it could become a hurricane as it moves over the warm waters of the Gulf of Mexico Loop current and remains in light-to-moderate wind shear conditions. However, there will be a fair amount of dry air surrounding Eta, and that should limit the amount of intensification. Beyond a couple of days, depending on exactly where Eta is, the models show an increase in westerly shear, which in combination with the dry air should cause a slow decay. The NHC intensity forecast is unchanged from the previous one, and is also of low confidence since its unclear where Eta will be located later in the week. Key Messages: 1. Heavy rainfall from Eta will continue across portions of Cuba, Jamaica, the Bahamas, and southern Florida and spread north into central Florida. Life-threatening flash flooding will be possible across inundated urban areas of southeast Florida today. Flash and urban flooding will also be possible for Cuba, Jamaica, the Bahamas and the remainder of southern Florida, along with potential minor river flooding in central Florida. 2. Tropical storm conditions will continue across portions of the Florida Keys, and south and central Florida today. 3. Water levels will gradually recede along portions of the southern coast of the Florida peninsula and Keys. Residents in these areas should follow any advice given by local officials. 4. 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 monitor the progress of Eta and updates to the forecast this week. FORECAST POSITIONS AND MAX WINDS INIT 09/0900Z 25.2N 82.0W 55 KT 65 MPH 12H 09/1800Z 24.6N 83.7W 60 KT 70 MPH 24H 10/0600Z 23.8N 84.7W 60 KT 70 MPH 36H 10/1800Z 23.9N 85.1W 65 KT 75 MPH 48H 11/0600Z 24.8N 85.1W 65 KT 75 MPH 60H 11/1800Z 26.0N 85.0W 65 KT 75 MPH 72H 12/0600Z 26.7N 84.7W 60 KT 70 MPH 96H 13/0600Z 27.8N 84.2W 55 KT 65 MPH 120H 14/0600Z 29.4N 83.2W 45 KT 50 MPH...INLAND $$ Forecaster Cangialosi

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