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Tropical Storm Nana Forecast Discussion Number 7

2020-09-02 22:36:29| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Wed Sep 02 2020 000 WTNT41 KNHC 022036 TCDAT1 Tropical Storm Nana Discussion Number 7 NWS National Hurricane Center Miami FL AL162020 400 PM CDT Wed Sep 02 2020 Nana continues to feel the effects of about 15 kt of northerly vertical shear, as the low-level center is located near the northern edge of the main convective mass. Data from the last Air Force Reserve Hurricane Hunter Mission showed that the flight-level winds at 850 mb were a little lower than earlier. However, incomplete SFMR data suggested surface winds near 50 kt, and the aircraft reported that the central pressure is near 999 mb. The initial intensity is held at 50 kt, although this could be a little generous. The initial motion is westward or 270/13 kt. A low- to mid-level ridge to the north of the cyclone is forecast to keep steering Nana toward the west, or maybe just south of west, for the remainder of the cyclone's life. The new forecast track, which again is changed only slightly from the previous forecast, calls for the cyclone to pass north of the Bay Islands this evening, then make landfall over central or southern Belize in about 12-18 h. Nana is running out of time to strengthen before landfall, and between that and the ongoing shear none of the intensity guidance forecasts it to become a hurricane before landfall. However, any strong convective burst could spin up the cyclone, and since the bursts have been frequent today the intensity forecast calls for Nana to strengthen to near hurricane strength at landfall. After landfall, steady weakening is expected. Several of the global models now show the remnants of Nana emerging over the Gulf of Tehuantepec in 48-60 h. However, these models continue to forecast dissipation even over water, so the forecast dissipation time is unchanged since the previous advisory. KEY MESSAGES: 1. Nana could bring hurricane conditions and dangerous storm surge tonight to portions of the coast of Belize, and a hurricane warning is in effect. Preparations to protect life and property should be rushed to completion. 2. Tropical storm conditions are expected in the tropical storm warning areas in Belize, the Bay Islands, Guatemala, and the Yucatan Peninsula of Mexico by tonight. 3. Heavy rainfall with isolated maximum amounts as high as 8 to 12 inches could result in flash flooding in Belize, Guatemala, and portions of southeastern Mexico and the Yucatan Peninsula. FORECAST POSITIONS AND MAX WINDS INIT 02/2100Z 17.0N 85.9W 50 KT 60 MPH 12H 03/0600Z 16.8N 87.8W 60 KT 70 MPH 24H 03/1800Z 16.6N 90.1W 45 KT 50 MPH...INLAND 36H 04/0600Z 16.4N 92.4W 30 KT 35 MPH...INLAND 48H 04/1800Z 16.2N 94.8W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 05/0600Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Omar Forecast Discussion Number 8

2020-09-02 16:48:26| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Wed Sep 02 2020 000 WTNT45 KNHC 021448 TCDAT5 Tropical Storm Omar Discussion Number 8 NWS National Hurricane Center Miami FL AL152020 1100 AM AST Wed Sep 02 2020 Although is seems as though we've been predicting it ad nauseam, the persistent, blistering shear has finally begun to dramatically affect the cyclone. What remains of the deep convective cloud mass is separated more than 100 miles east-southeast of the exposed surface circulation. The initial intensity is held at 35 kt based on the Dvorak satellite intensity estimates from TAFB and SAB using the shear scene-type. Omar's winds should begin to decrease later tonight as the exposed surface center further decouples from the convective mass. A few of the large-scale models show Omar dissipating as soon as Friday, but for this NHC intensity forecast, I'll indicate weakening to a depression Thursday, followed by degeneration to a remnant low in 36 hours, similar to the multi-model consensus intensity aids and the LGEM. Omar's initial motion is estimated to be eastward, or 080/11 kt, and is being steered by the deep-layer mid-latitude westerlies produced by a subtropical ridge anchored over the central Atlantic. Omar should continue on this general course through Friday. On Saturday, the remnant low is expected to slow a bit, and turn toward the east-northeast in response to an approaching frontal boundary moving away from the Canadian Maritimes. No significant changes were made to the previous track forecast, and the new NHC forecast is based on a blend of the HCCA and GFEX consensus models. FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 36.2N 67.4W 35 KT 40 MPH 12H 03/0000Z 36.5N 65.4W 35 KT 40 MPH 24H 03/1200Z 36.5N 62.4W 30 KT 35 MPH 36H 04/0000Z 36.3N 59.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 04/1200Z 36.2N 58.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 05/0000Z 36.4N 56.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 05/1200Z 37.0N 55.8W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 06/1200Z...DISSIPATED $$ Forecaster Roberts

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Tropical Storm Nana Forecast Discussion Number 6

2020-09-02 16:45:54| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Wed Sep 02 2020 000 WTNT41 KNHC 021445 TCDAT1 Tropical Storm Nana Discussion Number 6 NWS National Hurricane Center Miami FL AL162020 1100 AM EDT Wed Sep 02 2020 Ongoing northerly shear has caused the satellite appearance of Nana to degrade a little since the last advisory, with the low-level center located near the northern edge of an asymmetric convective burst. Reports from an NOAA Hurricane Hunter aircraft indicate that the maximum winds remain near 50 kt and the central pressure is near 998 mb. The initial motion is westward or 270/15 kt. A low- to mid-level ridge to the north of the cyclone is forecast to keep steering Nana toward the west, or maybe just south of west, for the remainder of the cyclone's life. The new forecast track, which is changed only slightly from the previous forecast, calls for the cyclone to pass north of the Bay Islands in about 12 h, then make landfall over Belize in 18-24 h. After that, a generally westward motion is expected until the cyclone dissipates over southeastern Mexico. The latest indications from the large-scale models are that light northerly shear should persist over Nana until landfall. However, the guidance is in good agreement that strengthening should occur before landfall. The new intensity forecast follows this trend, however, it is a little above the upper end of the intensity guidance. While not explicitly shown in the forecast, it is expected that Nana will reach hurricane intensity about the time it makes landfall in Belize. After landfall, the cyclone should steadily weaken until it dissipates by the 60 h forecast time. Observations from the NOAA aircraft indicate that Nana remains a rather small tropical cyclone, with tropical-storm-force winds extending 60 n mi or less from the center. KEY MESSAGES: 1. Nana is expected to bring hurricane conditions and dangerous storm surge tonight to portions of the coast of Belize, and a hurricane warning is in effect. Preparations to protect life and property should be rushed to completion. 2. Tropical storm conditions are expected in the tropical storm warning areas in Belize, Guatemala, and the Yucatan Peninsula of Mexico by tonight. 3. Heavy rainfall with isolated maximum amounts as high as 8 to 12 inches could result in flash flooding in Belize, Guatemala, and portions of southeastern Mexico and the Yucatan Peninsula. FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 17.1N 84.6W 50 KT 60 MPH 12H 03/0000Z 17.0N 86.5W 60 KT 70 MPH 24H 03/1200Z 16.8N 88.9W 60 KT 70 MPH...INLAND 36H 04/0000Z 16.6N 91.0W 35 KT 40 MPH...INLAND 48H 04/1200Z 16.5N 93.3W 25 KT 30 MPH...POST-TROP/INLAND 60H 05/0000Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Nana Forecast Discussion Number 5

2020-09-02 10:41:00| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Wed Sep 02 2020 000 WTNT41 KNHC 020840 TCDAT1 Tropical Storm Nana Discussion Number 5 NWS National Hurricane Center Miami FL AL162020 500 AM EDT Wed Sep 02 2020 Nana's Central Dense Overcast has become a little better defined, with cloud tops to -80 deg C or colder. Upper-level outflow continues to be somewhat inhibited over the northern portion of the circulation. SFMR-observed surface winds and adjusted flight-level winds from an Air Force Hurricane Hunter aircraft indicated an intensity of 50 kt. Although northerly shear is likely to limit intensification, the environment is conducive enough for Nana to strengthen into a hurricane later today. The official intensity forecast is close to the Decay-SHIPS prediction based on the ECMWF global model forecast fields. Center fixes from the aircraft indicate that the motion continues to be generally westward, or 270/16 kt, to the south of a strong mid-level high pressure area. Over the next couple of days, the ridge is expected to be maintained or to build a little more to the west. This should keep Nana on a westward or slightly south-of-west track until it moves into Central America. The official track forecast is close to the previous one and also very close to the latest Florida State University Superensemble prediction. Observations from the aircraft and NOAA data buoy 42057 indicate that Nana is a rather small tropical cyclone, with tropical-storm-force winds extending 60 n mi or less from the center. KEY MESSAGES: 1. Tropical Storm Nana is forecast to be a hurricane as it approaches the coast, and interests in Honduras, Guatemala, Belize, and the Yucatan Peninsula should closely monitor the progress of this storm. Strong winds, dangerous storm surge and very heavy rainfall causing flash flooding are becoming more likely from Nana. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 17.0N 82.7W 50 KT 60 MPH 12H 02/1800Z 16.9N 85.0W 60 KT 70 MPH 24H 03/0600Z 16.8N 87.6W 65 KT 75 MPH 36H 03/1800Z 16.5N 89.8W 50 KT 60 MPH...INLAND 48H 04/0600Z 16.4N 92.1W 30 KT 35 MPH...INLAND 60H 04/1800Z 16.1N 93.9W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 05/0600Z...DISSIPATED $$ Forecaster Pasch

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Tropical Storm Omar Forecast Discussion Number 7

2020-09-02 10:39:53| National Hurricane Center (Atlantic)

Issued at 500 AM AST Wed Sep 02 2020 000 WTNT45 KNHC 020839 TCDAT5 Tropical Storm Omar Discussion Number 7 NWS National Hurricane Center Miami FL AL152020 500 AM AST Wed Sep 02 2020 UW-CIMSS analyses and SHIPS model diagnostics all indicate that Omar is being blasted by 40-50 kt of northwesterly shear, and yet a sizable area of deep convection has been hanging close to the low-level center for some time now. Based on last evening's ASCAT pass and the most recent satellite classifications, Omar's initial intensity remains 35 kt. Despite the shear magnitude, Omar could maintain tropical storm intensity for another 24 hours or so while the shear direction remains out of the northwest and the cyclone is over the warm Gulf Stream. During this period, the NHC forecast is close to the ICON intensity consensus and the HCCA aid. After 24 hours, Omar should begin moving over waters with lower ocean heat content, and the shear is forecast to veer more out of the north. Both of these factors are expected to cause weakening, with Omar likely to lose its deep convection and become a remnant low in about 2 days. Global models indicate that the remnant low should dissipate ahead of a cold front by day 4, which is now shown in the NHC forecast, although the ECMWF shows this happening a day earlier. Omar is moving toward the east-northeast (070/12 kt), embedded within the mid-latitude westerlies north of the subtropical ridge. This steering pattern is forecast to keep Omar on an east-northeastward or eastward path until it dissipates, but a breakdown of the zonal flow should cause the cyclone's forward speed to slow down gradually during the next 2-3 days. The updated NHC track forecast has been shifted southward a bit in the direction of the multi-model consensus aids and the GFS and ECMWF solutions, which both lie along the southern edge of the guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 36.2N 68.7W 35 KT 40 MPH 12H 02/1800Z 36.6N 66.8W 35 KT 40 MPH 24H 03/0600Z 36.8N 64.0W 35 KT 40 MPH 36H 03/1800Z 36.6N 61.3W 30 KT 35 MPH 48H 04/0600Z 36.3N 59.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 04/1800Z 36.5N 57.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 05/0600Z 37.1N 56.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 06/0600Z...DISSIPATED $$ Forecaster Berg

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