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Tropical Depression Twenty-six Forecast Discussion Number 3

2020-10-05 10:56:23| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Mon Oct 05 2020 088 WTNT41 KNHC 050856 TCDAT1 Tropical Depression Twenty-Six Discussion Number 3 NWS National Hurricane Center Miami FL AL262020 500 AM EDT Mon Oct 05 2020 Deep convection has been steadily improving in both vertical depth and structure, with the cloud pattern becoming more circular with upper-level outflow now having become established in all quadrants. However, there are still some indications in satellite imagery that the low-level and the mid-/upper-level circulations are not yet vertically aligned, with the low-level center still located just inside the northern edge of the convective cloud shield. For now, the initial intensity remains at 30 kt based on Dvorak satellite classifications from TAFB and SAB and also UW-CIMSS ADT. However, SATCON estimates suggest that the cyclone is close to tropical storm status. The initial motion estimate is an uncertain 290/08 kt. Even after maintaining some continuity with the previous forecast, the initial position had to be adjusted a little farther to the south and west based on satellite animation, and the current position may have to be adjusted farther south on the next forecast cycle due to possible redevelopment of the center into the deep convective cloud mass. Otherwise, the previous forecast track reasoning remains essentially unchanged. The cyclone is expected to move west-northwestward to northwestward along the southern side of a deep-layer ridge for the next few days. By day 4 and beyond, a broad mid- to upper-level trough is forecast to develop across northern Mexico and Texas, which is expected to create a break in the ridge and turn the cyclone northward toward the north-central Gulf coast. The steering flow pattern becomes a little complex on days 2-3 due to expected binary interaction with Tropical Storm Gamma or its remnants, which could result in a sharp westward jog, after which a sharp turn back toward the northwest could occur. However, the latest NHC model guidance is in fairly good agreement that the cyclone will make a sharp northward turn between 90W-92W longitude around 96 hours or so. Thereafter, acceleration toward the north-northeast or northeast ahead of the approaching trough and frontal system is anticipated. The new NHC forecast track is a little to the left of the previous track through 72 hours, mainly to account for the more westward initial position, and lies down the middle of the tightly packed model guidance envelope. The northeasterly deep-layer vertical wind shear that has been plaguing the cyclone is finally showing signs of abating. The GFS- and ECMWF-based SHIPS model guidance shows the shear decreasing to near zero in the 24-48 hour period, which allow for some robust intensification to occur, assuming that the inner-core wind field becomes better defined later today. By 96 hours and beyond, the SHIPS models are forecasting the shear to increase 20-30 kt from the southwest, which would be expected to induce rapid weakening. However, the SHIPS models appear to be creating too much shear over the cyclone's center by incorporating jetstream winds of about 60 kt across Texas, whereas the 200-mb model fields only show winds of 10-15 kt over the center by 96 hours. As a result, the new intensity forecast calls for the cyclone to reach its peak intensity in the 72-96 hour period, followed by weakening due to likely cold upwelling of shallow cool shelf waters offshore the southwest coast of Louisiana and Mississippi. The new NHC intensity forecast is similar to the previous advisory, and lies near the upper end of the intensity guidance, similar to the corrected-consensus model HCCA. Users are reminded that the average 4- and 5-day NHC track forecast errors are about 160 to 200 miles at those time periods, respectively. Key Messages: 1. Tropical storm conditions are expected in the Cayman Islands beginning late today, and a Tropical Storm Warning is in effect. 2. Dangerous storm surge and hurricane conditions are possible in portions of western Cuba and the Isle of Youth by Tuesday afternoon, and a Hurricane Watch is in effect. 3. Heavy rainfall will affect portions of Jamaica, the Cayman Islands, and western Cuba during the next few days. This rainfall could lead to significant flash flooding and mudslides. 4. The system is forecast to approach the northern Gulf Coast late this week as a hurricane. While there is large uncertainty in the track and intensity forecasts at these time ranges, there is a risk of dangerous storm surge, wind, and rainfall hazards along the coast from Louisiana to the western Florida Panhandle. Residents in these areas should monitor the progress of the system and check for updates to the forecast during the week. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 17.0N 78.2W 30 KT 35 MPH 12H 05/1800Z 17.4N 79.1W 35 KT 40 MPH 24H 06/0600Z 18.6N 80.8W 45 KT 50 MPH 36H 06/1800Z 20.5N 83.3W 60 KT 70 MPH 48H 07/0600Z 22.5N 85.8W 70 KT 80 MPH 60H 07/1800Z 24.4N 88.1W 80 KT 90 MPH 72H 08/0600Z 25.9N 90.0W 90 KT 105 MPH 96H 09/0600Z 28.0N 91.0W 85 KT 100 MPH 120H 10/0600Z 32.4N 88.7W 55 KT 65 MPH...INLAND $$ Forecaster Stewart

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Tropical Storm Gamma Forecast Discussion Number 12

2020-10-05 10:50:29| National Hurricane Center (Atlantic)

Issued at 400 AM CDT Mon Oct 05 2020 000 WTNT45 KNHC 050850 TCDAT5 Tropical Storm Gamma Discussion Number 12 NWS National Hurricane Center Miami FL AL252020 400 AM CDT Mon Oct 05 2020 Gamma is a strongly sheared tropical storm that has been meandering just north of the northern coast of the Yucatan Peninsula of Mexico. Strong convective cloud tops colder than -80 deg C are located more than 60 nmi north and northeast of the well-defined and fully exposed low-level circulation center. An Air Force Reserve reconnaissance aircraft investigating the cyclone found maximum 850-mb flight-level winds of only 44 kt along with peak SFMR surface winds of 47 kt. In addition, a 0329Z ASCAT-A pass revealed peak surface winds of 41 kt. Allowing for undersampling by the scatterometer instrument and blending it with the SFMR winds supports lowering the initial intensity to 45 kt. The central pressure of 999 mb is based on a center dropsonde reports of 1000 mb and 10-12 kt winds. The initial motion estimate is an uncertain 240/02 kt. Gamma's future track remains highly uncertain after day 2. For the next 48 hours or so, the cyclone is forecast to move slowly southwestward to west-southwestward and weaken due to strong southerly vertical wind shear. Thereafter, TD-26 to the east is forecast to strengthen into a hurricane, accompanied by a corresponding increase in the size of that cyclone's circulation, which is expected to result in some binary interaction between the two tropical cyclones. Due to Gamma's expected smaller size on days 3 and 4, the cyclone is forecast to be pulled eastward and then northward, making landfall on and then moving over the northwestern Yucatan Peninsula. By day 5, Gamma is expected to degenerate into a remnant low and then be absorbed by the larger circulation of the expected central Gulf of Mexico hurricane that is currently TD-26. The new NHC track forecast is similar to but a little slower than the previous advisory track, and lies close to the southern edge of the guidance envelope. Strong southerly wind shear, combined with drier and more stable air just to the west of Gamma, is expected to weaken the cyclone further over the next few days. If land interaction occurs sooner than expected, then the cyclone could weaken and dissipate sooner than currently forecast. For now, the intensity forecast maintains continuity with the previous advisory and shows dissipation by 120 hours as Gamma interacts with what is expected to be a significant hurricane over the central Gulf of Mexico by 96 hours, which could end up absorbing the smaller Gamma tropical cyclone. Due to the uncertainty in the forecast at this time, future advisories may feature larger-than-normal changes to the track or intensity forecast. It is also worth noting that several dynamical model trackers appear to lose Gamma and jump to tracking Tropical Depression Twenty-Six when it reaches the Gulf of Mexico, resulting in unrealistic depictions of the model intensity and track forecasts. KEY MESSAGES: 1. Gamma is expected to produce heavy rainfall through midweek over portions of southeast Mexico, including the Mexican states of Yucatan, Campeche, and Tabasco. This rainfall could result in significant flash flooding. 2. Even though Gamma is just offshore, tropical storm conditions are likely along portions of the northern coast of the Yucatan Peninsula later today. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 22.7N 87.5W 45 KT 50 MPH 12H 05/1800Z 22.5N 88.0W 40 KT 45 MPH 24H 06/0600Z 22.1N 88.9W 40 KT 45 MPH 36H 06/1800Z 21.4N 89.8W 35 KT 40 MPH 48H 07/0600Z 20.8N 90.4W 35 KT 40 MPH 60H 07/1800Z 20.3N 90.5W 30 KT 35 MPH...INLAND YUCATAN 72H 08/0600Z 20.7N 90.1W 30 KT 35 MPH...INLAND YUCATAN 96H 09/0600Z 22.5N 90.0W 30 KT 35 MPH...OVER WATER 120H 10/0600Z...DISSIPATED $$ Forecaster Stewart

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Tropical Storm Marie Forecast Discussion Number 25

2020-10-05 10:32:24| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Mon Oct 05 2020 000 WTPZ43 KNHC 050832 TCDEP3 Tropical Storm Marie Discussion Number 25 NWS National Hurricane Center Miami FL EP182020 200 AM PDT Mon Oct 05 2020 Satellite images show the exposed low-level circulation well to the southwest of any remaining deep convection. ASCAT data, however, came in at 50-55 kt just before 06Z, which is higher than the satellite presentation alone would suggest. The initial wind speed is set to 55 kt on the basis of that data. Marie should weaken during the next few days due to cold waters and strong shear. Models are in very good agreement on a steady weakening, and little change was made to the previous forecast. Remnant low status is expected by 48 hours, and could even happen sooner based on current trends. The storm is moving west-northwestward at about 8 kt this morning. A ridge to the north should steer Marie to the northwest or west-northwest for the next couple of days, then the tropical cyclone is likely to turn westward as it becomes a more shallow feature. Model guidance has generally been adjusting to this scenario, with a mid-latitude trough no longer expected to exert much influence. The new forecast is shifted southward from 48 hours and beyond, near but a little slower than the model consensus. FORECAST POSITIONS AND MAX WINDS INIT 05/0900Z 20.8N 131.1W 55 KT 65 MPH 12H 05/1800Z 21.2N 132.2W 45 KT 50 MPH 24H 06/0600Z 21.8N 133.6W 35 KT 40 MPH 36H 06/1800Z 22.4N 135.2W 30 KT 35 MPH 48H 07/0600Z 22.9N 136.5W 30 KT 35 MPH...POST-TROP/REMNT LOW 60H 07/1800Z 23.2N 137.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 08/0600Z 23.2N 138.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 09/0600Z 23.0N 140.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 10/0600Z...DISSIPATED $$ Forecaster Blake

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Tropical Storm Gamma Forecast Discussion Number 11

2020-10-05 04:39:22| National Hurricane Center (Atlantic)

Issued at 1000 PM CDT Sun Oct 04 2020 000 WTNT45 KNHC 050239 TCDAT5 Tropical Storm Gamma Discussion Number 11 NWS National Hurricane Center Miami FL AL252020 1000 PM CDT Sun Oct 04 2020 Gamma has moved much farther northeast than previously anticipated. With little in the way of low or mid-level large-scale steering flow currently in place, it seems likely that upper-level southwesterlies, also responsible for the shear affecting the tropical storm, are causing this recent northeastward motion. This may be either through reformations of the center closer to the convection or a direct contribution to the net steering. An Air Force Reserve Hurricane Hunter aircraft is currently investigating Gamma and so far has found max SFMR winds of 47 kt but flight level winds of only 45 kt. Given the data so far, there is no indication that Gamma is any stronger than the 50 kt intensity, but it seems prudent to let the plane finish its pattern before lowering the winds at this time. Gamma's future track is highly uncertain. The track guidance spread is much higher than usual, and confidence in the forecast is accordingly low. Most of the global models indicate that Gamma will soon begin moving generally west-southwestward as a mid-level ridge briefly builds over the eastern and central Gulf of Mexico. Unfortunately, there is little agreement on exactly when this will happen or how fast Gamma will move once it turns. After about 48 h, Gamma could interact with Tropical Depression Twenty-Six, which is forecast to move over the northwestern Caribbean Sea at that time. Current dynamical models are notoriously bad at forecasting such interactions, but if the two systems do interact it will likely cause Gamma to move inland over the Yucatan. Whether such an interaction occurs or not, continued shear and proximity to land should cause the tropical storm to gradually weaken, and Gamma could dissipate if it moves over land (and stays there) in a few days, as shown by the GFS. The ECMWF shows the vortex remaining over water but dissipating nonetheless, while a couple other global models maintain the vortex longer and show it moving north over the central Gulf of Mexico late in the forecast period. The NHC forecast is a compromise between those solutions, showing a remnant low moving north over the southern Gulf of Mexico by day 5. In general, the NHC intensity forecast is similar to the previous one, but slightly lower to account for the system potentially moving inland. Due to the uncertainty in the forecast at this time, future advisories may feature larger than normal changes to the track or intensity forecast. It is also worth noting that several dynamical model trackers appear to lose Gamma and jump to tracking Tropical Depression Twenty-Six when it reaches the Gulf of Mexico, resulting in unrealistic depictions of the intensity and track forecasts. KEY MESSAGES: 1. Gamma is expected to produce heavy rainfall for several days over portions of southeastern Mexico, the Yucatan Peninsula, Central America, and far western Cuba. This rainfall could result in life-threatening flash flooding and mudslides, particularly in the mountainous regions of southeastern Mexico and Central America. 2. Even though Gamma is just offshore, tropical storm conditions are likely along portions of the northern coast of the Yucatan Peninsula on Monday. FORECAST POSITIONS AND MAX WINDS INIT 05/0300Z 22.8N 87.2W 50 KT 60 MPH 12H 05/1200Z 22.7N 87.7W 45 KT 50 MPH 24H 06/0000Z 22.3N 88.5W 45 KT 50 MPH 36H 06/1200Z 21.8N 89.5W 40 KT 45 MPH 48H 07/0000Z 21.3N 90.3W 40 KT 45 MPH 60H 07/1200Z 20.3N 90.7W 35 KT 40 MPH 72H 08/0000Z 20.2N 90.1W 30 KT 35 MPH...INLAND 96H 09/0000Z 22.0N 90.7W 30 KT 35 MPH...OVER WATER 120H 10/0000Z 24.0N 91.0W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Zelinsky

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Tropical Depression Twenty-Six Forecast Discussion Number 2

2020-10-05 04:34:49| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Sun Oct 04 2020 000 WTNT41 KNHC 050234 TCDAT1 Tropical Depression Twenty-Six Discussion Number 2 NWS National Hurricane Center Miami FL AL262020 1100 PM EDT Sun Oct 04 2020 Satellite data indicate that the disturbance just south of Jamaica has become better organized this evening. Microwave and shortwave infrared images indicate that the center is now well defined, and deep convection has been persisting near and to the south of the center. Based on this data, the system now meets the criteria to be considered a tropical depression. The initial intensity remains 30 kt for this advisory. The depression is currently moving west-northwestward at 8 kt on the southern side of an Atlantic subtropical ridge. A continued west-northwest to northwest motion at about the same forward speed is expected during the next 24 to 36 hours, taking the system across the Cayman Islands and toward western Cuba. Around the time the depression is expected to be near western Cuba, the models show it accelerating northwestward as it moves in the faster flow between the ridge and Tropical Storm Gamma. This motion should bring the tropical cyclone into the southern Gulf of Mexico Tuesday night or early Wednesday. Later on, a notable slow down and a turn to the north is forecast to occur late in the week when the depression will likely be approaching the northern Gulf coast. This change in the forecast motion is a result of the ridge weakening and a trough approaching the cyclone from the west. The models are in relatively good agreement, which is surprising since they often diverge for weak systems, and the NHC track forecast lies near the middle of the guidance envelope. There is currently some northeasterly shear affecting the depression, but the models all show the shear lessening by tomorrow and remaining fairly light for the next few days. These improving upper-level wind conditions combined with a moist air mass and warm waters should allow for at least steady strengthening during the next few days. By late in the week, when the storm is forecast to approach the U.S. Gulf coast, there could be an increase in southerly or southwesterly shear, which could limit additional strengthening by that time. The NHC intensity forecast lies fairly close to the IVCN and HCCA consensus aids, and shows the depression becoming a hurricane near western Cuba with additional strengthening over the Gulf of Mexico. Users are reminded that the average 4- and 5-day NHC track forecast errors are about 160 to 200 miles at those time periods, respectively. Key Messages: 1. Tropical storm conditions are expected in the Cayman Islands beginning late Monday, and a Tropical Storm Warning is in effect. 2. Dangerous storm surge and hurricane conditions are possible in portions of western Cuba and the Isle of Youth by Tuesday afternoon, and a Hurricane Watch is in effect. 3. Heavy rainfall will affect portions of Hispaniola, Jamaica, the Cayman Islands, and western Cuba during the next few days and could lead to life-threatening flash floods and mudslides. 4. The system is forecast to approach the northern Gulf Coast late this week as a hurricane. While there is large uncertainty in the track and intensity forecasts at these time ranges, there is a risk of dangerous storm surge, wind, and rainfall hazards along the coast from Louisiana to the western Florida Panhandle. Residents in these areas should monitor the progress of the system and check for updates to the forecast during the week. FORECAST POSITIONS AND MAX WINDS INIT 05/0300Z 17.0N 77.3W 30 KT 35 MPH 12H 05/1200Z 17.4N 78.3W 35 KT 40 MPH 24H 06/0000Z 18.3N 79.8W 40 KT 45 MPH 36H 06/1200Z 20.0N 81.9W 50 KT 60 MPH 48H 07/0000Z 22.2N 84.4W 65 KT 75 MPH 60H 07/1200Z 24.1N 87.0W 80 KT 90 MPH 72H 08/0000Z 25.4N 88.8W 85 KT 100 MPH 96H 09/0000Z 27.2N 90.4W 90 KT 105 MPH 120H 10/0000Z 30.0N 89.9W 75 KT 85 MPH...INLAND $$ Forecaster Cangialosi

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