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Tropical Storm Nana Forecast Discussion Number 1

2020-09-01 17:00:13| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Tue Sep 01 2020 000 WTNT41 KNHC 011500 TCDAT1 Potential Tropical Cyclone Sixteen Discussion Number 1 NWS National Hurricane Center Miami FL AL162020 1100 AM EDT Tue Sep 01 2020 An Air Force Reserve reconnaissance aircraft is currently investigating the vigorous tropical disturbance located south of Jamaica that the NHC has been tracking the past few days across the Caribbean Sea. An earlier report of 35 kt winds this morning from ship MAOR4 is the basis for the 35-kt initial intensity on Potential Tropical Cyclone Sixteen. (PTC-16) The initial motion estimate is an uncertain 285/16 kt, based mainly on past scatterometer surface wind data and passive microwave satellite fix data. A strong deep-layer to the north of the system is expected to build slowly westward over the next few days, keeping the disturbance moving in a general west-northwestward to westward direction through 48 hours, followed by a turn toward the west-southwest shortly thereafter when the system will be inland over Central America. The NHC official forecast track lies close to a blend of the simple consensus models TVCN and GFEX, and the corrected-consensus model HCCA. The disturbance has already developed an upper-level anticyclone, with outflow only restricted in the northeastern quadrant due to northeasterly shear of about 15 kt. This modest northeasterly to easterly vertical wind shear is expected to gradually subside to less than 10 kt by 48 hours, which should allow for at least some slow but steady strengthening, given that sea-surface temperatures (SST) will be 29.5-30.5 deg C along the track. Occasional intrusions of dry mid-level air might hinder development for the next 24-36 hours, but model guidance shows that the atmosphere is expected to moisten thereafter and right up until landfall. The official intensity forecast is similar to the consensus of the intensity guidance through 24 hours, and then is a little above the consensus at 36 h and 48 h due to the very warm waters, moistening atmosphere, and low shear conditions. The statistical SHIPS guidance suggests that the cyclone could reach hurricane strength just prior to landfall. This alternate scenario will be evaluated on the next advisory cycle. Due to the system's expected proximity to the north coast of Honduras and Roatan Island, a Tropical Storm Watch has been issued for those areas. Additional watches and possible warnings will likely be required later today for Guatemala, Belize, and the southern portion of the Yucatan Peninsula. FORECAST POSITIONS AND MAX WINDS INIT 01/1500Z 16.1N 77.5W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 02/0000Z 16.4N 79.6W 35 KT 40 MPH...TROPICAL STORM 24H 02/1200Z 16.6N 82.3W 40 KT 45 MPH 36H 03/0000Z 16.7N 85.1W 45 KT 50 MPH 48H 03/1200Z 16.8N 87.4W 50 KT 60 MPH 60H 04/0000Z 16.5N 89.6W 25 KT 30 MPH...INLAND 72H 04/1200Z 15.7N 91.7W 15 KT 15 MPH...POST-TROP/REMNT LOW 96H 05/1200Z...DISSIPATED $$ Forecaster Stewart

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Tropical Depression Fifteen Forecast Discussion Number 4

2020-09-01 16:41:41| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Tue Sep 01 2020 000 WTNT45 KNHC 011441 TCDAT5 Tropical Depression Fifteen Discussion Number 4 NWS National Hurricane Center Miami FL AL152020 1100 AM EDT Tue Sep 01 2020 The depression has become better organized this morning with the center embedded in the western side of a growing area of deep convection. An earlier buoy report recorded adjusted maximum winds of 32 kt, and roughly half of the estimates show that the depression is a tropical storm. Conservatively, the winds will be held at 30 kt until we will see what the scatterometer shows this afternoon. Increasing shear is expected to limit any significant strengthening, although it does seem more likely than not that this system will barely make it to a tropical storm later today. The shear is forecast to get quite strong by tomorrow afternoon, so weakening is anticipated by then, and persistence of that shear is expected to cause the cyclone to decay into a remnant low in about 48 hours. While the initial part of the forecast is a little higher than the last one, this change is really in the noise level for intensity prediction and is only noticeable because of the tropical storm threshold. The initial motion has turned toward the east-northeast or 060/12 kt. There is reasonable agreement among the guidance that this motion will continue through tomorrow, and then become eastward during the next 2-3 days around the northern side of the subtropical ridge, followed by a northeastward turn by day 4. Similar to the last forecast, most of the uncertainty is with the speed, rather than the direction of the cyclone. The new forecast is closer to the faster guidance and the previous NHC track prediction. Dissipation is shown at 120h, although a few models have it occuring as early as days 2-3. FORECAST POSITIONS AND MAX WINDS INIT 01/1500Z 34.7N 73.1W 30 KT 35 MPH 12H 02/0000Z 35.6N 71.1W 35 KT 40 MPH 24H 02/1200Z 36.5N 68.3W 35 KT 40 MPH 36H 03/0000Z 37.2N 65.1W 30 KT 35 MPH 48H 03/1200Z 37.3N 62.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 04/0000Z 37.0N 59.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 04/1200Z 37.0N 57.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 05/1200Z 39.5N 52.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 06/1200Z...DISSIPATED $$ Forecaster Blake

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Tropical Depression Fifteen Forecast Discussion Number 3

2020-09-01 10:35:31| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Tue Sep 01 2020 024 WTNT45 KNHC 010835 TCDAT5 Tropical Depression Fifteen Discussion Number 3 NWS National Hurricane Center Miami FL AL152020 500 AM EDT Tue Sep 01 2020 Deep convection remains displaced to the east and southeast of the depression's low-level center due to increasing west-northwesterly shear. Maximum winds are still estimated to be 30 kt, with no appreciable structural changes having occurred since last evening's ASCAT pass, which showed 25-30 kt winds. The initial motion is toward the northeast, or 050/11 kt, with the depression becoming embedded in the mid-latitude westerlies north of the subtropical ridge. Forecast guidance agrees that the cyclone will turn east-northeastward and then eastward during the next 2-3 days, moving farther out into the Atlantic, although there is greater-than-normal spread among the track models mainly due to speed differences. In particular, the ECMWF is one of the fastest models, while the GFS and UKMET hang the cyclone back to the west a little longer. Since many of the consensus aids, including HCCA, are closer to the faster model solutions, the NHC track forecast leans toward the quicker side of things. UW-CIMSS analyses indicate that the shear has increased to 20-25 kt, and it's only expected to get stronger from here. Some slight strengthening of the depression can't be ruled out during the next 12-24 hours while the shear vector is aligned with the storm motion, but by this time tomorrow the shear is likely to be as strong as 35-45 kt out of the northwest. Therefore, little change in strength is now indicated in the official forecast for the next day or two. The shear magnitude is expected to peak in about 48 hours, and the depression is forecast to be a remnant low at that point, if it hasn't become one already. The GFS dissipates the low in about 48 hours, but since the other models hang onto it a little bit longer, the NHC forecast shows dissipation by day 5. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 34.1N 74.4W 30 KT 35 MPH 12H 01/1800Z 35.0N 72.5W 30 KT 35 MPH 24H 02/0600Z 36.0N 69.7W 30 KT 35 MPH 36H 02/1800Z 36.9N 67.0W 30 KT 35 MPH 48H 03/0600Z 37.5N 63.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 03/1800Z 37.7N 61.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 04/0600Z 37.8N 58.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 05/0600Z 39.2N 53.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 06/0600Z...DISSIPATED $$ Forecaster Berg

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Tropical Depression Fifteen Forecast Discussion Number 2

2020-09-01 04:39:36| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Mon Aug 31 2020 000 WTNT45 KNHC 010239 TCDAT5 Tropical Depression Fifteen Discussion Number 2 NWS National Hurricane Center Miami FL AL152020 1100 PM EDT Mon Aug 31 2020 Tropical Depression Fifteen is a sheared tropical cyclone, with the edge of the deep convection displaced nearly 30 n mi to the east of the exposed low-level center. An Air Force Reserve Hurricane Hunter mission earlier this evening provided SFMR instrument measurements of nearly 30 kt in the southeast quadrant. And, a recent ASCAT overpass showed several wind vectors in that same general area between 25-30 kt. Therefore, the initial intensity is being held at 30 kt. The depression is moving northeast at 12 kt around the northwestern periphery of the subtropical ridge. A turn to the east-northeast is expected to occur on Tuesday as the cyclone moves along the northern periphery of the ridge. Late in the forecast period, what is left of the cyclone is forecast to get caught in the flow between building high pressure to its northeast, and an approaching trough to its northwest, which should induce a more northerly component of motion. The latest NHC forecast is little changed from the previous one, and is near the tightly clustered track guidance. The cyclone has moved into an environment with moderate westerly shear of about 20 kt, as indicated by the latest UW-CIMSS deep-layer shear analysis product. The SHIPS guidance by both the GFS and ECMWF indicate that the shear magnitude will more than double between 18 h and 36 h from now. So it is anticipated that the only window of time for the depression to strengthen will be tonight through tomorrow, while the system is over warm waters and in a moist atmospheric environment. After 48 h, when the vertical wind shear is forecast to be between 40 and 50 kt, the cyclone is forecast to weaken, and by 96 h the NHC forecast now shows the system degenerating into a remnant low. Exactly what transpires with the cyclone later in the forecast period remains uncertain, as some of the global models suggest that the cyclone will open into a trough in a few days, while others indicate that the system could be absorbed by a larger mid-latitude trough around day 5. Regardless of what occurs by those time frames, the models all suggest that the system should not gain much more strength than it currently has. The official NHC intensity forecast is in good agreement with the various intensity consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 01/0300Z 33.2N 75.7W 30 KT 35 MPH 12H 01/1200Z 34.2N 74.0W 35 KT 40 MPH 24H 02/0000Z 35.1N 71.2W 35 KT 40 MPH 36H 02/1200Z 36.0N 68.6W 35 KT 40 MPH 48H 03/0000Z 36.9N 65.9W 35 KT 40 MPH 60H 03/1200Z 37.7N 63.3W 30 KT 35 MPH 72H 04/0000Z 38.1N 60.5W 30 KT 35 MPH 96H 05/0000Z 38.7N 55.8W 30 KT 35 MPH...POST-TROP/REMNT LOW 120H 06/0000Z 41.6N 50.4W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Latto

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Tropical Depression Fifteen Forecast Discussion Number 1

2020-08-31 22:44:31| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Mon Aug 31 2020 000 WTNT45 KNHC 312044 TCDAT5 Tropical Depression Fifteen Discussion Number 1 NWS National Hurricane Center Miami FL AL152020 500 PM EDT Mon Aug 31 2020 Satellite images indicate that the area of low pressure offshore of the Carolinas has had convection organized in bands since before dawn, and scatterometer plus an Air Force Reserve Hurricane Hunter mission data confirm that the circulation is closed. Thus, this is now a tropical depression, and the initial wind speed is set to 30 kt in accordance with 25-30 kt ASCAT-A data plus buoy 42001 readings which earlier had an adjusted 10-m peak of 30 kt. The depression is moving northeastward at about 10 kt. The system should gradually turn toward the east-northeast by Wednesday due to it moving around the northwest side of the subtropical ridge, then move eastward in a few days around the flat ridge. By late week, the cyclone could slow and eventually turn back toward the northeast around a rather strong mid-latitude high pressure system over the northeast Atlantic. There is considerable spread in the guidance, which really seems to depend upon whether the system stays coherent, like the official forecast, or would become a shallow low-level swirl by 120h and end up slower and south of forecast track. This forecast is near the corrected-consensus guidance, leaning toward the ECMWF-based models, and it should be considered of low confidence. Gradual strengthening is expected over the next day or so while the depression remains in a low-to-moderate shear environment. Although the depression is expected to be traversing the warm Gulf Stream for the next several days, wind shear is expected to greatly increase by Wednesday, which should limit intensification. In fact there's some chance the system could decay and lose any deep convection in rather strong shear in a few days. However, since it likely will be moving near the Gulf Stream, I suspect it will continue to pulse thunderstorm activity and stay alive throughout the period. The NHC intensity forecast is near or just above the model consensus on that reasoning, closest to the HWRF model. The cyclone could become extratropical (or a remnant low) by the end of the forecast, but this is very uncertain. FORECAST POSITIONS AND MAX WINDS INIT 31/2100Z 32.6N 76.5W 30 KT 35 MPH 12H 01/0600Z 33.8N 75.0W 35 KT 40 MPH 24H 01/1800Z 34.9N 72.4W 40 KT 45 MPH 36H 02/0600Z 35.8N 69.7W 40 KT 45 MPH 48H 02/1800Z 36.7N 67.0W 40 KT 45 MPH 60H 03/0600Z 37.6N 63.7W 35 KT 40 MPH 72H 03/1800Z 38.2N 60.5W 35 KT 40 MPH 96H 04/1800Z 39.0N 55.0W 35 KT 40 MPH 120H 05/1800Z 41.5N 49.5W 35 KT 40 MPH...POST-TROP/EXTRATROP $$ Forecaster Blake

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