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Tropical Depression Seventeen Forecast Discussion Number 2

2020-09-07 10:37:11| National Hurricane Center (Atlantic)

Issued at 500 AM AST Mon Sep 07 2020 000 WTNT42 KNHC 070837 TCDAT2 Tropical Depression Seventeen Discussion Number 2 NWS National Hurricane Center Miami FL AL172020 500 AM AST Mon Sep 07 2020 Tropical Depression Seventeen is a large tropical cyclone, with its circulation evident several hundred miles from its center. An area of deep convection has developed tonight just over and to the east of its center with cloud top temperatures as low as -80 degrees C. However, there appears to be some dry air in the northwestern portion of the cyclone's circulation limiting convection there. The latest Dvorak satellite intensity estimates from TAFB and UW-CIMSS support keeping the initial intensity at 30 kt. The depression has been moving west-northwestward at about 5 kt over the past 12 hours. This general motion is expected to continue throughout the 5-day forecast period, with some increase in forward speed over the next couple of days as the cyclone is steered by a weak mid-level ridge to its north. The overall guidance shifted slightly to the north through day 3 and the latest NHC forecast lies in between the previous one and the consensus track guidance through this time period. For days 3 through 5, the forecast track is little changed from the previous one and is in good agreement with the fairly tightly clustered track guidance. Other than some dry air in the vicinity of the depression, the overall environment looks favorable for strengthening over the next few days. However, due to the large size of the circulation, strengthening may occur more slowly than what might be expected from a more compact cyclone. Therefore, only gradual strengthening is indicated in the official forecast during that time. By 72 h, increasing vertical wind shear should limit further intensification, and may cause some weakening to occur. The latest NHC intensity forecast is a blend of the previous one and the HFIP corrected consensus, HCCA. FORECAST POSITIONS AND MAX WINDS INIT 07/0900Z 17.3N 42.1W 30 KT 35 MPH 12H 07/1800Z 17.5N 42.6W 35 KT 40 MPH 24H 08/0600Z 17.8N 43.2W 35 KT 40 MPH 36H 08/1800Z 18.3N 44.1W 40 KT 45 MPH 48H 09/0600Z 19.1N 45.6W 45 KT 50 MPH 60H 09/1800Z 19.7N 47.1W 50 KT 60 MPH 72H 10/0600Z 20.1N 49.0W 50 KT 60 MPH 96H 11/0600Z 20.6N 52.7W 45 KT 50 MPH 120H 12/0600Z 21.7N 55.5W 45 KT 50 MPH $$ Forecaster Latto

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Tropical Depression Julio Forecast Discussion Number 6

2020-09-07 04:41:37| National Hurricane Center (East Pacific)

Issued at 900 PM MDT Sun Sep 06 2020 595 WTPZ45 KNHC 070241 TCDEP5 Tropical Depression Julio Discussion Number 6 NWS National Hurricane Center Miami FL EP152020 900 PM MDT Sun Sep 06 2020 Julio is losing organization. A small area of deep convection remains near the assumed center, which is best identified by SSMIS microwave data a few hours ago. However, Socorro Island, located about 30 miles south of the center, never showed any westerly winds. While it is possible the system has already opened up into a trough, advisories are being continued on the assumption that the island missed the small system's circulation. The initial wind speed is set to 30 kt due to the decay in overall satellite presentation. The depression is moving westward at a slower speed than before, roughly 11 kt. This motion is expected tomorrow at a slower speed as a ridge breaks down to the north of the cyclone. Afterwards, since the tropical cyclone is so small, a larger surface low to the southwest of the system seems likely to weaken Julio and then absorb the tropical cyclone in a couple of days. Thus the track forecast is adjusted south of the previous one, and the new intensity forecast is reduced as well. A plausible alternative scenario is that overnight scatterometer data will show that the system has already degenerated into a surface trough. FORECAST POSITIONS AND MAX WINDS INIT 07/0300Z 19.4N 111.3W 30 KT 35 MPH 12H 07/1200Z 19.5N 113.2W 30 KT 35 MPH 24H 08/0000Z 19.5N 114.7W 30 KT 35 MPH 36H 08/1200Z 19.3N 115.5W 25 KT 30 MPH 48H 09/0000Z...DISSIPATED $$ Forecaster Blake

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Tropical Depression Seventeen Forecast Discussion Number 1

2020-09-07 04:37:38| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sun Sep 06 2020 306 WTNT42 KNHC 070237 TCDAT2 Tropical Depression Seventeen Discussion Number 1 NWS National Hurricane Center Miami FL AL172020 1100 PM AST Sun Sep 06 2020 The tropical low that the National Hurricane Center has been tracking the past few days west of the Cabo Verde Islands has developed enough convective banding for the system to be classified as a tropical depression. A 06/2345Z ASCAT-B scatterometer pass indicated surface wind speeds of 30-31 kt north of the center, which are supported by a T2.0/30 kt Dvorak satellite intensity estimate from TAFB at 07/0000 UTC. The initial motion estimate is an uncertain 270/04kt. The center has been jumping around somewhat due to small, brief convective bursts that then dissipate, leaving small swirls rotating around the mean circulation center. However, the latest ASCAT data showed a much better defined overall circulation, especially in the inner core wind field region, so the forward motion should stabilize fairly soon. The depression is expected to remain caught in weak steering flow for the next 5 days, so only a slow westward motion is expected tonight and Monday, followed by a west-northwestward motion by later Monday and Tuesday that will continue through the remainder of the 120-h forecast period. Possible track forecast complications could develop by days 3-5 if binary interaction occurs between the depression and the large low (Invest 93L) currently located southeast of the Cabo Verde Islands, resulting in TD-17 not gaining as much latitude as currently indicated. The NHC forecast track lies down the middle of the simple- and corrected-consensus model guidance envelope. The depression is only forecast to slowly strengthen for the next 4 days or so due to expected intrusions of dry mid-level air. By day 5, increasing southerly vertical wind shear on the east side of a large upper-level low that is forecast to cut off north of Puerto Rico is expected to induce gradual weakening. However, both the timing of the development of the low and its west-southwestward motion will determine if sufficient shear will prevent further strengthening after 96 h; a slower formation of the low and/or a slower retrograding motion would result in at least less weakening. The NHC intensity forecast closely follows an average of the IVCN and NOAA-HCCA intensity consensus models. FORECAST POSITIONS AND MAX WINDS INIT 07/0300Z 17.0N 41.5W 30 KT 35 MPH 12H 07/1200Z 17.0N 42.1W 30 KT 35 MPH 24H 08/0000Z 17.2N 42.7W 35 KT 40 MPH 36H 08/1200Z 17.5N 43.4W 40 KT 45 MPH 48H 09/0000Z 18.0N 44.6W 45 KT 50 MPH 60H 09/1200Z 18.6N 46.3W 50 KT 60 MPH 72H 10/0000Z 19.2N 48.2W 55 KT 65 MPH 96H 11/0000Z 20.4N 51.9W 50 KT 60 MPH 120H 12/0000Z 21.2N 55.5W 45 KT 50 MPH $$ Forecaster Stewart

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Tropical Storm Julio Forecast Discussion Number 5

2020-09-06 22:35:06| National Hurricane Center (East Pacific)

Issued at 300 PM MDT Sun Sep 06 2020 000 WTPZ45 KNHC 062034 TCDEP5 Tropical Storm Julio Discussion Number 5 NWS National Hurricane Center Miami FL EP152020 300 PM MDT Sun Sep 06 2020 Julio remains a compact tropical storm that is producing a small area of deep convection near and to the west of the center. A recent ASCAT-A overpass showed maximum winds around 30 kt, which was lower than in previous passes. Based on that data and the latest Dvorak classifications, the initial intensity is lowered to 35 kt. The ASCAT data also suggest that the circulation of Julio is not as well defined on the south side as it was earlier. As mentioned in the previous discussion, Julio's future is somewhat unclear. The ECMWF and UKMET models show Julio becoming absorbed by a larger low just to its southwest in a couple of days. Conversely, the GFS, HWRF, and HMON models show Julio being the dominant feature, with the latter two aids even showing strengthening. Since confidence is low on which scenario will play out, it seems best to hold continuity for now, which ends up leaning closer to the ECMWF/UKMET solutions. The NHC intensity forecast is a little lower than the previous one given the lower initial intensity. The tropical storm is moving west-northwestward at about 17 kt. Julio should continue to move westward to west-northwestward at a fairly quick pace for another 12-24 hours. However, after that time, a notable slow down should occur as the ridge over the eastern Pacific breaks down and leaves the cyclone in weak steering currents. The new NHC track forecast is adjusted to the south of the previous one to come into better agreement with the latest models. FORECAST POSITIONS AND MAX WINDS INIT 06/2100Z 19.3N 110.0W 35 KT 40 MPH 12H 07/0600Z 19.6N 112.3W 40 KT 45 MPH 24H 07/1800Z 19.7N 114.2W 40 KT 45 MPH 36H 08/0600Z 19.6N 115.4W 40 KT 45 MPH 48H 08/1800Z 19.4N 116.1W 35 KT 40 MPH 60H 09/0600Z 19.2N 116.7W 35 KT 40 MPH...POST-TROPICAL 72H 09/1800Z...DISSIPATED $$ Forecaster Cangialosi

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Tropical Storm Julio Forecast Discussion Number 4

2020-09-06 16:34:33| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Sun Sep 06 2020 000 WTPZ45 KNHC 061434 TCDEP5 Tropical Storm Julio Discussion Number 4 NWS National Hurricane Center Miami FL EP152020 900 AM MDT Sun Sep 06 2020 Julio has surprisingly accelerated west-northwestward during the past few hours, moving around the northeast periphery of a broad trough of low pressure. The tiny tropical storm continues to produce small but frequent bursts of central convection and has not changed appreciably since the most recent ASCAT pass that supported an intensity of 40 kt. Julio's future is unusually unclear for a system that is forecast to dissipate. The global models are struggling to resolve Julio, and only the ECMWF has a realistic current depiction of the cyclone. Those models universally indicate that Julio will either dissipate or be absorbed by the aforementioned broad area of low pressure located to its southwest within the next 24 to 36 hours. This is certainly plausible given Julio's small size and limited convection. On the other hand, the HWRF and HMON models now indicate that Julio will not only persist through day 5, but potentially strengthen when upper-level winds are forecast to become less hostile in a couple days. The disagreement can not be explained entirely by model resolution as the COAMPS-TC also weakens Julio. I see no clear reason to support one solution over another at this point. Therefore, the official forecast continues to show dissipation for now, though the intensity has been hedged slightly higher at 48 h and 60 h. The NHC track forecast leans heavily on the HWRF and HMON models and has been adjusted significantly faster for the first 24 h due to Julio's recent acceleration. Much larger changes may be required to the forecast later today or tonight if it becomes more likely that Julio will persist longer than currently indicated. FORECAST POSITIONS AND MAX WINDS INIT 06/1500Z 19.2N 108.8W 40 KT 45 MPH 12H 07/0000Z 19.9N 111.1W 45 KT 50 MPH 24H 07/1200Z 20.3N 113.6W 45 KT 50 MPH 36H 08/0000Z 20.4N 115.1W 45 KT 50 MPH 48H 08/1200Z 20.2N 115.9W 40 KT 45 MPH 60H 09/0000Z 20.2N 116.8W 35 KT 40 MPH...POST-TROPICAL 72H 09/1200Z...DISSIPATED $$ Forecaster Zelinsky

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