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Tropical Depression Four Forecast Discussion Number 4

2017-07-06 22:32:49| National Hurricane Center (Atlantic)

Issued at 500 PM AST Thu Jul 06 2017 000 WTNT44 KNHC 062032 TCDAT4 Tropical Depression Four Discussion Number 4 NWS National Hurricane Center Miami FL AL042017 500 PM AST Thu Jul 06 2017 It's difficult to tell from visible and microwave satellite imagery whether or not the depression still has a closed surface circulation. In addition, the forward speed is now about 20 kt, which would make it difficult for a 25-kt depression to maintain westerly winds to the south of the center. For now, advisories will be maintained until/if there is stronger evidence that the circulation has opened up. Convective cloud tops have warmed during the past few hours, and Dvorak final-T numbers from TAFB and SAB have decreased. Therefore, the initial intensity will be held at 25 kt based on continuity. Multispectral satellite imagery shows the Saharan Air Layer is overtaking the depression, and it will be increasingly more difficult for the system to maintain organized deep convection. It has also become more likely that the cyclone will not strengthen beyond its current intensity. The global models indicate that southwesterly to westerly deep-layer shear will increase in about 36 hours, at which point the NHC official forecast now shows the depression degenerating to a remnant low. However, it's entirely possible that the cyclone could open up into a tropical wave at any time. Based on the estimated center location, the depression has not gained much latitude today, and the initial motion estimate is 275/20 kt. The global models remain persistent that the cyclone should turn west-northwestward in about 24 hours as it approaches a break in the subtropical ridge, and they are generally in very close agreement on the future track and forward speed through at least 48 hours. The updated NHC track forecast is nudged southward to account for the recent motion, but otherwise it is an update of the previous forecast. FORECAST POSITIONS AND MAX WINDS INIT 06/2100Z 13.1N 44.9W 25 KT 30 MPH 12H 07/0600Z 13.8N 47.8W 25 KT 30 MPH 24H 07/1800Z 14.9N 51.5W 25 KT 30 MPH 36H 08/0600Z 16.2N 55.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 08/1800Z 17.8N 58.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 09/1800Z 20.9N 64.1W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 10/1800Z...DISSIPATED $$ Forecaster Berg

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Tropical Depression FOUR Forecast Discussion Number 3

2017-07-06 16:34:21| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Thu Jul 06 2017 000 WTNT44 KNHC 061434 TCDAT4 Tropical Depression Four Discussion Number 3 NWS National Hurricane Center Miami FL AL042017 1100 AM AST Thu Jul 06 2017 A series of microwave images, an earlier NRL WindSAT scatterometer overpass, and recent ASCAT-B surface wind retrievals indicate that the surface circulation of the depression has become less organized. The center remains near the eastern edge of an amorphous blob of deep convection and is a bit farther south than previously thought. Enhanced BD-curve infrared imagery also shows that the cloud tops have warmed considerably during the past 6 hours. The initial intensity is held at 25 kt for this advisory based on the deteriorating cloud pattern and a blend of the TAFB and SAB satellite intensity estimates. Large-scale dry, sinking air associated with a previous Saharan Air Layer outbreak continues to be the primary inhibiting factor affecting the cyclone. Global and ensemble guidance show the system degenerating into a remnant low or trough in 3 days or less, due to the aforementioned harsh thermodynamic environment and increasing moderate westerly shear. The official intensity forecast is basically an update of the previous advisory and favors the dynamical models. The initial motion is estimated to be west-northwestward, or 285/18 kt. The circulation center continues to be difficult to locate, but the above-mentioned scatterometer pass was helpful in obtaining the position estimate. The depression is forecast to be steered by the low- to mid-level flow associated with a building subtropical ridge situated to its north over the next several days. The new official forecast is about a half a degree south of the previous one due to the initial position adjustment. The NHC track is close to the latest TVCN multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 06/1500Z 13.2N 42.6W 25 KT 30 MPH 12H 07/0000Z 14.0N 45.3W 30 KT 35 MPH 24H 07/1200Z 15.1N 49.3W 30 KT 35 MPH 36H 08/0000Z 16.2N 52.9W 30 KT 35 MPH 48H 08/1200Z 17.7N 56.6W 25 KT 30 MPH 72H 09/1200Z 20.6N 62.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 10/1200Z 23.0N 67.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 11/1200Z...DISSIPATED $$ Forecaster Roberts

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Tropical Depression Four Forecast Discussion Number 2

2017-07-06 10:34:49| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Jul 06 2017 000 WTNT44 KNHC 060834 TCDAT4 Tropical Depression Four Discussion Number 2 NWS National Hurricane Center Miami FL AL042017 500 AM AST Thu Jul 06 2017 The cloud pattern of the tropical cyclone consists mainly of a circularly shaped mass of deep convection that has persisted overnight. Since there has been little overall change in the satellite appearance of the system, the current intensity estimate remains 25 kt, which is also in agreement with a Dvorak estimate from SAB. Dry and dusty air related to the Saharan Air Layer, to the east of the tropical cyclone, is beginning to wrap around the northern part of the depression's circulation. Dynamical models indicate that this air mass will be partially entrained into the system over the next couple of days. This, combined with increasing vertical shear, should prevent significant strengthening of the system. Although the statistical-dynamical guidance, SHIPS and LGEM, forecast some modest intensification of the tropical cyclone, these models have been known to have a high bias at times. The GFS, UKMET, and ECMWF global models all show the system quickly degenerating to a wave. As a compromise between the global and the statistical-dynamical guidance, the official forecast more or less maintains the cyclone's intensity for a couple of days followed by weakening to a remnant low in 72 hours and dissipation after 96 hours. The center is not very easy to locate, but based on continuity with earlier data it is believed to be near the eastern edge of the convective mass. There has apparently been some acceleration and the motion is now estimated to be 290/15 kt. The flow on the southern side of a subtropical ridge should continue to steer the tropical cyclone, or its remnants, west-northwestward over the next few days. The official track forecast follows a trajectory very similar to the previous one, but is somewhat faster. This is close to the latest model consensus. FORECAST POSITIONS AND MAX WINDS INIT 06/0900Z 13.2N 40.0W 25 KT 30 MPH 12H 06/1800Z 14.0N 42.5W 30 KT 35 MPH 24H 07/0600Z 15.0N 46.2W 30 KT 35 MPH 36H 07/1800Z 16.3N 50.0W 30 KT 35 MPH 48H 08/0600Z 17.5N 53.5W 25 KT 30 MPH 72H 09/0600Z 20.5N 59.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 10/0600Z 23.0N 65.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 11/0600Z...DISSIPATED $$ Forecaster Pasch

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Tropical Depression FOUR Forecast Discussion Number 1

2017-07-06 04:32:47| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Wed Jul 05 2017 000 WTNT44 KNHC 060232 TCDAT4 Tropical Depression Four Discussion Number 1 NWS National Hurricane Center Miami FL AL042017 1100 PM AST Wed Jul 05 2017 The low pressure area west of the Cabo Verde Islands has a well-defined circulation based on a combination of surface observations and scatterometer data. In addition, there has been a persistent area of convection west of the center for the past 12 h or so. Based on these, advisories are initiated on the system as a tropical depression. The initial intensity is set to 25 kt based on the scatterometer data and satellite intensity estimates from TAFB and SAB. Experimental multispectral imagery shows a large area of dry and dusty air over the Atlantic near and east of the depression, and it appears likely this will entrain into the circulation during the next couple of days. The large-scale models forecast the system to dissipate very quickly due to this entrainment, while in contrast the statistical-dynamical guidance forecasts modest strengthening. Another factor is that the current environment of light to moderate easterly shear is expected to become moderate to strong southwesterly shear at about 48 h. As a compromise between the extremes in the guidance, the intensity forecast calls for little change in strength for 48 h, followed by the system degenerating to a remnant low by 72 h. The initial motion is 290/12. The depression is on the south side of a strong low- to mid-level ridge, and this feature should steer the cyclone or its remnants west-northwestward for the next 5 days. There should be an increase in forward speed during the next 24 h, with some decrease in forward speed after 72 h as the system approaches a weakness in the ridge. The forecast track lies close to the model consensus. FORECAST POSITIONS AND MAX WINDS INIT 06/0300Z 12.8N 38.4W 25 KT 30 MPH 12H 06/1200Z 13.4N 40.4W 30 KT 35 MPH 24H 07/0000Z 14.2N 43.8W 30 KT 35 MPH 36H 07/1200Z 15.2N 47.5W 30 KT 35 MPH 48H 08/0000Z 16.5N 51.1W 30 KT 35 MPH 72H 09/0000Z 19.5N 58.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 10/0000Z 22.0N 63.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 120H 11/0000Z 24.5N 68.0W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Beven

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Post-Tropical Cyclone DORA Forecast Discussion Number 15

2017-06-28 16:34:53| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Wed Jun 28 2017 000 WTPZ44 KNHC 281434 TCDEP4 Post-Tropical Cyclone Dora Discussion Number 15 NWS National Hurricane Center Miami FL EP042017 900 AM MDT Wed Jun 28 2017 Dora has been devoid of deep convection for more than 12 hours, and the cyclone has degenerated into a remnant low composed of mostly low-level stratocumulus clouds. Dvorak satellite classifications support an intensity of 30 kt, and that is the intensity used for this advisory. Redevelopment of significant convection is not expected due to unfavorable SSTs of 22C-23C, and dissipation is expected to occur in 36-48 h. The initial motion estimate is 290/09 kt. A low- to mid-level ridge to the north of the Dora should keep the weakening remnant low moving in a general west-northwestward motion for the next couple of days. The new NHC forecast track is an extension of the previous advisory track and lies close to the consensus model TCVN. This is the last advisory on Dora unless regeneration occurs. For additional information on the remnant low please see High Seas Forecasts issued by the National Weather Service, under AWIPS header NFDHSFEPI, WMO header FZPN02 KWBC, and on the web at http://www.opc.ncep.noaa.gov/shtml/NFDHSFEPI.shtml. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 20.1N 113.9W 30 KT 35 MPH...POST-TROP/REMNT LOW 12H 29/0000Z 20.7N 115.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 29/1200Z 21.3N 116.9W 20 KT 25 MPH...POST-TROP/REMNT LOW 36H 30/0000Z 21.8N 118.6W 15 KT 15 MPH...POST-TROP/REMNT LOW 48H 30/1200Z...DISSIPATED $$ Forecaster Stewart

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