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Potential Tropical Cyclone Three Forecast Discussion Number 3

2017-06-20 10:52:55| National Hurricane Center (Atlantic)

Issued at 400 AM CDT Tue Jun 20 2017 000 WTNT43 KNHC 200852 TCDAT3 Potential Tropical Cyclone Three Discussion Number 3 NWS National Hurricane Center Miami FL AL032017 400 AM CDT Tue Jun 20 2017 Nighttime infrared satellite imagery suggests that the circulation associated with the low pressure area in the Gulf of Mexico is gradually becoming better defined. However, the system is struggling to maintain organized convection near the center, and the radius of maximum winds remains large. Based on this, the system is still maintained as a potential tropical cyclone. The low is moving erratically northwestward around the southwestern portion of a deep-layer ridge located over the western Atlantic and Florida. The large-scale models suggest this ridge will strengthen some during the next 36 hours or so and cause the low to turn a little more westward. This would be followed by a northward turn around the western end of the ridge and eventual recurvature into the westerlies. Overall, there has been a left shift of the track guidance models since the previous advisory. The new forecast track is also shifted left, but it is to the right of the model consensus, especially at 36-48 h. Given the nature of the circulation, though, and the fact that the wind and rain hazards extend well north and east of the center, users are encouraged to not focus on the details of the track forecast. The initial intensity remains 35 kt based on partial scatterometer overpasses and continuity from the previous advisory. Significant strengthening is unlikely due to strong vertical shear caused by an upper-level trough over the northwestern Gulf of Mexico and entrainment of dry air into the system. However, the large-scale models suggest slight strengthening before landfall, and thus the intensity forecast is unchanged from the previous advisory. One change from the previous advisory is that it now appears more likely that the system would become a subtropical cyclone than a tropical cyclone due to the current structure of the low and interaction with the aforementioned trough. That being said, development into a tropical cyclone remains possible. The primary hazard from this disturbance is expected to be heavy rainfall over portions of the central and eastern Gulf Coast. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 24.8N 90.1W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 20/1800Z 25.7N 91.0W 35 KT 40 MPH...SUBTROPICAL CYCLONE 24H 21/0600Z 26.6N 92.1W 40 KT 45 MPH 36H 21/1800Z 27.5N 93.0W 40 KT 45 MPH 48H 22/0600Z 28.9N 93.5W 40 KT 45 MPH 72H 23/0600Z 32.5N 93.5W 25 KT 30 MPH...INLAND 96H 24/0600Z 36.5N 88.0W 20 KT 25 MPH...POST-TROP/INLAND 120H 25/0600Z...DISSIPATED $$ Forecaster Beven

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Potential Tropical Cyclone Three Forecast Discussion Number 2

2017-06-20 04:40:54| National Hurricane Center (Atlantic)

Issued at 1000 PM CDT Mon Jun 19 2017 000 WTNT43 KNHC 200240 TCDAT3 Potential Tropical Cyclone Three Discussion Number 2 NWS National Hurricane Center Miami FL AL032017 1000 PM CDT Mon Jun 19 2017 Earlier this afternoon, an Air Force Reserve reconnaissance aircraft investigating the central Gulf of Mexico disturbance found that the multiple swirls that this system possessed had consolidated into a single low-level circulation center with a pressure of about 1000 mb. However, since the circulation was and still is elongated north-to-south, the large low pressure system is being maintained as a potential tropical cyclone for this advisory cycle. The NWS now has the option to issue advisories, watches, and warnings for disturbances that are not yet a tropical cyclone, but which pose the threat of bringing tropical storm or hurricane conditions to land areas within 48 hours. Under previous policy this was not possible. These systems are known as Potential Tropical Cyclones in advisory products and are numbered from the same list as depressions. Because of the threat to the central Gulf coast, advisories have been initiated on Potential Tropical Cyclone Three and the appropriate watches/warnings have been issued. Users should be aware that forecast uncertainty for disturbances is generally larger than for tropical cyclones, especially beyond 48-72 hours. After stalling this afternoon due to re-organization of the system's center, the low now appears to be moving 330/07 kt based on recent conventional and microwave satellite fixes. The new forecast track has been shifted to the west of the previous advisory track through 48 hours, mainly due to the more westward initial position. Otherwise, no other significant changes were made to the previous forecast track. The low is expected to move generally northwestward through 48 hours around the southwestern portion of a deep-layer ridge located over the western Atlantic and Florida. After that, the system is forecast to turn northward around the ridge axis, moving into the south-central United States. The new NHC track forecast is a little to the right of the consensus model TVCA. Given the still disorganized nature of the circulation, and the fact that the wind and rain hazards extend well north and east of the center, users are encouraged to not focus on the details of the track forecast. The initial intensity remains 35 kt based on reconnaissance wind data and observations from nearby ships 3ETA7 and 3FZO8. Only slight strengthening is expected for the next 36 hours or so due to moderate to strong southwesterly vertical wind shear of 25-30 kt. By 48 hours, the shear is forecast to decrease to 20 kt or less, but the system will already be close to land at that time and probably not be able to take advantage of the lower shear conditions. The NHC intensity forecast is similar to the previous advisory and the intensity consensus model IVCN. The primary hazard from this disturbance is expected to be heavy rainfall over portions of the central and eastern Gulf Coast. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 24.4N 89.5W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 20/1200Z 25.4N 90.3W 35 KT 40 MPH...TROPICAL CYCLONE 24H 21/0000Z 26.4N 91.3W 40 KT 45 MPH 36H 21/1200Z 27.1N 92.1W 40 KT 45 MPH 48H 22/0000Z 28.3N 92.8W 40 KT 45 MPH 72H 23/0000Z 31.8N 93.1W 25 KT 30 MPH...INLAND 96H 24/0000Z 35.2N 90.1W 20 KT 25 MPH...POST-TROP/INLAND 120H 25/0000Z...DISSIPATED INLAND $$ Forecaster Stewart

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Potential Tropical Cyclone THREE Forecast Discussion Number 1

2017-06-19 22:47:28| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Mon Jun 19 2017 000 WTNT43 KNHC 192047 TCDAT3 Potential Tropical Cyclone Three Discussion Number 1 NWS National Hurricane Center Miami FL AL032017 400 PM CDT Mon Jun 19 2017 A pair of ASCAT passes between 15 and 16Z showed that the disturbance over the Gulf of Mexico has an area of tropical-storm- force winds within a large area of deep convection 100 to 150 n mi northeast of a broad low center. The ASCAT data and visible satellite imagery show that the center is not well defined, and in fact multiple low-level swirls are evident in the latest imagery. An Air Force Reserve Hurricane Hunter aircraft is currently investigating the system and will provide more data on its wind structure. The NWS now has the option to issue advisories, watches, and warnings for disturbances that are not yet a tropical cyclone, but which pose the threat of bringing tropical storm or hurricane conditions to land areas within 48 hours. Under previous policy this was not possible. These systems are known as Potential Tropical Cyclones in advisory products and are numbered from the same list as depressions. Because of the threat to the central Gulf coast, advisories have been initiated on Potential Tropical Cyclone Three and the appropriate watches/warnings have been issued. Users should be aware that forecast uncertainty for disturbances is generally larger than for tropical cyclones, especially beyond 48-72 hours. The initial intensity is set to 35 kt based on the ASCAT data. The initial pressure is set to 1002 mb based on extrapolated data from the aircraft. Only modest strengthening is expected given the moderate to strong southwesterly shear of around 30 kt expected to persist over the system during the next couple of days. The NHC intensity forecast is close to the latest intensity consensus aid. Given the broad nature of the low, the initial motion estimate is a rather uncertain 350/08. During the first 12 to 24 hours the system will move northward to north-northwestward due to the a weakness in the subtropical ridge over the northern Gulf and then the system should slow and turn more northwestward as it interacts with an upper-level low currently situated off the Texas coast. The details of this interaction will have significant implications on the track of the system, and there remains a fair amount of model spread. The HWRF is on the right side of the guidance envelope, with a track toward southeastern Louisiana while the UKMET and ECMWF show a solution toward southwest Louisiana or extreme eastern Texas. For now, the NHC forecast lies in the middle of the guidance envelope and a little left of the TCVA consensus aid. Given the disorganized nature of the circulation and the fact that the wind and rain hazards extend well north and east of the center, users are encouraged to not focus on the details of the track forecast. The primary hazard from this disturbance is expected to be heavy rainfall over portions of the central and eastern Gulf Coast. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 24.7N 88.7W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 20/0600Z 26.0N 89.5W 35 KT 40 MPH 24H 20/1800Z 27.2N 90.6W 40 KT 45 MPH...TROPICAL CYCLONE 36H 21/0600Z 27.9N 91.4W 40 KT 45 MPH 48H 21/1800Z 28.7N 92.3W 40 KT 45 MPH 72H 22/1800Z 32.0N 93.5W 30 KT 35 MPH...INLAND 96H 23/1800Z 35.5N 90.5W 25 KT 30 MPH...POST-TROP/INLAND 120H 24/1800Z...DISSIPATED $$ Forecaster Brennan

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Tropical Storm BRET Forecast Discussion Number 5

2017-06-19 22:42:22| National Hurricane Center (Atlantic)

Issued at 500 PM AST Mon Jun 19 2017 000 WTNT42 KNHC 192042 TCDAT2 Tropical Storm Bret Discussion Number 5 NWS National Hurricane Center Miami FL AL022017 500 PM AST Mon Jun 19 2017 An Air Force WC-130 invest mission this afternoon into the disturbance was able - after much effort - to find a well-defined closed circulation. The location of the center was substantially farther west than anticipated, as the system is now moving toward the west-northwest at a very quick 26 kt. Because the system has developed a well-defined center, it is now considered a tropical storm and has been named Bret. Convection continues to display robust bands on the northwestern and southeastern quadrants of the disturbance, as seen in the geostationary satellite and Trinidad radar imagery. The Hurricane Hunter mission also found maximum surface winds of about 35 kt, which agrees with the Dvorak 2.5 CI number from TAFB. These winds are only occurring in the northern semicircle due to the very quick translational velocity of the system. While the convective structure has been impressive today, it appears that the window of opportunity for the system to further develop is closing. Between interaction with the landmass of Venezuela and increasing vertical shear beginning in about a day, the intensity model guidance does not show any significant intensification. The official intensity forecast is steady state for about a day, followed by gradual weakening until dissipation in about three days. An alternative scenario - given that the disturbance has tracked left, or equatorward, of the guidance thus far - is that it could remain south of the forecast track and dissipate over Venezuela and never reach the Caribbean Sea. The system is anticipated to slow somewhat, but still move at a rapid rate toward the west-northwest until dissipation, steered by strong trade winds south of the Bermuda high. The official track forecast is based upon the HCCA corrected consensus technique and is faster compared with the previous advisory, due to the farther west initial position. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 9.4N 59.8W 35 KT 40 MPH 12H 20/0600Z 10.1N 62.1W 35 KT 40 MPH 24H 20/1800Z 11.3N 65.3W 35 KT 40 MPH 36H 21/0600Z 12.2N 68.5W 30 KT 35 MPH 48H 21/1800Z 13.0N 73.0W 30 KT 35 MPH...POST-TROP/REMNT LOW 72H 22/1800Z...DISSIPATED $$ Forecaster Landsea

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Potential Tropical Cyclone Two Forecast Discussion Number 4

2017-06-19 17:00:50| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Mon Jun 19 2017 000 WTNT42 KNHC 191500 TCDAT2 Potential Tropical Cyclone Two Discussion Number 4 NWS National Hurricane Center Miami FL AL022017 1100 AM AST Mon Jun 19 2017 The convective pattern of the disturbance has become better organized this morning with a ragged band that wraps more than half way around its center. The intensity is assessed at 35 kt based upon the Dvorak 2.5 CI number from TAFB. These winds are likely only occurring in the northern semicircle due to the very quick translational velocity of the system. Intensity model guidance is in good agreement that the environmental conditions may allow for some slight intensification in the next day or so with the system embedded within low vertical shear and an atmosphere that is marginally moist and unstable. Beginning in about 36 hours, however, the shear should go up to either moderate (if the GFS is correct) or high (if the ECMWF is correct) levels because of an approaching tropical upper- tropospheric trough. Because of this increasing shear, all guidance shows the system dissipating before 72 hr and so does the official forecast. An alternative scenario - given that the disturbance has tracked left, or equatorward, of the guidance thus far - is that it could remain south of guidance and dissipate over Venezuela and never reach the Caribbean Sea. The quick motion also makes it ambiguous as to whether the system has a closed, earth-relative circulation center. An Air Force Hurricane Hunter plane is scheduled to investigate the system later today to help in this determination. It is still anticipated that the system will become a tropical storm, but it may be running out of time if it interacts significantly with Venezuela. The system is moving toward the west at a very rapid 22 kt due to the fast low to mid-level tradewinds south of a strong Bermuda high. It is likely that a rapid motion toward the west or west-northwest will continue until dissipation. The official track forecast is based upon the HCCA corrected consensus technique and is slightly south compared with the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 19/1500Z 8.8N 56.8W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 20/0000Z 9.5N 59.7W 40 KT 45 MPH...TROPICAL CYCLONE 24H 20/1200Z 10.6N 63.1W 40 KT 45 MPH 36H 21/0000Z 11.7N 66.6W 35 KT 40 MPH 48H 21/1200Z 12.6N 70.6W 35 KT 40 MPH 72H 22/1200Z...DISSIPATED $$ Forecaster Landsea

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