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Hurricane Irma Forecast Discussion Number 16

2017-09-03 10:49:19| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sun Sep 03 2017 000 WTNT41 KNHC 030849 TCDAT1 Hurricane Irma Discussion Number 16 NWS National Hurricane Center Miami FL AL112017 500 AM AST Sun Sep 03 2017 Irma's convective pattern has improved slightly overnight. Very cold cloud tops completely encircle the eye, which has warmed and become a bit clearer in infrared satellite images. The initial intensity is raised to 100 kt based on a blend of the latest subjective and objective Dvorak estimates. Irma has been losing latitude since yesterday due to strong high pressure to its north, and the initial motion is now west- southwestward, or 255/13 kt. The hurricane is likely to continue moving on this trajectory for the next 36 hours, after which time it should gradually turn toward the west and then west-northwest on days 3-5 when it reaches the western extent of the ridge. The new NHC track forecast is very similar to the previous one during the first 48 hours, showing Irma bottoming out around 16.5N. However, the track guidance has shifted westward after 48 hours, delaying a turn toward the west-northwest, and this required a corresponding westward shift in the official forecast toward the multi-model consensus at the end of the forecast period. It should be noted that the official forecast still lies to the east of some of the better-performing models, such as the ECMWF, HWRF, and HCCA, so additional adjustment are possible in subsequent advisories. The environment ahead of Irma appears conducive for gradual strengthening for at least the next 2 to 3 days, with increasing sea surface temperatures and a moistening in the mid-levels of the atmosphere. We may still observe fluctuations in intensity, but overall, the model guidance seems to suggest a general upward trend with a peak in intensity possibly occurring around day 3. This type of intensification would coincide with the timing of Irma's west-southwest to westward motion, a pattern which we have observed in other west-southwestward-moving hurricanes in the past (i.e., Katrina, Joaquin, Fernanda, etc.). The NHC intensity forecast is bumped up slightly, showing a peak in intensity on day 3, and is largely a blend of the ICON intensity consensus and HCCA. While Irma is currently a small hurricane, the size guidance suggests it should grow in size during the next 72 h. This will affect how soon watches may be issued for portions of the Leeward and Virgin Islands, and interests on those islands should continue to monitor Irma's progress. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it moves closer to the Lesser Antilles over the next few days, producing rough surf and rip currents. Irma could also cause dangerous wind, storm surge, and rainfall impacts on some islands, although it is too soon to specify where and when those hazards could occur. Residents in the Lesser Antilles should monitor the progress of Irma through the weekend and listen to any advice given by local officials. 2. It is much too early to determine what direct impacts Irma will have on the Bahamas and the continental United States. Regardless, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place, as we are now near the peak of the season. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 18.0N 47.5W 100 KT 115 MPH 12H 03/1800Z 17.5N 49.1W 100 KT 115 MPH 24H 04/0600Z 16.8N 51.3W 105 KT 120 MPH 36H 04/1800Z 16.5N 53.4W 110 KT 125 MPH 48H 05/0600Z 16.5N 55.7W 115 KT 130 MPH 72H 06/0600Z 17.8N 60.6W 120 KT 140 MPH 96H 07/0600Z 20.0N 66.0W 115 KT 130 MPH 120H 08/0600Z 22.5N 71.5W 115 KT 130 MPH $$ Forecaster Berg

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Post-Tropical Cyclone Lidia Forecast Discussion Number 19

2017-09-03 10:43:18| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Sun Sep 03 2017 000 WTPZ44 KNHC 030843 TCDEP4 Post-Tropical Cyclone Lidia Discussion Number 19 NWS National Hurricane Center Miami FL EP142017 200 AM PDT Sun Sep 03 2017 Lidia has not produced any deep convection for the past 12-15 hours, which means it is no longer a tropical cyclone and has degenerated into a remnant low. Its initial intensity is estimated to be 30 kt based on a Dvorak CI number of 2.0 from TAFB, but Lidia's winds should gradually decrease as it moves over the cold waters west of Baja California. Model guidance indicates that Lidia's vorticity will merge with or become absorbed by a deep-layer low to its west in the coming days, and the surface low is expected to dissipate by 48 hours. Lidia's initial motion is toward the northwest, or 315/7 kt. High pressure over the southwestern United States should steer the remnant low toward the west-northwest beginning later today, and this motion should continue until the circulation dissipates off the coast of southern California. This is the last advisory issued by the National Hurricane Center on Lidia. 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 03/0900Z 29.6N 116.3W 30 KT 35 MPH...POST-TROP/REMNT LOW 12H 03/1800Z 30.3N 117.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 04/0600Z 31.2N 119.7W 20 KT 25 MPH...POST-TROP/REMNT LOW 36H 04/1800Z 32.2N 121.8W 15 KT 15 MPH...POST-TROP/REMNT LOW 48H 05/0600Z...DISSIPATED $$ Forecaster Berg

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Hurricane Irma Forecast Discussion Number 15

2017-09-03 04:32:14| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sat Sep 02 2017 000 WTNT41 KNHC 030232 TCDAT1 Hurricane Irma Discussion Number 15 NWS National Hurricane Center Miami FL AL112017 1100 PM AST Sat Sep 02 2017 The cloud pattern of Irma has not changed significantly in structure today. The eye continues to become apparent and then hide under the convective canopy, and this has been the observed pattern for the past 24 hours or so. Dvorak estimates go up and down with the presence of the eye, but an average of these numbers supports an initial intensity of 95 kt. A NOAA Hurricane Hunter plane will help with the intensity estimate on Sunday. I hesitate to speculate too much about the environment that Irma is embedded within. All of the standard ingredients necessary for strengthening are forecast to be at least marginally favorable, but none are expected to be hostile for intensification. The NHC forecast, which in fact is similar to the previous one, continues to be a blend of the statistical models and the explosive strengthening shown by the regional hurricane and global models. The subtropical ridge building to the north of Irma has been steering the hurricane toward the west or 260 degrees at 12 kt. The ridge is forecast to amplify even more, and this flow pattern will force the hurricane to dive west-southwestward for a couple of days. Irma should then begin to gain latitude once it reaches the southwestern edge of the ridge in about 3 days. The confidence in the track forecast is high for the next 72 hours since all of the reliable guidance is basically on top of each other. After 3 days, when the hurricane is forecast to be approaching the northern Leeward Islands, the guidance envelope spreads out and becomes bounded by the southernmost tracks of the HWRF, HCCA and the ECMWF models, and by the northernmost GFS and UK models. The confidence beyond 3 days is then much lower. Tonight's NHC forecast was adjusted a just little to the south of the previous one due to another small shift of the guidance envelope. The forecast is basically on top of the multi-model consensus TVCX. While Irma is currently a small hurricane, the size guidance suggests it should grow in size during the next 72 h. This will affect how soon watches may be issued for portions of the Leeward and Virgin Islands. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it moves closer to the Lesser Antilles early next week, producing rough surf and rip currents. Irma could also cause dangerous wind, storm surge, and rainfall impacts on some islands, although it is too soon to specify where and when those hazards could occur. Residents in the Lesser Antilles should monitor the progress of Irma through the weekend and listen to any advice given by local officials. 2. It is much too early to determine what direct impacts Irma will have on the Bahamas and the continental United States. Regardless, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place, as we are now near the peak of the season. FORECAST POSITIONS AND MAX WINDS INIT 03/0300Z 18.3N 46.2W 95 KT 110 MPH 12H 03/1200Z 17.8N 48.0W 95 KT 110 MPH 24H 04/0000Z 17.2N 50.1W 100 KT 115 MPH 36H 04/1200Z 16.7N 52.3W 105 KT 120 MPH 48H 05/0000Z 16.5N 54.5W 110 KT 125 MPH 72H 06/0000Z 17.7N 59.3W 110 KT 125 MPH 96H 07/0000Z 19.9N 64.5W 115 KT 130 MPH 120H 08/0000Z 22.5N 69.5W 115 KT 130 MPH $$ Forecaster Avila

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Tropical Storm Lidia Forecast Discussion Number 17

2017-09-02 22:42:16| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Sat Sep 02 2017 000 WTPZ44 KNHC 022042 TCDEP4 Tropical Storm Lidia Discussion Number 17 NWS National Hurricane Center Miami FL EP142017 200 PM PDT Sat Sep 02 2017 Lidia's deep convection is becoming less organized and located increasingly farther away from the center of the tropical storm. This is likely due to the system being disrupted while traveling up the spine of the mountainous Baja California peninsula as well as the cold waters on the west side of the peninsula. Despite this, the system still has a robust surface circulation and the ASCAT scatterometer passes this afternoon showed 35 kt peak winds within the Gulf of California, which is the value used for the initial intensity. The tropical storm should continue weakening as it traverses cold SSTs and a less unstable, dry atmosphere. It is likely that deep convection associated with Lidia will cease in 12-24 hr marking the end of the system as a tropical cyclone. The official intensity forecast is the based upon the tightly clustered statistical and dynamical model guidance and is the same as the previous advisory. Lidia is moving toward the northwest at 10 kt while moving around the southwestern periphery of a mid-level subtropical ridge. The tropical storm should turn toward the west-northwest at about the same rate of speed as it weakens and moves along in the low-level tradewind flow. The official track forecast is based upon the usually reliable global models and is slightly north of the previous advisory. Despite the forecast weakening, the heavy rainfall threat will continue over northwestern Mexico and the Baja California peninsula through Sunday. Moisture from the cyclone should spread over portions of the southwestern United States through the weekend. FORECAST POSITIONS AND MAX WINDS INIT 02/2100Z 28.8N 115.0W 35 KT 40 MPH 12H 03/0600Z 29.6N 116.2W 30 KT 35 MPH 24H 03/1800Z 30.9N 118.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 04/0600Z 31.8N 120.4W 20 KT 25 MPH...POST-TROP/REMNT LOW 48H 04/1800Z 32.4N 122.7W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 05/1800Z...DISSIPATED $$ Forecaster Landsea

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Hurricane Irma Forecast Discussion Number 14

2017-09-02 22:40:45| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 02 2017 000 WTNT41 KNHC 022040 TCDAT1 Hurricane Irma Discussion Number 14 NWS National Hurricane Center Miami FL AL112017 500 PM AST Sat Sep 02 2017 Hurricane Irma continues to display an eye within a small central dense overcast, although the eye has been going through periods where it becomes less well defined. The satellite intensity estimates from TAFB and SAB remain 90 kt, while the latest CIMMS ADT and satellite consensus technique estimates ARE 95-105 KT. Since there has been little overall change in organization since the last advisory, the initial intensity remains 95 kt. The environment in which Irma is embedded continues to show mixed signals during the forecast period, and the intensity guidance responds to this by ranging between little and slow intensification. The hurricane is currently suffering some impact of sea surface temperatures of about 27C and mid-level dry air entrainment. Later in the period, Irma should encounter warmer water and increasing moisture at a time when the vertical wind shear may be increasing. Given the uncertainty on when all of the ingredients may come together, the new intensity forecast is the same as the previous forecast and calls for gradual intensification through the next 5 days. An alternative forecast scenario is that Irma gets significantly stronger than forecast near the end of the forecast period if the shear is less than currently expected. The initial motion remains 265/13. A large and building subtropical ridge should steer Irma generally west-southwestward during the next two days or so. Between 72-120 h, Irma should be rounding the southwestern periphery of the ridge and start turning back toward the west-northwest. While the track guidance remains in good agreement with this scenario, from 72-120 h there has been a westward shift of the guidance that results in the new forecast track coming 30-60 n mi closer to the Leeward and Virgin Islands than in the previous advisory. This latter portion of the track lies near the center of the guidance envelope, but with the ECMWF and corrected consensus models to the south and the GFS to the north. While Irma is currently a small hurricane, the size guidance suggests it should grow in size during the next 72 h. This will affect how soon watches may be issued for portions of the Leeward and Virgin Islands. KEY MESSAGES: 1. Irma is expected to be a major hurricane when it moves closer to the Lesser Antilles early next week, producing rough surf and rip currents. Irma could also cause dangerous wind, storm surge, and rainfall impacts on some islands, although it is too soon to specify where and when those hazards could occur. Residents in the Lesser Antilles should monitor the progress of Irma through the weekend and listen to any advice given by local officials. 2. It is much too early to determine what direct impacts Irma will have on the Bahamas and the continental United States. Regardless, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place, as we are now near the peak of the season. FORECAST POSITIONS AND MAX WINDS INIT 02/2100Z 18.5N 44.6W 95 KT 110 MPH 12H 03/0600Z 18.0N 46.5W 95 KT 110 MPH 24H 03/1800Z 17.4N 48.8W 100 KT 115 MPH 36H 04/0600Z 17.0N 50.9W 105 KT 120 MPH 48H 04/1800Z 16.8N 53.0W 110 KT 125 MPH 72H 05/1800Z 17.5N 57.5W 110 KT 125 MPH 96H 06/1800Z 19.5N 62.5W 115 KT 130 MPH 120H 07/1800Z 22.0N 68.0W 115 KT 130 MPH $$ Forecaster Beven

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