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Hurricane Olaf Forecast Discussion Number 8

2021-09-09 16:51:10| National Hurricane Center (East Pacific)

Issued at 900 AM MDT Thu Sep 09 2021 000 WTPZ45 KNHC 091451 TCDEP5 Hurricane Olaf Discussion Number 8 NWS National Hurricane Center Miami FL EP152021 900 AM MDT Thu Sep 09 2021 Recent microwave imagery shows that Olaf's structure has improved considerably during the past 6 hours, and it now has well-defined eye. The eye is also now evident in radar imagery from Cabo San Lucas. Although an eye is not yet evident in infrared or visible imagery, satellite intensity estimates had increased and supported an intensity of 60-65 kt at 1200 UTC. Given the continued impressive satellite appearance since then, the estimated intensity is 65 kt for this advisory, making Olaf a hurricane. Olaf has a microwave ring structure that is known to indicate intensification in the short term. The wind shear that previously affected the hurricane seems to have lessened and the environment should also support further strengthening today. The hurricane is therefore forecast to quickly intensify during the next 12 h or so while it approaches the southern tip of the Baja California peninsula. After that, interaction with land and cold sea surface temperatures should cause the cyclone to quickly weaken over the weekend. Olaf is forecast to become post-tropical in about 60 h as it moves farther away from land. The NHC intensity forecast is above the previous advisory, and is above most of the guidance through 24 h. Beyond that, there have been only slight adjustments to the official intensity forecast, which is based on the multi- model consensus. The track forecast has shifted closer to the southern Baja California peninsula. In line with the intensity forecast, the NHC forecast now slightly favors the stronger GFS and regional hurricane models, compared to the weaker ECMWF, but is still near the HCCA and TVCN consensus aids. Olaf is moving north-northeast but should turn northwestward later today, steered by a mid-level ridge centered over the western United States. The models with a stronger hurricane show Olaf turning slightly slower, and bring the center very near or even over Baja California Sur tonight. As Olaf weakens it should begin to be steered by lower-level flow, which will cause it to eventually turn westward and then southwestward as it moves away from land early next week. Key Messages: 1. Olaf is forecast to pass very near or over the southern portion of Baja California Sur tonight and Friday, and hurricane conditions are likely within the hurricane warning area tonight. Tropical storm conditions are expected to begin within the hurricane warning area this afternoon, and preparations should be rushed to completion. 2. Heavy rains associated with Olaf are expected across portions of southern Baja California Sur through Friday. This will pose a threat of significant and life-threatening flash flooding and mudslides. FORECAST POSITIONS AND MAX WINDS INIT 09/1500Z 21.2N 108.3W 65 KT 75 MPH 12H 10/0000Z 22.2N 109.2W 75 KT 85 MPH 24H 10/1200Z 23.3N 110.7W 65 KT 75 MPH 36H 11/0000Z 24.0N 112.1W 60 KT 70 MPH 48H 11/1200Z 24.1N 113.5W 45 KT 50 MPH 60H 12/0000Z 23.8N 114.7W 35 KT 40 MPH...POST-TROPICAL 72H 12/1200Z 23.2N 116.0W 30 KT 35 MPH...POST-TROP/REMNT LOW 96H 13/1200Z 21.5N 119.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 14/1200Z 21.0N 122.0W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster D. Zelinsky

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Tropical Storm Olaf Forecast Discussion Number 7

2021-09-09 10:55:31| National Hurricane Center (East Pacific)

Issued at 300 AM MDT Thu Sep 09 2021 000 WTPZ45 KNHC 090855 TCDEP5 Tropical Storm Olaf Discussion Number 7 NWS National Hurricane Center Miami FL EP152021 300 AM MDT Thu Sep 09 2021 Olaf's convective canopy has been expanding somewhat and become a little more circular since last evening with clouds tops as cold as -80 degrees Celsius. Dvorak estimates have bounced up to T3.5 from TAFB, SAB, and the UW-CIMSS ADT, and Olaf's maximum winds are therefore now estimated to be 55 kt. Recent scatterometer data also indicated that the tropical-storm-force wind field is a little bigger than previously estimated. Also with the help of the scatterometer data, Olaf's center appears to have tracked a bit to the east, and the initial motion is estimated to be north-northwestward, or 340/5 kt. This has been the recent trend over the past day or so, with the motion having a more northward component than anticipated--but not surprising given what previous GFS model runs have been suggesting. A strong mid-level high over the western United States is expected to eventually force Olaf toward the northwest and then west, but probably not soon enough to avoid impacts on the southern Baja California peninsula. Mostly due to the adjusted initial position, the track models--and the NHC official forecast--have again shifted closer to the southern tip of the peninsula, with a closest approach in about 24 hours. This updated forecast is very close to the HCCA consensus aid and would be close enough for the core of the storm to at least graze land. It's also concerning that the GFS, HMON, and COAMPS-TC models are still to the right of the official forecast and the other models and show Olaf's center potentially moving over the southern tip of the peninsula in about 24 hours. Olaf should finally begin to move westward away from the peninsula in about 48 hours. Although satellite images and model analyses suggest that some moderate westerly shear is undercutting the outflow layer, Olaf has been able to strengthen nonetheless. If Olaf's center does not move over the Baja California peninsula, the storm will have the opportunity to strengthen for another 24-36 hours while ocean waters are warm and deep-layer shear is low. Therefore, the new NHC intensity forecast is near the upper end of the guidance envelope and shows Olaf becoming a hurricane later today. Further strengthening beyond what's shown in the official forecast is possible, with rapid intensification indices now close to a 50 percent chance of a 25-kt increase in winds over the next 24 hours. By 48 hours, colder waters should cause Olaf's convection to decrease, and global models suggest it could degenerate to a convection-less post-tropical low by day 3. The updated track and intensity forecasts have necessitated the issuance of a Hurricane Warning by the government of Mexico for southern portions of Baja California Sur, and an extension of Tropical Storm Warnings northward along both east and west coasts of the peninsula. Key Messages: 1. Olaf is forecast to strengthen to a hurricane later today and then pass very near or over the southern portion of Baja California Sur tonight and on Friday. Tropical storm conditions are expected to begin within the hurricane warning area this afternoon, and preparations should therefore be rushed to completion this morning. 2. Heavy rains associated with Olaf are possible across portions of far southern Baja California Sur today through Friday. This will pose a threat of flash flooding and mudslides. FORECAST POSITIONS AND MAX WINDS INIT 09/0900Z 20.5N 108.0W 55 KT 65 MPH 12H 09/1800Z 21.4N 108.6W 65 KT 75 MPH 24H 10/0600Z 22.6N 110.0W 70 KT 80 MPH 36H 10/1800Z 23.5N 111.7W 65 KT 75 MPH 48H 11/0600Z 23.7N 113.2W 50 KT 60 MPH 60H 11/1800Z 23.5N 114.5W 40 KT 45 MPH 72H 12/0600Z 23.1N 115.7W 35 KT 40 MPH...POST-TROPICAL 96H 13/0600Z 22.1N 118.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 120H 14/0600Z 21.3N 121.7W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Berg

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Hurricane Larry Forecast Discussion Number 35

2021-09-09 10:46:32| National Hurricane Center (Atlantic)

Issued at 500 AM AST Thu Sep 09 2021 652 WTNT42 KNHC 090846 TCDAT2 Hurricane Larry Discussion Number 35 NWS National Hurricane Center Miami FL AL122021 500 AM AST Thu Sep 09 2021 This morning's conventional satellite presentation appears to indicate a more banding eye feature rather than a previously noted irregular-type eye. A timely Advanced Technology Microwave Sounder (ATMS) image and an AMSR2 lower frequency pass, however, clearly shows the majority of the eyewall intact. The primary curved band wrapping around the west semicircle consists of -78 Celsius cloud tops. The western portion of the eyewall is now discernible on the Bermuda Weather Service Radar. The initial intensity is held at possibly generous 85 kt for this advisory and is based on a compromise of the subjective and objective satellite intensity estimates. A 53rd Air Force Reserve Unit reconnaissance mission is scheduled for this morning and will provide new information on Larry's intensity. The 0000 UTC GFS global model sounding revealed 15 to 20 kt of southwesterly effective shear impinging on the western half of the cyclone while the UW-CIMSS shear analysis showed 20-25 kt of bulk shear in the same area. This, along with a significantly higher statically stable surrounding environment (about 55 percent RH), has disrupted the inner core and more than likely created a SW to NW tilt with height. Larry is expected to change little in strength during the next 12 to 24 hours, then gradually weaken as the cyclone traverses sharply decreasing cooler water north of the gulf stream while the southwesterly shear increases significantly. The dynamic forcing, however, associated with a mid-latitude major shortwave trough near Atlantic Canada should aid in maintaining Larry as a hurricane while it passes near or over Newfoundland. Afterwards, Larry is expected to transition into a large extratropical cyclone, as indicated by the FSU Cyclone Phase Diagram. By early next week, Larry is expected to be absorbed by a larger extratropical low. The initial motion is estimated to be north-northwestward, or 330/14 kt. The track philosophy remains unchanged. Larry is expected to move around the western periphery of a subtropical high pressure system anchored over the central Atlantic. The hurricane should make its closest approach to the east of Bermuda later today while slowly turning north-northwestward and northward. Through the remaining portion of the forecast, Larry should accelerate generally northeastward in response to the aforementioned mid-latitude trough and move near or over southeastern Newfoundland Friday night. The NHC forecast has been nudged a bit to the left of the previous advisory and is close to the various multi-model consensus aids. The wind radii were decreased slightly in all quadrants based on earlier METOP A and B scatterometer passes. Key Messages: 1. Large swells generated by Larry will continue to affect the Leeward Islands, portions of the Greater Antilles, the Bahamas, Bermuda, and the east coast of the United States and Atlantic Canada through the end of the week. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the guidance of lifeguards and local officials this week. 2. The center of Larry is forecast to pass east of Bermuda later today, Tropical storm conditions are expected today, along with a risk of coastal flooding. A Tropical Storm Warning is in effect for Bermuda, and interests there should closely monitor the latest forecast updates. 3. Larry is forecast to move near or over portions of southeastern Newfoundland Friday night, early Saturday morning as it undergoes transition to a hurricane-force post-tropical cyclone. Hurricane and Tropical Storm Watches are in effect for southeastern Newfoundland. There is an increasing risk of impacts from high winds, rainfall, and storm surge in portions of Newfoundland, and interests there should monitor updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 09/0900Z 30.9N 61.1W 85 KT 100 MPH 12H 09/1800Z 32.9N 62.1W 85 KT 100 MPH 24H 10/0600Z 36.7N 62.0W 85 KT 100 MPH 36H 10/1800Z 41.8N 59.0W 80 KT 90 MPH 48H 11/0600Z 47.9N 52.5W 70 KT 80 MPH 60H 11/1800Z 54.1N 45.9W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 12/0600Z 59.2N 41.2W 60 KT 70 MPH...POST-TROP/EXTRATROP 96H 13/0600Z...ABSORBED $$ Forecaster Roberts

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Tropical Depression Mindy Forecast Discussion Number 3

2021-09-09 10:42:49| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Thu Sep 09 2021 000 WTNT43 KNHC 090842 TCDAT3 Tropical Depression Mindy Discussion Number 3 NWS National Hurricane Center Miami FL AL132021 500 AM EDT Thu Sep 09 2021 Doppler radar data and surface observations show that the center of Mindy has moved inland over southern Georgia overnight. Now that more of the circulation has moved over land, the cyclone has weakened and the latest observations indicate that Mindy has become a tropical depression with an estimated intensity of 30 kt. Mindy is forecast to move over the western Atlantic by this afternoon, but strong southwesterly shear is expected to prevent re-strengthening. In fact, the shear is forecast to increase to around 35 kt by tonight, and this is likely to strip away any remaining deep convection and cause gradual weakening. Mindy is forecast to become a remnant low in about 48 hours and dissipate by day 3, but given the expected shear both of those events could occur sooner. Mindy continues to move briskly northeastward or 055/17 kt. There has been no change to the track forecast philosophy since the previous advisory. Mindy should continue to move northeastward near the southern portion of a deep-layer trough that will be moving across the eastern United States today. Once the trough begins to lift northeastward late tonight and Friday, Mindy should slow down and turn east-northeastward until dissipation occurs. The guidance envelope has shifted slightly northward this cycle, and the updated NHC track forecast has been modified accordingly. The new NHC track lies between the GFEX consensus aid and the GFS ensemble mean. Key Messages: 1. Mindy is expected to produce heavy rainfall across portions of southern Georgia and coastal South Carolina this morning. This rainfall may produce isolated to scattered flash, urban, and small stream flooding. 2. Tropical-storm-force wind gusts are possible across portions of southeastern Georgia this morning. FORECAST POSITIONS AND MAX WINDS INIT 09/0900Z 30.8N 83.0W 30 KT 35 MPH 12H 09/1800Z 31.9N 80.0W 30 KT 35 MPH...OVER WATER 24H 10/0600Z 33.0N 75.8W 30 KT 35 MPH 36H 10/1800Z 33.9N 72.0W 25 KT 30 MPH 48H 11/0600Z 34.4N 69.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 11/1800Z 34.8N 67.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 12/0600Z...DISSIPATED $$ Forecaster Brown

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Hurricane Larry Forecast Discussion Number 34

2021-09-09 04:58:23| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Wed Sep 08 2021 000 WTNT42 KNHC 090258 TCDAT2 Hurricane Larry Discussion Number 34 NWS National Hurricane Center Miami FL AL122021 1100 PM AST Wed Sep 08 2021 The infrared satellite presentation of Larry this evening is giving the false impression of a well-organized hurricane, with a ring of colder cloud tops (-65 to -70 C) and a warm spot within. However, we are fortunate to have in-situ data provided by an Air Force Reconnaissance Hurricane Hunter mission investigating Larry this evening. Their observations show that Larry's center is actually southwest of the warm spot seen on satellite. In fact, the plane was unable to identify an eye with Larry, and the highest flight-level and SFMR winds were found nearly 80 nautical miles away from the center. Moreover, there remains a large discrepancy between the peak 700-mb flight level winds (108-kt) versus the much lower SFMR peak values (67-kt). This suggests that the 90 percent reduction factor that is typically applied to 700-mb flight level winds in the eyewall may not be appropriate for this hurricane given its very large radius of maximum winds more associated with weaker outer convection. Given these factors, the latest NHC initial intensity has been lowered to 85 kt for this advisory. This intensity is also a good compromise between the SAB and TAFB subjective Dvorak estimates. The hurricane's heading is still off to the northwest with the latest motion at 330/14 kt. The track forecast philosophy has not changed much for the last few days, with Larry moving around the western periphery of a deep-layer subtropical anticyclone over the central Atlantic. The hurricane should make its closest approach to the east of Bermuda tomorrow as it gradually turns north-northwest and north. After that, the hurricane will begin to dramatically accelerate to the northeast as Larry is picked up by a deep-layer trough moving offshore of the eastern United States. The latest forecast track is quite similar to the previous one, and takes Larry across the southeast portion of Newfoundland in 48-60 hours. The official forecast remains close to the tightly clustered track guidance consensus. The current structure of Larry appears to be somewhat tilted with height, with the low-level center identified by recon located to the southwest of the apparent center on IR satellite. While the shear as diagnosed by SHIPS appears to be lower, it appears dry air has significantly disrupted Larry's inner core structure, to the point that it likely will be unable to take advantage of the more favorable conditions. The latest NHC intensity forecast is lower than the previous one, owing to the lower initial intensity, but begins to show more pronounced weakening after 24 hours when the hurricane will accelerate poleward of the northern wall of the Gulf Stream. However, Larry is still forecast to be a hurricane as it passes near or over Newfoundland, though likely beginning to undergo extratropical transition. The models continue to maintain Larry as a large formidable cyclone after extratropical transition is complete while it moves into the far north Atlantic east of Greenland. This cyclone will eventually merge with another extratropical cyclone by the end of the forecast period. Key Messages: 1. Large swells generated by Larry will continue to affect the Leeward Islands, portions of the Greater Antilles, the Bahamas, Bermuda, and the east coast of the United States and Atlantic Canada through the end of the week. These swells will likely cause life-threatening surf and rip current conditions, and beachgoers and other interests along these coasts are urged to follow the guidance of lifeguards and local officials this week. 2. The center of Larry is forecast to pass east of Bermuda on Thursday, but given Larry's large size, tropical storm conditions are expected there on Thursday, along with a risk of coastal flooding. A Tropical Storm Warning is in effect for Bermuda, and interests there should closely monitor the latest forecast updates. 3. Larry is forecast to move near or over portions of southeastern Newfoundland late Friday and Friday night as it undergoes transition to a hurricane-force post-tropical cyclone. There is an increasing risk of impacts from high winds, rainfall, and storm surge in portions of Newfoundland, and interests there should monitor updates to the forecast. FORECAST POSITIONS AND MAX WINDS INIT 09/0300Z 29.7N 60.3W 85 KT 100 MPH 12H 09/1200Z 31.5N 61.5W 85 KT 100 MPH 24H 10/0000Z 34.9N 62.3W 85 KT 100 MPH 36H 10/1200Z 39.3N 60.8W 80 KT 90 MPH 48H 11/0000Z 45.2N 55.8W 75 KT 85 MPH 60H 11/1200Z 51.6N 48.2W 70 KT 80 MPH...POST-TROP/EXTRATROP 72H 12/0000Z 57.4N 41.6W 65 KT 75 MPH...POST-TROP/EXTRATROP 96H 13/0000Z 63.8N 35.6W 55 KT 65 MPH...POST-TROP/EXTRATROP 120H 14/0000Z...DISSIPATED $$ Forecaster Papin

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