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Tropical Depression Ten Forecast Discussion Number 3

2021-08-28 22:34:52| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Aug 28 2021 000 WTNT45 KNHC 282034 TCDAT5 Tropical Depression Ten Discussion Number 3 NWS National Hurricane Center Miami FL AL102021 500 PM AST Sat Aug 28 2021 This afternoon's satellite presentation consists of an exposed surface circulation with a rather shapeless convective mass displaced to the east and north of the center. Without question, the 15 to 20 kt of west-northwesterly shear is doing a number on the cloud pattern. Also evident, are a number of arc clouds propagating away from the system's deep convection, certainly indicative of dry air intrusion. Consequently, the initial intensity is once again held at 30 kt for this advisory. The global models as well as the Decay SHIPS (ECMWF/GFS) intensity guidance still show the shear relaxing a bit tonight and into Sunday, which should allow the depression to briefly become a tropical storm. By Monday, however, the shear is forecast to increase while the cyclone moves into an even more dry and stable air mass. These inhibiting contributions should stifle further strengthening and weaken the cyclone back to a depression through the remainder of the forecast period. It's worth mentioning that the GFS indicates that the depression will become a remnant low in 72 hours while turning northwestward to west-northwestward within the tradewind flow and dissipate by the end of the week. For now, the NHC forecast will stick with the consensus intensity models and the SHIPS guidance which agree on maintaining a depression through day 5. The initial motion is estimated to be northward, or 360/7 kt. The depression remains embedded in low- to mid-level southerly flow produced by a mid-Atlantic trough dominating the central Atlantic. This synoptic feature, along with a subtropical high pressure located over the east Atlantic and western Africa, should influence a generally northward motion through the 5-day period. The official track forecast again is adjusted a little to the right of the previous advisory beyond day 3, and lies between the TVCN and HCCA multimodel solutions. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 15.6N 50.0W 30 KT 35 MPH 12H 29/0600Z 16.6N 50.0W 35 KT 40 MPH 24H 29/1800Z 18.2N 49.9W 35 KT 40 MPH 36H 30/0600Z 19.8N 49.7W 30 KT 35 MPH 48H 30/1800Z 21.3N 49.3W 30 KT 35 MPH 60H 31/0600Z 22.5N 48.8W 30 KT 35 MPH 72H 31/1800Z 23.8N 49.0W 30 KT 35 MPH 96H 01/1800Z 27.2N 50.5W 30 KT 35 MPH 120H 02/1800Z 30.5N 51.5W 30 KT 35 MPH $$ Forecaster Roberts

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Hurricane Ida Forecast Discussion Number 9

2021-08-28 16:57:13| National Hurricane Center (Atlantic)

Issued at 1000 AM CDT Sat Aug 28 2021 000 WTNT44 KNHC 281457 TCDAT4 Hurricane Ida Discussion Number 9 NWS National Hurricane Center Miami FL AL092021 1000 AM CDT Sat Aug 28 2021 The satellite presentation of Ida has continued to improve this morning with the center embedded within a fairly symmetric Central Dense Overcast and hints of the eye in both visible in infrared satellite imagery. Earlier microwave imagery revealed a well- defined low- to mid-level eye and excellent spiral banding over the northern semicircle. Both NOAA and Air Force Reserve hurricane hunter aircraft, somewhat surprisingly, have not found a significantly deepening tropical cyclone yet, but given the very recent increase in organization and structure, the winds are likely to catch up to the satellite appearance very soon. Using the latest flight-level and SFMR wind data from the aircraft and satellite intensity estimates the initial wind speed remains 75 kt for this advisory. The aircraft data does indicate that Ida's wind field has expanded, and the NHC wind radii have been adjusted accordingly. Given the improved inner-core structure Ida appears poised to rapidly intensify during the next 12 to 24 hours as it remains within a favorable environment of low vertical wind shear and over warm water. Although the interpolated intensity guidance has lowered somewhat due to the fact that Ida has not significantly strengthened yet, the explicit forecasts from the dynamical models continue to support significant deepening, and the latest runs of the HWRF and HMON models indicate Ida will reach Category 4 strength before landfall. The new NHC intensity forecast calls for rapid intensification to Category 4 strength during the next 24 hours. After that time, fluctuations in intensity are possible due to eyewall replacement cycles as Ida approaches the northern Gulf coast. After landfall, rapid weakening is expected and Ida is forecast to weaken to a tropical depression by Monday night, and become a post-tropical cyclone as it moves over the Tennessee Valley by mid-week. Ida has been moving steadily northwestward or 315/14 kt. A deep layer ridge near the southeastern United States coast is forecast to shift westward during the next day or so, and this should continue to steer Ida northwestward through landfall on Sunday. The track guidance remains in remarkably good agreement through 36 hours, and the new NHC track forecast is essentially unchanged during that time, and continues to indicate that Ida will reach the coast of Louisiana on Sunday. After landfall, Ida will be near the western extent of the ridge and should turn northward and then northeastward as it recurves into the southern portion of the mid-latitude westerlies. The track guidance has shifted slightly westward beyond 48 hours, and the track forecast over the southern U.S. and the Tennessee Valley has been adjusted accordingly. Users are again reminded to not focus on the exact details of the track forecast as storm surge, wind, and rainfall impacts will extend far from the center. Wind and rainfall impacts will also penetrate inland through early next week after Ida makes landfall. Key Messages: 1. There is a danger of life-threatening storm surge inundation Sunday along the coasts of Louisiana and Mississippi within the Storm Surge Warning area. Extremely life-threatening inundation of 9 feet or greater above ground level is possible somewhere within the area from Morgan City, Louisiana, to the coast of Mississippi. Overtopping of local levees outside of the Hurricane and Storm Damage Risk Reduction System is possible where local inundation values may be higher. Interests throughout the warning area should follow any advice given by local officials. 2. Ida is expected to be an extremely dangerous major hurricane when it reaches the coast of Louisiana. Hurricane-force winds are expected Sunday in portions of the Hurricane Warning area along the Louisiana coast, including metropolitan New Orleans, with potentially catastrophic wind damage possible where the core of Ida moves onshore. Actions to protect life and property should be rushed to completion today in the warning area. 3. Ida is likely to produce heavy rainfall Sunday and Monday across the central Gulf Coast from southeast Louisiana to coastal Mississippi, resulting in life-threatening flash and urban flooding and significant river flooding impacts. As Ida moves inland, significant flooding impacts are likely across portions of the Lower Mississippi and Tennessee Valleys Monday and Tuesday. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 24.8N 86.1W 75 KT 85 MPH 12H 29/0000Z 26.3N 87.9W 100 KT 115 MPH 24H 29/1200Z 28.0N 89.8W 115 KT 130 MPH 36H 30/0000Z 29.4N 91.0W 115 KT 130 MPH...INLAND 48H 30/1200Z 31.0N 91.5W 60 KT 70 MPH...INLAND 60H 31/0000Z 32.7N 91.0W 30 KT 35 MPH...INLAND 72H 31/1200Z 34.4N 89.8W 25 KT 30 MPH...INLAND 96H 01/1200Z 36.8N 85.3W 20 KT 25 MPH...POST-TROP/INLAND 120H 02/1200Z 37.9N 79.9W 20 KT 25 MPH...POST-TROP/INLAND $$ Forecaster Brown

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Hurricane Nora Forecast Discussion Number 12

2021-08-28 16:50:44| National Hurricane Center (East Pacific)

Issued at 1000 AM CDT Sat Aug 28 2021 000 WTPZ44 KNHC 281450 TCDEP4 Hurricane Nora Discussion Number 12 NWS National Hurricane Center Miami FL EP142021 1000 AM CDT Sat Aug 28 2021 Nora continues to show signs of strengthening with the center embedded beneath a Central Dense Overcast, where clouds tops are now as cold as -85 degrees Celsius. Dvorak fixes from TAFB and SAB are T4.0/65 kt and T4.5/77 kt, and the most recent objective numbers are near 75 kt. Nora's initial intensity is conservatively estimated to be 70 kt, but an Air Force Reserve Hurricane Hunter aircraft should help us verify the maximum winds later this afternoon. The latest fixes indicate that Nora is moving west of due north, or 350/10 kt. A shortwave trough over the northern Rockies has created a weakness in the subtropical ridge over the southwestern U.S., which should allow Nora to continue moving northward or north-northwestward through tonight. This motion will take Nora's center and hurricane-force winds very close to or over portions of Jalisco, Mexico, later today and this evening. On Sunday, the track guidance suggests that Nora will make a turn back toward the northwest, bringing the center over the Gulf of California by Monday at a much slower forward speed. The track guidance is in good agreement on this scenario, but the narrow width of the Gulf of California means that hazards away from the center could affect both the coast of mainland Mexico and the east coast of Baja California Sur. Due to this uncertainty, new watches and warnings have been extended northward along both coasts. The track guidance--and the NHC official forecast--suggest that Nora's center could move inland over Sonora, Mexico, in 4 to 5 days. Assuming Nora's center remains over water, low shear and warm waters should allow for additional strengthening during the next 24-36 hours. After that time, possible land interaction, influences of nearby topography, and potential ingestion of dry air from the Mexican plateau could all conspire to cause at least gradual weakening while Nora begins to move into the Gulf of California. That said, there is still a lot of uncertainty in the intensity prediction, and Nora is forecast to remain as a hurricane through early Tuesday, a scenario supported by the ECMWF model. One thing the models do agree on is that Nora's wind field should contract after it passes Cabo Corrientes and heads into the Gulf of California, and this is reflected in the NHC forecast. Key Messages: 1. Nora is forecast strengthen through tonight as it moves near the coasts of the Mexican states of Colima, Jalisco, Nayarit, and southern Sinaloa, and hurricane warnings are in effect for portions of that coastline. Interests in these areas and points farther north should closely monitor the progress of Nora and subsequent updates to the forecast. 2. Heavy rain associated with Nora is expected across the west coast of Mexico from the Mexican states of Guerrero, northward to southern Sonora, including Baja California Sur. This rain will likely result in life-threatening flash flooding and mudslides across these regions. Rainfall from Nora may spread into the southwestern U.S. and central Rockies during the middle to latter portion of next week. 3. Nora is forecast to move over the Gulf of California as a hurricane by Monday, bringing a risk of wind impacts to portions of the Mexican states of Baja California Sur, Sinaloa, and Sonora. Given the above-average uncertainty in the forecast intensity, confidence is not high enough to determine the magnitude and location of these potential impacts. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 18.4N 105.4W 70 KT 80 MPH 12H 29/0000Z 20.1N 105.8W 75 KT 85 MPH 24H 29/1200Z 22.0N 106.5W 80 KT 90 MPH 36H 30/0000Z 23.1N 107.3W 80 KT 90 MPH 48H 30/1200Z 23.8N 108.0W 75 KT 85 MPH 60H 31/0000Z 24.5N 108.9W 70 KT 80 MPH 72H 31/1200Z 25.2N 109.6W 65 KT 75 MPH 96H 01/1200Z 26.9N 110.5W 60 KT 70 MPH 120H 02/1200Z 29.3N 111.0W 40 KT 45 MPH...INLAND $$ Forecaster Berg

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Tropical Depression Ten Forecast Discussion Number 2

2021-08-28 16:36:14| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Aug 28 2021 000 WTNT45 KNHC 281436 TCDAT5 Tropical Depression Ten Discussion Number 2 NWS National Hurricane Center Miami FL AL102021 1100 AM AST Sat Aug 28 2021 First light visible imagery shows that the rather disorganized surface center is exposed near the western edge of the cloud mass. The associated deep convection has been diminishing during the past 6 hours due to the stiff west-northwesterly shear and a rather parched surrounding thermodynamic environment. The initial intensity is held at 30 kt, consistent with the Dvorak satellite intensity estimates from TAFB and SAB and the 1156 UTC METOP-A scatterometer pass. The statistical SHIPS intensity guidance shows that the shear should decrease somewhat tonight and into Sunday, which should allow for some strengthening. By Monday, the aforementioned model, along with the deterministic guidance, indicates an increase in the shear magnitude and even lower mid- tropospheric relative humidity values, which should cap further strengthening and weaken the cyclone back to a depression through the remainder of the forecast period. There's more agreement in the large-scale models this morning indicating that the depression will degenerate into a remnant low toward the end of the week, and the NHC forecast follows suit. This is the only change from the previous advisory, and the NHC intensity forecast closely resembles the skilled IVCN and HCCA intensity consensus aids. The initial motion is estimated to be northward, or 350/7 kt. The depression is embedded in low- to mid-level southerly flow produced by a mid-Atlantic trough stretching from the northeast Atlantic to the central tropical Atlantic. This feature, along with a subtropical ridge situated over the east Atlantic and western Africa, should steer the depression generally toward the north through the 5-day period. The official track forecast has been nudged to the right of the previous advisory beyond day 3, and is close to the TVCN and NOAA HCCA multimodel guidance. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 14.8N 50.0W 30 KT 35 MPH 12H 29/0000Z 15.7N 50.2W 30 KT 35 MPH 24H 29/1200Z 17.1N 50.1W 35 KT 40 MPH 36H 30/0000Z 18.6N 50.0W 35 KT 40 MPH 48H 30/1200Z 20.1N 49.7W 30 KT 35 MPH 60H 31/0000Z 21.4N 49.4W 30 KT 35 MPH 72H 31/1200Z 22.7N 49.4W 30 KT 35 MPH 96H 01/1200Z 25.4N 50.6W 30 KT 35 MPH 120H 02/1200Z 28.6N 51.6W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Roberts

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Hurricane Nora Forecast Discussion Number 11

2021-08-28 11:00:40| National Hurricane Center (East Pacific)

Issued at 400 AM CDT Sat Aug 28 2021 000 WTPZ44 KNHC 280900 TCDEP4 Hurricane Nora Discussion Number 11 NWS National Hurricane Center Miami FL EP142021 400 AM CDT Sat Aug 28 2021 The inner-core structure appears to be tightening up with Nora this morning. Deep convection with tops colder than -70 C has been blossoming and wrapping around the center, first around the eastern side, but more recently in all quadrants as a distinct warm spot denotes the center. Earlier, there was a helpful SSMIS microwave pass at 0143 UTC that showed a closed cyan ring on the 37 GHz channel, which is a signal often related to a formative low-level eyewall and is often a harbinger for significant intensification. An ASCAT-B pass at 0403 UTC also suggested the wind-radii were gradually contracting, and also that the center position is a little further east of the short-term track. The latest subjective Dvorak estimates at 0600 UTC from SAB and TAFB have both increased to T4.0/65 kt. These estimates, in addition to the recent structural improvements in IR satellite and microwave imagery, support raising the intensity to 65-kt for this advisory, making Nora a hurricane. Nora continues to move a little more poleward this morning, with the latest motion estimate at 345 degrees at 9 kt. While the track forecast philosophy hasn't changed much over the last day, as a weakening mid-level ridge provides a weakness for Nora to escape north-northwestward, there has been a notable eastward shift in the track guidance. The deterministic ECMWF run, for instance, is now showing the hurricane brushing the shoreline of Mainland Mexico near Cabo Corrientes in 24 hours, and is now quite similar to the latest GFS forecast. The ECMWF ensembles have also shifted further east, with around half of the members explicitly showing landfall over mainland Mexico. The latest NHC track forecast has thus been shifted eastward, especially during the first 24-48 hours of the forecast, but I did not want to shift Nora's track as far east as the latest HCCA and TVCE consensus aids and further eastward adjustments may be necessary in subsequent forecasts. Afterwards, if Nora remains intact, the guidance shows the system bending back to the northwest and running up the gap of the Gulf of California between Baja California Sur and Mainland Mexico. Now that Nora appears to developing an inner core, at least steady intensification is expected in the short-term while the hurricane remains far enough offshore of mainland Mexico in a favorable dynamic and thermodynamic environment. After 12 hours, the intensity forecast becomes very tricky, due to just how much land interaction Nora will encounter as it brushes by Mexico. Once again, a lot of the reliable intensity guidance shows substantial land interaction and are sharply lower after 24 hours. Because the latest forecast track has been shifted further east, it increases the chance that significant disruption of the storm may occur by the mountainous terrain located near the coast of Mexico in that location. The latest intensity forecast was lowered after 24 hours and held at 70 kt through 96 hours due to this large uncertainty tied to possible land interaction. The latest forecast remains higher than the majority of the guidance, but is lower than the previous forecast. The wind radii have also been adjusted smaller after 24 hours, as the global model guidance suggests that the TC wind field will shrink quite dramatically as the system tracks near mainland Mexico. Tropical Storm and Hurricane Warnings have been extended further north along the mainland Mexico coastline given the eastward adjustments in the latest forecast track. Key Messages: 1. Nora is forecast strengthen today as it approaches the coasts of the Mexican states of Jalisco and Nayarit, and hurricane warnings are in effect for portions of that coastline. Interests in these areas and points further north should closely monitor the progress of Nora and subsequent updates to the forecast. 2.Heavy rain associated with Nora is expected across coastal sections of the Mexican states of Guerrero, Michoacan, Colima, Jalisco and Nayarit, as well as Baja California Sur. Life-threatening flash flooding and mudslides are expected. As Nora lifts northwest, heavy rainfall will spread into the Mexican states of Sinaloa and Sonora as well as the southwestern U.S. during the middle to latter portion of next week. 3. Nora is forecast to move over the Gulf of California as a hurricane early next week, bringing a risk of wind and rain impacts to portions of the Mexican states of Baja California Sur, Sinaloa, and Sonora. Given the above-average uncertainty in the forecast intensity, confidence is not high enough to determine the magnitude and location of these potential impacts. FORECAST POSITIONS AND MAX WINDS INIT 28/0900Z 17.1N 105.2W 65 KT 75 MPH 12H 28/1800Z 18.8N 105.5W 75 KT 85 MPH 24H 29/0600Z 20.8N 106.1W 75 KT 85 MPH...NEAR MAINLAND MEXICO 36H 29/1800Z 22.4N 107.0W 70 KT 80 MPH 48H 30/0600Z 23.3N 107.8W 70 KT 80 MPH 60H 30/1800Z 24.1N 108.6W 70 KT 80 MPH 72H 31/0600Z 24.9N 109.4W 70 KT 80 MPH 96H 01/0600Z 26.4N 110.6W 70 KT 80 MPH 120H 02/0600Z 28.7N 111.0W 45 KT 50 MPH...INLAND $$ Forecaster Papin/Beven

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