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Tropical Depression Ten Forecast Discussion Number 8

2021-08-30 04:37:43| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sun Aug 29 2021 000 WTNT45 KNHC 300237 TCDAT5 Tropical Depression Ten Discussion Number 8 NWS National Hurricane Center Miami FL AL102021 1100 PM AST Sun Aug 29 2021 The depression is looking poorly organized this evening, with the low-level center exposed for the past several hours, and only a small area of convection located over 50 n mi east of the center. Strong west-northwesterly shear on the order of 20 to 30 kt due to a nearby subtropical jet stream has been impacting the cyclone since earlier today. A recent ASCAT pass showed peak winds of 29 kt, so the initial advisory intensity is being kept at 30 kt. The shear is forecast to increase even more later tonight and persist through Monday night. If the depression survives through that time period, then it may have an opportunity to strengthen in a few days when the cyclone moves north of the jet stream. The official NHC intensity forecast was little changed from the previous one, and is close to the HCCA and IVCN consensus. The lack of deep convection has likely been the cause of a wobble to the northwest over the past several hours, as the shallow depression has been steered primarily the low-level trade wind flow. The overall motion over the past 12 h is 350/7 kt. A weakness in the subtropical ridge to the north of the depression should induce a generally northward motion over the next few days. By the middle of this week, the weakness in the ridge is forecast to fill in, and force the cyclone on more of a northwestward path. The guidance continues to shift to the west with their solutions, as they come into better agreement on the strength of the ridge, and the NHC foreast has followed suit with a shift to the west. However, the latest NHC track forecast remains to the east of all of the consensus track guidance. Therefore, further future adjustments to the track may be necessary. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 20.1N 50.6W 30 KT 35 MPH 12H 30/1200Z 21.1N 50.8W 30 KT 35 MPH 24H 31/0000Z 22.4N 50.4W 30 KT 35 MPH 36H 31/1200Z 23.5N 50.0W 30 KT 35 MPH 48H 01/0000Z 24.5N 50.3W 30 KT 35 MPH 60H 01/1200Z 25.6N 51.0W 35 KT 40 MPH 72H 02/0000Z 27.0N 51.9W 40 KT 45 MPH 96H 03/0000Z 30.0N 53.7W 45 KT 50 MPH 120H 04/0000Z 34.0N 53.3W 55 KT 65 MPH $$ Forecaster Latto

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Hurricane Ida Forecast Discussion Number 15

2021-08-29 22:52:12| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Sun Aug 29 2021 000 WTNT44 KNHC 292052 TCDAT4 Hurricane Ida Discussion Number 15 NWS National Hurricane Center Miami FL AL092021 400 PM CDT Sun Aug 29 2021 NWS Doppler radar imagery and data from an Air Force Reserve Reserve Hurricane Hunter aircraft showed that Ida made landfall around 1655 UTC along the southeastern coast of Louisiana near Port Fourchon with estimated maximum winds of 130 kt and a minimum pressure around 930 mb. Since that time, Ida made a second landfall southwest of Galliano, Louisiana, and with the eyewall now onshore weakening has begun. Based on the latest Doppler velocities the initial wind speed has been conservatively reduced to 115 kt for this advisory. As Ida's circulation moves farther inland this evening and overnight a faster rate of weakening is expected, and Ida is forecast to become a tropical depression over Mississippi by late Monday. Although weakening is forecast, damaging winds, especially in gusts, are expected to spread inland over southeastern Louisiana and southwestern Mississippi through Monday morning. To account for this, the gust factor in the Forecast/Advisory has been adjusted accordingly. By 72 hours, Ida is predicted to merge with a frontal zone over the eastern United States and become an extratropical low, and this low is forecast to strengthen into a gale center near Atlantic Canada by the end of the forecast period. Radar fixes indicate that Ida's forward motion has slowed and the initial motion estimate is 325/9 kt. The hurricane should turn northward tonight around the western periphery of a deep-layer ridge near the southeastern United States coast. Ida is forecast to turn northeastward and recurve over the eastern United States as it enters the mid-latitude westerlies. The GFS is a bit faster in ejecting the post-tropical cyclone northeastward on days 3 through 5, and the NHC forecast follows a blend of the various consensus models and the GFS ensemble mean. Key Messages: 1. Extremely life-threatening storm surge inundation of 9 feet or greater above ground level will continue through early this evening along portions of the coast between Burns Point, Louisiana, to Ocean Springs, Mississippi. Overtopping of local levees outside of the Hurricane and Storm Damage Risk Reduction System is possible where local inundation values may be higher. 2. Catastrophic wind damage will occur near the core of Ida as it moves inland over southeastern Louisiana during the next few hours. Hurricane-force winds and damaging wind gusts are expected through tonight within the Hurricane Warning area in southeastern Louisiana, including metropolitan New Orleans. 3. Damaging winds, especially in gusts, will spread inland near the track of the center of Ida into southwestern Mississippi tonight and early Monday. These winds will likely lead to widespread tree damage and power outages. 4. Ida will continue to produce heavy rainfall through Monday across the central Gulf Coast across southeast Louisiana, coastal Mississippi, and southwestern Alabama, resulting in considerable to life-threatening flash and urban flooding and significant river flooding impacts. As Ida moves inland, significant flooding impacts are possible across portions of the Lower Mississippi Valley, Tennessee Valley, Upper Ohio Valley, Central Appalachians, and Mid-Atlantic through Wednesday. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 29.5N 90.6W 115 KT 130 MPH...INLAND 12H 30/0600Z 30.7N 90.9W 65 KT 75 MPH...INLAND 24H 30/1800Z 32.4N 90.7W 35 KT 40 MPH...INLAND 36H 31/0600Z 34.0N 89.4W 25 KT 30 MPH...INLAND 48H 31/1800Z 35.6N 87.2W 25 KT 30 MPH...INLAND 60H 01/0600Z 37.1N 84.2W 20 KT 25 MPH...POST-TROP/INLAND 72H 01/1800Z 38.6N 80.5W 20 KT 25 MPH...POST-TROP/EXTRATROP 96H 02/1800Z 41.3N 72.5W 30 KT 35 MPH...POST-TROP/EXTRATROP 120H 03/1800Z 43.8N 67.0W 35 KT 40 MPH...POST-TROP/EXTRATROP $$ Forecaster Brown

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Tropical Storm Nora Forecast Discussion Number 17

2021-08-29 22:52:12| National Hurricane Center (East Pacific)

Issued at 300 PM MDT Sun Aug 29 2021 000 WTPZ44 KNHC 292052 TCDEP4 Tropical Storm Nora Discussion Number 17 NWS National Hurricane Center Miami FL EP142021 300 PM MDT Sun Aug 29 2021 Visible satellite imagery and surface observations along the west-central coast of Mexico suggest that the center of Nora has re-formed closer to a mass of deep convection that has persisted today over the southern Gulf of California. Various satellite data sets indicate that Nora is weakening. The latest SSMIS microwave data reveal that the structure of Nora has degraded since this morning, especially in the mid-levels where an eyewall structure is no longer evident. Additionally, a 15 UTC ASCAT-A pass shows the winds are substantially weaker than previously estimated. Thus, the initial intensity has been lowered to 50 kt for this advisory. The initial motion is somewhat uncertain given the earlier center relocation, but it is estimated to be north-northwestward, or 330/10 kt. The global models suggest the tropical cyclone will move generally northwestward over the next couple of days, between an upper-level low offshore the Baja California peninsula and a mid-level ridge to the east of Nora. This motion should keep the center of Nora along the coast, or just inland over mainland west-central or northwestern Mexico. The official NHC track forecast is once again shifted a little to the right based on the latest track guidance, which brings the cyclone further inland by Tuesday. Nora is forecast to gradually weaken over the next few days as the system continues to interact with the coastline of Mexico. Despite warm SSTs in the Gulf of California and weak environmental shear, it appears unlikely that Nora will move far enough away from the coast to take advantage of this favorable environment. In fact, it is certainly plausible that the center of Nora could move well inland earlier than forecast and weaken more rapidly. The NHC intensity forecast has been adjusted about 10 kt weaker at all forecast hours based on today's data supporting a much weaker initial intensity. Key Messages: 1. Nora is forecast to be at or near tropical storm strength while it moves near the coast of Sinaloa and southern Sonora through early Tuesday, and tropical storm warnings and watches are in effect for portions of that coastline. Interests in these areas should closely monitor the progress of Nora. 2. Heavy rain associated with Nora is expected across the west coast of Mexico from the states of Colima northward to southern Sonora. This rain will likely result in life-threatening flash flooding and mudslides across these regions. Rainfall from Nora is likely to spread into the southwestern U.S. and central Rockies during the middle to latter portion of the week bringing the potential for flash flooding to the region. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 24.3N 107.5W 50 KT 60 MPH...NEAR THE COAST 12H 30/0600Z 25.1N 108.2W 45 KT 50 MPH...NEAR THE COAST 24H 30/1800Z 25.8N 108.8W 40 KT 45 MPH...NEAR THE COAST 36H 31/0600Z 26.5N 109.3W 40 KT 45 MPH...NEAR THE COAST 48H 31/1800Z 27.2N 109.9W 35 KT 40 MPH...INLAND 60H 01/0600Z 28.2N 110.6W 30 KT 35 MPH...INLAND 72H 01/1800Z 28.9N 111.1W 30 KT 35 MPH...INLAND 96H 02/1800Z...DISSIPATED $$ Forecaster Reinhart

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Tropical Storm Julian Forecast Discussion Number 4

2021-08-29 22:39:43| National Hurricane Center (Atlantic)

Issued at 900 PM GMT Sun Aug 29 2021 000 WTNT41 KNHC 292039 TCDAT1 Tropical Storm Julian Discussion Number 4 NWS National Hurricane Center Miami FL AL112021 900 PM GMT Sun Aug 29 2021 The convective pattern and structure of Julian has remained consistent throughout today, with the low-level center remaining tucked under the southwestern edge of a deep convective mass and good outflow in the northeastern semicircle. A late morning ASCAT-C overpass sampled peak winds of 45 to 47 kt, and since the cyclone has likely intensified slightly since that time, the initial intensity has been increased to 50 kt. Julian is now moving quickly to the northeast, or 050/21 kt in southwesterly flow to the south of a deep-layer area of low pressure located just east of Newfoundland. The storm is forecast to continue to move northeastward at a similar forward speed through Monday, and then begin to turn northward then northwestward Monday night through Tuesday as the cyclone moves around the periphery of the larger low. The model track guidance continues to be in excellent agreement on this scenario, and the latest NHC track forecast is little changed from the previous one. A cold front associated with the low east of Newfoundland has already begun to interact with the circulation of Julian, as noted by dry air being pulled into the southern portion of the circulation, and linear banding developing to the south of the cyclone. Baroclinic forcing could allow for some slight additional strengthening through tonight. However, Julian should cross the 26 C isotherm this evening, and reach waters of 22 C by 24 h while interacting with the cold front and associated mid- to upper level trough to its north. These factors should cause the cyclone to go through extratropical transition tonight through tomorrow morning, with the transition complete by late tomorrow. The latest SHIPS guidance as well as the FSU phase-space diagrams depict this transition scenario. Once extratropical, the system should dissipate within a couple of days over the north Atlantic. The latest NHC intensity forecast was essentially an update of the previous one, and is close to the HCCA and IVCN solutions. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 36.7N 44.6W 50 KT 60 MPH 12H 30/0600Z 38.8N 41.4W 55 KT 65 MPH 24H 30/1800Z 42.2N 38.1W 55 KT 65 MPH...POST-TROP/EXTRATROP 36H 31/0600Z 46.3N 36.8W 50 KT 60 MPH...POST-TROP/EXTRATROP 48H 31/1800Z 51.0N 37.7W 45 KT 50 MPH...POST-TROP/EXTRATROP 60H 01/0600Z 55.0N 39.6W 40 KT 45 MPH...POST-TROP/EXTRATROP 72H 01/1800Z...DISSIPATED $$ Forecaster Latto

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Tropical Depression Ten Forecast Discussion Number 7

2021-08-29 22:33:43| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sun Aug 29 2021 000 WTNT45 KNHC 292033 TCDAT5 Tropical Depression Ten Discussion Number 7 NWS National Hurricane Center Miami FL AL102021 500 PM AST Sun Aug 29 2021 The depression's center has again scooted northwestward away from the associated deep convection due to continuing 20-25 kt of northwesterly shear. Since there has been no appreciable change to the various intensity estimates from this morning, the estimated maximum winds remain 30 kt for this advisory. The source of the shear can be seen in water vapor imagery, with high-level clouds emanating from the eastern Caribbean Sea and blowing directly into the depression. In fact, model guidance is now indicating that the shear may increase as high as 35-40 kt during the next 24 hours while the depression moves beneath the subtropical jet stream, and the NHC intensity forecast therefore now shows no intensity change through 36 hours. By 48 hours, the system should have moved north of the subtropical jet, and lower shear and relatively warm waters should finally allow for some strengthening. During the latter part of the forecast period, the NHC intensity forecast is very close to the HCCA and IVCN consensus aids. Much of this forecast is contingent on the depression actually surviving the next 24-36 hours, however. The depression is moving just west of due north (355/10 kt), but the system is expected to slow down and turn slightly toward the north-northeast within 24 hours, ahead of the tail end of a deep-layer trough which extends over much of the central Atlantic. Beginning in about 48 hours, a piece of the east Atlantic subtropical ridge is forecast to pinch off and block the depression, causing it to turn back to the north and north-northwest through the end of the 5-day forecast period. This developing high looks like it will be a little stronger than previously thought, and most of the models have shifted significantly westward after 48 hours. The new NHC track forecast has been adjusted westward as well, but it is not as far west as the deterministic models and the consensus aids. Therefore, additional adjustment may be required in subsequent advisories. FORECAST POSITIONS AND MAX WINDS INIT 29/2100Z 19.7N 50.4W 30 KT 35 MPH 12H 30/0600Z 20.7N 50.3W 30 KT 35 MPH 24H 30/1800Z 22.1N 49.9W 30 KT 35 MPH 36H 31/0600Z 23.3N 49.4W 30 KT 35 MPH 48H 31/1800Z 24.5N 49.3W 35 KT 40 MPH 60H 01/0600Z 25.8N 49.8W 40 KT 45 MPH 72H 01/1800Z 27.4N 50.6W 40 KT 45 MPH 96H 02/1800Z 30.4N 51.9W 50 KT 60 MPH 120H 03/1800Z 33.7N 52.0W 55 KT 65 MPH $$ Forecaster Berg

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