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Tropical Depression Seven Forecast Discussion Number 3

2021-08-14 04:59:57| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Fri Aug 13 2021 000 WTNT42 KNHC 140259 TCDAT2 Tropical Depression Seven Discussion Number 3 NWS National Hurricane Center Miami FL AL072021 1100 PM AST Fri Aug 13 2021 While Tropical Depression Seven has maintained a irregularly shaped cirrus canopy of deep cold cloud tops near its estimated center, the convection does not appear well organized. Several SSMIS microwave passes between 2011 UTC and 2205 UTC did not reveal much organization under the cirrus, with just a few patches of deeper convection contributing to the larger stratiform region. A helpful ASCAT-B pass at 0030 UTC showed that the center was near the southeastern end of this cirrus canopy, and found peak winds lower than earlier today at only 27 kt. The 0000 UTC subjective Dvorak estimates were T2.5/35 kt from SAB and T2.5/30 kt from TAFB and the most recent objective ADT estimate was in between at T2.2/32 kt. A blend of these data support keeping the initial intensity at 30-kt for this advisory. The small cyclone continues to move quickly off to the west at 280/18 kt. A large low- to mid-level ridge draped across the central and western North Atlantic is expected to maintain the system on a general west-northwest heading, though with gradual deceleration as the ridge is eroded some by a mid- to upper-tropospheric trough. The latest track guidance remains tightly clustered but a bit more poleward through the first 72 hours. Afterwards, more track guidance spread becomes apparent. A quick look at the latest ECMWF ensemble guidance suggest that some of this spread is driven by the forecast intensity of the system, with stronger members taking the cyclone on a more poleward track. For now, the latest NHC track forecast is fairly close to, but a little poleward of the previous track. This track remains close to the HCCA and TVCN consensus aids, and roughly splits the difference between the deterministic GFS and ECMWF model solutions. The intensity forecast is somewhat conflicting. Even though both the GFS & ECMWF based SHIPS guidance depict low 200-850 hPa vertical wind shear between 5-10 knots over the next 48 hours, the depression is also embedded in very dry mid-level air, with 700-500 hPa layer mean relative humidity as low as 44 percent currently in the ECMWF-SHIPS. In addition, the system is moving rapidly westward, and a continued fast motion in the short-term may result in higher westerly mid-level shear which may have a larger than normal effect to a small tropical cyclone in a very dry environment. After 48 hours, vertical wind shear out of the northwest is expected to increase, ahead of a large upper-level trough digging southwestward, upstream of the cyclone. Moreover, land interaction with both Puerto Rico and Hispaniola remains a distinct possibility, especially if the cyclone tracks left of the current forecast track. It is worth noting the latest HWRF run continues to be a extreme outlier with a much higher intensity than the remaining guidance. In fact, much of global model guidance and COAMPS-TC regional hurricane model barely maintains a closed circulation over the next 36-48 hours. I have elected to maintain a very similar forecast to the previous advisory, with peak winds of only 45 kt in 48-60 hours. This forecast remains conservative and is still lower than the SHIPS and HCCA intensity guidance. Key Messages: 1. Tropical storm conditions are expected in portions of the Leeward Islands late Saturday or early Sunday, and are possible over the Virgin Islands and Puerto Rico on Sunday. The risk of strong winds will then spread westward to the Dominican Republic Sunday night and Monday. 2. Heavy rainfall could lead to flash and urban flooding over the Leeward and Virgin Islands. Across Puerto Rico, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides. 3. There is a risk of wind and rainfall impacts across Haiti, the Turks and Caicos Islands, the southeastern Bahamas, and Cuba next week, and interests in those areas should monitor the progress of this system. FORECAST POSITIONS AND MAX WINDS INIT 14/0300Z 15.5N 53.8W 30 KT 35 MPH 12H 14/1200Z 16.0N 56.8W 35 KT 40 MPH 24H 15/0000Z 16.7N 60.3W 35 KT 40 MPH 36H 15/1200Z 17.5N 63.5W 40 KT 45 MPH 48H 16/0000Z 18.3N 66.2W 45 KT 50 MPH...INLAND 60H 16/1200Z 18.9N 68.2W 45 KT 50 MPH...OVER WATER 72H 17/0000Z 19.6N 70.4W 35 KT 40 MPH...INLAND 96H 18/0000Z 21.7N 74.3W 40 KT 45 MPH...OVER WATER 120H 19/0000Z 24.3N 78.6W 40 KT 45 MPH $$ Forecaster Papin/Brown

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Tropical Depression Fred Forecast Discussion Number 18

2021-08-14 04:51:25| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Fri Aug 13 2021 239 WTNT41 KNHC 140251 TCDAT1 Tropical Depression Fred Discussion Number 18 NWS National Hurricane Center Miami FL AL062021 1100 PM EDT Fri Aug 13 2021 Interaction with land and southwesterly shear has continued to take a toll on Fred this evening. It is very difficult to determine in infrared satellite imagery and recent surface observations from Cuba if a closed circulation still exists, however the system is maintained as a tropical depression for now. Scatterometer data, which should arrive very soon, and reconnaissance aircraft observations on Saturday morning should provide additional information on the system's intensity and structure. The system still appears to be moving moving westward or 280/11 kt. There has been no change to the track forecast reasoning from the previous advisory. The cyclone is approaching the western periphery of a subtropical ridge extending over the western Atlantic, and Fred should turn west-northwestward overnight, and then northwestward on Saturday. The track guidance has again shifted westward, but the shift is not as large as was noted on the previous cycle. The NHC track forecast has been adjusted to the left, closer to the consensus aids, but it still remains to the east of those models. Some additional westward adjustments in subsequent forecasts may be required. As mentioned above, land interaction and southwesterly vertical will limit any attempt of re-organization overnight, however the system is expected to move off the northern coast of Cuba Saturday morning and the global model guidance suggests that a center re-formation could occur over the western portion of the Straits of Florida. The center re-formation shown by the guidance appears to be aided by an area of upper-level diffluent flow to the southeast of an upper-level trough currently located over the eastern Gulf of Mexico. Although only a little strengthening is indicated in the official forecast through 36 hours, the environment could become a little more conducive on Sunday, and the NHC wind speed forecast again calls for gradual strengthening while Fred moves northwestward over the eastern Gulf. The updated intensity forecast is unchanged from the previous advisory and is a blend of the statistical guidance and the HCCA model. The intensity forecast continues to be of lower-than-normal confidence given the current disorganized structure of the system. KEY MESSAGES: 1. Tonight through Monday, heavy rainfall could lead to areal, urban, and small stream flooding impacts, and cause new and renewed river flooding, across southern and central Florida into the Big Bend. From Sunday onward, heavy rain and flood impacts could extend into other portions of the Southeast and into the southern and central Appalachians and Piedmont as Fred interacts with a front in the area. 2. Tropical storm conditions are expected in portions of the Lower Florida Keys on Saturday, where a Tropical Storm Warning is in effect. 3. There is a risk of tropical storm conditions in the Florida Panhandle beginning on Monday. Watches may be required for a portion of this area on Saturday. FORECAST POSITIONS AND MAX WINDS INIT 14/0300Z 22.7N 80.6W 30 KT 35 MPH 12H 14/1200Z 23.4N 81.9W 30 KT 35 MPH...OVER WATER 24H 15/0000Z 24.8N 83.6W 35 KT 40 MPH 36H 15/1200Z 26.3N 85.0W 35 KT 40 MPH 48H 16/0000Z 27.8N 86.0W 40 KT 45 MPH 60H 16/1200Z 29.2N 86.6W 45 KT 50 MPH 72H 17/0000Z 30.6N 86.8W 50 KT 60 MPH...INLAND 96H 18/0000Z 34.0N 86.5W 25 KT 30 MPH...INLAND 120H 19/0000Z 37.5N 84.0W 20 KT 25 MPH...POST-TROP/INLAND $$ Forecaster Brown

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Tropical Depression Seven Forecast Discussion Number 2

2021-08-13 22:52:54| National Hurricane Center (Atlantic)

Issued at 500 PM AST Fri Aug 13 2021 000 WTNT42 KNHC 132052 TCDAT2 Tropical Depression Seven Discussion Number 2 NWS National Hurricane Center Miami FL AL072021 500 PM AST Fri Aug 13 2021 Over the past few hours, the system moved just to the north of a buoy owned by the Woods Hole Oceanographic Institute. The buoy's winds backed from northwest to west to south, indicating that the system has a closed surface circulation. In addition, Dvorak estimates from TAFB and SAB were a unanimous T2.0, and the system is therefore being designated as a tropical depression on this advisory. Maximum winds remain 30 kt based on earlier ASCAT data and the Dvorak estimates. The circulation has closed off despite the depression moving quickly westward (275 degrees) at about 19 kt. The track guidance is tightly clustered during the first 48-60 hours, with the depression being driven quickly westward across the Leeward Islands and toward the Greater Antilles by ridging to the north. After 60 hours, there is considerably more spread, with the regional dynamical models keeping the system farther south over the Caribbean Sea, and most of the other models indicating a turn toward the west-northwest, following a track similar to Tropical Depression Fred. The NHC track forecast generally favors the latter scenario and is very close to the HCCA and TVCN consensus aids. During the next 2 days, the depression is expected to move beneath an upper-level ridge axis, which should allow the deep-layer shear to fall below 10 kt, with the system also moving over warmer waters and through an environment of increased moisture. However, the depression's fast motion, as well as the possible development of some mid-level westerly shear, could stunt the rate of strengthening. Due to these conflicting factors, the NHC intensity forecast remains on the conservative side and is not quite as high as the solutions shown by the SHIPS and HCCA models. The HWRF model is quite aggressive, bringing the depression to hurricane strength by day 3, but that model is an extreme outlier compared to the other guidance. After 48 hours, the current forecast takes the center of the depression over Puerto Rico and Hispaniola, a scenario that would cause weakening and suppress the system's intensity. As is typically the case, the system could get stronger than shown in the official forecast if it ends up moving over less land, or dissipate entirely if it moves over land for too long. Key Messages: 1. Tropical storm conditions are possible in portions of the Leeward Islands late Saturday or early Sunday, and over the Virgin Islands and Puerto Rico on Sunday. The risk of strong winds will then spread westward to the Dominican Republic Sunday night and Monday. 2. Heavy rainfall could lead to flash and urban flooding over the Leeward and Virgin Islands. Across Puerto Rico, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides. 3. There is a risk of wind and rainfall impacts across Haiti, the Turks and Caicos Islands, the southeastern Bahamas, and Cuba next week, and interests in those areas should monitor the progress of this system. FORECAST POSITIONS AND MAX WINDS INIT 13/2100Z 15.4N 51.8W 30 KT 35 MPH 12H 14/0600Z 15.9N 54.7W 35 KT 40 MPH 24H 14/1800Z 16.4N 58.5W 35 KT 40 MPH 36H 15/0600Z 17.0N 61.9W 40 KT 45 MPH 48H 15/1800Z 17.7N 65.0W 45 KT 50 MPH 60H 16/0600Z 18.4N 67.7W 45 KT 50 MPH 72H 16/1800Z 19.0N 70.3W 40 KT 45 MPH...OVER HISPANIOLA 96H 17/1800Z 20.9N 74.7W 35 KT 40 MPH...OVER WATER 120H 18/1800Z 23.6N 79.1W 40 KT 45 MPH $$ Forecaster Berg

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Hurricane Linda Forecast Discussion Number 15

2021-08-13 22:46:48| Tropical Depression LIDIA

Issued at 300 PM MDT Fri Aug 13 2021

Tags: number discussion linda forecast

 

Potential Tropical Cyclone Seven Forecast Discussion Number 1

2021-08-13 17:02:17| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Fri Aug 13 2021 000 WTNT42 KNHC 131502 TCDAT2 Potential Tropical Cyclone Seven Discussion Number 1 NWS National Hurricane Center Miami FL AL072021 1100 AM AST Fri Aug 13 2021 A tropical wave and small area of low pressure moving quickly westward across the central tropical Atlantic has been producing bursts of deep convection near and to the west of the wave axis. Scatterometer data from last evening indicated that the system did not quite have a closed circulation, but it is close. In addition, the convective organization has increased a bit since yesterday, with TAFB and SAB providing Dvorak classification of T1.5 and T2.0. Therefore, only an additional slight increase in convective organization and the low-level circulation closing off would make the system a tropical depression, and it could be at tropical storm strength when it approaches the Leeward Islands during the next couple of days. Therefore, advisories are being initiated on the system in order to issue Tropical Storm Watches for portions of the Leeward Islands at this time. Subtropical ridging extending across nearly the entire Atlantic is pushing the low along quickly toward the west, or 280/18 kt. In general this ridging is expected to persist into the middle part of next week. However, around the Sunday-Monday time frame, some mid-level troughing over the western Atlantic could allow the system to gain a little more latitude before it runs into additional ridging located near the east coast of the United States at the end of the forecast period. The GFS, which has perhaps one of the strongest solutions, is a northern outlier and shows the system turning farther into the weak troughing, while the ECMWF has a weaker solution and keeps the system on a westward track across the Lesser and Greater Antilles. At this time, the NHC official forecast lies close to the TVCN and HCCA consensus aids, and it is also relatively close to the ECMWF ensemble mean, which is farther north than its parent model. The biggest negative to the system becoming a tropical cyclone and strengthening is its fast motion of 15-20 kt. However, that motion is expected to gradually decrease in 2 to 3 days. At that same time, deep-layer shear has decreased a bit, and the system will be moving over warmer waters and toward a slightly more moist environment. Therefore, gradual strengthening is anticipated, and the NHC intensity forecast closely follows the HCCA and IVCN consensus aids through 72 hours, up until the system is near Hispaniola. After that time, land interaction is likely to disrupt the circulation, and the official forecast is below the model consensus aids on days 4 and 5. Key Messages: 1. Tropical storm conditions are possible in portions of the Leeward Islands late Saturday or early Sunday, and the risk of tropical storm conditions will spread westward to the Virgin Islands and Puerto Rico late Sunday and Sunday night. 2. Heavy rainfall with this system may produce areas of scattered flash flooding over the northern Leeward Islands Saturday into Sunday. 3. There is a risk of wind and rainfall impacts in portions of Hispaniola, the Turks and Caicos Islands, the southeastern Bahamas, and Cuba next week, and interests in those areas should monitor the progress of this system. FORECAST POSITIONS AND MAX WINDS INIT 13/1500Z 15.3N 49.3W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 12H 14/0000Z 15.7N 52.0W 30 KT 35 MPH...TROPICAL DEPRESSION 24H 14/1200Z 16.2N 55.9W 35 KT 40 MPH 36H 15/0000Z 16.7N 59.6W 35 KT 40 MPH 48H 15/1200Z 17.2N 63.0W 40 KT 45 MPH 60H 16/0000Z 17.8N 66.0W 40 KT 45 MPH 72H 16/1200Z 18.4N 68.7W 45 KT 50 MPH...OVER HISPANIOLA 96H 17/1200Z 20.0N 73.5W 35 KT 40 MPH...OVER WATER 120H 18/1200Z 23.0N 77.0W 40 KT 45 MPH $$ Forecaster Berg

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