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Tropical Storm Sam Forecast Discussion Number 5
2021-09-23 22:59:56| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Sep 23 2021 000 WTNT43 KNHC 232059 TCDAT3 Tropical Storm Sam Discussion Number 5 NWS National Hurricane Center Miami FL AL182021 500 PM AST Thu Sep 23 2021 Sam could be in the beginning stages of developing a small inner-core this afternoon as a central dense overcast is beginning to take shape. On geostationary visible satellite imagery, the storm has occasionally exhibited a clear area, though this feature has been tilted a bit southeast of the estimated low-level center position. In addition, overshooting convective cloud tops have been recirculating cyclonically around this feature, suggesting convective symmetrization that may help shield the low-level vortex from dry-air intrusion as seen earlier on the northwest side of Sam. Unfortunately, I have not received a high resolution microwave pass over Sam in 9-12 hours, so there remains some uncertainty on the structure underneath the cirrus canopy. The latest subjective Dvorak estimates from SAB and TAFB were 55-kt and 45-kt respectively. Taking a blend of these intensity estimates yields 50-kt for this advisory. Sam's estimated motion is nearly identical to this morning, moving north of due west at 280/14 kt. A large and anomalously strong mid-level ridge is located poleward of Sam and should maintain the current west to west-northwest heading over the next 2-3 days. However, this ridge-axis is forecast to build in to the northwest ahead of Sam, which is likely to gradually slow down this forward motion over the next few days. In fact, Sam is forecast to move unusually slow for a tropical cyclone in the deep tropics east of the Lesser Antilles. Afterwards, a weakness in the ridge to the northwest is forecast to gradually develop as a broad deep-layer trough parks itself just offshore of the eastern United States by early next week. This could potentially allow Sam to gain a bit more latitude at a somewhat faster forward motion as the ridge axis shifts back east. The track guidance this cycle has slowed down a bit compared to this morning and is also a bit further south in the early portion of the forecast. The NHC official track will follow suit this advisory, though is not quite as slow as the GFS model. It is interesting to note that both the GFS and ECMWF ensemble guidance this afternoon shows a significant spread in track solutions by day 5 in the cross-track direction, so this track forecast is a bit more uncertain than usual. With the apparent improvement in inner-core structure inferred on satellite today, Sam appears poised to rapidly intensify in the short-term. In fact, the latest GFS-SHIPS guidance is now explicitly forecasting rapid intensification over the next 24-48 hours. This seems feasible given that the guidance is forecasting 5 kt or less vertical wind shear over the next 60 hours as the cyclone traverses over warm 28-29 C sea-surface temperatures. Thus, the intensity guidance in the short-term has been raised once again, now showing a 50-kt increase in intensity over the next 48 hours taking Sam to major hurricane intensity by Saturday afternoon. Remarkably, this rapid intensification is still below the latest HCCA and LGEM guidance over this time span. After this period, Sam is likely to start undergoing inner core fluctuations, related to eyewall replacement cycles that are challenging to forecast far in advance. A modest increase in southwesterly vertical wind shear could also occur in the day 3-5 period. Thus, the NHC intensity forecast still tops off Sam as 110-kt hurricane by the end of the forecast period, which is on the high end of the intensity guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 11.1N 39.7W 50 KT 60 MPH 12H 24/0600Z 11.5N 41.9W 60 KT 70 MPH 24H 24/1800Z 11.9N 44.2W 75 KT 85 MPH 36H 25/0600Z 12.3N 46.0W 90 KT 105 MPH 48H 25/1800Z 12.6N 47.4W 100 KT 115 MPH 60H 26/0600Z 13.1N 48.7W 105 KT 120 MPH 72H 26/1800Z 13.7N 50.0W 110 KT 125 MPH 96H 27/1800Z 15.2N 52.5W 110 KT 125 MPH 120H 28/1800Z 17.5N 55.5W 110 KT 125 MPH $$ Forecaster Papin
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Tropical Storm Sam Forecast Discussion Number 4
2021-09-23 17:00:20| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Thu Sep 23 2021 000 WTNT43 KNHC 231500 TCDAT3 Tropical Storm Sam Discussion Number 4 NWS National Hurricane Center Miami FL AL182021 1100 AM AST Thu Sep 23 2021 Overnight and this morning, the satellite structure of the tropical cyclone has been improving, with prominent curved bands, and obvious low-level cloud motions that indicate the circulation is becoming established within the convection. In particular, an SSMIS microwave pass at 0653 UTC indicated a substantial improvement in the convective structure, with a well-defined curved band wrapping three-quarters of the way around the center in both the 91- and 37-GHz channels. ASCAT-B wind retrievals at 1234 UTC also indicated a tight, well-defined circulation had formed, with peak winds of 44-kt on the north side of the vortex. Subjective Dvorak satellite intensity estimates are now T3.5/55-kt from SAB and T2.5/35-kt from TAFB. In addition, the latest objective intensity estimates from UW-CIMSS ADT and SATCON were at 35-kt and 43-kt respectively. Given the recent scatterometer data, the intensity has been set to 45-kt for this advisory. Thus, Tropical Depression 18 has been upgraded to Tropical Storm Sam. It is noteworthy that this is the 2nd earliest formation of the 18th named storm in the Atlantic basin, moving ahead of the 2005 hurricane season, and only trailing last year. Sam continues to move to the west-northwest with an estimated motion at 280/14 kt. A prominent mid-level ridge remains entrenched to the north of the cyclone, and this feature should continue to steer Sam to the west-northwest over the next 2-3 days. Over this period, the ridge orientation actually shifts more westward ahead of the cyclone as a deep-layer trough digs in well northeast of Sam. This gradual change in the synoptic pattern should also result in a slowdown in Sam's forward motion over the next 72 hours. While the track guidance remains in good agreement over this period, larger spread begins to take shape beyond the day 3 forecast. Similar to yesterday, the ECMWF model and its ensemble mean flanks the southwest side of the guidance envelop, while the GFS is on the northeast side by day 5. These differences appear to be partially related to the mid-level ridge intensity on the western end, which could begin to be eroded by a mid-latitude trough off the eastern United States in 120-h. In addition, differences in both the size and intensity of Sam may also play a role in its ultimate track evolution. For this advisory, the latest NHC track forecast remains close to the consensus aids TCVN and TCVE, which represents a slight shift westward and a bit faster motion compared to the previous forecast. Because Sam is forecast to slow down, it remains too early to determine what impacts, direct or indirect, could be felt by the Lesser Antilles by this cyclone. The previously mentioned SSMIS microwave imagery also indicated that the low-level center has become better aligned with the mid-level vortex, perhaps a bit earlier than anticipated given the scatterometer data from last night. This improvement in structure, combined with favorable low vertical wind shear under 10 kt and warm 28-29 C sea-surface temperatures, argues for significant intensification. In fact, the GFS-SHIPS rapid intensification index now give Sam a 39 percent chance of a 65-kt increase over the next 72 hours, which is more than 7 times the climatological value. Thus, the NHC intensity forecast has been raised quite a bit from the prior one, especially in the short-term. The latest forecast now makes Sam a hurricane in 36 hours, and a major hurricane in 72 hours. This intensity forecast is just a shade under the latest HCCA consensus aid. Thereafter, a more gradual intensification rate is forecast. By this period it is possible that Sam could undergo hard to predict inner-core changes such as eyewall replacement cycles. In addition, the ECMWF-SHIPS suggests that the shear may also increase a little in days 4-5 which also argues for a bit slower rate of intensification for this period. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 10.9N 38.1W 45 KT 50 MPH 12H 24/0000Z 11.3N 40.3W 50 KT 60 MPH 24H 24/1200Z 11.9N 42.9W 60 KT 70 MPH 36H 25/0000Z 12.3N 45.0W 70 KT 80 MPH 48H 25/1200Z 12.7N 46.7W 80 KT 90 MPH 60H 26/0000Z 13.2N 48.2W 90 KT 105 MPH 72H 26/1200Z 13.7N 49.4W 100 KT 115 MPH 96H 27/1200Z 15.3N 52.1W 105 KT 120 MPH 120H 28/1200Z 17.4N 55.2W 110 KT 125 MPH $$ Forecaster Papin
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Tropical Depression Rose Forecast Discussion Number 16
2021-09-23 04:38:42| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Sep 22 2021 000 WTNT42 KNHC 230238 TCDAT2 Tropical Depression Rose Discussion Number 16 NWS National Hurricane Center Miami FL AL172021 1100 PM AST Wed Sep 22 2021 Rose continues to be devoid of deep convection as shear and dry mid-level air has taken their toll on the system. Although Rose appears to be well on its way to post-tropical cyclone status, it seems prudent to maintain advisories a little longer since the system is over warm waters and some convection could return overnight. If that does not occur, Rose is likely to become a remnant low by tomorrow morning, and that is what is indicated in the official forecast. The remnant low is forecast to continue to gradually spin down over the next day or two. The GFS and ECMWF suggest that some convection could re-develop in the 48-96 h time period when the remnants of Rose interact with an upper-level trough, but the NHC forecast once again does not call for re-generation at that time. In fact, the latest forecast calls for the system to become an open trough by 96 h, which is supported by the GFS. Rose has continued to move west-northwestward or 285/9 kt. The cyclone should turn northwestward very soon as it moves around the western flank of a low- to mid-level ridge. Rose or its remnants should then turn northward, northeastward, and then eventually east-northeastward as a deep-layer trough digs southward over the east-central Atlantic. The latest guidance envelope has again shifted westward in the short term, and the updated NHC track forecast has been adjusted accordingly. After 36 h, the official forecast lies near the previous advisory and is close to the various consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 23/0300Z 24.6N 40.9W 30 KT 35 MPH 12H 23/1200Z 25.7N 41.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 24H 24/0000Z 27.2N 42.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 36H 24/1200Z 28.4N 41.4W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 25/0000Z 29.0N 39.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 25/1200Z 29.5N 36.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 26/0000Z 30.2N 33.5W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 27/0000Z...DISSIPATED $$ Forecaster Brown
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Tropical Depression Eighteen Forecast Discussion Number 2
2021-09-23 04:38:42| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Sep 22 2021 000 WTNT43 KNHC 230238 TCDAT3 Tropical Depression Eighteen Discussion Number 2 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Wed Sep 22 2021 The depression's organization has not improved since earlier this afternoon. An analysis of the ambiguities from 0000 UTC ASCAT-B data suggests that the surface circulation is just barely closed, with the center still attached to the northern end of a trough. Peak wind retrievals from the scatterometer pass were 25-30 kt, so the initial intensity remains 30 kt. Based on the ASCAT data, the depression appears to be moving just north of due west, or 275/13 kt. Low- to mid-level ridging to the north is expected to maintain the cyclone on a westward heading for the next 36 hours, followed by a path toward the west-northwest from day 2 through day 5. The more notable part of the forecast is the forward speed. Global models are showing anomalously strong 500-mb ridging developing over the eastern and central Caribbean Sea in 2 to 3 days, which is likely to block the depression's forward progress toward the end of the forecast period. In fact, the current NHC forecast has the system moving west-northwestward at only 7 or 8 kt well east of the Lesser Antilles on days 3 through 5. All this means that it will probably be a few more days before we have a better idea if and how the system might potentially threaten areas farther west. For the 5-day forecast period, there is a normal amount of spread among the track guidance, and the updated NHC forecast is very close to a blend of the TVCA and HCCA consensus aids. This solution is not too different from the previous forecast. Overall, an environment of low shear, warm sea surface temperatures, and a moist, unstable atmosphere appears conducive for strengthening. However, there is some sort of signal being conveyed by the global models (e.g., the GFS, ECMWF, and UKMET) whereby the cyclone's surface circulation does not really tighten up for another 24 to 48 hours. Given the system's current structure, this scenario seems plausible. Therefore, the updated NHC intensity forecast shows a slower rate of strengthening during the first 48 hours, just a bit below the intensity consensus in deference to the global model solutions. After 48 hours, the official forecast converges on top of the previous NHC forecast and still brings the cyclone to hurricane, and then possibly major hurricane, strength by the end of the forecast period. FORECAST POSITIONS AND MAX WINDS INIT 23/0300Z 10.2N 35.0W 30 KT 35 MPH 12H 23/1200Z 10.4N 37.0W 30 KT 35 MPH 24H 24/0000Z 10.9N 39.6W 35 KT 40 MPH 36H 24/1200Z 11.4N 42.1W 45 KT 50 MPH 48H 25/0000Z 12.0N 44.3W 55 KT 65 MPH 60H 25/1200Z 12.6N 46.0W 70 KT 80 MPH 72H 26/0000Z 13.2N 47.5W 80 KT 90 MPH 96H 27/0000Z 14.5N 50.2W 90 KT 105 MPH 120H 28/0000Z 16.0N 53.3W 100 KT 115 MPH $$ Forecaster Berg
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Remnants of Peter Forecast Discussion Number 18
2021-09-23 04:37:12| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Wed Sep 22 2021 000 WTNT41 KNHC 230237 TCDAT1 Remnants Of Peter Discussion Number 18 NWS National Hurricane Center Miami FL AL162021 1100 PM AST Wed Sep 22 2021 Although a low-level swirl can still be seen in infrared satellite imagery, this feature has continued to lose definition. A recently arriving partial ASCAT-A overpass shows that the circulation has become more elongated, and Peter lacks a well-defined center. In addition, the system has not produced any organized deep convection in quite some time. The cloudiness and convective activity that has been occuring over the western Atlantic has been located along a trough axis well northeast of the decaying circulation center. As a result, Peter no longer meets the definition of a tropical cyclone, and this will be the final NHC advisory on this system. The initial intensity is set at 25 kt in accordance with the ASCAT data. The remnants of Peter are expected to remain within an area of strong upper-level westerly winds, and further weakening should occur over the next day or two. The system has been moving slowly north-northwestward or 335/4 kt. A weakness in the low-level ridge should allow the remnants to turn northward tomorrow, and this general motion should continue through the end of the week. This is the last NHC advisory on this system. Additional information on the remnants of Peter can be found in High Seas Forecasts issued by the National Weather Service, under AWIPS header NFDHSFAT1, WMO header FZNT01 KWBC, and online at ocean.weather.gov/shtml/NFDHSFAT1.php FORECAST POSITIONS AND MAX WINDS INIT 23/0300Z 22.1N 67.0W 25 KT 30 MPH...REMNANTS 12H 23/1200Z...DISSIPATED $$ Forecaster Brown
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