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Hurricane Sam Forecast Discussion Number 15

2021-09-26 10:47:43| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sun Sep 26 2021 000 WTNT43 KNHC 260847 TCDAT3 Hurricane Sam Discussion Number 15 NWS National Hurricane Center Miami FL AL182021 500 AM AST Sun Sep 26 2021 Sam remains a small but intense hurricane this morning, with a well-defined 10 n mi-wide eye. A 0500 UTC AMSR-2 microwave pass reveals Sam has a compact inner core of deep convection surrounding its eye, with curved convective bands primarily extending around the northern and eastern portions of the circulation. Infrared cloud top temperatures briefly warmed overnight in portions of Sam's central dense overcast, perhaps due to some intrusions of drier mid-level air. The 06z objective and subjective satellite estimates ranged from 113-127 kt, but Sam's satellite presentation has improved in recent infrared imagery as cloud top temperatures are cooling around the eye. Thus, the initial intensity is held at 125 kt for this advisory. NOAA aircraft are scheduled to investigate Sam later this afternoon, which should provide helpful information about the structure and intensity of the hurricane. Sam is moving toward the west-northwest, or 295/7 kt. A deep-layer subtropical ridge to the north and northeast of Sam should generally steer the hurricane west-northwestward to northwestward for the next several days. By midweek, an amplifying upper-level trough will move off the east coast of the U.S. and dig southward over the western Atlantic, which will erode the western extent of the steering ridge. The deep-layer southerly flow ahead of the trough should cause Sam to begin recurving, with a somewhat faster north-northwestward motion forecast by day 5. The latest NHC track forecast is very similar to the previous one and generally lies between the HFIP corrected consensus approach (HCCA) and TVCA aids. At days 4-5, there is more spread in the track guidance, with the ECMWF on the far left side of the guidance envelope and the GFS on the far right. The NHC forecast track has been nudged just slightly to the left at 96 and 120 h, but it still lies to the right of HCCA. The hurricane will likely experience some fluctuations in intensity during the next day or two. On the one hand, Sam remains over warm sea-surface temperatures of around 28.5 deg C, with vertical wind shear less than 10 kt as diagnosed from the SHIPS guidance. However, GOES-16 water vapor imagery shows some drier mid-level air in the surrounding environment that may periodically entrain into the inner core of Sam. Additionally, the onset of an eyewall replacement cycle remains a distinct possibility at this stage of Sam's life cycle, although the latest microwave data does not suggest one is imminent. The official NHC intensity forecast lies on the high end of the guidance for the first 36-48 h of the forecast period. Thereafter, the NHC forecast trends closer to the HCCA and IVCN consensus aids. Some gradual weakening is forecast later in the period as the southwesterly vertical wind shear begins to increase, but Sam is still forecast to remain a major hurricane through the 5-day period. FORECAST POSITIONS AND MAX WINDS INIT 26/0900Z 13.8N 49.7W 125 KT 145 MPH 12H 26/1800Z 14.3N 50.6W 125 KT 145 MPH 24H 27/0600Z 15.0N 51.8W 120 KT 140 MPH 36H 27/1800Z 15.8N 52.9W 120 KT 140 MPH 48H 28/0600Z 16.6N 53.9W 115 KT 130 MPH 60H 28/1800Z 17.5N 55.0W 115 KT 130 MPH 72H 29/0600Z 18.4N 56.3W 115 KT 130 MPH 96H 30/0600Z 20.8N 59.3W 110 KT 125 MPH 120H 01/0600Z 24.5N 62.0W 110 KT 125 MPH $$ Forecaster Reinhart

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Hurricane Sam Forecast Discussion Number 14

2021-09-26 04:51:08| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sat Sep 25 2021 000 WTNT43 KNHC 260251 TCDAT3 Hurricane Sam Discussion Number 14 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Sat Sep 25 2021 It's been an interesting evening with regards to analyzing the various data from a NOAA Hurricane Hunter research flight into Hurricane Sam. Dropsonde data in both the southeast and northwest quadrants indicate that small eyewall mesovortices and possibly even tornado-scale vortices were present based on the wind profiles showing sharply opposite-direction winds from what would normally be expected in those regions of the hurricane. Some dropsonde surface winds have been as high as 162 kt, which is more representative of a gust, while SFMR surface winds have been as high as 133 kt. However, the strongest 700-mb flight-level winds have been 138-139 kt in the northeastern quadrant, which equates to about 125-kt tangential surface winds. Three dropsondes released in the eye indicate that the pressure had remained steady at 943-944 mb during the duration of the aircraft reconnoiter. That pressure equates to about 125 kt based on various pressure-wind relationships. Based on that estimate and the 700-mb flight-level to surface-wind conversion, the advisory intensity is 125 kt, which is representative of the mean tangential winds and no localized wind perturbations. The initial motion is toward the west-northwest, or 295/07 kt. Not to sound like a broken record, but no significant changes were made to the previous track forecast and reasoning. Sam is expected to move slowly west-northwestward and northwestward over the next few days around the southwestern periphery of a deep-layer subtropical ridge that is situated to the north and northeast of the small hurricane. On days 4 and 5, an upper-level trough/low is forecast to dig southward and amplify off the U.S. east coast and extend all the way to the Bahamas. This feature should act to lift Sam northward at a faster forward speed. The latest NHC model guidance based on 12Z and 18Z model runs has shifted noticeably to the east of the previous runs, and the new NHC track forecast has been nudged in that direction as well. However, since the NOAA G-IV jet aircraft has been out there sampling the environment around Sam, it's best to remain conservative and not shift the track any farther to the east until the new 00Z model runs with that new aircraft data come in for the next advisory package at 0600Z. The new NHC track forecast lies about halfway between the previous advisory track on the left and the tightly packed consensus track models on the right. The radar images from the reconnaissance aircraft indicated that the eyewall was thin in many locations due to dry-air intrusions, and the latest SHIPS intensity output indicates that Sam will remain within a fairly dry mid-level environment. Also, the depth of the warm water beneath the hurricane isn't overly deep, which could result in cold upwelling owing to Sam's slow forward motion of only 5-7 kt during the next couple of days. Eyewall replacement cycles are also likely now due to the hurricane's small size and strong intensity. Thus, fluctuations in intensity seem likely for the next couple of days even though the vertical wind shear is expected to remain quite low at only 5-10 kt. On days 3-5, however, the shear is forecast to increase to 15-20 kt from the southwest, which is expected to induce a slow weakening trend. However, it is likely that Sam will remain a major hurricane through 120 h, even on days 3-5 due to the cyclone moving over warmer and deeper water during that 3-day period. The new official intensity forecast is essentially the same as the previous advisory, and remains above the consensus model and is near the higher end of the intensity guidance. FORECAST POSITIONS AND MAX WINDS INIT 26/0300Z 13.5N 49.0W 125 KT 145 MPH 12H 26/1200Z 14.0N 49.9W 130 KT 150 MPH 24H 27/0000Z 14.7N 51.0W 125 KT 145 MPH 36H 27/1200Z 15.4N 52.1W 120 KT 140 MPH 48H 28/0000Z 16.3N 53.2W 120 KT 140 MPH 60H 28/1200Z 17.1N 54.3W 115 KT 130 MPH 72H 29/0000Z 18.0N 55.6W 115 KT 130 MPH 96H 30/0000Z 20.1N 58.5W 110 KT 125 MPH 120H 01/0000Z 23.7N 61.4W 110 KT 125 MPH $$ Forecaster Stewart

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Post-Tropical Cyclone Teresa Forecast Discussion Number 5

2021-09-25 22:44:37| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 25 2021 000 WTNT44 KNHC 252044 TCDAT4 Post-Tropical Cyclone Teresa Discussion Number 5 NWS National Hurricane Center Miami FL AL192021 500 PM AST Sat Sep 25 2021 Teresa has continued to consist of just a swirl of low-level clouds since last night. Although a convective band persists a couple hundred miles northeast of the low center, the system no longer meets the definition of a tropical cyclone. Strong west-southwesterly wind shear should prevent any regeneration of convection near the center. All of the global models show the remnant low degenerating into an open trough by Sunday morning. Teresa has been moving eastward or 090/4 kt during the past 12 hours. A turn to the northeast is expected within the next couple of hours as the cyclone moves in the southwesterly flow ahead of a deep-layer trough. The northeastward motion should continue until the low dissipates Sunday morning. Additional information on Post-Tropical Cyclone Teresa 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 25/2100Z 34.4N 64.3W 30 KT 35 MPH...POST-TROPICAL 12H 26/0600Z 35.7N 63.2W 30 KT 35 MPH...POST-TROP/REMNT LOW 24H 26/1800Z...DISSIPATED $$ Forecaster Hagen/Latto

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Hurricane Sam Forecast Discussion Number 13

2021-09-25 22:40:07| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 25 2021 000 WTNT43 KNHC 252039 TCDAT3 Hurricane Sam Discussion Number 13 NWS National Hurricane Center Miami FL AL182021 500 PM AST Sat Sep 25 2021 After a brief hiccup, Sam has resumed rapid strengthening and now shows a 12-15 n mi wide well-defined eye embedded in a central dense overcast with cloud tops colder than -70C. Subjective satellite intensity estimates from TAFB and SAB were 115 kt at 18Z, and objective intensity estimates have also been near 115 kt. Since the organization has increased some since 18Z, the initial intensity is increased to 120 kt. A NOAA Hurricane Hunter aircraft is currently enroute to Sam to provide a better estimate of the intensity. The hurricane has good cirrus outflow in all quadrants. However, a large arc cloud to the northwest of the central core suggests dry air is present in that area. The initial motion is still west-northwestward or 290/9. There are again no changes to the forecast track reasoning. The weak subtropical ridge currently to the north of Sam is forecast to shift eastward as a large deep-layer mid-latitude trough develops over the western Atlantic. This evolution should cause Sam to continue west-northwestward with some decrease in forward speed during the next couple of days, followed by a turn toward the northwest. Some increase in forward speed is likely by 120 h as Sam encounters stronger steering flow. The track guidance has shifted a little to the north and east since the previous advisory, and the new forecast track is also nudged in that direction, with the new track lying between the HCCA corrected consensus model and the other consensus models. Based on the NHC forecast, and all of the other guidance, Sam is expected to still be well to the east or northeast of the northern Leeward Islands through day 5. The current strengthening is expected to continue for the next 6-12 h, and based on this the new intensity forecast now calls for a peak intensity of 130 kt. After the peak, Sam is expected to be in an environment of warm sea surface temperatures and light wind shear for at least the next 3-4 days. This suggests the intensity will be controlled mainly by eyewall replacement cycles and possible intrusions of dry air, which would cause some short-term fluctuations in intensity. In the bigger picture, the intensity guidance calls for a slow weakening during the 24-120 h forecast period, and the new intensity forecast does the same. However, it is likely that Sam will remain a major hurricane through 120 h. FORECAST POSITIONS AND MAX WINDS INIT 25/2100Z 13.3N 48.5W 120 KT 140 MPH 12H 26/0600Z 13.7N 49.4W 130 KT 150 MPH 24H 26/1800Z 14.3N 50.5W 125 KT 145 MPH 36H 27/0600Z 14.9N 51.6W 120 KT 140 MPH 48H 27/1800Z 15.7N 52.7W 120 KT 140 MPH 60H 28/0600Z 16.6N 53.9W 115 KT 130 MPH 72H 28/1800Z 17.4N 55.0W 115 KT 130 MPH 96H 29/1800Z 19.5N 58.0W 110 KT 125 MPH 120H 30/1800Z 22.5N 61.0W 110 KT 125 MPH $$ Forecaster Beven

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Subtropical Depression Teresa Forecast Discussion Number 4

2021-09-25 16:45:28| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Sep 25 2021 897 WTNT44 KNHC 251445 TCDAT4 Subtropical Depression Teresa Discussion Number 4 NWS National Hurricane Center Miami FL AL192021 1100 AM AST Sat Sep 25 2021 Teresa has been devoid of deep convection for more than 12 hours now. The cloud pattern consists of a swirl of low-level clouds and a band of showers and thunderstorms located about 250 n mi north of the center. This patch of convection is detached from the low-level circulation, and it appears to be more involved with an upper-level trough to the northeast of Teresa. A 1210 UTC ASCAT-A pass shows that the winds within that convection have decreased, with several vectors in the 25 to 30 kt range. Based on that data, the intensity is lowered to 30 kt, making Teresa a subtropical depression. Teresa is unlikely to redevelop any convection near its center as strong west-southwesterly wind shear continues over the cyclone. This will prevent any opportunity for strengthening. Teresa is expected to become a remnant low later today or this evening and dissipate on Sunday. The global models are in good agreement with this scenario. Teresa has moved little during the past 12 hours, so a stationary motion is currently indicated for this advisory. However, a motion toward the northeast should begin within the next few hours and continue through dissipation as the cyclone moves in the flow ahead of a deep-layer trough. The track forecast was adjusted a little to the east of the previous advisory, and is in agreement with the model consensus. FORECAST POSITIONS AND MAX WINDS INIT 25/1500Z 34.3N 65.0W 30 KT 35 MPH 12H 26/0000Z 35.4N 64.4W 30 KT 35 MPH...POST-TROP/REMNT LOW 24H 26/1200Z 37.2N 62.6W 30 KT 35 MPH...POST-TROP/REMNT LOW 36H 27/0000Z...DISSIPATED $$ Forecaster Hagen/Beven

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