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Tropical Storm Iselle Forecast Discussion Number 12

2020-08-29 10:37:45| National Hurricane Center (East Pacific)

Issued at 200 AM PDT Sat Aug 29 2020 000 WTPZ44 KNHC 290837 TCDEP4 Tropical Storm Iselle Discussion Number 12 NWS National Hurricane Center Miami FL EP142020 200 AM PDT Sat Aug 29 2020 Iselle has become less organized during the past several hours. The low-level center is exposed to the east of a decaying and disheveled area of deep convection. An ASCAT-B overpass from around 0430 UTC showed maximum winds in the 40-45 kt range, but since the system's appearance has degraded since that time, the initial intensity is set at 40 kt. This value is also in agreement with an average of the latest satellite intensity estimates, which range from 33 to 45 kt. Iselle is feeling the effects of about 25 kt of easterly vertical wind shear. Since the shear is expected to remain strong while Iselle heads for cooler waters and a drier air mass, steady weakening is anticipated. Iselle will likely become a tropical depression by tonight and a remnant low on Sunday. Most of the models show the remnant low dissipating in 3 days, and so does the official forecast. The compact tropical storm is moving northward at about 5 kt. A continued northward motion is expected for another day or so, followed by a turn to the northwest when Iselle becomes a weak and shallow system and moves in the low-level flow. The new NHC track forecast is a little to the east of the previous one to come into better agreement with the latest consensus aids. The initial 34-kt wind radii have been modified slightly based on the aforementioned ASCAT data. FORECAST POSITIONS AND MAX WINDS INIT 29/0900Z 19.7N 115.1W 40 KT 45 MPH 12H 29/1800Z 20.7N 114.9W 35 KT 40 MPH 24H 30/0600Z 22.2N 114.9W 30 KT 35 MPH 36H 30/1800Z 23.5N 115.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 48H 31/0600Z 24.5N 115.5W 20 KT 25 MPH...POST-TROP/REMNT LOW 60H 31/1800Z 25.3N 116.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 01/0600Z...DISSIPATED $$ Forecaster Cangialosi

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Tropical Storm Iselle Forecast Discussion Number 11

2020-08-29 04:32:14| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Fri Aug 28 2020 000 WTPZ44 KNHC 290232 TCDEP4 Tropical Storm Iselle Discussion Number 11 NWS National Hurricane Center Miami FL EP142020 800 PM PDT Fri Aug 28 2020 Persistent bursts of deep convection continue to develop just to the west of Iselle's center, which occasionally becomes hidden beneath the cirrus outflow (as it is now). Dvorak final-T numbers from TAFB and SAB are 3.0/45 kt based on a shear pattern, and along with what was shown by an afternoon ASCAT pass, Iselle's intensity remains 45 kt. Strong shear from the east-northeast is forecast to continue for the next 24 hours or so. After that time, the shear should relax, but then the thermodynamic environment becomes more limiting with Iselle moving over sub-26C waters and into a much more stable environment. All told, Iselle is expected to steadily weaken during the next several days, and it is likely to degenerate into a remnant low on Sunday. The NHC forecast is very similar to the previous prediction and is close to HCCA and the IVCN intensity consensus aid. The remnant low is expected to dissipate in about 4 days. Iselle is moving slowly north-northeastward (015/5 kt) on the western periphery of a low- to mid-level area of high pressure centered over Mexico. This high, along with a trough extending off the coast of California, should cause Iselle to gradually turn toward the north and then north-northwest with some increase in forward speed during the next couple of days. There is a typical amount of spread among the track models, and they generally agree on this northward motion with a slight bend to the left through the forecast period. The regional hurricane models appear almost in a cluster all to themselves on the eastern side of the guidance envelope, and the NHC forecast therefore closely follows the TVDG model consensus aid, which gives double weight to the global models. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 19.3N 115.2W 45 KT 50 MPH 12H 29/1200Z 20.2N 115.0W 40 KT 45 MPH 24H 30/0000Z 21.7N 115.0W 35 KT 40 MPH 36H 30/1200Z 23.1N 115.4W 30 KT 35 MPH 48H 31/0000Z 24.2N 115.9W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 31/1200Z 24.9N 116.4W 20 KT 25 MPH...POST-TROP/REMNT LOW 72H 01/0000Z 25.6N 116.8W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 02/0000Z...DISSIPATED $$ Forecaster Berg

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Post-Tropical Cyclone Hernan Forecast Discussion Number 11

2020-08-28 22:33:38| National Hurricane Center (East Pacific)

Issued at 300 PM MDT Fri Aug 28 2020 000 WTPZ43 KNHC 282033 TCDEP3 Post-Tropical Cyclone Hernan Discussion Number 11 NWS National Hurricane Center Miami FL EP132020 300 PM MDT Fri Aug 28 2020 Satellite imagery, scatterometer data, and surface observations indicate that Hernan has degenerated into a broad low pressure area near the southern end of the Baja California peninsula. The remnant low is forecast to move generally westward for the next 12-24 h until it is absorbed into the monsoon gyre that includes Tropical Storm Iselle. This is the last advisory issued by the National Hurricane Center on this system. For additional information on the remnant low please see High Seas Forecasts issued by the National Weather Service, under AWIPS header NFDHSFEPI, WMO header FZPN02 KWBC, and on the web at ocean.weather.gov/shtml/NFDHSFEPI.php FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 23.2N 110.1W 25 KT 30 MPH...POST-TROP/REMNT LOW 12H 29/0600Z 23.3N 111.8W 20 KT 25 MPH...POST-TROP/REMNT LOW 24H 29/1800Z...DISSIPATED $$ Forecaster Beven

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Tropical Storm Iselle Forecast Discussion Number 10

2020-08-28 22:32:37| National Hurricane Center (East Pacific)

Issued at 200 PM PDT Fri Aug 28 2020 000 WTPZ44 KNHC 282032 TCDEP4 Tropical Storm Iselle Discussion Number 10 NWS National Hurricane Center Miami FL EP142020 200 PM PDT Fri Aug 28 2020 First light visible imagery shows that Iselle's center of circulation has once again reappeared to the northeast of the deep convective mass. Last night's briefly diminished shear interlude probably resulted in Iselle's peak intensity. For this advisory, the initial intensity is lowered to 45 kt which is based on a blend of the Dvorak intensity estimates from TAFB and SAB, a METOP-B ASCAT overpass containing 40-45 kt winds and a SATCON analysis of 42 kt. The northeasterly shear is forecast to persist through the 24 hour period and then decrease and veer from the southeast. At that time, however, Iselle will be traversing sub 25C sea surface temperatures and will be moving into a more dry and stable surrounding environment. As a result, Iselle should weaken to a tropical depression Saturday night and degenerate to a remnant low on Sunday. The initial motion is estimated to be north-northeastward, or, 020/6 kt. A northward direction should commence Saturday morning, then a turn north-northwestward is forecast Sunday morning. As Iselle continues to weaken and degenerates to a remnant low, a turn toward the northwest, well offshore of the southern coast of the Baja California peninsula, is expected to occur Sunday late night. The NHC forecast is an update of the previous advisory and is close to the GFEX (FV3/ECMWF mean) and the NOAA HCCA consensus model. FORECAST POSITIONS AND MAX WINDS INIT 28/2100Z 18.8N 115.2W 45 KT 50 MPH 12H 29/0600Z 19.6N 114.9W 40 KT 45 MPH 24H 29/1800Z 21.0N 114.7W 35 KT 40 MPH 36H 30/0600Z 22.6N 115.0W 30 KT 35 MPH 48H 30/1800Z 23.7N 115.7W 25 KT 30 MPH...POST-TROP/REMNT LOW 60H 31/0600Z 24.4N 116.3W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 31/1800Z 25.1N 117.0W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 01/1800Z...DISSIPATED $$ Forecaster Roberts

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Tropical Storm Iselle Forecast Discussion Number 9

2020-08-28 16:47:33| National Hurricane Center (East Pacific)

Issued at 800 AM PDT Fri Aug 28 2020 000 WTPZ44 KNHC 281447 TCDEP4 Tropical Storm Iselle Discussion Number 9 NWS National Hurricane Center Miami FL EP142020 800 AM PDT Fri Aug 28 2020 GOES-16 shortwave infrared imagery and a recent AMSR2 microwave overpass indicate that deep convection has been developing near the surface center during the past several hours. In fact, the microwave image showed a small, compact inner core defined by a partially closed eye-like feature. My initial thoughts were that this cloud feature is in the mid-portion of the atmosphere, but the lower 37 GHZ frequency confirmed very little vertical tilt. Subjective and objective satellite intensity estimates are based on the shear scene-type which would yield a slightly lower intensity estimation. Consequently, the initial intensity is held at 50 kt, but it could certainly be a little stronger based on the aforementioned polar low-orbiter pass. The FV3/GFS and ECMWF SHIPS statistical-dynamical intensity models show 20 to 30 kt of northeasterly shear persisting through the next few days, however, the UW-CIMSS shear analysis reveals less than 20 kt. For now, based on what the large-scale models and the SHIPS models agree on, gradual weakening should begin by early Saturday morning and continue through Monday morning as the cyclone traverses decreasing oceanic temperatures and moves into a more thermodynamically stable surrounding environment. The NHC intensity is an update of the previous advisory, and calls for Iselle to weaken to a tropical depression on Sunday and degenerate into a remnant low on Sunday evening. Based on the 0920 UTC AMSR2 pass, the initial position was adjusted to the northwest of the previous position and the forward motion is estimated to be northeastward, or 035/4 kt. A northward direction should commence by early Saturday morning, then a turn north-northwestward to northwestward is forecast during the 48-60 hr period. As Iselle continues to weaken and become a more shallower system, a turn toward the west-northwest, well offshore of the southern coast of the Baja California peninsula, is forecast to occur Monday morning. The NHC forecast is nudged to the left of the previous track forecast due the adjusted position, and is based on the HCCA and TVCE multi-model consensus aids. FORECAST POSITIONS AND MAX WINDS INIT 28/1500Z 18.3N 115.3W 50 KT 60 MPH 12H 29/0000Z 19.2N 114.8W 50 KT 60 MPH 24H 29/1200Z 20.6N 114.6W 45 KT 50 MPH 36H 30/0000Z 22.2N 114.9W 35 KT 40 MPH 48H 30/1200Z 23.5N 115.7W 30 KT 35 MPH 60H 31/0000Z 24.1N 116.6W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 31/1200Z 24.4N 117.3W 20 KT 25 MPH...POST-TROP/REMNT LOW 96H 01/1200Z...DISSIPATED $$ Forecaster Roberts

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