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Tropical Storm Henri Forecast Discussion Number 17

2021-08-20 05:00:01| National Hurricane Center (Atlantic)

Issued at 1100 PM EDT Thu Aug 19 2021 000 WTNT43 KNHC 200259 TCDAT3 Tropical Storm Henri Discussion Number 17 NWS National Hurricane Center Miami FL AL082021 1100 PM EDT Thu Aug 19 2021 The satellite presentation on Henri this evening continues to exhibit a persistent bursting pattern, with the center estimated to be just to the north and west of the coldest cloud tops which occasionally have been below -80 C in the overshooting tops. This current satellite presentation is primarily due to continued moderate to strong northerly vertical wind shear, which is forcing the convection underneath the cirrus canopy down-shear of the low-level center, as seen on a 2230 UTC SSMIS microwave pass. While the mid-level vortex with the convection also remains tilted down-shear of the low-level center, it has not completely separated due to the persistent convection, preventing the low-level center from escaping poleward in more shallow low-level steering. Tonight's subjective Dvorak estimates from SAB/TAFB are in agreement with T3.5/55 kt and given that this value is near what the earlier Air Force Reconnaissance mission found, the latest intensity is being maintained at 55 kt for this advisory. There is a bit of uncertainty determining if Henri has begun a more poleward motion since the center remains under the convective cirrus plume, but my best guess is now 285/9 kt. Over the next 12-24 hours, the mid- to upper-level ridging that has dominated the synoptic steering pattern for Henri the last few days will quickly break down, as a shortwave trough drops in from the Great Lakes into the Mid-Atlantic and cuts off. This feature is now forecast to continue digging in to the west of Henri. To the east, a new mid-level ridge is also forecast to build in to the right of Henri. This synoptic pattern should draw the cyclone poleward with an acceleration to the north-northeast in the 24-48 h period. Afterwards, the aforementioned trough takes on a negative tilt to the southwest of Henri, helping to reorient the mid- to upper-level flow out of the south-southeast, and this flow could result in a slight leftward bend in the track between 48-72 h. The majority of guidance this cycle now is forecasting the mid-level ridge east of Henri to build poleward with the storm, blocking an easy path for the storm to stay on a more northeast heading out to sea. Consequently, the latest NHC forecast track now explicitly shows landfall in southeast Massachusetts at 72 h. The track guidance this cycle has come into better agreement, though there remain some leftward (UKMET) and rightward (ECMWF) outliers. The latest forecast track lies very close to the HFIP corrected consensus approach (HCCA) guidance, which is also very near the latest GFS forecast track. Data from the NOAA G-IV synoptic mission around Henri shows that just north of the tropical cyclone there remains some very dry mid-latitude air, which is being advected into the storm by 20-25 kt of northerly vertical wind shear. Over the next 24-36 hours, this shear is forecast to gradual subside, as Henri moves near the center of an upper-level ridge axis. By 36-48 hours, the vertical wind shear is forecast to be under 10-kt by both the GFS- and ECWMF-based SHIPS guidance, while the storm is also traversing 28-29 C sea-surface temperatures (SSTs). Thus, the latest NHC intensity forecast still calls for strengthening beginning after 12 hours, and the rate of strengthening could be a bit quicker as the storm moves over the warm gulf stream waters between 36-48 hours. Thereafter, Henri will cross a very sharp SST gradient with sea-surface temperatures down below 23 C near the New England coast to the east of Long Island. Henri is forecast to begin weakening after 48 hours, but the storm could still be near hurricane intensity by the time Henri is forecast to be near the Northeast coastline. Transition to a post-tropical storm is expected to begin shortly thereafter which should be sometime in the 96-h to 120-h points as deep convection ceases over the storm over cold SSTs As noted previously, the wind field of Henri is expected to expand, especially as it interacts with a mid-latitude trough located to its west. Therefore, users are reminded to not focus on the exact forecast points as impacts will extend far from the center. Key Messages: 1. Henri is forecast to be near the northeast coast of the U.S. on Sunday and Monday, and the risks of storm surge, wind, and rain impacts in portions of southern New England and eastern Long Island are increasing. Watches will likely be required for portions of this area early Friday. 2. Swells from Henri will begin to reach much of the east coast of the U.S. and Atlantic Canada by the end of the week and continue through the weekend. These swells could cause life-threatening surf and rip currents. 3. Heavy rainfall may lead to flash, urban, and small stream flooding over portions of southeastern New England Sunday into Monday. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 29.8N 72.3W 55 KT 65 MPH 12H 20/1200Z 30.3N 73.2W 55 KT 65 MPH 24H 21/0000Z 31.8N 73.3W 60 KT 70 MPH 36H 21/1200Z 34.4N 72.2W 65 KT 75 MPH 48H 22/0000Z 37.6N 71.0W 80 KT 90 MPH 60H 22/1200Z 40.1N 70.6W 75 KT 85 MPH 72H 23/0000Z 41.7N 70.8W 60 KT 70 MPH...INLAND 96H 24/0000Z 42.5N 70.1W 40 KT 45 MPH...OVER WATER 120H 25/0000Z 43.7N 65.4W 35 KT 40 MPH...POST-TROPICAL $$ Forecaster Papin/Brown

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Tropical Storm Grace Forecast Discussion Number 26

2021-08-19 22:57:55| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Thu Aug 19 2021 000 WTNT42 KNHC 192057 TCDAT2 Tropical Storm Grace Discussion Number 26 NWS National Hurricane Center Miami FL AL072021 400 PM CDT Thu Aug 19 2021 Grace has now been inland for almost 12 hours and the inner core structure has been gradually decaying. In fact, the low-level center is now partially exposed to the north of the mid-level vortex which still has the majority of the deep convection. The initial intensity has been lowered to 45 kt this advisory, assuming a bit more weakening of the low-level center has occurred since 1800 UTC. Environmental conditions appear favorable for Grace to restrengthen after the storm moves offshore this evening. However, it will take a bit of time for the inner-core to reorganize before a faster rate of intensification can occur. After that process occurs, significant intensification is possible, and the latest intensity forecast makes Grace a hurricane again by 24 hours and is near the upper end of the intensity guidance by 36 hours when Grace will be just inland over Mexico. However, it remains possible that the storm could intensify more than indicated between the 24 and 36 hour points, when both the global and regional high-res hurricane model guidance suggests Grace will be near peak intensity. Grace is still moving a bit north of due west at 280/13 kt. A continued westward to west-northwestward motion is expected for the next 6-12 hours and Grace should be emerging offshore of the western Yucatan Peninsula later this evening. Thereafter, a strong mid-level ridge oriented southwest-to-northeast ahead of Grace is likely to steer the cyclone towards the west, and then west-southwest as it nears mainland Mexico. The track guidance remains in good agreement, and the latest track forecast is quite similar to the previous one, staying close to the multi-model consensus aids. Although Grace is likely to dissipate by 60 hours over the high terrain of Central Mexico, the mid-level circulation should survive and emerge into the East Pacific. This feature could contribute to the formation of a new tropical cyclone in that basin. Key Messages: 1. Tropical storm conditions will continue this afternoon and evening across the northern Yucatan Peninsula but should subside later tonight. 2. Hurricane conditions and dangerous storm surge are likely along portions of eastern mainland Mexico beginning late Friday. A Hurricane Warning is in effect from Puerto Veracruz northward to Cabo Rojo. 3. Through the weekend, heavy rainfall across central and northern portions of the Yucatan Peninsula and Veracruz State should lead to flash and urban flooding. In addition, the heavy rainfall from Grace will be capable of producing mudslides in Veracruz. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 20.6N 90.2W 45 KT 50 MPH...INLAND 12H 20/0600Z 20.8N 92.5W 55 KT 65 MPH...OVER WATER 24H 20/1800Z 20.7N 94.9W 70 KT 80 MPH 36H 21/0600Z 20.4N 97.0W 75 KT 85 MPH...INLAND 48H 21/1800Z 19.6N 99.4W 35 KT 40 MPH...INLAND 60H 22/0600Z...DISSIPATED $$ Forecaster Papin/Cangialosi

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Tropical Storm Henri Forecast Discussion Number 16

2021-08-19 22:39:53| National Hurricane Center (Atlantic)

Issued at 500 PM EDT Thu Aug 19 2021 000 WTNT43 KNHC 192039 TCDAT3 Tropical Storm Henri Discussion Number 16 NWS National Hurricane Center Miami FL AL082021 500 PM EDT Thu Aug 19 2021 The Air Force Hurricane Hunters investigated Henri earlier this afternoon and found maximum flight-level winds and SFMR values to support an intensity of about 55 kt and a minimum pressure of 997 mb. The storm's appearance in satellite images has been generally steady throughout the day with the low-level center located on the northern side of the convective mass, and banding features confined to the south side of the circulation. This somewhat asymmetric cloud pattern is due to about 25 kt of north-northeasterly shear. The tropical storm is now moving due westward at 9 kt. A mid-level ridge should keep Henri on a westward to west-northwestward track through tonight. On Friday, however, a trough is expected to cut off over the Ohio Valley and mid-Atlantic region of the U.S. while a ridge amplifies to the east of Henri over the western Atlantic. A combination of these features should cause Henri to turn northward on Friday and accelerate in that direction over the weekend. The exact location of the northward turn will be very critical in determining Henri's future track and how close it gets to New England. The latest GFS run has shifted eastward and the UKMET and ECMWF have shifted westward. Consequently, the net result is very little change in the consensus models and the new NHC track forecast is just a little faster than the previous one. Based on this forecast, Henri is expected to be very near southern New England on Sunday and Monday. The NOAA Gulfstream IV jet will be flying around Henri this evening to help assess the environmental conditions and gather data for the numerical models. It is hoped that these data will help the models more accurately predict the future track of the storm. Although Henri is a bit weaker now, the overall intensity forecast philosophy has not changed. The shear is expected to persist for another 12 to 24 hours, so little change in strength seems likely during that time period. However, the global models show a more conducive upper-level wind pattern over Henri shortly after that, and in fact, all of the environmental conditions appear favorable for strengthening on Friday and Saturday. Therefore, steady intensification is expected during the 24-60 h time period, and Henri is forecast to become a hurricane during that time. Henri is expected to cross the north wall of the Gulf Stream and move over much cooler SSTs on Sunday, and that should end the opportunity for strengthening and induce a steady weakening trend from days 3 to 5. Post-tropical transition is now shown to occur by the end of the forecast period when Henri is expected to be over SSTs below 20 deg C. It should be noted that as Henri gains latitude and moves near New England, the wind field is expected to expand. Therefore, users are reminded to not focus on the exact forecast points as impacts will extend far from the center. Key Messages: 1. Henri is forecast to be near the northeast coast of the U.S. on Sunday and Monday, and the risks of storm surge, wind, and rain impacts in portions of southern New England and eastern Long Island are increasing. Watches will likely be required for portions of this area early Friday. 2. Swells from Henri will begin to reach much of the east coast of the U.S. and Atlantic Canada by the end of the week and continue through the weekend. These swells could cause life-threatening surf and rip currents. 3. Heavy rainfall may lead to flash, urban, and small stream flooding over portions of southeastern New England Sunday into Monday. FORECAST POSITIONS AND MAX WINDS INIT 19/2100Z 29.7N 71.4W 55 KT 65 MPH 12H 20/0600Z 30.1N 72.6W 55 KT 65 MPH 24H 20/1800Z 31.0N 73.2W 55 KT 65 MPH 36H 21/0600Z 33.0N 72.6W 65 KT 75 MPH 48H 21/1800Z 36.0N 71.3W 75 KT 85 MPH 60H 22/0600Z 38.9N 70.3W 80 KT 90 MPH 72H 22/1800Z 40.8N 70.1W 65 KT 75 MPH 96H 23/1800Z 42.3N 69.6W 50 KT 60 MPH 120H 24/1800Z 43.6N 65.7W 40 KT 45 MPH...POST-TROPICAL $$ Forecaster Cangialosi

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Tropical Storm Linda Forecast Discussion Number 39

2021-08-19 22:33:51| Tropical Depression LIDIA

Issued at 1100 AM HST Thu Aug 19 2021

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Tropical Storm Grace Forecast Discussion Number 25

2021-08-19 16:44:18| National Hurricane Center (Atlantic)

Issued at 1000 AM CDT Thu Aug 19 2021 000 WTNT42 KNHC 191444 TCDAT2 Tropical Storm Grace Discussion Number 25 NWS National Hurricane Center Miami FL AL072021 1000 AM CDT Thu Aug 19 2021 Grace made landfall as a hurricane near Tulum, Mexico around 0945 UTC (445am CDT). A storm chaser in Tulum reported a minimum pressure of 983 mb at the time of landfall. Since then, the cyclone has moved further inland where observations are far more sparse, and we have no recent in situ observations to assist the intensity analysis. The intensity is therefore set at 55 kt, based in part on the inland decay wind model built into the SHIPS model. The central pressure estimate of 995 mb is likewise uncertain. Some additional weakening is likely this afternoon while Grace continues to cross the Yucatan peninsula. The tropical storm should emerge over the southern Gulf of Mexico this evening, and re-strengthening is anticipated shortly thereafter. Environmental conditions are expected to be quite conducive for strengthening, but it will likely take Grace at least a little time to redevelop its inner-core after disruption by land. That could act to limit the rate at which the cyclone will intensify at first. The HWRF and HMON both suggest that the most significant reintensification could occur in the last 12 hours before final landfall occurs in mainland Mexico, and that scenario seems likely. It would not be surprising if Grace strengthened slightly more between the 36 h forecast point over water and the 48 h point inland, particularly if the hurricane moves slightly slower, giving it more time over the warm waters of the southern Gulf of Mexico. After landfall, Grace should quickly weaken and dissipate over the mountainous terrain of Mexico. Overall, the NHC intensify forecast is very similar to the previous advisory. Grace continues to move to the west near 16 kt. A westward or west-northwestward motion is likely for the next 12 hours or so, after which a strong-deep layer ridge extending well over the northern Gulf of Mexico and southern U.S. will steer Grace westward until landfall. Only minor adjustments were made to the official track forecast, which remains near the multi-model consensus. Although the tropical cyclone will quickly dissipate after it moves inland, the ridge will likely steer its remnants further westward toward the eastern North Pacific, where it could contribute to the formation of a new tropical cyclone there. Key Messages: 1. Tropical storm conditions will continue today across the northern Yucatan Peninsula. 2. Hurricane conditions and dangerous storm surge is likely along portions of eastern mainland Mexico beginning late Friday. A Hurricane Warning is in effect from Puerto Veracruz northward to Cabo Rojo. 3. Through the weekend, heavy rainfall across central and northern portions of the Yucatan Peninsula and Veracruz State should lead to flash and urban flooding. In addition, the heavy rainfall from Grace will be capable of producing mudslides in Veracruz. FORECAST POSITIONS AND MAX WINDS INIT 19/1500Z 20.2N 88.8W 55 KT 65 MPH 12H 20/0000Z 20.6N 91.2W 55 KT 65 MPH...OVER WATER 24H 20/1200Z 20.6N 94.0W 65 KT 75 MPH 36H 21/0000Z 20.4N 96.3W 75 KT 85 MPH 48H 21/1200Z 20.0N 98.7W 40 KT 45 MPH...INLAND 60H 22/0000Z 19.6N 101.7W 25 KT 30 MPH...POST-TROP/INLAND 72H 22/1200Z...DISSIPATED $$ Forecaster Zelinsky

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