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Tropical Storm Michael Forecast Discussion Number 6

2018-10-08 04:57:13| National Hurricane Center (Atlantic)

Issued at 1000 PM CDT Sun Oct 07 2018 000 WTNT44 KNHC 080257 TCDAT4 Tropical Storm Michael Discussion Number 6 NWS National Hurricane Center Miami FL AL142018 1000 PM CDT Sun Oct 07 2018 On the last outbound leg to the north, the Air Force Reserve reconnaissance aircraft measured a peak 925-mb flight-level wind of 60 kt and an SFMR surface wind of 53 kt in very light rain. Since that time, deep convection has developed in the same area where those peak wind values were measured, so the initial intensity has been increased to a conservative 50 kt. Another recon aircraft is scheduled to investigate Michael around 0600 UTC. The initial motion estimate is northward, or 005/03 kt based on the recon and microwave satellite data. The bulk of the forecast rationale remains unchanged, although there remains some notable differences between the models that will ultimately determine where and when Michael will make landfall in about 72 h or so. The GFS, ECMWF, HWRF, and HMON models are tightly clustered along the western side of the guidance envelope. In contrast, the UKMET model is the farthest east of the dynamical models. Interestingly, the NOAA corrected-consensus HCCA model and the simple consensus IVCN models are close to each other and lie between the aforementioned model track dichotomy. Given that modest westerly vertical wind shear is expected to affect Michael throughout the forecast period until landfall occurs, which should keep the strongest convection, associated latent heat release, and pressure falls occurring in the eastern semicircle, the official forecast track leans more toward the HCCA/IVCN and UKMET model solutions. As a result, a slight eastward shift to the previous forecast track was made on this advisory through 48 hours, with little change made to the previous advisory track on days 3-5. Michael's overall cloud pattern remains unchanged, with the bulk of the convection being displaced into the eastern semicircle due to about 20 kt of westerly shear. However, the inner-core convective pattern within about 60 nmi of the center has been improving over the past few hours, and a recent SSMI/S microwave pass reveled a tightly curved band wrapping about 75 percent around the center. This better structure combined with very warm sea-surface temperatures of at least 29 deg C supports at least gradual strengthening until landfall despite the expected persistent westerly shear conditions. The HWRF model is the most robust of the intensity guidance and makes Michael a category 4 hurricane just prior to landfall. However, all of global models and the HWRF and HMON regional models are indicating westerly to west-northwesterly shear keeping the upper-level outflow restricted to the eastern semicircle, which is not a pattern conducive for the development of an intense hurricane. Therefore, the HRWF intensity solution has been discounted, and the new official intensity forecast remains similar to the previous advisory, which is close to but a little lower than the IVCN and ICON intensity consensus models. Although the 72-h forecast shows a decrease to 75 kt, this is due to the cyclone being inland, and should not be interpreted as being an indication of a weakening trend prior to landfall. Key Messages: 1. Michael is expected to produce heavy rainfall and flash flooding over portions of western Cuba and the northeastern Yucatan Peninsula of Mexico during the next couple of days. 2. Tropical storm conditions are expected tonight over portions of western Cuba and the northeastern Yucatan Peninsula, where tropical storm warnings are in effect. 3. Michael is forecast to be a hurricane when it reaches the northeastern Gulf Coast by mid-week, and the risk of dangerous storm surge, rainfall, and wind impacts continues to increase. In addition, Michael is expected to affect portions of the Florida Gulf Coast that are especially vulnerable to storm surge, regardless of the storm's exact track or intensity. Residents in these areas should monitor the progress of this system and follow any advice given by local officials. FORECAST POSITIONS AND MAX WINDS INIT 08/0300Z 20.0N 85.4W 50 KT 60 MPH 12H 08/1200Z 21.1N 85.3W 60 KT 70 MPH 24H 09/0000Z 22.7N 85.6W 65 KT 75 MPH 36H 09/1200Z 24.4N 86.0W 75 KT 85 MPH 48H 10/0000Z 26.3N 86.1W 85 KT 100 MPH 72H 11/0000Z 30.4N 84.5W 75 KT 85 MPH...INLAND ERN FL PANHNDL 96H 12/0000Z 34.9N 78.4W 55 KT 65 MPH...INLAND 120H 13/0000Z 40.7N 64.4W 55 KT 65 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart

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Tropical Storm Leslie Forecast Discussion Number 47

2018-10-08 04:42:08| National Hurricane Center (Atlantic)

Issued at 1100 PM AST Sun Oct 07 2018 000 WTNT43 KNHC 080242 TCDAT3 Tropical Storm Leslie Discussion Number 47 NWS National Hurricane Center Miami FL AL132018 1100 PM AST Sun Oct 07 2018 Leslie's cloud pattern consists of a large and vigorous circulation, but the convection is shallow at this time. Dvorak numbers only support an initial intensity of 45 kt. Some fluctuations in intensity are expected over the next 24 to 36 hours, while Leslie is over fairly cool SSTs. After that time, Leslie will encounter warmer waters, and since the shear is low, some re-intensification is anticipated. Leslie will likely become a hurricane once again by the end of the week as indicated by the guidance. Leslie is still moving east-southeastward at about 11 kt. Since Leslie is embedded within the mid-latitude westerly flow, this general motion will likely continue for the next 2 days or so. This is consistent with most of the track guidance, which continue to show a southeastward motion of the cyclone during that period. Another shortwave trough in the westerlies will bypass the cyclone, but will modify the steering flow, and Leslie should then turn to the east-northeast. It should be emphasized that confidence in the track forecast remains low. FORECAST POSITIONS AND MAX WINDS INIT 08/0300Z 35.7N 49.1W 45 KT 50 MPH 12H 08/1200Z 35.2N 47.4W 45 KT 50 MPH 24H 09/0000Z 33.8N 45.0W 45 KT 50 MPH 36H 09/1200Z 32.0N 43.0W 50 KT 60 MPH 48H 10/0000Z 30.5N 41.5W 55 KT 65 MPH 72H 11/0000Z 28.5N 39.0W 60 KT 70 MPH 96H 12/0000Z 30.0N 33.0W 65 KT 75 MPH 120H 13/0000Z 32.0N 24.0W 60 KT 70 MPH $$ Forecaster Avila

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Hurricane Sergio Forecast Discussion Number 35

2018-10-08 04:36:06| National Hurricane Center (East Pacific)

Issued at 800 PM PDT Sun Oct 07 2018 000 WTPZ41 KNHC 080236 TCDEP1 Hurricane Sergio Discussion Number 35 NWS National Hurricane Center Miami FL EP212018 800 PM PDT Sun Oct 07 2018 Sergio still has its annular-like structure, but the surrounding convection is not as strong as earlier today. Dvorak objective and subjective numbers only support 80 kt at this time. Little significant change in strength is anticipated during the next 12 to 24 hours while the waters are still relatively warm. In about 2 days, the cyclone should encounter cooler waters, and a weakening trend at a faster rate is anticipated. Sergio is forecast to be a tropical storm by the time it is approaching the Baja California peninsula. Sergio is currently embedded within light steering currents and is barely moving toward the northwest at 3 kt. A northward drift is expected during the next 24 hours, but after that time, a broad mid-level trough is forecast to amplify over the west coast of the U.S., and this flow pattern should force Sergio to recurve northeastward with increasing forward speed. The track guidance is in remarkably good agreement, basically on top of each other for the next 3 days, increasing the confidence in the track forecast. Beyond 3 days, models are in less agreement, but still bring the cyclone very near the central portion of the Baja California peninsula in 4 days or so. The NHC forecast is not very different from the previous one and is in the middle of the tight guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 08/0300Z 14.9N 127.9W 80 KT 90 MPH 12H 08/1200Z 15.3N 128.3W 80 KT 90 MPH 24H 09/0000Z 16.0N 128.3W 75 KT 85 MPH 36H 09/1200Z 16.9N 127.3W 65 KT 75 MPH 48H 10/0000Z 17.7N 125.8W 60 KT 70 MPH 72H 11/0000Z 20.5N 121.5W 60 KT 70 MPH 96H 12/0000Z 25.5N 115.5W 45 KT 50 MPH 120H 13/0000Z 32.0N 109.0W 25 KT 30 MPH...INLAND $$ Forecaster Avila

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Tropical Storm Michael Forecast Discussion Number 5

2018-10-07 23:01:14| National Hurricane Center (Atlantic)

Issued at 400 PM CDT Sun Oct 07 2018 000 WTNT44 KNHC 072101 TCDAT4 Tropical Storm Michael Discussion Number 5 NWS National Hurricane Center Miami FL AL142018 400 PM CDT Sun Oct 07 2018 Deep convection has continued to develop over the eastern semicircle of the cyclone, and data from the reconnaissance aircraft indicate that the center has re-formed farther east, closer to the convection. The Air Force aircraft has measured peak 925-mb flight-level winds of 56 kt, and believable SFMR winds of 40-45 kt. Based on these data, the initial wind speed has been increased to 45 kt. Due to the center reformation, the initial motion estimate is a highly uncertain 020/3 kt. The overall forecast reasoning has not changed much since the previous advisory. Although there could be some additional eastward re-formation of the center, the tropical storm is forecast to begin moving northward between a ridge over the western Atlantic and a deep-layer trough over the west-central United States. A general northward motion at around 10 kt is then expected to continue during the next 2 to 3 days. After that time, Michael should turn northeastward ahead of an approaching trough. The track guidance remains in overall agreement on this scenario, however, significant along-track (forward speed and timing) differences remain. The HWRF brings Michael onshore the northern Gulf coast within 72 hours, while the ECMWF is much slower and has Michael still offshore at day 4. The new NHC track has been shifted eastward primarily in the short term due to the more eastward initial position. The latter portion of the track forecast is again close to the consensus aids due to the large along- and cross-track guidance spread. Michael has strengthened today despite moderate westerly shear. The shear is forecast to gradually decrease over the next couple of days while the system moves over warm waters. This should allow for steady strengthening and most of the intensity models bring Michael to hurricane strength within the next couple of days. It should also be noted that the global models also significantly deepen the storm over the next 72 hours to pressures below 970 mb. The new NHC intensity forecast calls for Michael to become a hurricane in about 36 hours when the storm reaches the southeastern Gulf of Mexico. Additional strengthening is indicated through 72 hours when the storm is forecast to be near the northern Gulf coast, and the NHC forecast is near the higher SHIPS and HWRF models. Key Messages: 1. Michael is expected to produce heavy rainfall and flash flooding over portions of western Cuba and the northeastern Yucatan Peninsula of Mexico during the next couple of days. 2. Tropical storm conditions are expected tonight over portions of western Cuba and the northeastern Yucatan Peninsula, where tropical storm warnings are in effect. 3. Michael is forecast to be a hurricane when it reaches the northeastern Gulf Coast by mid-week, and the risk of dangerous storm surge, rainfall, and wind impacts continues to increase. In addition, Michael is expected to affect portions of the Florida Gulf Coast that are especially vulnerable to storm surge, regardless of the storm's exact track or intensity. Residents in these areas should monitor the progress of this system and follow any advice given by local officials. FORECAST POSITIONS AND MAX WINDS INIT 07/2100Z 19.2N 85.5W 45 KT 50 MPH 12H 08/0600Z 20.1N 85.6W 50 KT 60 MPH 24H 08/1800Z 21.5N 85.8W 60 KT 70 MPH 36H 09/0600Z 23.2N 86.2W 70 KT 80 MPH 48H 09/1800Z 25.0N 86.7W 80 KT 90 MPH 72H 10/1800Z 29.2N 85.7W 85 KT 100 MPH 96H 11/1800Z 33.7N 80.4W 50 KT 60 MPH...INLAND 120H 12/1800Z 39.0N 68.5W 55 KT 65 MPH...POST-TROP/EXTRATROP $$ Forecaster Brown

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Tropical Storm Leslie Forecast Discussion Number 46

2018-10-07 22:36:38| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sun Oct 07 2018 000 WTNT43 KNHC 072036 TCDAT3 Tropical Storm Leslie Discussion Number 46 NWS National Hurricane Center Miami FL AL132018 500 PM AST Sun Oct 07 2018 Late-arriving ASCAT data from shortly after 1200 UTC indicated that Leslie is still producing winds of just below 45 kt, primarily in the southwest and northeast quadrants of the tropical storm. Assuming at least a little undersampling has occurred, this would support an initial intensity of 45 or 50 kt, while an average of more recent intensity estimates from SAB, TAFB, and the UW-CIMSS is 50 kt. The initial value is therefore held at 50 kt. Regardless of the exact initial intensity of Leslie, only slight fluctuations are expected over the next 24 to 36 hours while Leslie is over fairly cool SSTs, in part because it will be crossing its own previous track multiple times. Beyond 48 h, the intensity guidance is now in better agreement that some intensification will occur, and Leslie is forecast to become a hurricane once again by the end of the week. The NHC intensity forecast is a little higher than the previous advisory by the end of the forecast period, and is close to IVCN at all times. Leslie is still moving east-southeastward with an initial speed of 9 kt, and this general motion will likely continue for at least the next day or two while the tropical storm is steered by westerly flow associated with a mid-latitude trough to its north. Most of the global models still forecast that Leslie will separate from the trough in a few days and turn southeastward, and there are still large differences on exactly when that will occur. The deterministic runs of the GFS and ECMWF have come into better agreement on the future track of Leslie, but the ensembles of those models indicate that the range of possible tracks hasn't actually decreased. Confidence in the track forecast therefore remains quite low. In general, the guidance suite has shifted to the south and west, particularly between 36 and 96 h. The official track forecast has been shifted in that direction to bring it closer to the track consensus. However, it should be emphasized that confidence in the track forecast remains low at this time. FORECAST POSITIONS AND MAX WINDS INIT 07/2100Z 35.9N 49.9W 50 KT 60 MPH 12H 08/0600Z 35.4N 48.2W 50 KT 60 MPH 24H 08/1800Z 34.3N 45.7W 50 KT 60 MPH 36H 09/0600Z 32.8N 43.2W 55 KT 65 MPH 48H 09/1800Z 31.2N 41.3W 55 KT 65 MPH 72H 10/1800Z 28.2N 38.8W 60 KT 70 MPH 96H 11/1800Z 28.6N 33.0W 65 KT 75 MPH 120H 12/1800Z 30.5N 27.0W 60 KT 70 MPH $$ Forecaster Zelinsky

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