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Tropical Depression Eighteen-E Forecast Discussion Number 1

2017-09-23 22:43:23| National Hurricane Center (East Pacific)

Issued at 400 PM CDT Sat Sep 23 2017 000 WTPZ43 KNHC 232042 TCDEP3 Tropical Depression Eighteen-E Discussion Number 1 NWS National Hurricane Center Miami FL EP182017 400 PM CDT Sat Sep 23 2017 ASCAT-A/-B scatterometer data from around 16-17Z indicated that the low pressure system located just offshore of the southwestern coast of Mexico that the NHC has been tracking the past several days has become much better defined, and it also possessed surface winds of 30-33 kt. As a result, the system has been upgraded to a tropical depression, the eighteenth of the eastern Pacific hurricane season. The initial motion estimate is 335/04, based primarily on microwave satellite fixes. The latest NHC model guidance is in fairly good agreement on the cyclone moving slowly in a general north-northwestward direction around the western periphery of a deep-layer ridge for the next 5 days. Some of the models like the GFS, Canadian, and HCCA take the system just inland near Cabo Corrientes in about 24 hours, whereas the remainder of the guidance, especially the UKMET and ECMWF, keep the cyclone just offshore of the southwestern coast of Mexico. The forecast motion for the next few days is expected to be 5 kt or less, an indication that steering currents will be weak. Since there is no strong forcing that would want to drive the depression inland over the mountainous terrain of southwestern Mexico, the official forecast calls for the center of the cyclone to remain just offshore of the coast throughout the forecast period, similar to the ECMWF and UKMET solutions. Given the well-defined circulation noted in the aforementioned scatterometer data, along with vertical wind shear decreasing to less than 10 kt by 24 hours, steady strengthening is forecast for the next 24-36 hours. Afterwards, southeasterly to southerly shear is expected to gradually increase to 25 kt by 72 hours and more than 30 kt in the 96-120 hour period, which should induce steady to possible rapid weakening. The NHC intensity forecast is above most of the guidance through 48 hours, and then is a little lower than the guidance after that. Due to the depression being forecast to become a tropical storm by tonight, along with its proximity to the coast of Mexico, a tropical storm warning has been issued from Manzanillo northward to El Roblito, including the Islas Marias. Heavy rainfall causing flash floods and mudslides will also be possible within the warning area and extending well inland. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 18.4N 105.3W 30 KT 35 MPH 12H 24/0600Z 18.8N 105.5W 40 KT 45 MPH 24H 24/1800Z 19.4N 105.7W 50 KT 60 MPH 36H 25/0600Z 20.2N 106.0W 50 KT 60 MPH 48H 25/1800Z 21.2N 106.2W 45 KT 50 MPH 72H 26/1800Z 22.7N 107.0W 40 KT 45 MPH 96H 27/1800Z 24.0N 108.4W 35 KT 40 MPH...POST-TROPICAL 120H 28/1800Z 26.0N 110.0W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart

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Tropical Storm Lee Forecast Discussion Number 22

2017-09-23 22:36:18| National Hurricane Center (Atlantic)

Issued at 500 PM AST Sat Sep 23 2017 000 WTNT44 KNHC 232036 TCDAT4 Tropical Storm Lee Discussion Number 22 NWS National Hurricane Center Miami FL AL142017 500 PM AST Sat Sep 23 2017 Compact Lee continues to produce a small cluster of central deep convection, along with several small curved bands in all quadrants. Ship LAOX5 traversed through the center of Lee around 1400Z, and at 1500Z reported a north wind of 30 kt about 20 nmi west of the center. Based on that ship observation, along with satellite intensity estimates of T2.5/35 kt from TAFB and UW-CIMSS ADT, the initial intensity remains at 40 kt for this advisory. The initial motion is a slow drift toward the north-northwest or 335/03 kt. The global and regional models have come into much better agreement on this cycle and now show a much tighter anticyclonic loop occurring during the forecast period, similar to the current and previous runs of the ECMWF model. As result, the official NHC forecast is west of the previous advisory track, and lies close to a blend of the TCVA, TVCX, and HCCA consensus models. There is no significant change to the previous forecast or reasoning. Due to the tighter loop that Lee is expected to make within the col region between an upper-level low to the south and a mid-latitude trough to the north, the deep-layer vertical wind shear is now forecast to remain less than 10 kt throughout the forecast period. Since the small cyclone will remain over SSTs near 27.5 C within a region of below-average upper-level temperatures, strong instability should persist for the next 4 days. The only inhibiting factor during that time will continue to be occasional intrusions of very dry mid-level air that will temporarily disrupt the inner-core convection. By 120 hours, gradual weakening is expected to begin due to increasing westerly shear. The new NHC intensity forecast is unchanged from the previous advisory, and remains close to a blend of the IVCN and HCCA intensity consensus models. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 32.1N 49.8W 40 KT 45 MPH 12H 24/0600Z 32.3N 49.3W 45 KT 50 MPH 24H 24/1800Z 32.2N 48.7W 50 KT 60 MPH 36H 25/0600Z 31.8N 48.4W 55 KT 65 MPH 48H 25/1800Z 31.1N 48.2W 60 KT 70 MPH 72H 26/1800Z 30.5N 49.0W 70 KT 80 MPH 96H 27/1800Z 31.0N 50.5W 70 KT 80 MPH 120H 28/1800Z 32.4N 51.7W 65 KT 75 MPH $$ Forecaster Stewart

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Hurricane Maria Forecast Discussion Number 30

2017-09-23 16:55:53| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Sat Sep 23 2017 000 WTNT45 KNHC 231455 TCDAT5 Hurricane Maria Discussion Number 30 NWS National Hurricane Center Miami FL AL152017 1100 AM EDT Sat Sep 23 2017 Data from this morning's Air Force Hurricane Hunter flight suggests that Maria's maximum winds may be decreasing a bit--a trend which was noted in the previous advisory package. The plane measured a maximum 700-mb flight level wind of 102 kt and SFMR winds as high as 85 kt, neither of which supports an intensity of 105 kt. The initial intensity is therefore conservatively lowered to 100 kt. Maria is now located between a mid-level high centered near Bermuda and a cut-off low over the northeastern Gulf of Mexico, both of which are steering the hurricane north-northwestward, or 340/7 kt. Maria should turn northward between these two features beginning in about 12 hours, but its forward motion is likely to be impeded in a couple of days by ridging developing over the northeastern United States. As a result, the hurricane's forward speed will decrease to 5 kt or less from day 2 and beyond. The track guidance has continued to trend toward slower and farther westward solutions, and as we had foreshadowed in previous discussions, an additional westward adjustment to the new NHC track forecast was required on this cycle. The updated forecast is between the GFS and ECMWF solutions and close to the consensus aids and HCCA through day 3. After that time, the new forecast is slower and southwest of the consensus aids, but still not quite as far west as the GFS, ECMWF, and UKMET solutions. Based on various analyses, the southwesterly shear over Maria has decreased since yesterday, and it appears to remain relatively low for the next 2 days or so. Maria will be moving over very warm waters during that time, although the depth of the warm pool does decrease, and Maria's slower motion could increase the effects of colder upwelled water. After 48 hours, vertical shear is forecast to increase again, and a critical part of the intensity forecast will be whether or not Maria moves over the cold wake left behind by Jose. The recent westward shifts in the forecast track make it more likely that Maria would avoid the cold wake and move over the warmer Gulf Stream waters. Since the environment may not be as hostile as the dynamical models are assuming, the NHC intensity forecast is a little above the intensity consensus for much of the forecast period. To increase the sampling of the environment upstream and north of Maria, supplemental 0600/1800 UTC upper-air soundings are scheduled to begin from the eastern and southeastern United States this afternoon. In addition, NOAA G-IV missions are scheduled to begin sampling the environment around Maria on Sunday. KEY MESSAGES: 1. Maria's forecast track has shifted closer to the U.S. east coast, and it is becoming increasingly likely that some direct impacts will occur along portions of the coast next week. Interests along the coast of the Carolinas and the Mid-Atlantic should monitor the progress of Maria, as tropical storm or hurricane watches may be needed for part of this area on Sunday. 2. Swells from Maria are increasing along the coast of the southeastern United States and are expected to reach the Mid-Atlantic coast tonight and on Sunday. These swells will likely cause dangerous surf and rip currents at the beach through much of next week. For more information, please monitor information from your local National Weather Service office at www.weather.gov. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 25.4N 72.3W 100 KT 115 MPH 12H 24/0000Z 26.7N 72.6W 105 KT 120 MPH 24H 24/1200Z 28.3N 73.0W 105 KT 120 MPH 36H 25/0000Z 29.5N 73.3W 100 KT 115 MPH 48H 25/1200Z 30.5N 73.4W 95 KT 110 MPH 72H 26/1200Z 32.1N 73.4W 85 KT 100 MPH 96H 27/1200Z 33.5N 73.5W 75 KT 85 MPH 120H 28/1200Z 34.5N 72.5W 65 KT 75 MPH $$ Forecaster Berg

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Tropical Storm Lee Forecast Discussion Number 21

2017-09-23 16:40:20| National Hurricane Center (Atlantic)

Issued at 1100 AM AST Sat Sep 23 2017 000 WTNT44 KNHC 231440 TCDAT4 Tropical Storm Lee Discussion Number 21 NWS National Hurricane Center Miami FL AL142017 1100 AM AST Sat Sep 23 2017 Lee remains a compact tropical cyclone as indicated by a 20-kt wind report at 1200Z from ship LAQX5, located only 70 nmi east of the center. A 0943Z WindSAT pass had a few 40-45 uncontaminated wind vectors in the southwestern quadrant, so the intensity has been conservatively increased to 40 kt, which is closer to the satellite intensity estimates of T2.5/35 kt from TAFB and UW-CIMSS ADT. The initial motion is a slow drift toward the north or 360/02 kt, based primarily on microwave satellite fixes. The global and regional models are in general agreement that Lee will move slowly at 5 kt or less for the next 5 days, and either make a wide or a tight anticyclonic turn back toward the west after 72 h due to a building high pressure ridge to the north of the cyclone, and a stationary upper-level low located a few hundred nmi to the south. The GFS makes the widest turn, whereas the ECMWF makes the tightest turn; the remainder of the model guidance lies somewhere between those two extremes. Until the model guidance becomes more convergent, the official NHC forecast track will remain basically down the middle of the guidance envelope, close to the blend of the TCVA, TVCX, and HCCA consensus models. Earlier WindSAT and SSMI/S microwave data showed that Lee possessed an 8-10 nmi diameter eye-like feature in both the low- and mid-levels, indicating that the cyclone has a well-defined inner-core wind field, albeit quite small. The deep-layer vertical wind shear is forecast by both the GFS and ECMWF models to decrease to 5-10 kt during the next 48 hours or so while Lee remains over SSTs of about 27.5 deg C. In addition, upper-level temperatures are currently running, and are expected to remain, about 3 deg C colder than normal, which should ensure sufficient instability for the generation of inner-core thunderstorm activity through the entire forecast period. Based on the combination of these favorable dynamic and thermodynamic conditions, Lee is forecast to steadily develop into a hurricane by 48-72 hours, with the only inhibiting factor being occasional intrusions of very dry mid-level air. However, the strong instability/CAPE that will be available should allow for deep convection to more or less persist near the center until modest westerly shear begins to affect the cyclone by 72-120 hours, during which time a slow weakening trend is expected. The official intensity forecast is very similar to the previous advisory and is close to a blend of the IVCN and HCCA intensity consensus models. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 31.9N 49.4W 40 KT 45 MPH 12H 24/0000Z 32.2N 49.1W 45 KT 50 MPH 24H 24/1200Z 32.3N 48.3W 50 KT 60 MPH 36H 25/0000Z 31.9N 47.2W 55 KT 65 MPH 48H 25/1200Z 31.3N 46.2W 60 KT 70 MPH 72H 26/1200Z 30.5N 45.3W 70 KT 80 MPH 96H 27/1200Z 29.9N 45.9W 70 KT 80 MPH 120H 28/1200Z 30.5N 47.1W 65 KT 75 MPH $$ Forecaster Stewart

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Tropical Storm Lee Forecast Discussion Number 20

2017-09-23 10:55:55| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sat Sep 23 2017 000 WTNT44 KNHC 230855 TCDAT4 Tropical Storm Lee Discussion Number 20 NWS National Hurricane Center Miami FL AL142017 500 AM AST Sat Sep 23 2017 Not much has changed with Lee over the past six hours. The convection previously observed in the SE quadrant appears to have rotated around the tropical storm and faded, but a 0609 UTC SSMI overpass indicates that a new burst has since taken its place. Dvorak intensity estimates range from 30 to 35 kt, so the intensity has been held at 35 kt for this advisory. Every aspect of the forecast is highly uncertain. With regards to intensity, the model forecasts range from dissipation (GFS) to a category 2 or 3 hurricane (HWRF, HMON). Since the environment still appears to be generally favorable for intensification, my forecast remains close to the HWRF, and a little above the intensity consensus through 72 h. The forecast shows gradual weakening after that time as the shear increases. Because Lee is so small, it is very possible that the tropical storm could strengthen far more quickly than indicated here, at just about any time during the forecast period. Likewise, rapid weakening could occur later in the period if the shear increases like the SHIPS diagnostics indicate it will. As uncertain as the intensity forecast is, the track forecast may be even more so. The ECMWF now shows a relatively deep Lee drifting south for the next 72 hours, followed by a gradual turn toward the northwest. On the other hand, the GFS continues to depict a shallow Lee (or its remnants) moving steadily eastward through the forecast period. As a result, these two typically reliable models differ by more than 900 miles at day 5. The new official track forecast is generally close to the corrected consensus models, FSSE and HCCA, but hedges somewhat toward the ECMWF since it's version of Lee is more in line with the NHC intensity forecast. As a result, the NHC track forecast has been shifted significantly to the west, especially at 72 h and beyond, but still lies nearly 400 miles to the east of the ECMWF. Needless to say, confidence in the forecast is low, and significant changes to the track or intensity forecast may be required during the next day or two. FORECAST POSITIONS AND MAX WINDS INIT 23/0900Z 31.9N 49.2W 35 KT 40 MPH 12H 23/1800Z 32.3N 49.1W 40 KT 45 MPH 24H 24/0600Z 32.5N 48.4W 45 KT 50 MPH 36H 24/1800Z 32.2N 47.4W 50 KT 60 MPH 48H 25/0600Z 31.7N 46.2W 60 KT 70 MPH 72H 26/0600Z 30.5N 44.5W 75 KT 85 MPH 96H 27/0600Z 29.8N 45.0W 70 KT 80 MPH 120H 28/0600Z 30.5N 46.5W 60 KT 70 MPH $$ Forecaster Zelinsky

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