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Hurricane Maria Forecast Discussion Number 34

2017-09-24 16:43:22| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Sun Sep 24 2017 000 WTNT45 KNHC 241443 TCDAT5 Hurricane Maria Discussion Number 34 NWS National Hurricane Center Miami FL AL152017 1100 AM EDT Sun Sep 24 2017 NOAA and Air Force Hurricane Hunter aircraft have measured 700-mb flight-level winds of 101 to 117 kt over the eastern and southeastern portion of the Maria this morning. However, surface wind estimates from the SFMR suggest that these winds are not mixing to the surface at the usual efficiency, as the highest SFMR winds on both flights have been around 80 kt. Based on a blend of the flight-level and SFMR winds the initial wind speed has been reduced to 90 kt. The initial motion estimate is 350/8 kt. A north-northwestward to northward motion is expected over the next 2 to 3 days while Maria is steered between a cut-off low/trough over the southeastern U.S. and eastern Gulf of Mexico, and a subtropical ridge over the southwestern Atlantic. Maria is predicted to slow down within the next couple of days as a high pressure ridge builds to the north of the hurricane. After 72 h, Maria should turn east-northeastward as the ridge to the north weakens and a broad mid-latitude trough begins to move eastward over the northern United States. The dynamical model guidance is in good agreement on this overall scenario, but there remains a fair amount of spread on the timing of recurvature, with the ECMWF a little farther west and slower than much of the remainder of the guidance. The updated NHC track is between the ECWMF and the various consensus aids, and very close to the previous official forecast. Since Maria is a large hurricane, the associated tropical-storm-force winds could reach a portion of the North Carolina coast by mid-week regardless of the exact forecast track. Some fluctuations in intensity could still occur during the next day or so while Maria moves over warm water and remains in a low shear environment. Later in the forecast period, cooler waters from the wake of Hurricane Jose that traversed the same area last week will likely cause a gradual decrease in intensity. KEY MESSAGES: 1. Maria's is forecast to continue moving northward, paralleling the U.S. east coast, and it is likely that some direct impacts will occur along portions of the coast by midweek. 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 later today. 2. Swells from Maria are increasing along the coast of the southeastern United States and are expected to reach the Mid- Atlantic coast today. These swells will likely cause dangerous surf and rip currents at beaches in these areas through much of the week. For more information, please monitor information from your local National Weather Service office at www.weather.gov. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 28.7N 72.9W 90 KT 105 MPH 12H 25/0000Z 29.7N 73.1W 90 KT 105 MPH 24H 25/1200Z 30.8N 73.2W 90 KT 105 MPH 36H 26/0000Z 31.9N 73.3W 85 KT 100 MPH 48H 26/1200Z 32.9N 73.4W 80 KT 90 MPH 72H 27/1200Z 34.4N 73.5W 75 KT 85 MPH 96H 28/1200Z 35.5N 71.8W 70 KT 80 MPH 120H 29/1200Z 36.5N 68.0W 65 KT 75 MPH $$ Forecaster Brown

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Tropical Storm Pilar Forecast Discussion Number 4

2017-09-24 16:33:53| National Hurricane Center (East Pacific)

Issued at 1000 AM CDT Sun Sep 24 2017 000 WTPZ43 KNHC 241433 TCDEP3 Tropical Storm Pilar Discussion Number 4 NWS National Hurricane Center Miami FL EP182017 1000 AM CDT Sun Sep 24 2017 Pilar has been a difficult tropical storm to analyze and forecast this morning. The overnight short-wave and clean infrared channels from GOES-16 were suggestive that the system had accelerated north-northwestward unexpectedly. However, the available microwave imagery was ambiguous and the spread between the SAB and TAFB Dvorak fixes was a degree. I've re-positioned Pilar north-northwestward significantly, but not as much as may be needed later once additional visible imagery becomes available. The initial motion, then, is a very uncertain 335 degrees at 7 kt, as Pilar is being advected around a deep-layer ridge located over central Mexico. Despite the initial speed up in movement, the guidance insists that Pilar should slow down shortly. The official forecast track is quite similar to the previous one, with a course along or just west of the coast of southwestern Mexico. This forecast is based upon a blend of the GFS, HWRF, and UKMET models. However, the guidance has a large spread (and some of the models could not explicitly track Pilar), and a plausible alternative scenario is that the center of Pilar makes landfall and dissipates shortly thereafter. This alternative is supported by the ECMWF, COAMPS, and HMON models. Hopefully, the 12Z model runs will be more consistent. The initial intensity of Pilar is unchanged at 40 kt, as the SAB and TAFB Dvorak Current Intensity classifications remained the same. While the SSTs are warm and the vertical shear is only moderate for the next 24 to 36 hours, interaction with the high topography of southwestern Mexico may prevent any additional intensification. After about 24 to 36 hours, the vertical shear should go way up as Pilar is affected by strong southerlies associated with a vigorous mid- to upper-tropospheric trough over the southwestern United States. The official intensity forecast is closest to the HWRF model, though none of the guidance now shows any significant intensification. This new forecast is slightly below that from the previous advisory and now calls for Pilar to become a remnant low in about two days. An alternative scenario, mentioned above, is for Pilar to make landfall and dissipate sooner. FORECAST POSITIONS AND MAX WINDS INIT 24/1500Z 19.9N 105.9W 40 KT 45 MPH 12H 25/0000Z 20.4N 106.0W 40 KT 45 MPH 24H 25/1200Z 21.1N 106.0W 35 KT 40 MPH 36H 26/0000Z 21.8N 106.0W 30 KT 35 MPH 48H 26/1200Z 22.5N 106.2W 25 KT 30 MPH...POST-TROP/REMNT LOW 72H 27/1200Z...DISSIPATED $$ Forecaster Landsea

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Hurricane Maria Forecast Discussion Number 33

2017-09-24 10:57:55| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Sun Sep 24 2017 000 WTNT45 KNHC 240857 TCDAT5 Hurricane Maria Discussion Number 33 NWS National Hurricane Center Miami FL AL152017 500 AM EDT Sun Sep 24 2017 A NOAA Hurricane Hunter aircraft investigating Maria has reported 700-mb flight-level winds of 107 kt in the southeastern eyewall, with surface wind estimates from the Stepped Frequency Microwave Radiometer between 75-80 kt. The plane also reported that the central pressure has risen to 948 mb inside a 30 n mi wide eye. Based on these data, the initial intensity is nudged downward to 95 kt, and it is possible that this is a bit generous. The initial motion is 355/8, with Maria currently being steered by the flow between the Atlantic subtropical ridge and a cut-off low/trough over the eastern Gulf of Mexico and the southeastern United States. A general motion toward the north or north- northwest should continue for the next 3 days or so, with some decrease in forward speed as a mid-latitude westerly ridge moves through the New England states to the north of the hurricane. After 72 h, the westerlies move south and erode the subtropical ridge, which should allow Maria to recurve to the northeast. The track guidance supports this scenario, although there is some disagreement on the timing and the location of the recurvature. The new forecast track lies a little to the north and west of the previous track in best agreement with the HFIP Corrected consensus model. However, the 72 and 96 h points lie a little to the east of the forecasts of the GFS and ECMWF models. Regardless of where the recurvature occurs, Maria is a large cyclone and the associated tropical storm force winds could eventually reach a portion of the North Carolina coast. Fluctuations in intensity appear likely during the next 24-36 h as Maria remains over warm water and in an environment of light or moderate shear. After that time, the hurricane is likely to encounter the colder water left by Hurricane Jose, which should cause a weakening trend. The new intensity forecast follows the overall trend of the guidance and, except for a downward nudge at 12 and 24 h, is similar to the previous forecast. KEY MESSAGES: 1. Maria's forecast track continues to be northward, paralleling the U.S. east coast, and it is 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 later today. 2. Swells from Maria are increasing along the coast of the southeastern United States and are expected to reach the Mid-Atlantic coast today. 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 24/0900Z 27.9N 72.7W 95 KT 110 MPH 12H 24/1800Z 28.9N 72.8W 95 KT 110 MPH 24H 25/0600Z 30.1N 73.0W 95 KT 110 MPH 36H 25/1800Z 31.2N 73.1W 90 KT 105 MPH 48H 26/0600Z 32.1N 73.2W 85 KT 100 MPH 72H 27/0600Z 34.0N 73.5W 75 KT 85 MPH 96H 28/0600Z 35.0N 72.5W 70 KT 80 MPH 120H 29/0600Z 36.0N 69.0W 65 KT 75 MPH $$ Forecaster Beven

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Tropical Storm Pilar Forecast Discussion Number 3

2017-09-24 10:56:22| National Hurricane Center (East Pacific)

Issued at 400 AM CDT Sun Sep 24 2017 000 WTPZ43 KNHC 240856 TCDEP3 Tropical Storm Pilar Discussion Number 3 NWS National Hurricane Center Miami FL EP182017 400 AM CDT Sun Sep 24 2017 Recent microwave imagery from an AMSU overpass indicate that the low-level structure of Pilar is rather messy. It is possible that the low-level center may be reforming under the convection to the west, but there have been no recent higher resolution microwave or ASCAT overpasses to confirm this. Despite this, a blend of Dvorak estimates from TAFB and SAB now supports an initial intensity of 40 kt. Given a lack of any other data to go off of, that has been used as the basis for the initial intensity. The initial motion estimate is a highly uncertain 335/4 kt, based in part on extrapolation from the previous motion. There is still a high spread among the models in the first 24 h of the forecast. Several models, including the GFS, forecast that Pilar will move quickly inland and dissipate. A few others, like the HWRF, keep the center of Pilar just far enough offshore to maintain a coherent vortex for about 72 hours. Finally, the ECMWF is an outlier in showing Pilar reforming to the west and stalling off the west coast of Jalisco for 24 h before moving inland and dissipating. In the interest of continuity, the NHC forecast for this advisory does not account for any reformation of the center, and shows Pilar moving steadily northward to north-northwestward until dissipation, like the HWRF. The NHC forecast is close to the multi-model consensus, though in this case so many members quickly show dissipation, that relatively few members are averaged together. The intensity forecast is also complicated by land interaction. SSTs near the southwest coast of Mexico are very warm, and the shear is forecast to remain low for about 36 h. The NHC intensity forecast only shows slight intensification since significant land interaction is still expected. In the event that Pilar stays farther offshore, it could become stronger than currently forecast. Regardless of how much Pilar intensifies, a sharp increase in shear around 48 h should quickly cause the cyclone to weaken to a remnant low and eventually dissipate. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 19.1N 105.6W 40 KT 45 MPH 12H 24/1800Z 19.5N 105.8W 45 KT 50 MPH 24H 25/0600Z 20.2N 105.9W 45 KT 50 MPH 36H 25/1800Z 21.0N 105.8W 40 KT 45 MPH 48H 26/0600Z 22.0N 106.0W 30 KT 35 MPH 72H 27/0600Z 24.0N 107.0W 25 KT 30 MPH...POST-TROP/REMNT LOW 96H 28/0600Z...DISSIPATED $$ Forecaster Zelinsky

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Hurricane Lee Forecast Discussion Number 25

2017-09-24 10:55:26| National Hurricane Center (Atlantic)

Issued at 500 AM AST Sun Sep 24 2017 000 WTNT44 KNHC 240855 TCDAT4 Hurricane Lee Discussion Number 25 NWS National Hurricane Center Miami FL AL142017 500 AM AST Sun Sep 24 2017 Since the earlier special advisory issued to make Lee a hurricane, the eye has become much better defined in IR imagery, indicating that the hurricane has continued to rapidly intensify. The initial intensity has therefore been increased to 75 kt, based on an application of the Dvorak technique to the latest IR imagery. It should be noted that in this case, the intensity is fairly uncertain given the lack of ground truth and the small size of the cyclone. At this point, it is hard to judge how long this period of rapid intensification will continue. On one hand, Lee is expected to remain in a relatively unstable and low-shear environment for the next several days. On the other, despite the clearing of the eye, cloud tops have not cooled significantly overnight, and the slow motion of Lee allows the possibility that the storm could begin to interact with its own cold wake. The NHC forecast brings Lee to near major hurricane strength in 24 h, and then keeps Lee around that intensity through day 4, similar to the FSSE model. By the end of the forecast, higher shear, due in part to the outflow of Hurricane Maria, could cause Lee to weaken, so the NHC forecast is a little lower at that point. Lee is drifting southward for now, but a southeastward motion is still forecast to begin later today. Very little change has been made to the NHC forecast, which is mainly just an update of the previous one. The models have come into better agreement that Lee will be primarily steered for the next several days by a mid-level ridge building to the northwest. By day 5, a trough associated with Maria and the subtropical ridge should cause Lee to begin recurving. Since the model spread has decreased, confidence in that aspect of the forecast has increased this morning. FORECAST POSITIONS AND MAX WINDS INIT 24/0900Z 31.8N 50.1W 75 KT 85 MPH 12H 24/1800Z 31.6N 49.6W 85 KT 100 MPH 24H 25/0600Z 31.1N 48.9W 95 KT 110 MPH 36H 25/1800Z 30.6N 48.8W 95 KT 110 MPH 48H 26/0600Z 30.2N 49.4W 95 KT 110 MPH 72H 27/0600Z 30.5N 51.3W 90 KT 105 MPH 96H 28/0600Z 32.0N 52.5W 90 KT 105 MPH 120H 29/0600Z 34.5N 51.4W 80 KT 90 MPH $$ Forecaster Zelinsky

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