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Tropical Storm OLAF Forecast Discussion Number 12

2015-10-17 22:33:17| National Hurricane Center (East Pacific)

Issued at 200 PM PDT SAT OCT 17 2015 000 WTPZ44 KNHC 172033 TCDEP4 TROPICAL STORM OLAF DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 200 PM PDT SAT OCT 17 2015 There appears to have been a temporary interruption to Olaf's intensification this morning, which could be related to a couple of different factors. Microwave imagery from earlier suggested that the cyclone ingested a tongue of relatively drier air. There is also moderate northwesterly shear still affecting the system. Nevertheless, there are some signs that Olaf could be about to resume an intensification. The cyclone has maintained a small CDO, but there is now greater definition to inner-core features than 12 to 24 hours ago, and the convective bands have lengthened and gained greater curvature. While satellite classifications remain a consensus T3.0/45 kt at 1800 UTC, the initial intensity estimate of 50 kt from the previous advisory is maintained. The initial motion estimate is 265/11. The mid-level subtropical ridge steering Olaf westward should weaken soon in response to the long tail of a mid- to upper-level trough near the U.S. west coast settling southward to the north of the cyclone. The weaker ridge should induce a west-northwestward course at a reduced forward speed during the 2 to 3 days. After about 72 hours, a large anticyclone forming in the east-central subtropical Pacific near 140W is forecast to shift eastward while a weakness develops along farther west. Global models show Olaf moving in the flow between these two synoptic features, resulting in a northwestward and then north- northwestward turn after 96 hours. The guidance has shifted toward the left again this cycle, especially later in the forecast period, and the official track forecast is moved in that direction. Since most of the global models depicted Olaf farther north than where it currently is, it would not be surprising if further leftward adjustments are necessary. Other than some northwesterly shear over Olaf during the next day or so, the large-scale environment is conducive for Olaf to intensify. The shear could slow the rate of intensification, but it would not appear strong enough not to allow Olaf to strengthen at least at the climatological rate of development of one Dvorak T-number per day. A diminution of the shear by 36 hours, in combination with other very conducive environmental factors, should allow Olaf to strengthen to near major hurricane strength in 2 to 3 days. Olaf is also expected to become a much larger cyclone during the next several days, with an impressive expansion of the wind field depicted in global models. Late in the forecast period, once Olaf gains enough latitude, a drier environment with an increase in southerly or south-southwesterly shear should result in weakening. The new intensity forecast is identical to the previous one and is closest to the FSU Superensemble output and near the upper edge of the guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 17/2100Z 9.2N 130.5W 50 KT 60 MPH 12H 18/0600Z 9.3N 131.8W 60 KT 70 MPH 24H 18/1800Z 9.8N 133.5W 75 KT 85 MPH 36H 19/0600Z 10.3N 135.2W 90 KT 105 MPH 48H 19/1800Z 11.0N 136.9W 100 KT 115 MPH 72H 20/1800Z 12.9N 139.9W 100 KT 115 MPH 96H 21/1800Z 15.2N 142.0W 95 KT 110 MPH 120H 22/1800Z 17.3N 142.8W 85 KT 100 MPH $$ Forecaster Kimberlain

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Tropical Storm OLAF Forecast Discussion Number 11

2015-10-17 16:46:46| National Hurricane Center (East Pacific)

Issued at 800 AM PDT SAT OCT 17 2015 000 WTPZ44 KNHC 171446 TCDEP4 TROPICAL STORM OLAF DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 800 AM PDT SAT OCT 17 2015 Olaf's cloud pattern has not exhibited much overall change since the last advisory in conventional satellite imagery. The cyclone is characterized by a small CDO with one primary band over the western half of the circulation, though the band is detached from the main convective mass and fragmented. The convective distribution is suggestive of some northwesterly shear, as shown in GFS- and ECMWF-based SHIPS output, but a partial 1030 UTC AMSU pass indicated a mid-level eye, perhaps not completely coincident with the low-level center. Dvorak intensity estimates are consensus T3.0/45 kt, but the initial intensity is increased to 50 kt on the basis of the greater organization seen in microwave imagery. The initial motion estimate is 270/11. The mid-level subtropical ridge steering Olaf westward is about to weaken in response to the long tail of a mid- to upper-level trough near the U.S. west coast settling southward to the north of the cyclone. The weaker ridge should induce a slower forward speed with a gradual increase in latitude during the next couple of days. After 48 hours, a large anticyclone forming in the east-central subtropical Pacific near 140W is forecast to shift eastward while a weakness develops along 150W. Global models show Olaf moving in the flow between these two synoptic features, which should result in a northwestward and then north-northwestward turn after 96 hours. The official track forecast is nearly unchanged through 36 hours. After that time, the track forecast is shifted westward in agreement with a leftward- shifting multi-model consensus and a majority of ECMWF ensemble members that were farther west than the operational ECMWF solution that lies east of the multi-model consensus. The new track forecast is on the western side of the guidance envelope. Except for the northwesterly shear, there are no obvious impediments to continued strengthening for the next few days. Exactly how much the shear is a factor, however, remains to be seen, but its most likely effect would be to possibly slow the rate of intensification. According to the SHIPS output, the northwesterly shear diminishes after 36 hours, and with all other large-scale factors favoring intensification, Olaf is likely to become a major hurricane in 2 or 3 days. Late in the forecast period, once Olaf gains enough latitude, a drier environment with an increase in southerly or south-southwesterly shear should result in weakening. The official intensity forecast is near the multi-model consensus through 36 hours and generally above it after that time. FORECAST POSITIONS AND MAX WINDS INIT 17/1500Z 9.4N 129.5W 50 KT 60 MPH 12H 18/0000Z 9.5N 130.9W 60 KT 70 MPH 24H 18/1200Z 9.8N 132.5W 75 KT 85 MPH 36H 19/0000Z 10.3N 134.1W 90 KT 105 MPH 48H 19/1200Z 11.0N 135.8W 100 KT 115 MPH 72H 20/1200Z 12.6N 139.0W 100 KT 115 MPH 96H 21/1200Z 14.7N 141.4W 95 KT 110 MPH 120H 22/1200Z 16.9N 142.5W 85 KT 100 MPH $$ Forecaster Kimberlain

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Tropical Storm OLAF Forecast Discussion Number 10

2015-10-17 10:33:45| National Hurricane Center (East Pacific)

Issued at 200 AM PDT SAT OCT 17 2015 000 WTPZ44 KNHC 170833 TCDEP4 TROPICAL STORM OLAF DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 200 AM PDT SAT OCT 17 2015 The convective organization of Olaf has continued to improve since the previous advisory. A ragged CDO feature has developed, but very cold overshooting cloud tops with temperatures of -87C to -88C have also developed near the low-level circulation center as depicted in recent passive microwave satellite images. In addition, outer curved band features have also improved to the north and south of the center while the upper-level outflow pattern has expanded and become more symmetrical. The initial intensity has been increased to 45 kt based on a consensus satellite intensity estimate of T3.0/45 kt from TAFB and SAB, which is supported by a UW-CIMSS ADT estimate of T3.4/53 kt and an 0134 UTC AMSU intensity estimate of 59 kt. The initial motion estimate is now 270/10 kt. The NHC model guidance remains in excellent agreement on Olaf rounding the southwestern and western periphery of a deep-layer ridge during the next five days, accompanied by a gradual decrease in forward speed. On day 5, the cyclone could begin to recurve and accelerate toward the north-northeast or northeast as suggested by the GFS model, but the preponderance of the guidance keeps Olaf south of ridge axis and moving generally northward at that time, and this is reflected in the official forecast. The new track forecast is similar to the previous advisory, and essentially lies down the middle of the guidance envelope and close to the consensus models. Microwave imagery suggests that the aforementioned intense burst of convection has likely tightened up the inner-core wind field and has reduced the size of the radius of maximum winds (RMW). Given the small RMW of about 20 nmi, the expanding outflow pattern, continued deep-layer vertical wind shear of less than 10 kt, and mid-level humidity values expected to increase to around 70 percent, a period of rapid intensification is forecast to occur for the next 24 hours. After that, the intensity forecast calls for steady strengthening through 72 hours, followed by a leveling off in the intensity on days 4 and 5 due to possible eyewall replacement cycles that are impossible to forecast that far in advance. The new intensity forecast is close to the LGEM intensity model through 72 hours, and then is lower than the LGEM model and closer to the SHIPS model on days 4 and 5. As mentioned in previous advisories, it would not be surprising to see more strengthening than is currently forecast given the very conducive large-scale environment, similar to the 96-h GFS and ECMWF model forecasts of a 938-mb and 956-mb hurricane, respectively. FORECAST POSITIONS AND MAX WINDS INIT 17/0900Z 9.5N 128.5W 45 KT 50 MPH 12H 17/1800Z 9.5N 129.8W 60 KT 70 MPH 24H 18/0600Z 9.7N 131.5W 75 KT 85 MPH 36H 18/1800Z 10.1N 133.0W 85 KT 100 MPH 48H 19/0600Z 10.7N 134.6W 95 KT 110 MPH 72H 20/0600Z 12.2N 137.7W 100 KT 115 MPH 96H 21/0600Z 14.2N 140.3W 100 KT 115 MPH 120H 22/0600Z 16.3N 141.6W 100 KT 115 MPH $$ Forecaster Stewart

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Tropical Storm OLAF Forecast Discussion Number 9

2015-10-17 04:34:40| National Hurricane Center (East Pacific)

Issued at 800 PM PDT FRI OCT 16 2015 000 WTPZ44 KNHC 170234 TCDEP4 TROPICAL STORM OLAF DISCUSSION NUMBER 9 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 800 PM PDT FRI OCT 16 2015 The cloud pattern of Tropical Depression Nineteen-E has continued to become better organized during the past several hours. There is now a persistent convective area near the center and ragged outer bands in the western semicircle. Satellite intensity estimates have increased to 45 kt from SAB and 35 kt from TAFB. In addition, Rapidscat data from the International Space Station showed an area of 30-35 kt winds just north of the central convection. Based on these data, the depression is upgraded to Tropical Storm Olaf with 35 kt winds. The initial motion estimate is now 270/12. Olaf is currently being steered by a mid-level subtropical ridge. The dynamical models forecast the ridge to persist but weaken during the next 72-96 hours due to the influence of a strong deep-layer trough forecast to develop over the southwestern United States. This evolution should cause Olaf to continue westward with a decrease in forward speed during the first 36-48 hours, followed by a west-northwestward motion from 48-96 hours. After that time, the ridge is forecast to break to the east of the Hawaiian Islands, which should allow the storm to turn more northward. The track guidance is generally in good agreement with this scenario, with the ECMWF, GFS, and the various consensus models rather tightly clustered. The new forecast track is similar to the previous track through 72 hours, then is nudged slightly westward thereafter. The new track is close to the model consensus. Olaf is expected to remain in an environment of good moisture, light shear, and warm sea surface temperatures for at least the next three days, so there is no obvious reason it should not strengthen. On one side of the guidance, the LGEM forecasts Olaf to become a major hurricane in 72 hours, and the Rapid Intensification Index of the SHIPS model shows a 30-35 percent chance of 30 kt of strengthening during the next 24 hours. On the other side, the GFDL and HWRF forecast a much slower rate of intensification. The new intensity forecast is nudged upward from the previous forecast based on current trends and is a compromise between the SHIPS model and the Florida State Superensemble. As stated in the previous advisory, given the very conducive large-scale environment, it would not be surprising to see more strengthening than is currently forecast. FORECAST POSITIONS AND MAX WINDS INIT 17/0300Z 9.7N 127.6W 35 KT 40 MPH 12H 17/1200Z 9.7N 129.1W 45 KT 50 MPH 24H 18/0000Z 9.7N 130.8W 55 KT 65 MPH 36H 18/1200Z 10.0N 132.4W 65 KT 75 MPH 48H 19/0000Z 10.5N 133.9W 75 KT 85 MPH 72H 20/0000Z 12.0N 137.0W 95 KT 110 MPH 96H 21/0000Z 14.0N 140.0W 95 KT 110 MPH 120H 22/0000Z 16.5N 141.5W 90 KT 105 MPH $$ Forecaster Beven

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Tropical Depression NINETEEN-E Forecast Discussion Number 8

2015-10-16 22:33:42| National Hurricane Center (East Pacific)

Issued at 200 PM PDT FRI OCT 16 2015 000 WTPZ44 KNHC 162033 TCDEP4 TROPICAL DEPRESSION NINETEEN-E DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 200 PM PDT FRI OCT 16 2015 The depression's cloud pattern has increased in organization since yesterday. A small CDO-like feature appears to be forming over the estimated low-level center. There has also been an increase in banding features and their associated curvature, especially over the western half of the circulation. Dvorak intensity estimates have increased some, and the initial intensity estimate is raised to 30 kt. Recent fixes suggest that the depression may be slowing down, and the initial motion estimate could be slightly slower than the longer-term motion of 265/13. The cyclone is forecast to maintain a general westward course underneath a mid-level subtropical ridge during the next 24 to 36 hours but decelerate as the tail of a mid- to upper- level trough, extending from the U.S. west coast, causes the ridge to weaken. As the ridge weakens further by 36 hours, the cyclone should begin to gain more latitude. An even greater in increase in latitude is expected after 96 hours when the cyclone encounters a more significant weakness west of 140W. The official track forecast has changed very little compared to the previous one and is near the multi-model consensus throughout the forecast period. With the increased organization of the cyclone's central features, intensification seems likely in an environment characterized by very warm SSTs, low shear, and a moist mid-troposphere. In fact, the SHIPS RI index shows a 65 percent chance of a 25-kt increase in intensity during the next 24 hours. The official intensity forecast is raised relative to the previous one, especially during the early part of the forecast. Given the very conducive large-scale environment for intensification, it would not be surprising to see more occur than what is currently forecast. FORECAST POSITIONS AND MAX WINDS INIT 16/2100Z 9.5N 126.2W 30 KT 35 MPH 12H 17/0600Z 9.4N 127.7W 40 KT 45 MPH 24H 17/1800Z 9.4N 129.5W 50 KT 60 MPH 36H 18/0600Z 9.5N 131.1W 60 KT 70 MPH 48H 18/1800Z 10.0N 132.6W 70 KT 80 MPH 72H 19/1800Z 11.3N 135.7W 90 KT 105 MPH 96H 20/1800Z 13.4N 138.6W 95 KT 110 MPH 120H 21/1800Z 15.5N 140.6W 90 KT 105 MPH $$ Forecaster Kimberlain

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