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Hurricane OLAF Forecast Discussion Number 17

2015-10-19 04:42:29| National Hurricane Center (East Pacific)

Issued at 800 PM PDT SUN OCT 18 2015 000 WTPZ44 KNHC 190242 TCDEP4 HURRICANE OLAF DISCUSSION NUMBER 17 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 800 PM PDT SUN OCT 18 2015 Olaf's inner-core convective pattern has improved significantly since the previous advisory. A cloud-filled eye about 15 n mi in diameter has become apparent in visible, infrared, and passive microwave satellite data since around 18/2300Z, and cloud tops have cooled to below -80C almost encircling the eye during the past hour. The initial intensity has been increased to 85 kt based on a blend of Dvorak satellite intensity estimates of T5.0/90 kt from TAFB and T4.5/77 kt from SAB. This intensity is supported by NHC AODT satellite intensity estimates of T4.8/85 kt and T5.2/95 kt using embedded-center and obscured-eye patterns, respectively. The initial motion estimate is 275/10 kt based on microwave and visible eye fixes. Other than a slight southward adjustment due to the recent eye positions, the previous forecast track and reasoning essentially remain unchanged. Olaf is expected to gradually round the southwestern and western portion of a deep-layer subtropical ridge, turning northward and slowing down considerably on days 4 and 5 as the hurricane moves into a weakness in the ridge. The westward shift in the guidance as mentioned in the previous discussion appears to have settled down on this latest cycle with the UKMET and the other models now having moved closer together. The new NHC track forecast has been adjusted slightly to the left of the previous advisory track due to the more southward initial position, and lies between the consensus models and the more westward UKMET solution. Although Olaf has made a sharp intensity comeback, the proximity of a pronounced dry slot that has worked its way completely around the small inner-core convective region of the hurricane precludes forecasting rapid intensification at this time. However, with the vertical shear expected to be near or below 5 kt for the next 72 hours while Olaf remains over 29C or greater SSTs, at least steady strengthening appears to be in order for the next 36 hours or so. Although the LGEM intensity guidance brings Olaf to category 4 strength (115-120 kt) in the 36-72 h period, the official forecast levels off the intensity at 110 kt due to the likelihood of one or more eyewall replacement cycles occurring during that time. By days 4 and 5, a gradual weakening trend is forecast to occur as Olaf moves into a region of much drier mid-level air with humidity values less than 50 percent, along with cold upwelling occurring beneath the cyclone due to the hurricane's expected slower forward speed of around 5 kt. FORECAST POSITIONS AND MAX WINDS INIT 19/0300Z 9.7N 135.4W 85 KT 100 MPH 12H 19/1200Z 9.9N 136.8W 95 KT 110 MPH 24H 20/0000Z 10.3N 138.6W 105 KT 120 MPH 36H 20/1200Z 11.0N 140.2W 110 KT 125 MPH 48H 21/0000Z 11.9N 141.6W 110 KT 125 MPH 72H 22/0000Z 13.7N 144.1W 110 KT 125 MPH 96H 23/0000Z 16.1N 145.4W 100 KT 115 MPH 120H 24/0000Z 18.2N 145.4W 90 KT 105 MPH $$ Forecaster Stewart

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Hurricane OLAF Forecast Discussion Number 16

2015-10-18 22:55:23| National Hurricane Center (East Pacific)

Issued at 200 PM PDT SUN OCT 18 2015 000 WTPZ44 KNHC 182055 TCDEP4 HURRICANE OLAF DISCUSSION NUMBER 16 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 200 PM PDT SUN OCT 18 2015 Olaf hasn't changed much in organization during the past few hours, with satellite imagery still showing a CDO and a large, but broken, convective band in the western and southern semicircles of the cyclone. There appears to be a dry slot working into the western side of the circulation, and the eye has been less distinct in recent microwave imagery than it was earlier. The initial intensity remains 70 kt, a little higher than the latest Dvorak estimates from TAFB, SAB, and the UW-CIMSS ADT. The initial 34- and 50-kt wind radii were adjusted based on a timely 1815Z ASCAT-B overpass. The pause in strengthening could be due to the aforementioned dry air and moderate west-northwesterly to northwesterly shear analyzed over the cyclone by the SHIPS model and UW-CIMSS. Given the shear and an inner-core that may be a little less organized, the new NHC intensity forecast shows a slower rate of strengthening in the first couple of days, and is now closer to the bulk of the intensity guidance. The forecast still shows Olaf becoming a major hurricane in 2-3 days followed by a slow decay as the hurricane moves over somewhat cooler SSTs late in the period. The NHC prediction is close to the SHIPS model through much of the period, but a little above it at peak intensity. Olaf continues to move quickly westward, with a motion now estimated at 280/12. This continues a trend of the hurricane moving faster and to the left of the previous couple of forecasts. While the subtropical ridge to the north weakens is still expected to weaken and lift northward, the track model guidance continues to shift westward given the initial motion and Olaf failing to gain as much latitude as previously thought in the short term. The new NHC track forecast has been adjusted to the left of the previous one, especially at days 3 through 5, and is now close to a blend of the latest ECMWF and GFS forecasts. FORECAST POSITIONS AND MAX WINDS INIT 18/2100Z 9.7N 134.3W 70 KT 80 MPH 12H 19/0600Z 9.9N 135.7W 75 KT 85 MPH 24H 19/1800Z 10.3N 137.5W 85 KT 100 MPH 36H 20/0600Z 11.0N 139.1W 95 KT 110 MPH 48H 20/1800Z 11.8N 140.6W 105 KT 120 MPH 72H 21/1800Z 13.7N 143.4W 105 KT 120 MPH 96H 22/1800Z 16.0N 145.0W 95 KT 110 MPH 120H 23/1800Z 18.0N 145.0W 85 KT 100 MPH $$ Forecaster Brennan

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Hurricane OLAF Forecast Discussion Number 15

2015-10-18 16:36:53| National Hurricane Center (East Pacific)

Issued at 800 AM PDT SUN OCT 18 2015 000 WTPZ44 KNHC 181436 TCDEP4 HURRICANE OLAF DISCUSSION NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 800 AM PDT SUN OCT 18 2015 The could pattern of Olaf features a CDO with a primary convective band extending into the southern and western semicircles. A 0726Z GPM image showed a well-defined low- and mid-level eye, and the initial intensity has been nudged upward to 70 kt, a little above the latest Dvorak estimates from TAFB and SAB. Further strengthening appears likely for the next couple of days while the hurricane moves over warm waters and through a generally low-shear environment. The SHIPS RI index continues to suggest that rapid intensification is possible, with a 47 percent chance of a 25-kt increase in the next 24 hours. The new NHC intensity forecast is similar to the previous one and is along the upper edge of the guidance, closest to the LGEM forecast. By 72 hours, the cyclone will be moving over somewhat cooler waters with a slight increase in shear, which should result in slow weakening late in the period. Olaf continues to move quickly westward, with an initial motion estimate of 275/10. The track forecast reasoning remains unchanged, with Olaf expected to gradually turn poleward during the forecast period as the subtropical ridge initially to the northwest of Olaf lifts northward and weakens. The track model guidance is in good agreement on this scenario, but continues to trend toward a more gradual northward turn. The new NHC track forecast has been adjusted a little to the left of the previous one and is close to the latest multi-model consensus. FORECAST POSITIONS AND MAX WINDS INIT 18/1500Z 9.5N 133.2W 70 KT 80 MPH 12H 19/0000Z 9.7N 134.5W 80 KT 90 MPH 24H 19/1200Z 10.2N 136.3W 90 KT 105 MPH 36H 20/0000Z 10.8N 138.0W 100 KT 115 MPH 48H 20/1200Z 11.6N 139.5W 110 KT 125 MPH 72H 21/1200Z 13.5N 142.4W 105 KT 120 MPH 96H 22/1200Z 15.5N 143.9W 95 KT 110 MPH 120H 23/1200Z 17.8N 144.1W 85 KT 100 MPH $$ Forecaster Brennan

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Hurricane OLAF Forecast Discussion Number 14

2015-10-18 10:51:47| National Hurricane Center (East Pacific)

Issued at 200 AM PDT SUN OCT 18 2015 000 WTPZ44 KNHC 180851 TCDEP4 HURRICANE OLAF DISCUSSION NUMBER 14 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 200 AM PDT SUN OCT 18 2015 Olaf has intensified overnight. Satellite images show that convection has deepened with a sizable area of -90C cloud tops near the center, along with an eye occasionally appearing on the night-visible channel. Microwave data also show that the eye has become better defined, and the feature has even become more distinct in the lower resolution AMSU data. With the appearance of the eye in conventional satellite data and the increase in convection near the center, the initial wind speed is set to 65 kt, near the Dvorak estimate from SAB. It is also worth noting that ASCAT data show that Olaf has also grown in size significantly from yesterday, and this is reflected in the larger initial and forecast wind radii. Olaf should continue to intensify during the next couple of days with favorable environmental conditions of low shear, warm water and high mid-level moisture in the forecast. Rapid intensification is a distinct possibility, with the SHIPS-RI index showing about a 40 percent chance of a 30-kt increase in the next 24 hours. The intensity forecast will bring Olaf up to major hurricane strength in 36 hours, in line with the highest guidance from the LGEM model and the Florida State Superensemble. In a few days, some southerly shear and dry air entrainment is possible, which could lead to Olaf beginning a weakening trend. Little change was made to the long range intensity forecast, close to a blend of the previous NHC prediction and the intensity consensus. ASCAT and satellite data were very helpful in determining an initial motion of 275/8. A motion to the west and then west- northwest is expected for the next two days or so while the cyclone moves on the south side of the subtropical ridge. After that time, a mid- to upper-level trough passing north of the Hawaiian Islands is forecast to cause a break in the ridge, allowing Olaf to turn northwestward by 96 hours and northward by 120 hours. Although guidance is in general agreement on the large-scale pattern, the timing of the northward turn is in some question, especially since Olaf has been struggling to gain latitude. Overall, the guidance has generally been showing a later turn, with the ECMWF/UKMET models having a weaker trough and less of a ridge breakdown. The NHC track forecast is shifted west toward the latest consensus aids, although it still lies east of the model consensus. At 9.4N, Olaf is the lowest-latitude hurricane formation in the eastern Pacific dataset. Reliable records in the basin go back to about 1971. FORECAST POSITIONS AND MAX WINDS INIT 18/0900Z 9.4N 131.9W 65 KT 75 MPH 12H 18/1800Z 9.6N 133.2W 75 KT 85 MPH 24H 19/0600Z 10.0N 135.1W 90 KT 105 MPH 36H 19/1800Z 10.6N 136.9W 100 KT 115 MPH 48H 20/0600Z 11.3N 138.6W 110 KT 125 MPH 72H 21/0600Z 13.3N 141.7W 105 KT 120 MPH 96H 22/0600Z 15.0N 143.5W 95 KT 110 MPH 120H 23/0600Z 17.5N 143.5W 80 KT 90 MPH $$ Forecaster Blake

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Tropical Storm OLAF Forecast Discussion Number 13

2015-10-18 04:39:49| National Hurricane Center (East Pacific)

Issued at 800 PM PDT SAT OCT 17 2015 000 WTPZ44 KNHC 180239 TCDEP4 TROPICAL STORM OLAF DISCUSSION NUMBER 13 NWS NATIONAL HURRICANE CENTER MIAMI FL EP192015 800 PM PDT SAT OCT 17 2015 After the pause noted in the previous advisory, Olaf has resumed intensification this evening. Well-defined convective bands with cloud tops colder than -80C have formed around the center, and microwave data show a developing 15 n mi wide eye. Satellite intensity estimates have increased to 55 kt from TAFB and SAB, and there is a recent CIMSS AMSU intensity estimate of 52 kt. Based on these data, the initial intensity is increased to 55 kt. The initial motion is 270/8. Olaf is now moving slower as the subtropical ridge to the north weakens in response to the influence of a mid- to upper-level trough near the U. S. west coast. A general west-northwestward motion is expected for the next two days or so as the cyclone moves on the south side of the weakened ridge. After that time, a second mid- to upper-level trough passing north of the Hawaiian Islands is forecast to cause a break in the ridge, allowing Olaf to turn northwestward by 72 hours and northward by 120 hours. There is little overall change in the guidance since the last advisory. Thus, the new forecast track is very similar to the previous track and lies near the center of the track guidance envelope. There is still some light northwesterly shear over Olaf. However, this is forecast to diminish during the next 24 hours, and improving cirrus outflow suggests this may already be occurring. Based on this, warm sea surface temperatures, and a moist environment, continued strengthening is expected during the next 48 hours. After 72 hours, developing southerly shear and dry air entrainment are expected to cause Olaf to weaken. The new intensity forecast is similar to the previous forecast and is in best agreement with the Florida State Superensemble. It should be noted that the small inner core seen in microwave imagery could allow rapid intensification, and the Rapid Intensification Index of the SHIPS model has a 25-30 percent chance of a 30-kt increase in intensity during the next 24 hours. Should this occur, Olaf could get significantly stronger than currently forecast. FORECAST POSITIONS AND MAX WINDS INIT 18/0300Z 9.2N 131.0W 55 KT 65 MPH 12H 18/1200Z 9.4N 132.3W 65 KT 75 MPH 24H 19/0000Z 9.9N 134.1W 80 KT 90 MPH 36H 19/1200Z 10.4N 135.8W 90 KT 105 MPH 48H 20/0000Z 11.2N 137.4W 100 KT 115 MPH 72H 21/0000Z 13.5N 140.5W 100 KT 115 MPH 96H 22/0000Z 15.5N 142.0W 95 KT 110 MPH 120H 23/0000Z 18.0N 142.5W 80 KT 90 MPH $$ Forecaster Beven

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