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Hurricane ISELLE Forecast Discussion Number 8

2014-08-02 16:51:35| National Hurricane Center (East Pacific)

Issued at 800 AM PDT SAT AUG 02 2014 000 WTPZ44 KNHC 021451 TCDEP4 HURRICANE ISELLE DISCUSSION NUMBER 8 NWS NATIONAL HURRICANE CENTER MIAMI FL EP092014 800 AM PDT SAT AUG 02 2014 Iselle has maintained an eye during the past several hours, but the deepest convection has been unable to persist within the western eyewall. According to UW-CIMSS shear analyses, about 15 kt of north-northeasterly shear is affecting the hurricane, and a recent TRMM pass indicated that the mid-level center is displaced about 10-15 n mi south of the low-level center. The maximum winds are still estimated to be 70 kt based on Dvorak estimates of T4.5/77 kt from SAB and T4.0/65 kt from TAFB. Moderate vertical shear is expected to persist for another 12 hours or so and then become negligible between 24-72 hours. Even though sea surface temperatures are expected to stay near or just above 26C through the next 5 days, oceanic heat content is forecast to drop significantly in about 24-36 hours. Therefore, mixing and upwelling of colder water is likely to limit significant additional strengthening. The intensity models are in good agreement in showing Iselle leveling off around 75 kt during the next couple of days, and the official forecast maintains that scenario. Cooler ocean water and increased shear are expected to cause weakening beyond day 3. Iselle is located to the south of a mid-level ridge, which is steering the hurricane westward with an initial motion of 280/9 kt. A mid-level shortwave trough is still expected to develop to the north of Iselle in about 48 hours, but its only real effect will be to weaken the steering flow for a day or two. After day 3, mid-level ridging is forecast to restrengthen to the north and northeast of Iselle, causing the cyclone to accelerate westward by the end of the forecast period. There continues to be some latitudinal spread in the track guidance, especially on days 4 and 5. The updated NHC track forecast is very near the previous forecast and the model consensus TVCE for the sake of continuity. FORECAST POSITIONS AND MAX WINDS INIT 02/1500Z 15.0N 129.2W 70 KT 80 MPH 12H 03/0000Z 15.2N 130.5W 75 KT 85 MPH 24H 03/1200Z 15.6N 132.4W 75 KT 85 MPH 36H 04/0000Z 15.9N 134.2W 75 KT 85 MPH 48H 04/1200Z 16.0N 135.9W 75 KT 85 MPH 72H 05/1200Z 16.0N 139.0W 70 KT 80 MPH 96H 06/1200Z 16.0N 143.0W 60 KT 70 MPH 120H 07/1200Z 16.5N 148.5W 45 KT 50 MPH $$ Forecaster Berg

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Hurricane ISELLE Forecast Discussion Number 7

2014-08-02 10:59:48| National Hurricane Center (East Pacific)

Issued at 200 AM PDT SAT AUG 02 2014 000 WTPZ44 KNHC 020859 TCDEP4 HURRICANE ISELLE DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL EP092014 200 AM PDT SAT AUG 02 2014 Satellite images indicate that Iselle continues to become better organized during the past several hours with a ragged eye feature present on the shortwave infrared channel. Microwave data also show that the inner core has become better defined on the last SSMI/S pass. There is quite a disparity, however, of current satellite intensity estimates, ranging from 55 to 90 kt. Since the cyclone appears somewhat better organized than 6 hours ago, the wind speed is raised to 70 kt, but this estimate is more uncertain than average. Iselle has turned westward and appears to be moving 280/9, although recently has wobbled due westward. The subtropical ridge to the northwest of the cyclone should remain firm for the next several days, continuing the general westward motion. There has been a notable southward shift to most of the guidance overnight with less influence now expected from a digging trough along 130W in a couple of days. The ridge is expected to strengthen at long range, causing a faster westward motion at that time. The new NHC track forecast is shifted southward at all times, especially at Day 3 and beyond, close to the model consensus. Although Iselle has good inner core structure, a combination of moderate northeasterly shear and decreasing SSTs is expected to limit the chance for significant strengthening over the next couple of days. However, the overall environment is not that unfavorable, so little change in wind speed is forecast through 48 hours. After that time, the cyclone will likely encounter drier air aloft, marginal SSTs and more significant shear by Day 5, which should cause weakening. The new forecast is kept very similar to the previous forecast, which lies near the model consensus. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 14.8N 128.4W 70 KT 80 MPH 12H 02/1800Z 15.0N 129.6W 75 KT 85 MPH 24H 03/0600Z 15.4N 131.4W 75 KT 85 MPH 36H 03/1800Z 15.7N 133.2W 75 KT 85 MPH 48H 04/0600Z 16.0N 134.8W 75 KT 85 MPH 72H 05/0600Z 16.0N 138.0W 70 KT 80 MPH 96H 06/0600Z 16.0N 141.5W 60 KT 70 MPH 120H 07/0600Z 16.5N 147.0W 45 KT 50 MPH $$ Forecaster Blake

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Tropical Storm BERTHA Forecast Discussion Number 6

2014-08-02 10:51:19| National Hurricane Center (Atlantic)

Issued at 500 AM AST SAT AUG 02 2014 000 WTNT43 KNHC 020850 TCDAT3 TROPICAL STORM BERTHA DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL AL032014 500 AM AST SAT AUG 02 2014 Bertha is a disorganized tropical cyclone, but based on earlier reconnaissance aircraft data, it is still producing winds of 40 to 45 knots. Satellite data show that the low-level center is exposed with most of the showers and squalls to the north and east. San Juan Puerto Rico radar also shows the circulation associated with the cyclone. Another plane will be investigating Bertha in a few hours. The shear has not decreased, but global models insist on forecasting a little more conducive upper-level environment in 24 hours or so. If the cyclone survives the current shear, and the interaction with Puerto Rico and Hispaniola, it could strengthen slightly over the open Atlantic as indicated in the NHC forecast. In fact, this is the solution of the SHIPS and HWRF models. Bertha continues to be embedded within a strong easterly flow south of the subtropical ridge, and the cyclone is racing toward the west-northwest of 295 degrees at about 19 knots. However, in about 24 to 36 hours, Bertha will reach the southwestern edge of the ridge and will likely slow down a little. The cyclone will then move northward and northeastward over the Atlantic, steered by the southerly flow between the ridge and a trough over the eastern United States. This is the solution of most of the dynamical models which indeed are in pretty good agreement. The NHC forecast follows the multi-model consensus and is very similar to the previous one. FORECAST POSITIONS AND MAX WINDS INIT 02/0900Z 16.3N 64.9W 45 KT 50 MPH 12H 02/1800Z 17.8N 67.3W 45 KT 50 MPH 24H 03/0600Z 20.0N 70.0W 45 KT 50 MPH 36H 03/1800Z 22.5N 72.3W 45 KT 50 MPH 48H 04/0600Z 25.0N 73.9W 50 KT 60 MPH 72H 05/0600Z 31.0N 72.5W 60 KT 70 MPH 96H 06/0600Z 36.0N 67.0W 65 KT 75 MPH 120H 07/0600Z 40.0N 58.0W 55 KT 65 MPH $$ Forecaster Avila

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Tropical Storm BERTHA Forecast Discussion Number 5

2014-08-02 04:54:59| National Hurricane Center (Atlantic)

Issued at 1100 PM AST FRI AUG 01 2014 000 WTNT43 KNHC 020254 TCDAT3 TROPICAL STORM BERTHA DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL AL032014 1100 PM AST FRI AUG 01 2014 While some convection developed near the estimated center during the past few hours, the circulation of Bertha remains disorganized. In fact, the Hurricane Hunter aircraft has been unable to find a center at 5000 ft this evening. However, surface observations suggested that there was still a small closed surface circulation when Bertha moved between Martinique and Dominica a few hours ago. The estimated center position is on the southwestern edge of the convective canopy, consistent with the 16 kt of southwesterly shear analyzed by the SHIPS model. The initial intensity remains 45 kt based on several SFMR winds of 40-45 kt reported by the aircraft east and northeast of the center. While the SHIPS model shows the shear decreasing on Saturday, Bertha will continue moving through a dry environment during the next 24 to 36 hours. There is also the potential for land interaction with Puerto Rico and Hispaniola during this time, which could disrupt the small circulation. Given all of these factors, little change in intensity is expected through 36 hours. After that time, if Bertha survives, the environment is expected to become more favorable for intensification with warming SSTs, increasing moisture, and lower vertical shear. Much of the intensity guidance shows Bertha reaching hurricane strength in 72 to 96 hours, and the official forecast follows suit. The new NHC intensity forecast has been adjusted upward from 48 to 96 hours, but remains a little below the IVCN intensity consensus. Note that it is possible that the combination of shear, dry air, and land interaction could cause Bertha to degenerate to a tropical wave during the next 36 hours, followed by possible regeneration when the system reaches the more favorable environment later in the forecast period. The initial motion estimate is 290/19, as Bertha is being steered west-northwestward by a deep-layer subtropical ridge over the western Atlantic. The ridge will begin to erode after 24 hours as a mid- to upper-level trough moves through the eastern United States, which should allow Bertha to turn northwestward and then northward by 3 days. After that time, Bertha is expected to complete recurvature and accelerate northeastward into the north Atlantic. The track model guidance remains in good agreement on this general scenario, however, there is a fair bit of spread in how sharply Bertha will recurve. The GFS and GEFS ensemble mean lie on the eastern edge of the guidance envelope, with the ECMWF, HWRF, and GFDL showing a more gradual turn and a track a little farther west. Through 36 hours the new NHC track is an update of the previous one. After that time, the official forecast has been nudged toward the left, but lies a little to the right of the middle of the guidance envelope. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 15.7N 63.0W 45 KT 50 MPH 12H 02/1200Z 16.9N 65.5W 45 KT 50 MPH 24H 03/0000Z 19.0N 68.6W 45 KT 50 MPH 36H 03/1200Z 21.4N 71.2W 45 KT 50 MPH 48H 04/0000Z 24.0N 73.2W 50 KT 60 MPH 72H 05/0000Z 30.0N 73.5W 60 KT 70 MPH 96H 06/0000Z 35.5N 68.0W 65 KT 75 MPH 120H 07/0000Z 40.0N 59.0W 60 KT 70 MPH $$ Forecaster Brennan

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Hurricane ISELLE Forecast Discussion Number 6

2014-08-02 04:38:56| National Hurricane Center (East Pacific)

Issued at 800 PM PDT FRI AUG 01 2014 000 WTPZ44 KNHC 020238 TCDEP4 HURRICANE ISELLE DISCUSSION NUMBER 6 NWS NATIONAL HURRICANE CENTER MIAMI FL EP092014 800 PM PDT FRI AUG 01 2014 Iselle's cloud pattern has increased in organization since the last advisory, transitioning from a banding to an eye pattern. Cloud top temperatures have also generally cooled in a small central dense overcast that has recently formed. Though ragged and occasionally obscured by high clouds, the 20 to 25 n mi diameter eye appeared well defined in an earlier AMSR-2 microwave overpass. Satellite intensity estimates were 3.5 and 4.0 from TAFB and SAB at 0000 UTC, respectively, with the latest ADT CI values at 4.0. Based on these data, the initial intensity is raised to 65 kt. The initial motion estimate is 295/10. Iselle should move steadily west-northwestward on the south side of a subtropical ridge until a mid- to upper-level trough digging along 130W in 2-3 days causes the ridge to weaken. Only a modest decrease in forward speed is expected in at that time before the ridge re-strengthens over the central Pacific by day 4, resulting in Iselle's moving faster toward the west. The track guidance is tightly clustered through 2 days but diverges some after that time, with the guidance shifting noticeably to the south at later times this cycle. The official track has been nudged a bit south in the short term and southward even more by days 3-5, but is not as far south as the multi-model consensus TVCE. Iselle is likely to continue to intensify during the next day or two. The closed low-level ring of convection in microwave imagery could portend a period of rapid intensification in the short term. However, moderate north-northeasterly shear and marginal thermodynamic parameters suggest this to be a lower probability scenario. After about 2 days, sea surface temperature drop below 26.5C and the environment is expected to gradually become even drier and more stable later in the forecast period. The shear could also increase substantially by days 4-5 in association with an upper-level trough, but this will depend on how far north Iselle tracks. The intensity forecast is higher than the previous one through about 2 days but is unchanged at later times, representing a blend of the multi-model consensus and the FSU Superensemble output. FORECAST POSITIONS AND MAX WINDS INIT 02/0300Z 14.7N 127.3W 65 KT 75 MPH 12H 02/1200Z 15.1N 128.7W 70 KT 80 MPH 24H 03/0000Z 15.5N 130.4W 75 KT 85 MPH 36H 03/1200Z 15.8N 132.3W 75 KT 85 MPH 48H 04/0000Z 16.2N 134.1W 75 KT 85 MPH 72H 05/0000Z 16.4N 137.0W 65 KT 75 MPH 96H 06/0000Z 16.5N 139.8W 60 KT 70 MPH 120H 07/0000Z 17.0N 144.5W 50 KT 60 MPH $$ Forecaster Kimberlain

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