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Hurricane ISELLE Forecast Discussion Number 12

2014-08-03 16:34:40| National Hurricane Center (East Pacific)

Issued at 800 AM PDT SUN AUG 03 2014 000 WTPZ44 KNHC 031434 TCDEP4 HURRICANE ISELLE DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL EP092014 800 AM PDT SUN AUG 03 2014 It hasn't been particularly steady, but Iselle continues on a gradual strengthening trend. The eye has cleared out in infrared satellite imagery, and the overall presentation has become a little more symmetric. Dvorak estimates are now T5.5/102 kt from SAB, T5.0/90 kt from TAFB, and near 110 kt from the UW-CIMSS ADT. Therefore, the maximum winds are set at 100 kt as a blend of these numbers, making Iselle a major hurricane. The intensity forecast has been complicated. Iselle continues to push through the peak intensities shown in previous model cycles, so apparently the environment has been a little more favorable than expected. Vertical shear has decreased since yesterday, but still the latest model runs do not show any additional intensification, presumably because the thermodynamic environment is becoming more marginal. The NHC forecast maintains Iselle at 100 kt for the next 12 hours and then only gradually weakens it through 48 hours, but given the hurricane's history, I can't rule out some additional strengthening in the short term. Interestingly, the statistical models are stronger than the dynamical models during the first 48 hours, but the scenario flip-flops on days 3 through 5 with the dynamical models maintaining a stronger cyclone as Iselle approaches the Hawaiian Islands. For now, the NHC forecast closely follows the intensity consensus ICON and is unchanged from the previous advisory beyond 48 hours. The initial motion is 280/9 kt. Ridging to the north of Iselle is expected to maintain a westward heading for the next 3 days, but the forward motion will decrease in about 36 hours as that ridge weakens. After day 3, Iselle is forecast to turn west- northwestward and accelerate when a mid-level high develops halfway between Hawaii and California. The track models are in good agreement for the entire forecast period, but as mentioned in the previous advisory, a slight northward adjustment was needed in the official forecast on days 4 and 5 to account for recent model trends. FORECAST POSITIONS AND MAX WINDS INIT 03/1500Z 15.5N 132.8W 100 KT 115 MPH 12H 04/0000Z 15.6N 134.3W 100 KT 115 MPH 24H 04/1200Z 15.7N 136.1W 95 KT 110 MPH 36H 05/0000Z 15.7N 137.6W 90 KT 105 MPH 48H 05/1200Z 15.8N 139.1W 80 KT 90 MPH 72H 06/1200Z 16.5N 143.5W 65 KT 75 MPH 96H 07/1200Z 18.0N 149.5W 55 KT 65 MPH 120H 08/1200Z 20.0N 156.0W 45 KT 50 MPH $$ Forecaster Berg

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Tropical Storm BERTHA Forecast Discussion Number 10

2014-08-03 11:00:04| National Hurricane Center (Atlantic)

Issued at 500 AM AST SUN AUG 03 2014 000 WTNT43 KNHC 030859 TCDAT3 TROPICAL STORM BERTHA DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL AL032014 500 AM AST SUN AUG 03 2014 The overall convective pattern of Bertha has changed little since the previous advisory. The center position has been difficult to determine and is based on a blend of the past motion and available reconissance data. An Air Force Reserve reconnaissance was able to close off an 850 mb center around 0200 UTC, and a dropsonde released to the southeast of the alleged center indicated a surface wind of 210 degrees at 21 kt, which implies a west wind of at least 10 kt somewhere in the southern quadrant. These data are the basis for keeping Bertha as a tropical cyclone for this advisory. Also during the outbound leg, the aircraft found rain-contaminated SFMR surface winds of 53, 57, and 48 kt in the southeastern quadrant. After adjusting those wind speeds due to rain enhancement yields a consecutive 3-bin average speed of about 42 kt, which justifies maintaining the intensity at 40 kt. The initial motion estimate is 310/18 kt. The NHC model guidance remains in excellent agreement with Bertha gradually moving northward and then northeastward around the periphery of a deep-layer subtropical ridge located along 28N latitude and ahead of an broad mid-level trough that is located over the eastern United States through 72 hours. After that, the cyclone is expected to become embedded in the mid-latitude westerlies and accelerate northeastward and east-northeastward as it becomes extratropical or merges with a frontal system in 96-120 hours. The NHC track forecast is essentially just an update of the previous advisory, only slightly faster, and follows the trend of the consensus model TVCA. It is possible that Bertha could briefly open up into a wave this morning due to strong northwesterly shear of at least 25 kt. However, the environment ahead of the cyclone for about the next 48 hours is expected to become increasingly more conducive for at least gradual strengthening to occur, especially at 36-48 hours when the vertical shear is expected to decrease to near 10 kt while SSTs and mid-level moisture reach their peak. By 96 hours, extratropical transition over sub-23C SSTs and/or merger with a frontal system is expected. The ECMWF indicates frontal merger in about 72 hours, but this appears be premature based on other available model guidance and input from the NOAA Ocean Prediction Center (OPC). The new NHC intensity forecast is consistent with the previous advisory, which now calls for Bertha to reach hurricane strength by 72 hours. However, this could occur sooner than indicated given that the most favorable oceanic and atmospheric conditions become juxtaposed by 48 hours or so. Intensity forecasts during the extratropical phase are based on input from OPC. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 20.9N 71.1W 40 KT 45 MPH 12H 03/1800Z 22.8N 72.7W 45 KT 50 MPH 24H 04/0600Z 25.6N 74.0W 50 KT 60 MPH 36H 04/1800Z 28.8N 74.0W 55 KT 65 MPH 48H 05/0600Z 32.0N 72.7W 60 KT 70 MPH 72H 06/0600Z 37.8N 66.8W 65 KT 75 MPH 96H 07/0600Z 42.5N 58.0W 60 KT 70 MPH...POST-TROP/EXTRATROP 120H 08/0600Z 46.0N 47.3W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart

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Hurricane ISELLE Forecast Discussion Number 11

2014-08-03 10:52:04| National Hurricane Center (East Pacific)

Issued at 200 AM PDT SUN AUG 03 2014 000 WTPZ44 KNHC 030851 TCDEP4 HURRICANE ISELLE DISCUSSION NUMBER 11 NWS NATIONAL HURRICANE CENTER MIAMI FL EP092014 200 AM PDT SUN AUG 03 2014 The overall satellite presentation of Iselle hasn't changed much during the past several hours with strong convection in the eyewall and an eye which is intermittently obscured by cirrus clouds. Satellite intensity estimates have not changed significantly, so the initial wind speed will remain 95 kt. Although Iselle is probably near peak intensity, it still has a short window of time today to become a major hurricane. It is not likely to strengthen much, however, due to Iselle moving closer to a less favorable thermodynamic environment. Since wind shear is forecast to remain low, only gradual weakening is anticipated after today while Iselle moves over marginal SSTs for the next couple of days. One possible pitfall of this forecast is that, given the forecast storm environment, Iselle could evolve into an annular hurricane, which could delay the weakening during that time. Late in the period, an increase in both shear and drier air aloft will probably help to weaken the storm while it approaches the Hawaiian Islands. The official NHC intensity forecast is close to the previous one and the intensity consensus. The cyclone continues to move westward or 275/9. A subtropical ridge north of Iselle should steer the storm on this general path for the next few days, with a slight deceleration expected in a day or so due to the ridge temporarily weakening. Guidance is in very good agreement through day 3, and the NHC forecast through that time us basically an update of the previous one. After that time, Iselle should move on a more west-northwestward heading at a faster forward speed due to the ridge rebuilding. The model guidance has shifted northward on this cycle, which seems reasonable given the forecast synoptic pattern, and the official forecast is moved in that direction. It should be noted that the bulk of the guidance is north of the latest NHC forecast at long range, and further adjustments in that direction could be required on later forecasts. FORECAST POSITIONS AND MAX WINDS INIT 03/0900Z 15.3N 131.8W 95 KT 110 MPH 12H 03/1800Z 15.4N 133.4W 95 KT 110 MPH 24H 04/0600Z 15.6N 135.4W 90 KT 105 MPH 36H 04/1800Z 15.7N 137.0W 85 KT 100 MPH 48H 05/0600Z 15.7N 138.6W 80 KT 90 MPH 72H 06/0600Z 16.0N 142.2W 65 KT 75 MPH 96H 07/0600Z 17.3N 147.8W 55 KT 65 MPH 120H 08/0600Z 19.0N 154.0W 45 KT 50 MPH $$ Forecaster Blake

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Tropical Storm BERTHA Forecast Discussion Number 9

2014-08-03 04:46:07| National Hurricane Center (Atlantic)

Issued at 1100 PM AST SAT AUG 02 2014 000 WTNT43 KNHC 030245 TCDAT3 TROPICAL STORM BERTHA DISCUSSION NUMBER 9 NWS NATIONAL HURRICANE CENTER MIAMI FL AL032014 1100 PM AST SAT AUG 02 2014 While there has been some increase in the coverage of cold convective tops associated with Bertha this evening, the cyclone remains disorganized. Surface observations from the Dominican Republic have not definitively shown a closed circulation, and it is possible that what surface circulation there was has been disrupted by land interaction. We will maintain Bertha as a tropical cyclone for now, but advisories could be discontinued on Sunday if the organization of the system does not improve. The initial intensity remains 40 kt based on rain-adjusted SFMR winds reported by the Air Force Hurricane Hunter aircraft well east of the center prior to 00 UTC. The initial motion estimate of 310/19 is based on a blend of the latest satellite imagery and continuity. The synoptic reasoning for the track forecast remains unchanged. Bertha will continue moving northwestward along the southwestern periphery of the subtropical ridge tonight and Sunday. Then the cyclone will turn northward on Monday between the ridge and a deep-layer trough over the southeastern United States, followed by a northeastward acceleration into the mid-latitude westerlies. The track model guidance remains in good agreement on this general scenario. However, the models have shifted to the left this cycle at 48 hours and beyond, showing a more gradual recurvature. The NHC track through 36 hours is largely an update of the previous one. After that time, the official forecast has been adjusted to the left of the previous one, but now lies along the right side of the guidance envelope. There continue to be three possible scenarios for the intensity of Bertha. The official forecast assumes that Bertha will survive its current lack of structure, land interaction, shear, and dry air entrainment long enough to reach a more favorable environment in 24-36 hours. At that point, the environment would likely allow for intensification until extratropical transition begins after 96 hours. The new NHC intensity forecast is similar to the previous one and calls for Bertha to become a hurricane in 4 days, and is close to the latest IVCN intensity consensus. An alternate scenario is that Bertha degenerates to a tropical wave in the next 12 to 24 hours, with possible regeneration later when the system reaches the more favorable environment. A third possibility involves a trough of low pressure currently situated over the central and northwestern Bahamas. The 1200 UTC runs of the UKMET and NAVGEM forecast the low to absorb Bertha in 48 hours or so, while the latest GFS and ECMWF runs keep this system weaker and maintain Bertha through the forecast period. Later model runs should help refine the likelihood of this scenario. FORECAST POSITIONS AND MAX WINDS INIT 03/0300Z 19.5N 69.7W 40 KT 45 MPH 12H 03/1200Z 21.5N 71.8W 40 KT 45 MPH 24H 04/0000Z 24.1N 73.8W 45 KT 50 MPH 36H 04/1200Z 27.2N 74.4W 45 KT 50 MPH 48H 05/0000Z 30.4N 73.8W 50 KT 60 MPH 72H 06/0000Z 36.0N 69.0W 60 KT 70 MPH 96H 07/0000Z 41.0N 60.0W 65 KT 75 MPH 120H 08/0000Z 45.0N 50.0W 50 KT 60 MPH...POST-TROP/EXTRATROP $$ Forecaster Brennan

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Hurricane ISELLE Forecast Discussion Number 10

2014-08-03 04:38:41| National Hurricane Center (East Pacific)

Issued at 800 PM PDT SAT AUG 02 2014 000 WTPZ44 KNHC 030238 TCDEP4 HURRICANE ISELLE DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL EP092014 800 PM PDT SAT AUG 02 2014 Iselle is still strengthening. The central dense overcast has become more symmetric since the last advisory, with cloud top temperatures cooling slightly. The eye was also becoming better defined but has recently become obscured by overshooting convective tops. Satellite intensity estimates from TAFB and SAB at 0000 UTC were T5.0/90 kt and UW-CIMSS ADT values are at 5.4/99 kt. These data support increasing the initial intensity estimate to 95 kt. The initial motion estimate is just north of due west or 275/09. A subtropical ridge north of Iselle should allow the current motion to persist in the short term. However, a mid-latitude trough digging roughly along 130W should temporarily weaken the ridge during the next day or two, causing the forward speed of Iselle to nominally decrease. When the subtropical ridge rebuilds over the central Pacific after day 3, Iselle should move on a westward and then west-northwestward heading a faster forward speed. The track guidance is in relatively good agreement throughout the forecast period, and the forecast track is adjusted only slightly to the south in the direction of the multi-model consensus TVCE. It is difficult to see Iselle strengthening much further. Although northeasterly shear over the cyclone is forecast to decrease during the next couple of days, the thermodynamic contribution from the environment should gradually become less favorable. This would suggest that some additional intensification is possible in the very short term, followed by slow weakening after 24 hours. The one caveat would be that with little shear and marginally favorable thermodynamic conditions Iselle could evolve into an annular hurricane and maintain a somewhat higher intensity during the next few days. By days 4 and 5, even less favorable thermodynamic factors and an uptick in south-southwesterly shear should result in quicker weakening as Iselle approaches the Hawaiian Islands. The intensity forecast is somewhat higher than the previous one in the short term and represents a blend of the multi-model consensus and FSU Superensemble output. FORECAST POSITIONS AND MAX WINDS INIT 03/0300Z 15.2N 131.0W 95 KT 110 MPH 12H 03/1200Z 15.4N 132.6W 100 KT 115 MPH 24H 04/0000Z 15.6N 134.6W 95 KT 110 MPH 36H 04/1200Z 15.7N 136.4W 90 KT 105 MPH 48H 05/0000Z 15.8N 137.9W 85 KT 100 MPH 72H 06/0000Z 15.9N 141.2W 75 KT 85 MPH 96H 07/0000Z 16.6N 146.2W 60 KT 70 MPH 120H 08/0000Z 17.8N 152.4W 45 KT 50 MPH $$ Forecaster Kimberlain

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