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Tropical Storm ERIKA Forecast Discussion Number 10
2015-08-27 10:56:32| National Hurricane Center (Atlantic)
Issued at 500 AM AST THU AUG 27 2015 000 WTNT45 KNHC 270856 TCDAT5 TROPICAL STORM ERIKA DISCUSSION NUMBER 10 NWS NATIONAL HURRICANE CENTER MIAMI FL AL052015 500 AM AST THU AUG 27 2015 Data from a NOAA Hurricane Hunter aircraft indicate that Erika has strengthened overnight, although it is not clear as to how much. The central pressure dropped to around 1001 mb around 0540Z, but was up to 1003 mb on the next fix about an hour later. The highest 850-mb flight-level winds sampled by the aircraft were 48 kt, which would correspond to an intensity of around 40 kt. However, the aircraft found SFMR winds of 45 to 55 kt near the center. Based on a blend of these data, the initial intensity has been conservatively raised to 45 kt for this advisory. An Air Force Hurricane Hunter aircraft will be investigating Erika again later this morning. Aircraft data and satellite imagery show that Erika is still a sheared tropical cyclone, with the deepest convection found east and southeast of the center due to about 20 kt of westerly shear. The models show the environment remaining unfavorable for significant strengthening in the first 48 hours, with shear associated with an upper-level trough west of Erika expected to increase to 25 to 30 kt. Given this, most of the intensity guidance shows little change during the first couple of days and so does the NHC forecast. After that time, the upper trough weakens and Erika should encounter a more favorable upper-level pattern and warmer SSTs, which should support intensification assuming that the cyclone survives the next 48 hours. The HWRF and GFDL are much stronger than the statistical models this cycle, but the GFS and ECMWF now keep Erika weaker than they did previously. The NHC forecast has been adjusted upward slightly late in the period, but is well below the intensity consensus given the large uncertainty and spread in the guidance. Aircraft fixes and radar data from Guadeloupe were helpful in finding the low-level center and determining an initial motion of 280/14. The steering flow from the subtropical ridge to the north should result in a west-northwestward heading for the next 2 to 3 days. After that time, the spread in the guidance increases as the cyclone moves between the southwestern edge of the ridge and a mid/upper-level trough over the southeastern U.S. and Gulf of Mexico. The models are in poor agreement on the eventual structure and track of Erika late in the period. The ECMWF is weaker this cycle and is on the left side of the dynamical model envelope. The GFS, HWRF, and UKMET are all farther east but have trended west this cycle. Given the large spread and the continued run-to-run variability, little change was made to the NHC track forecast. The new NHC forecast is closest to the latest GFS model prediction at day 3 and beyond. One should remember to not focus on the exact forecast track, especially at the long range where the average NHC track errors during the past 5 years are about 180 miles at day 4 and 240 miles at day 5. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 16.8N 61.5W 45 KT 50 MPH 12H 27/1800Z 17.6N 63.5W 50 KT 60 MPH 24H 28/0600Z 19.0N 66.2W 50 KT 60 MPH 36H 28/1800Z 20.3N 69.0W 50 KT 60 MPH 48H 29/0600Z 21.5N 71.6W 50 KT 60 MPH 72H 30/0600Z 24.0N 76.0W 55 KT 65 MPH 96H 31/0600Z 26.5N 78.5W 65 KT 75 MPH 120H 01/0600Z 29.0N 79.5W 75 KT 85 MPH $$ Forecaster Brennan
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