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Tropical Storm ORLENE Forecast Discussion Number 5
2016-09-12 04:48:37| National Hurricane Center (East Pacific)
Issued at 800 PM PDT SUN SEP 11 2016 000 WTPZ41 KNHC 120248 TCDEP1 TROPICAL STORM ORLENE DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL EP162016 800 PM PDT SUN SEP 11 2016 Visible, infrared, and microwave satellite imagery indicate that the cyclone has continued to become more organized, with a well-defined low- to mid-level eye noted in a 2336Z SSMI/S overpass. Orlene now has a large, circular CDO along with a symmetrical and expanding upper-level outflow pattern. Dvorak intensity estimates from TAFB and SAB are a consensus T3.5/55 kt while the objective UW-CIMSS ADT is 60-65 kt. Given the rather large average eye diameter of about 35 nmi, the initial intensity is set below the ADT values and follows the subjective estimate of 55 kt. The initial motion is 305/08 kt based heavily on microwave eye position estimates. Although there is some divergence in the models, they are good agreement overall on Orlene gradually slowing down and moving north-northwestward and northward into a break in the subtropical ridge during the next 48 hours. After that, the break in the ridge along 120W longitude is forecast to fill back in as a shortwave trough lifts out quickly to the northeast. The re-strengthening ridge to the north of Orlene is then expected to accelerate the cyclone on a westward track on days 3-5. The new NHC forecast track is essentially just an update and extension of the previous advisory track, and lies close to a consensus of the GFS-ECMWF model solutions. Orlene is expected to remain in a very low-shear environment for the next 72 hours, which typically favors rapid intensification. However, the cyclone is also expected to remain embedded within a fairly dry mid-level environment with humidity values of 50-55 percent. Given that Orlene has already developed a solid eyewall structure, little if any dry air intrusions should be able to penetrate into the eye in the short term. Based on the favorable low-shear and already impressive outflow pattern, a period of rapid intensification is forecast to occur for the next 24 hours, followed by a leveling off in the intensity due to cooler waters and possible cold upwelling occurring beneath the cyclone. By 48 hours and beyond, gradual weakening is expected due to Orlene moving into a considerably drier and more stable environment characterized by mid-level humidities in the 20- to 30-percent range. The intensity forecast remains near the upper end of the guidance envelope close to the LGEM and Florida State Superensemble models, and is just a little higher than the previous advisory. FORECAST POSITIONS AND MAX WINDS INIT 12/0300Z 16.6N 118.3W 55 KT 65 MPH 12H 12/1200Z 17.3N 119.1W 70 KT 80 MPH 24H 13/0000Z 18.0N 119.7W 85 KT 100 MPH 36H 13/1200Z 18.6N 119.8W 85 KT 100 MPH 48H 14/0000Z 19.0N 120.0W 80 KT 90 MPH 72H 15/0000Z 19.2N 121.7W 70 KT 80 MPH 96H 16/0000Z 18.9N 125.7W 65 KT 75 MPH 120H 17/0000Z 18.9N 130.8W 60 KT 70 MPH $$ Forecaster Stewart
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