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Subtropical Storm ALEX Forecast Discussion Number 3
2016-01-14 09:49:35| National Hurricane Center (Atlantic)
Issued at 500 AM AST THU JAN 14 2016 000 WTNT41 KNHC 140849 TCDAT1 SUBTROPICAL STORM ALEX DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL012016 500 AM AST THU JAN 14 2016 Since the previous advisory, the convective structure of Alex has continued to improve with a 15-nmi clear eye now embedded within a solid cloud shield of tops colder than -50C, with a ring of cloud tops near -60C surrounding the eye in the northern semicircle. The intensity of Alex is difficult to ascertain due to its subtropical characteristics, and satellite intensity estimates range from ST3.5/55 kt from TAFB to a tropical T4.5/77 kt from SAB. NHC AODT intensity estimates range from T4.0/65 kt to T4.5/77 kt using a tropical pattern and various eye scenes. Since the overall cloud pattern of Alex has improved markedly since the earlier 50-kt ASCAT-B scatterometer wind data, and a clear and distinct eye feature is now evident, the intensity is being raised to 60 kt. Alex continues to gradually turn toward the left and the initial motion is now 020/16 kt. The cyclone is forecast to turn northward during the next 12 hours or so as it is steered by deep southerly flow between a large extratropical low centered over the northwest Atlantic and a subtropical ridge over northwestern Africa. A general northward motion is expected to continue through at least 36 hours, which should bring the center of Alex through the central Azores in about 30 hours or 1200 UTC 15 January. By 48 hours, Alex should start to turn more toward the northwest and west as it moves closer to the center of a larger extratropical low forecast to be south of Greenland on days 3 and 4. The new forecast track is essentially just an update of the previous advisory track through 36 hours, and lies close to the consensus model TVCA. Global and regional models, plus geostationary and microwave satellite data, indicate that Alex remains a vertically stacked low pressure system up to the 200 mb level, suggesting that the cyclone is still primarily a subtropical storm. However, water vapor imagery during the past few hours has been hinting that some weak upper-level outflow might be trying to develop. GFS and NAM model forecast soundings indicate that the current inner-core region of Alex with Lifted Indices (LI) of -2 and CAPE values of 400-500 are only expected to decrease to LI of -1 and CAPE near 300 as the cyclone nears the Azores islands in 24-30 hours. In addition, the inner-core region is forecast to remain saturated up to the 200 mb level, along with precipitable water values around 1.60 inches. This would suggest that Alex could remain as a subtropical cyclone by the time it reaches the Azores, and some slight strengthening is even possible as 300 mb temperatures are forecast to decrease from -40C to around -42C in the inner core. The official intensity has been nudged upward and is similar to a blend of the ECMWF and HRWF model intensity forecasts. Gale- and storm-force winds, as well as locally heavy rains, are likely to affect portions of the Azores beginning tonight and continuing into early Friday. Recent communications with the Azores Meteorological Office indicate that wind gusts to a least 70 kt (130 km/h) are forecast to occur across portions of the central and eastern Azores islands. However, stronger gusts will be possible at higher elevations, especially over mountain tops and ridges. Interests in the Azores should closely monitor the progress of Alex and official forecasts issued by the Azores weather office. FORECAST POSITIONS AND MAX WINDS INIT 14/0900Z 30.1N 29.3W 60 KT 70 MPH 12H 14/1800Z 32.4N 28.3W 60 KT 70 MPH 24H 15/0600Z 36.4N 27.6W 60 KT 70 MPH 36H 15/1800Z 42.0N 28.2W 60 KT 70 MPH...POST-TROP/EXTRATROP 48H 16/0600Z 49.1N 29.7W 60 KT 70 MPH...POST-TROP/EXTRATROP 72H 17/0600Z 60.0N 40.0W 60 KT 70 MPH...POST-TROP/EXTRATROP 96H 18/0600Z...MERGED WITH EXTRATROPICAL LOW $$ Forecaster Stewart
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