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Tropical Storm IDA Forecast Discussion Number 12
2015-09-21 10:58:47| National Hurricane Center (Atlantic)
Issued at 500 AM AST MON SEP 21 2015 000 WTNT45 KNHC 210858 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST MON SEP 21 2015 A significant increase in the amount and organization of deep convection has occurred during the past 6 hours. A large CDO-like feature with a connecting curved band in the southern quadrant has developed, and cloud tops have cooled significantly to colder than -80C. However, the low-level circulation center is not embedded in the core of the main convective cloud mass, and is instead located about 30 n mi inside the western portion of the cloud shield. This suggests that some westerly vertical shear still exists. The initial intensity has been increased to 45 kt based on a Dvorak satellite intensity estimate of T3.0/45 kt from TAFB using a shear pattern. Recent AMSU and AMSR-2 microwave satellite fixes indicate that Ida's forward motion has continued to decrease and is now 330/07 kt. Global models indicate that the subtropical ridge to the north of Ida is rapidly eroding as a broad mid-/upper-level trough to the northeast and east of the cyclone slowly retrogrades westward. Steering currents are forecast to collapse within the next 12-24 hours, resulting in Ida becoming nearly stationary during that time. By 36-48 hours, Ida is expected to become embedded within the western portion of the aforementioned east-west oriented trough and move slowly southeastward through 72 hours or so. After that time, the trough is forecast to weaken, allowing the cyclone to move slowly north-northwestward to northward. The resultant model guidance more closely resembles the proverbial squashed spider pattern, which is usually indicative of slow and meandering motion. The NHC official track forecast follows suit, lying close to the previous one and the consensus model TVCA. Once the low-level center becomes juxtaposed with the mid-/and upper-level circulations, significant strengthening should occur. Given that vertical wind shear values are forecast by the GFS and ECMWF models to decrease to near 5 kt within the next 12 hours or so, at least slow and steady intensification should continue for the next 24-36 hours. After that time, the vertical shear is forecast to increase from the west and northwest as the aforementioned trough begins to interact with Ida. The intensity is forecast to decrease from 48-72 hours, followed by re-strengthening as the trough and its associated upper-level shear lifts out to the northeast, leaving Ida in a weak col region. Given that Ida will be over SSTs of at least 29C and within an upper-level cold pool by days 4 and 5, instability should increase and act to regenerate vigorous convection despite mid-level humidity values of near 50 percent. The official intensity forecast is slightly higher than the previous forecast and the intensity consensus model IVCN, and is similar to a blend of the SHIPS/LGEM, GFDL, and ECMWF intensity forecasts. FORECAST POSITIONS AND MAX WINDS INIT 21/0900Z 19.8N 47.3W 45 KT 50 MPH 12H 21/1800Z 20.7N 47.9W 50 KT 60 MPH 24H 22/0600Z 21.3N 48.1W 60 KT 70 MPH 36H 22/1800Z 21.4N 48.0W 60 KT 70 MPH 48H 23/0600Z 21.1N 47.7W 55 KT 65 MPH 72H 24/0600Z 20.8N 47.2W 50 KT 60 MPH 96H 25/0600Z 21.8N 47.6W 60 KT 70 MPH 120H 26/0600Z 23.3N 48.5W 65 KT 75 MPH $$ Forecaster Stewart
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