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Subtropical Depression Four Forecast Discussion Number 1
2020-06-22 22:45:27| National Hurricane Center (Atlantic)
Issued at 500 PM AST Mon Jun 22 2020 276 WTNT44 KNHC 222045 TCDAT4 Subtropical Depression Four Discussion Number 1 NWS National Hurricane Center Miami FL AL042020 500 PM AST Mon Jun 22 2020 The non-tropical low pressure system that the National Hurricane Center has been following for the past couple of days off of the U.S. east coast has developed enough organized convection near the center to be classified as subtropical depression. The subtropical status is due to the low-level circulation center being co-located beneath an upper-level cold low as seen in water vapor imagery. The initial intensity of 30 kt is based on earlier ASCAT wind data indicating numerous surface wind vectors of 26-28 kt in the southern semicircle, along with a TAFB subtropical satellite classification of ST1.5/25-30 kt. The initial motion estimate is 075/08 kt. Subtropical Depression Four is located north of a deep-layer ridge and is being influenced by weak westerly mid- to upper-level flow. The cyclone is forecast by all of the global and regional models to move east-northeastward tonight and then turn northeastward on Tuesday. A northeastward motion at a faster forward speed is expected Tuesday night through Thursday when the system is forecast to be located over the cold waters of the far north Atlantic. The cyclone should be absorbed by a larger extratropical low or dissipate on Thursday. The NHC track forecast is close to a blend of the simple consensus aids TVCN and GFEX and the NOAA-HCCA corrected consensus. The cyclone is beginning to move over a ridge of higher SSTs of 26.0-26.5 deg C in the northern extent of the Gulf Stream. The forecast track takes the subtropical depression down the length of the axis of warmer water during the next 12-18 h, so there is the potential for the cyclone to become a subtropical storm during that time, especially given the large pool of cold air aloft, with 200-mb temperatures of near -58 deg C and 500-mb temperatures of about -10 deg C which is creating a lot of instability. After the system moves off of the warm ridge and into sharply cooler water around 36 hours or so, gradual weakening is expected, with transition to an extratropical cyclone forecast by 48 hours. The NHC intensity forecast closely follows a blend of the consensus models ICON, IVCN, and HCCA. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 38.2N 65.7W 30 KT 35 MPH 12H 23/0600Z 38.5N 64.0W 35 KT 40 MPH 24H 23/1800Z 39.0N 62.0W 30 KT 35 MPH 36H 24/0600Z 40.5N 59.3W 30 KT 35 MPH 48H 24/1800Z 42.5N 56.7W 30 KT 35 MPH...POST-TROP/EXTRATROP 60H 25/0600Z 45.0N 53.3W 30 KT 35 MPH...POST-TROP/EXTRATROP 72H 25/1800Z 47.8N 50.4W 30 KT 35 MPH...POST-TROP/EXTRATROP 96H 26/1800Z...DISSIPATED $$ Forecaster Stewart
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