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| #904179 (Received by flhurricane at: 4:40 PM 22.Sep.2017) TCDAT4 Tropical Depression Lee Discussion Number 18 NWS National Hurricane Center Miami FL AL142017 500 PM AST Fri Sep 22 2017 The mid-level circulation from Lee separated from the low-level center two days ago and moved northward around the eastern periphery of a large upper-level trough. A large convective burst over the mid-level center yesterday then caused a new low-level center to develop while it moved on the northern side of the upper trough. ASCAT and ship data indicate that the small circulation is now well defined, and since deep convection has persisted near the center, advisories on Lee are being restarted. The initial wind speed is 30 kt, which could be conservative given the healthy structure seen on the last-light visible and recent microwave data. Cold upper-level temperatures from the upper trough should mitigate the effect of marginally warm waters for the next few days while Lee remains in a low-shear environment. The depression is expected to strengthen over the next few days until westerly shear is forecast to increase. The official forecast is similar to the model guidance, although it shows an earlier peak more coincident with the lower-shear, warmer-water environment. This wind speed forecast is difficult because Lee is a very small tropical cyclone, which notoriously have rapid changes, both up and down, in intensity. Lee is moving northward at about 6 kt. The depression should turn to the northeast and east over the next couple of days while it moves around a ridge over the tropical central Atlantic. Lee is forecast to become trapped beneath a mid-latitude ridge in a few days, which should cause the cyclone to drift at long range. It should be noted that models have a rather weak representation of Lee at the present time, which causes this to be an uncertain forecast. For now, the forecast is closest to the ECMWF, which has the most coherent cyclone to follow. FORECAST POSITIONS AND MAX WINDS INIT 22/2100Z 30.8N 48.9W 30 KT 35 MPH 12H 23/0600Z 31.7N 48.7W 40 KT 45 MPH 24H 23/1800Z 32.3N 48.2W 45 KT 50 MPH 36H 24/0600Z 32.3N 47.2W 50 KT 60 MPH 48H 24/1800Z 32.1N 45.8W 55 KT 65 MPH 72H 25/1800Z 31.4N 43.2W 60 KT 70 MPH 96H 26/1800Z 30.5N 42.0W 55 KT 65 MPH 120H 27/1800Z 29.5N 43.0W 50 KT 60 MPH $$ Forecaster Blake |
