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Tropical Storm Lee Forecast Discussion Number 21
2017-09-23 16:40:20| National Hurricane Center (Atlantic)
Issued at 1100 AM AST Sat Sep 23 2017 000 WTNT44 KNHC 231440 TCDAT4 Tropical Storm Lee Discussion Number 21 NWS National Hurricane Center Miami FL AL142017 1100 AM AST Sat Sep 23 2017 Lee remains a compact tropical cyclone as indicated by a 20-kt wind report at 1200Z from ship LAQX5, located only 70 nmi east of the center. A 0943Z WindSAT pass had a few 40-45 uncontaminated wind vectors in the southwestern quadrant, so the intensity has been conservatively increased to 40 kt, which is closer to the satellite intensity estimates of T2.5/35 kt from TAFB and UW-CIMSS ADT. The initial motion is a slow drift toward the north or 360/02 kt, based primarily on microwave satellite fixes. The global and regional models are in general agreement that Lee will move slowly at 5 kt or less for the next 5 days, and either make a wide or a tight anticyclonic turn back toward the west after 72 h due to a building high pressure ridge to the north of the cyclone, and a stationary upper-level low located a few hundred nmi to the south. The GFS makes the widest turn, whereas the ECMWF makes the tightest turn; the remainder of the model guidance lies somewhere between those two extremes. Until the model guidance becomes more convergent, the official NHC forecast track will remain basically down the middle of the guidance envelope, close to the blend of the TCVA, TVCX, and HCCA consensus models. Earlier WindSAT and SSMI/S microwave data showed that Lee possessed an 8-10 nmi diameter eye-like feature in both the low- and mid-levels, indicating that the cyclone has a well-defined inner-core wind field, albeit quite small. The deep-layer vertical wind shear is forecast by both the GFS and ECMWF models to decrease to 5-10 kt during the next 48 hours or so while Lee remains over SSTs of about 27.5 deg C. In addition, upper-level temperatures are currently running, and are expected to remain, about 3 deg C colder than normal, which should ensure sufficient instability for the generation of inner-core thunderstorm activity through the entire forecast period. Based on the combination of these favorable dynamic and thermodynamic conditions, Lee is forecast to steadily develop into a hurricane by 48-72 hours, with the only inhibiting factor being occasional intrusions of very dry mid-level air. However, the strong instability/CAPE that will be available should allow for deep convection to more or less persist near the center until modest westerly shear begins to affect the cyclone by 72-120 hours, during which time a slow weakening trend is expected. The official intensity forecast is very similar to the previous advisory and is close to a blend of the IVCN and HCCA intensity consensus models. FORECAST POSITIONS AND MAX WINDS INIT 23/1500Z 31.9N 49.4W 40 KT 45 MPH 12H 24/0000Z 32.2N 49.1W 45 KT 50 MPH 24H 24/1200Z 32.3N 48.3W 50 KT 60 MPH 36H 25/0000Z 31.9N 47.2W 55 KT 65 MPH 48H 25/1200Z 31.3N 46.2W 60 KT 70 MPH 72H 26/1200Z 30.5N 45.3W 70 KT 80 MPH 96H 27/1200Z 29.9N 45.9W 70 KT 80 MPH 120H 28/1200Z 30.5N 47.1W 65 KT 75 MPH $$ Forecaster Stewart
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