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Subtropical Storm Andrea Forecast Discussion Number 3
2019-05-21 10:35:39| National Hurricane Center (Atlantic)
Issued at 500 AM AST Tue May 21 2019 000 WTNT41 KNHC 210835 TCDAT1 Subtropical Storm Andrea Discussion Number 3 NWS National Hurricane Center Miami FL AL012019 500 AM AST Tue May 21 2019 During the past few hours, deep convection has waned considerably, likely due to the entrainment of dry mid-level from the south and modest southerly vertical wind shear as indciated by GOES-16 mid- and upper-level water vapor imagery. As mentioned in the previous advisory discussion, an earlier ASCAT overpass just barely caught 35-kt winds about 50-55 nmi northeast of the low-level center, which was likely just outside the radius of maximum winds. Since slightly stronger winds could have existed closer to the center, the initial intensity is being maintained at 35 kt for this advisory despite the aforementioned degradation in the convective pattern. The initial motion estimate is 360/05 kt. It appears that Andrea is slowing down, implying that a turn toward the northeast should occur within the next 6-12 h as a deep-layer trough and cold front steadily approach the cyclone from the northwest. The global and regional models remain in good agreement that Andrea will turn northeastward later today, and then move eastward by tonight and Wednesday when the cyclone becomes embedded within the approaching deep-layer mid-latitude westerly flow. The NHC track forecast is essentially unchanged from the previous advisory and lies a little south of the middle of the track guidance envelope. Although deep convection has decreased markedly, thunderstorm activity should redevelop some by this afternoon and tonight when Andrea will be moving eastward over an SST thermal ridge and into a region of convective instability that exist between 65W-68W longitude. which will also be coincident with the vertical wind shear decreasing to less than 5 kt as per SHIPS model output. However, some slight spin down of the vortex will likely occur before convection can re-fire and re-strengthens the system. Thus, the intensity forecast essentially calls for little change in strength for the next 24 h or so, followed by weakening due to increasing northwesterly vertical wind shear, and then merger with a cold front on Wednesday. The official intensity forecast closely follows the IVCN consensus model and the weakening trend noted in the global models. FORECAST POSITIONS AND MAX WINDS INIT 21/0900Z 30.0N 69.0W 35 KT 40 MPH 12H 21/1800Z 30.7N 68.3W 35 KT 40 MPH 24H 22/0600Z 31.2N 66.7W 35 KT 40 MPH 36H 22/1800Z 31.3N 65.0W 30 KT 35 MPH 48H 23/0600Z...DISSIPATED $$ Forecaster Stewart
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