WTPZ42 KNHC 242031
TROPICAL STORM SANDRA DISCUSSION NUMBER 5
NWS NATIONAL HURRICANE CENTER MIAMI FL EP222015
200 PM MST TUE NOV 24 2015
Visible satellite imagery suggests that Sandra is strengthening quickly. Deep convection has recently blossomed in a ring around a relatively clear region near the center, with a warm spot also noted in infrared imagery. Dvorak intensity estimates have risen to T3.0/45 kt from SAB, T3.5/55 kt from TAFB, and T4.0/65 kt from the UW-CIMSS ADT. Therefore, the initial intensity is raised to 55 kt.
Sandra has turned west-northwestward with an initial motion of 290/12 kt. A mid-level ridge currently extends from southern Mexico westward over the Pacific Ocean but an amplifying mid- to upper- level trough near the U.S. west coast should erode the western portion of the ridge during the next 24-48 hours. This should cause Sandra to recurve to the north and northeast during the forecast period. For the most part, the track guidance has been consistent and has not shifted much to the right or left from cycle to cycle. However, the latest guidance has trended a little faster than in previous runs. The updated NHC track forecast is pushed a little faster close to the model consensus aids, and also a little bit westward after 48 hours in deference to the ECMWF model, which is on the western edge of the guidance envelope.
Based on the recent evolution of Sandra's cloud pattern, it appears the storm could be going through a period of rapid intensification. Sandra is over sea surface temperatures around 30 degrees Celsius and in an environment of low shear, and the SHIPS RI index is around 50 percent for a 30-kt increase in winds during the next 24 hours. Therefore, the NHC official forecast now shows the cyclone reaching hurricane intensity tonight and peaking as a category 2 hurricane in 36-48 hours. Thereafter, vertical shear is expected to increase from around 10 kt at 48 hours to 50-60 kt at 96 hours, which should cause Sandra to weaken quickly as it approaches the coast of Mexico. The NHC intensity forecast is a little higher than the previous forecast and is close to the HWRF model, which shows the highest peak intensity among the guidance suite.
It is too soon to know exactly how Sandra will affect portions of the southern Baja California peninsula or the west coast of mainland Mexico. However, a watch may be required on Wednesday, and interests in those areas should monitor the progress of Sandra.
FORECAST POSITIONS AND MAX WINDS
INIT 24/2100Z 11.9N 107.2W 55 KT 65 MPH 12H 25/0600Z 12.5N 108.6W 65 KT 75 MPH 24H 25/1800Z 13.5N 110.1W 80 KT 90 MPH 36H 26/0600Z 15.1N 110.9W 90 KT 105 MPH 48H 26/1800Z 16.9N 111.3W 90 KT 105 MPH 72H 27/1800Z 21.1N 110.3W 65 KT 75 MPH 96H 28/1800Z 25.5N 106.5W 30 KT 35 MPH...INLAND 120H 29/1800Z...DISSIPATED
$$ Forecaster Berg