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Tropical Storm Alvin Discussion Number 4

Wiki-Hurricanes Forecast Center

8:00 AM PDT Wed Jun 26 2019

Satellite imagery shows continued convective organization of Alvin, which remains a compact tropical storm this morning. The convective centroid has moved farther southwards, though the current location of the actual low-level center is ambiguous at this time and there is a possibility that the mid-level circulation has gone rogue. An SSMIS microwave pass at 1109z showed a tightly wound central curved band with secondary rainbands developing north of the center, though without consistent scatterometer profiles it is difficult to determine whether these are associated with a tilted mid-level circulation or rooted to the low-level center. The earlier GMI pass at 0841z was more indicative of a tilted circulation, but the storm was less organized as a whole then. Regardless, active convective growth continues throughout the circulation this morning, and if anything the surface circulation will gravitate towards the convection. A -75C central dense overcast also appears to be developing. SAB continues to assess T2.5/35kt while CIMSS-ADT yielded higher at T2.9/43kt, though recent ADT assessments have been ambiguous due to the uncertainty of the storm's positioning. Based on fairly low temperature-derived pressures from microwave analyses and the small size of the system, I have elected to raise the intensity of the storm further to 40 kt, despite rather meager analyses of ASCAT-C and ScatSat-1 passes from about nine hours prior.

Conditions presently remain favorable for intensification as Alvin tracks towards the west beneath a large anticyclone. This general motion should continue throughout the forecast window as upper-level ridging remains in place over the Mexican Plateau. Track guidance has shifted slightly westward with the latest early-cycle package and the latest motion favors the more zonal solutions, though model consensus remains north of the ECMWF-IFS ensemble plume. A stronger Alvin in the short-term would provide a more poleward trajectory, and HWRF is quite aggressive in this respect in showing a hurricane. This is the highest end of intensity guidance. In any case, supportive ocean waters are expected to remain available to Alvin for at least the next 36 or so hours before the storm reaches the East Pacific surface thermocline where conditions are much more stable and dry. Alvin is expected to intensify given the soundness of short-term parameters and coherence of vorticity seen on MIMIC-TPW imagery, though a rapid demise is forecast after two days once Alvin reaches cooler waters. If the system is able to gain vertical coherence today, a quick rise in strength may be in short order.

FORECAST MAX WINDS

INIT 26/1500Z 40 KT 45 MPH

12H 27/0000Z 45 KT 50 MPH

24H 27/1200Z 45 KT 50 MPH

36H 28/0000Z 40 KT 45 MPH

48H 28/1200Z 30 KT 35 MPH

72H 29/1200Z 20 KT 25 MPH...POST-TROP/REMNT LOW

96H 30/1200Z...DISSIPATED

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