SYNOPTIC DISCUSSION

TUESDAY 20th MAY 2025 PM SHIFT.

TONIGHT INTO TOMORROW:

A detached area of high pressure located south of Niue, which sustains the moderate southeasterly wind flow prevailing over the area of concern. Early in the afternoon, the island chain experienced partly cloudiness, primarily over highlands because of surface heating and orographic lifting. Yet, the satellite images captured low-to-mid-level clouds at elevated locations. Evidently, both the integrated water vapor and sounding profile show a slight change of moisture content within the atmosphere. In addition, the sounding data again illustrates an inversion layer prohibiting vertical cloud growth extending to the upper atmosphere. On that note, the upper trough of low pressure drifts eastward while the left flank of the upper system is characterized as the upstream directing pole descending airflow. As a result, it forms an upper convergence with the persistence of the unfavorable wind shear supporting the dominant surface and thus suppressing convective clouds over Samoa for tonight.  

Tomorrow, the prevailing moderate southeasterly wind flow will slowly degrade since the area of high pressure located south of Niue relaxes. So, expect the wind speed to become light while remaining persistent. Unfavorable wind shear remains over the area of concern as the upper trough of low pressure projects eastward. So anticipate shallow showery activities along the dominant wind flow and possible thunderstorms.  

THURSDAY THROUGH TUESDAY:

The global weather model implies an accumulated amount of rainfall on Thursday to Sunday. Another area of high pressure lies west of New Zealand and is expected to steadily intensify and generate a convergence zone on Thursday. Simultaneously, it will withhold the trough of low pressure across Tokelau until it descends over Samoa on Saturday. Concurrently, a pronounced upper diffluence will support and enhance the predicted surface systems. Later in the outlook period, an approached upper trough of pressure from the west may influence the evolution of the surface trough of low pressure into a weak circulation. Henceforth, an issued Severe Weather Information (SWI) will disseminate for these expected weather conditions in the next 12-24 hours. 

MARINE OUTLOOK:

The most recent marine observational data still indicate wave activities across Samoa's marine areas fluctuate below 2.5 meters, and such wave trend will likely to extend through Thursday. However, the global wave model  on Friday, another high-pressure system is expected to form west of New Zealand, potentially generating wind waves.

A thorough evaluation is recommended for these expected wave conditions, then advise accordingly for proper actions. 

ADVISORIES/WARNINGS:

NONE

Weather On-Duty Team:

A. Liu, L. I. Tuatipi & P. Huch.