SYNOPTIC DISCUSSION
MONDAY 23rd DECEMBER 2024 PM SHIFT

TONIGHT, INTO TOMORROW:
The SPCZ with associated major thunderstorms and rain bands lie further southwest of Samoa. Meanwhile, a moist northeasterly wind flow prevails over the group causing light showery activities in most locations apart from the afternoon convections mainly over highlands. Convections this afternoon were mostly towering cumulus along the southern coast of Upolu due to sea breeze and central Savai'i due to the orographic effect as mountains are higher thus air parcels were lifted to condensation level. The dominance of a ridge on the mid-level and polar southerly airmass on the uppers led to these suppressing activities. Surface winds are expected to become more northerly by tomorrow and thus more moisture advection that may trigger afternoon thunderstorms and therefore brief heavy falls. This situation keeps the risk of flooding likely in rivers and vulnerable low-lying areas leading to the extension of the heavy rain and flood advisories. However, for tonight anticipate brief showers and a chance of thunderstorms in some areas. But we’ll continue to monitor this situation and update accordingly.

WEDNESDAY THROUGH SUNDAY:
The ridge of high pressure will remain over Samoa through Thursday, thus keeping the major convective activities to the south of the group. This high-pressure ridge extends through the mid-levels with the strong southerly polar descending airmass at the uppers, which would lead to suppressing activities. However, afternoon thunderstorms are still possible due to moist northerly wind flow which would bring moderate falls, particularly in central and southern areas of both main islands. By Friday, the ridge is expected to drift northward resulting in major convections currently to the south, to move over Samoa and landfall late Saturday through Sunday. For such reason, expect scattered showers with heavy falls possible for the later days of the outlook period.

MARINE OUTLOOK:
Current marine observations indicate waves and swell heights fluctuating below the advisory threshold of 2.5 metres. Global Wave models predict a low chance of wave activities to reach 2.5 metres throughout the outlook period over the marine waters of Samoa.

ADVISORIES/WARNINGS:
VALID: HEAVY RAIN ADVISORY
VALID: FLOOD ADVISORY

Weather Duty Team:
F. Kalapani, G. Niko & A. Uili