Area Forecast Discussion

Marianas Update

Satellite shows deep convection over Guam and partly to mostly cloudy skies across the CNMI. Scattered to numerous showers are seen across the region. Altimetry shows combined seas are currently between 4 and 6 feet.

The forecast remains complicated for the Marianas over the next day or two. A strong Mid-Level Circulation (MLC) associated with Invest 90W is producing widespread showers and isolated thunderstorms just east of the coastal waters and scattered to numerous showers within the coastal waters. Models have performed undesirably with this persistent convection and this forecast relies heavily on satellite trends. PoPs were increased significantly across the Marianas with numerous showers expected across the region today. There is some uncertainty in how long the MLC will remain convectively active. We are approaching diurnal maximum (DMAX), which is when the difference in air temperature and water temperature is greatest. This helps convection remain intense in the early morning hours. Once we move beyond DMAX, then convection needs some sort of feedback look to remain intense. We see this feedback loop develop within tropical cyclones, but MLCs separated from the main system (90W) typically collapse after DMAX. In this case, there are some signs that the MLC has begun to decay, however, convection has increased along the western flank, this puts the Marianas in the crosshairs for some heavy showers today and tonight. This forecast will likely need to be updated if convection begins to collapse today.

The only change to the marine forecast was a slight bump in combined seas from 3 to 5 feet to 4 to 6 feet based on altimetry. Seas look to remain benign except for some choppy seas near heavier showers.

Tropical Systems Update

Joint Typhoon Warning Center's Invest 90W is located near 14N151E and is still rated as medium. 90W continues to remain poorly organized as the system remains stretched with a broad circulation. Convection has increased over night and evidence points towards the eastern side of the circulation beginning to take over. At this time direct impacts to the Marianas are not expected, but heavy rainfall from displaced convection will be possible today and tonight.

For more information on Invest 90W, see bulletins issued by the Joint Typhoon Warning Center under WMO header ABPW10 PGTW.

Eastern Micronesia Update

Satellite imagery shows isolated showers and thunderstorms across Pohnpei and isolated showers across Kosrae and Majuro. Altimetry shows combined seas between 5 and 7 feet.

No changes were made with this forecast package. Isolated showers are expected today across eastern Micronesia. Latest satellite trends show an isolated thunderstorm in the western coastal waters of Pohnpei. Trends will need to be watched this morning for a potential increase in shower coverage.

No changes were made to the marine forecast. Combined seas are currently between 5 and 7 feet and should decrease to 4 to 6 feet by tonight.

Western Micronesia Update

Satellite imagery shows scattered showers near Palau and isolated showers across Yap and Chuuk. Combined seas are between 4 and 7 feet across Palau and Yap, and 6 to 8 feet across Chuuk.

Overall, no changes were made with this forecast. Satellite shows a slight break in convection across Palau, but showers and thunderstorms should build this morning. Showers are expected to build over Yap today and especially tonight as 90W moves northward.

The Small Craft Advisory and High Surf Advisory remain in effect for Chuuk. Winds are expected to fall today and conditions may fall below hazardous levels for seas and surf late this afternoon. Choppy seas will be possible across Palau near heavier showers while benign conditions are expected for Yap.

Prev discussion

/issued 630 PM ChST Mon Sep 14 2026/

Marianas Synopsis... Visible satellite imagery depicts mostly cloudy skies and scattered showers. Altimetry indicates combined seas of 3 to 5 ft.

Discussion... A surge in moisture has moved over the region which can be attributed to Invest 90W, the monsoon trough, and a surface trough. Model guidance indicates showers to continue over the region with a heavier concentration over Saipan and Tinian tonight and Tuesday, so the forecast zones were split to account for this spatial and slight coverage difference from Rota and Guam. Then, depending on Invest 90W's evolution (discussed in the Tropical Systems section below), a dry spell may occur over the region should 90W track northwest toward the northern CNMI. The monsoon trough will follow, bringing increasing showers and winds possibly this weekend.

Marine... Combined seas of 3 to 5 ft will slowly build, possibly peaking up to 7 ft around the latter half of the week. There is a moderate risk of rip currents along north- and east-facing reefs at least through Tuesday night. The primary northeast trade swell will persist, potentially building by a foot for Saipan and Tinian should 90W pass through the northern CNMI. This will also increase the period of said swell, so surf along north- and east-facing reefs may build as well. Then, a secondary southeast swell will reach the region due to the monsoonal inflow into 90W. This swell may eventually build and shift to the southwest by the end of the week should the monsoon trough shift north over the region.

Tropical Systems... A developing disturbance, Invest 90W, is centered east of Guam along the monsoon trough near 13N154E. Invest 90W is currently moving slowly northwest and has become a bit more organized over the last 24 hours. 90W is now rated medium due to favorable conditions for development. Medium indicates that development is likely, but not within the next 24 hours. It is still difficult to identify a good Low-Level Circulation Center (LLCC), however, satellite shows that a weak, poorly defined circulation has slowly consolidated over the past 12 hours. Both deterministic and ensemble model guidance agree with a slow development over the next 1 to 2 days as it tracks northwest toward the northern CNMI. Invest 90W will be monitored closely over the next few days.

For more information on Invest 90W, see bulletins issued by the Joint Typhoon Warning Center under WMO header ABPW10 PGTW.

Eastern Micronesia... A very dry pattern is now in place across eastern Micronesia as Invest 90W moves more northwest, slowly pulling the monsoon trough along with it. This dry pattern looks to continue for the next several days for Pohnpei and Kosrae, with only a brief increase in POPs (Probability of Precipitation) to around 30 percent on Thursday, with the dry pattern returning Friday into the weekend. For Majuro, there will be periods of increased showers as the monsoon trough continues to affect the Marshalls, with weak transient circulations passing through and enhancing convection at times.

For the marine forecast, not much of a change. Seas are expected to remain between 3 and 5 feet across eastern Micronesia, with mostly light to gentle winds expected through the week at Pohnpei and Majuro, and occasional moderate winds midweek at Kosrae.

Western Micronesia... Weather in Western Micronesia this week will continue to be dominated by the monsoon trough. Koror Palau will be getting some much needed rainfall with scattered showers expected through Wednesday. The showers will then thin a bit for the rest of the forecast period, which extends through Saturday. Winds will be from the west or southwest at 10 to 20 knots for the entire period. Seas will be 5 to 7 feet tonight, then gradually diminish to between 3 and 5 feet by Saturday. This will make for marine conditions that could be challenging tonight and Wednesday through Friday, but are expected to stay just below Small Craft Advisory criteria.

Yap will have scattered showers for the rest of this week, with the possible exception of Wednesday night, when the showers could increase to numerous. Winds will be from the west at 5 to 15 knots most of the period, however they could increase from Wednesday night through Friday and reach 10 to 20 knots. Seas will be 5 to 7 feet through Thursday, then build a foot Friday to between 6 and 8 feet, then diminish back to between 5 and 7 feet Saturday. This will result in challenging marine conditions Wednesday night through Friday, while they will be benign through Wednesday and on Saturday.

Chuuk will have southwest winds of 15 to 25 knots through Tuesday, which makes marine conditions hazardous and has required a Small Craft Advisory through Tuesday. Also, southeast to southwest swells of 5 to 7 feet are requiring a High Surf Advisory through Tuesday, which may need to be extended further, possibly through Wednesday. Chuuk will have scattered showers tonight and Tuesday night, with isolated on Tuesday. Isolated showers will then dominate Wednesday through Friday, then weather could moisten again Saturday. After Tuesday, winds will be 10 to 15 knots from the southwest, with gusts to 25 knots possible Tuesday night. Seas of 6 to 8 feet tonight could build to between 7 and 9 feet Tuesday, then diminish to between 6 and 8 feet again Tuesday night. They could diminish further to between 5 and 7 feet Wednesday night and 4 to 6 feet on Thursday, then remain at that level through Saturday. This would result in marine conditions that are hazardous through at least Tuesday, challenging on Tuesday night and perhaps Wednesday, then becoming benign Wednesday night.

GUM WATCHES / WARNINGS / ADVISORIES

GU...None. MP...None. Marianas Waters...None.